Introduction
Coal mining practices
Coal mining can occur through surface methods, known as opencast mining, or underground techniques, depending on the depth of the coal seams Globally, about 60% of coal production comes from underground mines; however, in the United States and Australia—ranked as the second and fourth largest coal producers—over 65% of coal is extracted through surface mining.
Surface mining is viable when coal is relatively near to the surface, typically less than 100 m
Opencast mining involves the removal of overburden soil and rock using large draglines or shovels and trucks, reaching depths of up to 350 feet The exposed coal is then drilled, fractured, and excavated in strips, with the coal transported to preparation facilities via trucks or conveyors This method allows for the extraction of coal seams as thin as 100 mm (4 in) while recovering over 90% of the coal deposit Opencast mines can span extensive areas, covering many square kilometers.
Underground mining is carried out by two principle methods: longwall mining, and room and pillar mining Almost all modern, high-production mines use a retreat longwall method of mining
Longwall mining is a method used to extract coal from extensive panels within a targeted seam This process employs continuous miners that create parallel tunnels, known as entries or gates, extending from the main mine entries into the seam The longwall panel is finalized with a connecting tunnel between the gates, which serves as the working face In optimal geological conditions found in the U.S., longwall panels can reach widths of approximately 440 meters (1,450 feet) and lengths of about 3,960 meters (13,000 feet) [Karacan et al, 2007].
A mechanical shearer, equipped on self-advancing, hydraulically powered ceiling supports, efficiently shears coal from the longwall face in multiple passes The extracted coal then falls onto a conveyor for transport to the surface As the shearer advances to cut the next section, the ceiling supports move forward, causing the roof behind to collapse and create the gob, also referred to as goaf This method of mining, which involves retreating towards the main entries, is known as retreat mining.
‘retreat longwall mining’ and over 75% of the coal in the deposit can be extracted with this method
Room and pillar mining is an effective method for extracting coal at shallower depths and in areas with complex geological formations that are unsuitable for longwall mining This technique involves the use of a continuous miner to create a network of rectangular 'rooms' within the coal seam, allowing for the recovery of up to 60% of the coal The remaining 40% is left as 'pillars' that provide structural support for the mined-out areas, and these pillars can be extracted during the final phase of the mining process.
Methane generation, retention and migration in coal
This section briefly summarizes the key factors that influence methane’s formation and movement through coal seams
Coal formation and methane generation
Coal seams are created over millions of years from decaying plant material in swampy environments, forming peat As this peat is buried under sediments, heat and pressure drive out gases like water, oxygen, and carbon dioxide, ultimately increasing its carbon content and transforming it into coal During this coalification process, significant amounts of methane are produced, with much of it escaping at shallow depths, while deeper layers retain the methane due to higher pressure.
Cleat is a coal miners’ term for the natural system of vertical fractures generated by local tectonic forces, and shrinkage of the source plant material, during the coalification process
The primary fracture orientation in coal is known as the "face cleat," while the secondary, perpendicular fractures are referred to as "butt cleats." Face cleats can range in spacing from one-tenth of an inch to several inches apart, serving as crucial channels for the migration of methane from the coal.
Methane is stored mainly in the matrix of the coal and partly in the fracture spaces (cleat)
Matrix porosity largely determines the ability of coal to retain methane [Steidl, 1996]
Methane molecules are densely packed in a monolayer on the extensive internal surface area of coal, held in place by hydrostatic pressure Remarkably, one cubic foot of coal can store six to seven times the volume of natural gas found in a cubic foot of conventional sandstone reservoirs.
Reducing the hydrostatic pressure in coal, whether through mining or drainage boreholes, causes methane to desorb from the coal's micropores This released methane then diffuses through the coal matrix and flows through the cleats, facilitating its movement.
Exhibit 3: Methane migration in coal
Emissions of methane in coal mines
Methane release in coal seams is primarily influenced by the mining method, gas location, and the permeability of geological materials In room-and-pillar mining, methane is released from the coal during the development of entries and crosscuts, and additionally from the roof and floor during pillar recovery as the overlying strata subsides Similarly, in longwall mining, the geometry of the panels plays a significant role in affecting methane emissions.
In longwall mining, methane emissions occur from the longwall face and the coal being transported to the surface The lower pressures in the mining area facilitate the migration of gas from the surrounding strata into the mine Additionally, a significant source of emissions arises from the gob, which is created when the overlying strata collapse into the void left by the mining process.
Methane emissions in mines typically occur at a consistent rate; however, geological discontinuities like faults, clay veins, and igneous intrusions, along with features such as floor feeders, sandstone paleochannels, and localized folding, can lead to sudden and potentially hazardous spikes in emissions.
Control by ventilation
All the major coal-producing countries mandate maximum methane concentrations of 1.0
Coal mine operators manage methane emissions, which can reach levels of 1.25% at the coal face and within mine workings, by utilizing large fans to circulate substantial volumes of air throughout the mine This ventilation process effectively dilutes methane concentrations, allowing the gas to be transported to the surface through bleeder entries and ventilation shafts.
Exhibit 4: Typical ventilation configuration at a U.S longwall mine
Overview of methane drainage practices
This report categorizes methane drainage techniques into two primary groups: those aimed at reducing gas content in coal seams before and during mining, and those focused on minimizing gob gas volume entering mine workings during and after mining Furthermore, these techniques can be divided into surface-originating methods and those that originate from within the mine itself.
Techniques that reduce coal seam gas content include:
Vertical boreholes drilled from the surface
In-seam boreholes drilled from within the mine – “short hole” and “long hole”
Superjacent boreholes drilled directionally from within the mine
Horizontal in-seam boreholes drilled directionally from the surface Chapter 2 describes each of these techniques to reduce coal seam gas content
Techniques capturing gob gas include:
Cross-measure boreholes Chapter 3 summarizes each of the technologies that capture gob gas
To effectively reduce emissions from coal seams before mining, a combination of degasification methods is employed The design of the methane drainage system must consider factors such as the volume of methane generated, the geology of the coal seam and surrounding strata, emission patterns, mining-related costs, and the potential for market income from the captured gas Continuous adjustments to the drainage system design may be necessary to optimize methane capture as the mine evolves.
CMM drainage techniques which reduce in-situ gas content
Vertical wells
A vertical well refers to a well drilled from the surface through targeted coal seams, which is then cased and hydraulically fractured to maximize methane pre-drainage before mining These wells are typically operational between 2 to 10 years prior to the commencement of mining activities.
To facilitate the extraction of methane from coal seams, it is essential to remove water, which reduces hydrostatic pressure and enables methane to desorb from the coal matrix and flow through the cleat system to the well The extracted water is separated from the produced gas and subsequently treated or disposed of in an environmentally responsible manner Near the well head, the gas undergoes separation to eliminate any remaining water before it is transported to a processing facility for compression and dehydration, ultimately being fed into commercial pipelines.
Vertical wells provide a significant advantage in pre-mining drainage by effectively draining multiple coal seams at once When conditions are optimal, these wells can generate pipeline-quality gas with minimal processing, producing sufficient quantities to ensure economic viability.
Exhibit 5: Typical vertical well setup 1
In the major coal bed methane basins across the U.S., vertical wells are employed to extract methane from un-mined coalbeds, independent of coal mining activities Additionally, six out of the twenty-three most gassy underground mines in the country utilize vertical wells for pre-mining degasification projects, particularly in Alabama and Virginia, as reported by the USEPA in 2008.
Vertical wells face challenges globally due to less permeable, deeper, and more geologically complex coal seams, resulting in increased drilling costs and reduced hydraulic fracturing success According to Thakur (2006), the costs for drilling, completion, and fracturing in Europe and Australia are three times higher than in the U.S., compounded by elevated permitting and site preparation expenses Additionally, high population density and environmental concerns complicate the identification of suitable surface drilling sites in many coal mining regions worldwide Furthermore, the scarcity of appropriate drilling, completion, and fracturing equipment in several potential coal mine methane (CMM) areas significantly impedes the success of vertical methane drainage wells.
1 Source: Hartman et al., 1997 Copyright 1997, John Wiley & Sons, Inc Reprinted with permission of John Wiley & Sons, Inc
A comprehensive analysis of coal geology, utilizing coal thickness and structure maps, is essential for strategically planning pre-mining drainage well locations An optimized well pattern considers various factors, including the mine development plan, the timeline for mining operations, reservoir characteristics, completion effectiveness, well stimulation impacts, and associated drilling, completion, and operational costs Ultimately, the finalized well pattern represents a balance between the ideal theoretical approach and the practical economic and technical constraints.
A study by Zuber, Kuuskraa, and Sawyer (1990) investigated the optimal spacing of vertical wells in the Oak Grove field of Alabama, revealing that well spacing between 40 to 160 acres is ideal, particularly with closer spacing benefiting lower permeabilities under 10.0 md In the Alabama coal fields, the standard well spacing is approximately 40 acres, with 3 to 7 wells typically situated in each longwall panel These findings align with similar conclusions drawn by Richardson, Sparks, and Burdett (1991).
Spacing of vertical wells in CBM projects tends to be larger than spacing for CMM projects
Vertical CBM wells in the San Juan Basin are commonly drilled on a spacing of 160 to 320 acres, with 160 acres being the standard in the Unita and Raton Basins.
In the Arkoma and Powder River Basins, the standard spacing for wells is typically 80 acres Operators of CMM vertical wells face the challenge of balancing economic viability with the goal of minimizing methane content in the target coal seam When the time available before mining is limited, closer well spacing is necessary to expedite gas drainage; however, this approach results in a higher number of wells and increased overall project costs.
Vertical wells drilled into untouched coal seams typically yield significant water quantities and minimal methane during the initial months of operation However, as water extraction continues and the pressure within the coal seam decreases, methane production subsequently rises.
Vertical wells are strategically arranged in a regular grid pattern to optimize drainage overlap, thereby improving the dewatering process and lowering hydrostatic pressure in coal seams Adjustments to this grid may be necessary to address site-specific challenges posed by surface topography or nearby habitation.
CNX Gas Corporation is addressing the challenge of optimizing well spacing, timing before mining, and associated costs at the Buchanan and VP 8 mines in Virginia's Oakwood coal field They have chosen a proactive approach with a three to five-year advance degasification program that aligns with Consol Energy's mining plans while mitigating investment risks in a vertical pre-mine drainage system The company has implemented a 40-acre well spacing in the Oakwood field and 60-acre spacing in the Middle Ridge field, and is currently analyzing data from 53 wells drilled at 30-acre spacing.
2007 The viability of drilling on 20 acre spacing is also being investigated [CNX, 2007]
After drilling a well bore, it is essential to complete the process by lining the hole with steel casing This crucial step prevents the well bore from collapsing and effectively seals it against potential water intrusion.
Completions are broadly classified as either "open hole" or "cased hole"
An open-hole completion is the simplest form of wellbore completion, involving drilling through the target coal seam and casing only to a point just above it This method is applicable when the uncased wellbore wall is composed of stable geological formations that are not prone to collapse.
A vertical CMM drainage well is drilled through the target coal seam and cased with steel pipe, incorporating a fiberglass joint to ensure borehole stability during stimulation This fiberglass section allows for safe mining through the seam when production reaches the wellbore Following the mining process, the vertical well may continue to effectively drain methane, functioning as a gob web.
Final casing sizes and completion type depends on a number of factors including the following:
Depth of the targeted coals
Number of seams to be stimulated
Maximum water production required to dewater the coals
Reservoir pressure of each drained coal seam
Horizontal in-seam boreholes
Coal seams typically appear as extensive horizontal beds, with a thickness ranging from 1 to 10 meters while spanning thousands of meters in area Consequently, vertical wells only access a limited portion of these seams, necessitating hydraulic fracturing to improve horizontal permeability and facilitate gas flow to the borehole.
An effective pre-mining drainage technique involves drilling horizontal boreholes, extending up to 1,525 meters (5,000 feet) within the coal seam This method significantly enhances the volume of coal impacted by the drainage borehole, thereby minimizing or potentially eliminating the necessity for hydraulic fracturing of the well.
Boreholes can be drilled into coal seams either directly from mine workings or from the surface, using a method that allows for horizontal drilling through the coal By employing directional drilling techniques, these horizontal boreholes can be strategically placed to intersect the face cleats of the coal seam at optimal angles, enhancing methane drainage efficiency.
The main types of in-seam boreholes described in the following sections are as follows:
“Short holes” – typically drilled parallel to the face of a longwall panel
“Long holes” – drilled longitudinally through the panel or can be drilled across multiple panels
Superjacent boreholes – used to pre-drain methane from over- and under-lying gassy strata adjacent to the target coal seam
Directional surface boreholes – start at the surface and turn through varying radii to drill horizontally through the coal
Among the twenty-three U.S gassy mines recognized by the USEPA for utilizing methane drainage systems, seven employ horizontal in-seam boreholes for methane extraction before mining operations commence In Australia, in-seam boreholes are widely implemented for methane drainage, with approximately 100 kilometers drilled annually in the coal basins of New South Wales and Queensland.
4 In reality, boreholes are never completely horizontal, as coal seams are rarely completely flat, but "dip" (slope)
Short horizontal boreholes, drilled parallel to coal seams, effectively drain methane before mining, minimizing its flow into mine workings These boreholes, typically less than 305 meters (1000 feet), can be drilled using straightforward equipment, unlike long-hole drilling that requires complex steerable systems An excellent overview of horizontal borehole drilling advancements in the U.S., where this methane drainage method is well established, is provided by Diamond (1994).
Short holes, measuring 5-8 mm (2-3 inches) in diameter and spaced 30-122 m (100-400 ft) apart, are commonly utilized in longwall panels, drilled to within 15 m (50 ft) of the panel's opposite side To optimize drainage time from the future panel and minimize methane flow into adjacent development entries during mining, boreholes are typically drilled from tail gate entries, as indicated in Exhibit 9.
Exhibit 9: Schematic plan view of short horizontal boreholes in longwall panels
Coal permeability, gas content, time to mining, and drilling economics significantly influence borehole spacing decisions Closer borehole spacing is often required for effective degassing of longwall panels, particularly when there is limited time before mining, high seam gas content, or low permeability A study by Aul & Ray (1991) in Virginia's Pocahontas #3 seam demonstrated that short-hole boreholes could extract 30% of in-situ gas in under two months and 80% within ten months, leading to a 79% reduction in ventilation air volume Additionally, gas produced from horizontal boreholes is generally of high quality, suitable for pipeline use The typical cost for drilling boreholes using a rotary drill, including utilities and logistical support, ranges from $50 to $65 per meter ($15 to $20 per foot).
At the Blue Creek mines in Alabama, Jim Walters Resources reports producing 22.6 MMcm/year (800 MMcf/year) from short, across panel, in-seam degasification boreholes [JWR, 2008]
Long in-seam boreholes, drilled from existing mine entries into target coal seams (Exhibit
Drilling long holes with down-hole motors at least twelve months prior to mining can greatly decrease the in-situ gas content of coal This directionally drilled method allows for the effective degassing of longwall panels well in advance of mining operations and facilitates the drainage of methane from coal near development entries during the mining process.
These "shielding" boreholes reduce the volumes of methane entering the development entry
Exhibit 10: Longhole drilling from within a mine entry
Positioning – Advances in drilling technology over the last decade allow long boreholes over
Recent advancements in drilling technology have enabled accurate and rapid drilling at depths of 1525 m (5000 ft) within coal seams The combination of stronger drilling equipment and real-time drill bit navigation has achieved impressive drilling accuracies of +/- 8 m (26 ft) over distances of 915 m (3,000 ft) These innovations have not only reduced the costs associated with directional drilling but have also expanded the potential for utilizing this technique in Coal Mine Methane (CMM) drainage.
A study by the National Institute for Occupational Safety and Health (NIOSH) utilized a three-dimensional numerical simulator to analyze the effectiveness of various in-seam borehole layouts for methane drainage The findings revealed that dual and trilateral boreholes (layouts A and B) significantly reduce emissions and provide better protection for entries compared to shorter, cross-panel boreholes (layouts C and D) Specifically, the tri-lateral layout demonstrated a 38.6% reduction in methane emissions over 12 months, while the cross-panel boreholes achieved only a 23% reduction [Karacan et al, 2007].
Exhibit 11: Plan view of horizontal methane drainage borehole patterns modeled for degasification of a longwall panel (not to scale)
Layout A, as depicted in Exhibit 11, represents a widely used in-seam borehole configuration in the U.S In this method, boreholes are initiated from the tailgate side of a longwall panel and drilled parallel to the tailgate entry A secondary borehole extends from the first, traversing the panel and aligning parallel to the headgate entry This approach effectively shields the development entries while simultaneously facilitating the drainage of the panel [Karacan et al, 2007].
Long in-seam boreholes can be drilled into future longwall panels well in advance of mining operations When these boreholes are directionally drilled, they can be strategically placed in coal seams that are already being degassed by existing vertical surface wells.
Exhibit 12: Schematic plan view showing in-fill drilling of in-seam boreholes between hydraulically stimulated vertical wells
CNX Gas has successfully drilled thirteen in-seam long holes at their southern Virginia mining operations, with the longest measuring 1,569 meters (5,148 feet) These directionally drilled boreholes were strategically placed into untouched coal sections that were already being drained by hydraulically fractured vertical wells, ensuring they were positioned perpendicularly to future longwall panels while avoiding major fracture zones This approach mitigated potential stability and fluid circulation issues associated with hydraulic fractures during drilling The total drilled distance, including sidetracks, reached 22,960 meters (75,327 feet), resulting in the production of 31 million cubic meters (1.1 billion cubic feet) of methane, all without adversely affecting the output of the vertical wells.
Long-hole degasification significantly enhances mine production by reducing gas content and associated costs According to Brunner and Schwoebel (2005), a shielding borehole measuring 885 m (2,900 ft) at a mine in northern Mexico decreased methane emissions into nearby development entries by 30% within two months This reduction led to a 30% decrease in ventilation needs and an impressive 78% increase in mining advance rates.
Horizontal in-seam boreholes can effectively drain up to 50% of in-situ gas from high permeability coals in the U.S before mining operations commence However, the total gas drainage is constrained by the available time for degasification Once the development entries are finished and the longwall panel is prepared for extraction, usually within six months to a year, the shielding boreholes will be mined through.
Specialist, in-mine drilling contractors report that long, directionally drilled boreholes cost
$65-100 per meter ($20-30 per foot) A 1,370 m (4,500 ft) shielding borehole would cost approximately $150,000 including wellhead and mine staff support costs
Surface-drilled directional boreholes
Surface-drilled directional boreholes have been a fundamental technique in conventional oil and gas drilling for decades The drilling process begins like that of a vertical well, but at a specific "kick-off" point (KOP), the trajectory is intentionally deviated from vertical This deviation creates an arc that allows the well bore to align closely with the target formation's bedding plane The effectiveness of surface-drilled directional holes is characterized by the radius of their turn from the vertical.
Zero 0 3 / 10 Telescopic probe with hydraulic jet
Ultra-short 0.3-0.6 / 1-2 60 / 200 Coiled tubing with hydraulic jet
Short 1-12 / 3-40 460 / 1,500 Curved drilling guide with flexible drill pipe; entire drill string rotated from the surface
Steerable mud motor used with compressive drill pipe; conventional drilling technology can also be used
Conventional directional drilling equipment used; very long curve length of 850-1,350 m (2,800-4,400 ft) needed to be drilled before achieving horizontal
Exhibit 14: Surface-drilled directional oil & gas well types defined by radius size
Surface directional drilling in the CBM and CMM industries effectively combines the advantages of vertical and horizontal in-seam drilling This method is safer than in-mine drilling, does not disrupt mining operations, and allows for pre-mining drilling A long horizontal borehole can access a larger volume of the coal seam, often eliminating the need for hydraulic fracturing and enabling precise control over the borehole trajectory to optimize coal seam permeability Additionally, a single surface site can drain an area of 2.6 km² (640 acres), replacing the need for 16 vertical wells on a much smaller 0.16 km² (40 acres) spacing This approach significantly reduces the environmental impact of methane drainage projects while also lowering drilling, infrastructure, and maintenance costs.
Medium radius boreholes are the predominant type of surface directional boreholes used for methane drainage from coal seams, particularly in the U.S and Australia Over the past decade, advancements in this technique have led to its increased adoption within the Coal Bed Methane (CBM) and Coal Mine Methane (CMM) sectors Initial attempts at horizontal drainage boreholes faced challenges with produced water removal due to the wellbore's curved configuration, which complicated conventional pumping methods and made more intricate solutions cost-prohibitive To address these issues, U.S and Australian drilling companies developed innovative directional drilling techniques that enable horizontal wells to intersect existing vertical wells that extract gas and water.
In Australia, the technique known as surface to in-seam drilling (SIS) involves directionally drilling multiple boreholes into the same vertical well This method typically drains a single coal seam but can also target various coal seams located at different depths.
Exhibit 15: Schematic of multiple horizontal wells drilled to a single vertical well
A magnetic guidance tool is utilized to direct horizontal drillers to intersect production wells by being lowered down the vertical well to the target coal seam In Australia, traditional directional drilling methods using standard oil field equipment have proven too costly for shallow, low-producing coal seams To address this, Australian drilling companies have adopted small, modified mineral drill rigs and implemented "slant-hole" drilling techniques This method involves drilling from the surface at angles between 60-90 degrees, which minimizes the angle needed to transition to horizontal drilling and enables targeting of shallower coal seams more efficiently than conventional vertical drilling.
In the U.S., vertical production wells are strategically located near the transition point of horizontal boreholes, which can extend up to 1,525 meters (5,000 feet) These horizontal boreholes intersect with the vertical well or its lateral extensions, allowing for the drilling of additional lateral holes in various configurations to enhance the area of coal drainage The design of these laterals ensures that produced water efficiently drains to the vertical well for surface pumping.
CDX Gas in the U.S employs a dual well technique to create laterals in a unique "pinnate" drainage pattern, as illustrated in Exhibit 18 This method allows for the drilling of four sets of main laterals along with side laterals, resulting in a comprehensive 360-degree drainage pattern capable of efficiently draining 1,280 acres and replacing 16 existing wells.
Exhibit 18: Top view of CDX Pinnate drainage pattern
CDX Gas (2005) highlights the effectiveness of standard 80-acre drilling locations in the Appalachian and Arkoma Basins, where successful pinnate configurations have demonstrated impressive initial production rates These configurations efficiently dewater coal, achieving a remarkable drainage of 80-90% of in-situ methane within just two to three years.
Maricic et al (2005) found that the ideal configuration for multi-lateral drainage patterns can be optimized by analyzing total horizontal length, lateral spacing, and the number of laterals While longer horizontal lengths enhance contact with coal seams and boost gas recovery yields, they also elevate drilling costs and risks Consequently, operators often favor a straightforward three to four lateral pattern for each horizontal well to strike a balance between efficiency and cost-effectiveness.
In 2007 and 2008, CNX Gas drilled 176 horizontal wells at its Mountaineer CBM field, targeting the Freeport coal seam in southern Pennsylvania and northern West Virginia, with depths averaging 180-240 meters (600-800 feet) The company improved its drilling technique by transitioning from a simple three-lateral design covering 2.6 km² (640 acres) to an asymmetrical quad design, known as the "turkey foot," which enhanced methane drainage and reduced well spacing to 1.9 km² (480 acres) This innovation decreased drilling times from 21 to 15 days, significantly boosting well economics Additionally, employing a gamma detector near the drill bit allowed for more precise steering of the horizontal borehole in the coal, further cutting drilling times to just 10 days.
One of the first wells brought on line produced at 25 Mcmd (900 Mcfd) [CNX, 2008]
To maintain borehole stability and prevent shallow water ingress, wells are cased in their vertical sections and, in some instances, along their arcs.
To prevent borehole collapse, main laterals can be equipped with slotted pipes During the directional drilling of horizontal laterals, if the wellbore intersects the coal seam's roof or floor, the drill string can be retracted, allowing drilling to continue at an angle away from the coal boundary This technique, known as "sidetracking," ensures that the lateral remains entirely within the coal seam.
Surface-drilled horizontal borehole techniques are rarely used outside the U.S and Australia due to lower coal permeabilities and more complex geological conditions in other countries However, China has made notable progress, having drilled twenty-five multi-branch horizontal wells by 2007.
2.3.2 Gas content reduction and production
Horizontal wells drilled into high permeability coals prior to mining can effectively drain over 80% of in-situ methane, achieving drainage efficiencies comparable to vertical wells but with higher production rates These wells extract gas from untouched coal seams without contamination from mine ventilation air, and after processing to eliminate excess carbon dioxide, nitrogen, or water, the gas typically meets the quality standards for injection into commercial pipelines.
Target Drilling's report indicates an average initial gas production of 18-21 Mcmd (650-750 Mcfd) from over 1220 meters (4000 feet) horizontal wells in Pennsylvania, which feature relatively high cleat permeability After two years, the production remains steady at 11 Mcmd (400 Mcfd).
Water disposal
Pre-mining drainage of coal mine methane (CMM) typically entails extracting water from the coal seam to reduce reservoir pressure, facilitating methane desorption and flow to the surface through wellbores The volume of water extracted varies across global coal basins, influenced by factors such as reservoir thickness, porosity, permeability, well spacing, pumping rates, and the proximity to aquiferous sandstones or meteoric recharge sources.
In the United States, the average daily water production from coalbed methane (CBM) wells ranges from 2 to 5 cubic meters (17 to 42 barrels) up to over 60 cubic meters (500 barrels) per day Large-scale coal mine methane (CMM) drainage projects can generate significant total production, necessitating careful management to comply with local environmental regulations.
The quality of coal seam water differs significantly across various coal basins, with some regions producing water suitable for beneficial uses like irrigation, drinking, and industrial applications However, in areas with poor quality water, high salt concentrations—up to five times that of seawater—necessitate extensive treatment before the water can be utilized or require reinjection into appropriate aquifers for disposal.
Currently, there is no proven technology available that can effectively reduce water production without negatively impacting gas production rates As a result, mitigation strategies have primarily concentrated on either disposing of produced water through underground injection or surface evaporation, or treating the produced water at the surface for disposal or potential reuse.
In the U.S., the disposal of produced water from coalbed methane (CBM) and coal mine methane (CMM) operations involves various methods tailored to specific conditions The choice of disposal technique is influenced by factors such as water volume, salinity, chemical composition, local climate, surface drainage, and environmental regulations As a result, water disposal technology is highly site-specific and must be customized for each application.
The most commonly used water disposal options include:
Shallow and deep re-injection
Active treatment using Reverse Osmosis (RO)
Surface discharge - Surface discharge is the least expensive of the water disposal options
Surface discharged water can be utilized for crop irrigation or livestock watering, contingent on its quality However, these uses are typically secondary to more beneficial applications at mining sites or in industrial processes that do not require potable water Such applications encompass ore washing, cooling for power plants, drilling or fracturing fluids, and dust suppression Depending on the intended use, some level of water treatment may be necessary.
Disposing of produced water through evaporation ponds is a straightforward process that involves creating and maintaining a shallow, impermeable pond with a large surface area Produced water is introduced into the pond, where it is allowed to evaporate The rate of evaporation and the salinity of the water determine how often salt deposits need to be removed In the San Juan Basin, this accumulation is roughly 5 cm every 20 years of continuous operation.
Using a pump-and-spray system in active evaporation ponds can significantly enhance evaporation rates, allowing for a reduced surface area requirement for disposing of a specific volume of water, albeit at a higher operational cost.
If produced water is of sufficient quality, impoundment ponds can also be used for beneficial uses such as fishponds, livestock and wildlife watering ponds or recreation
In the U.S., underground re-injection of water requires that the salinity of the re-injected water matches that of the aquifer it enters For instance, in the Powder River Basin of Wyoming and Montana, the produced water from coalbed methane (CBM) is relatively fresh, allowing for shallow re-injection wells that typically range from 90 to 300 meters (approximately 300 to 1,000 feet) in depth.
Downhole gas/water separation is an innovative method for water disposal that involves drilling well boreholes deeper than initially planned to inject water into a permeable layer beneath coal seams This technique utilizes a pump positioned below the coal seams to draw water down while enabling gas to flow upward to the surface Under specific conditions, downhole water separation can enhance gas flow rates and reduce transportation costs associated with water disposal, making it an economically viable solution.
Exhibit 19: Forced evaporation pond technique, however, requires an adequately permeable zone located below the coal that can take substantial volumes of fluid
Reverse osmosis (RO) is an effective method for treating brackish produced water, utilizing a permeable membrane to separate fresh water from waste brine This process can significantly reduce the salinity of the product water with each pass through the membrane, enhancing the overall efficiency of water purification.
An RO system is designed based on the specific requirements for product water chemistry, typically processing produced coal seam water to create fresh product water This process generates a small waste stream of highly saline water, which can either be injected into a conventional underground disposal well or transported to a permitted disposal site.
CMM drainage techniques which recover gob gas
Vertical gob wells
In the U.S., the primary technique for gob degasification involves drilling wells from the surface to just above the coal seam Typically, gob wells are drilled before mining operations commence, but they are activated only after the longwall mining has progressed past the wellbore, allowing the gob to form.
Methane released from fractured strata in and above the gob travels into wells and rises to the surface In the U.S., vertical gob wells effectively lower methane levels in shallow, fast-moving longwall faces, although their use is limited globally due to challenges such as deeper, less permeable coals and surface access issues, necessitating alternative gob degasification methods.
The number of vertical gob wells required on a longwall panel varies significantly, depending on factors such as mining rate, longwall length, and gas content in the caved strata Typically, the first borehole is located 50-170 m (150-500 ft) from the longwall face, and for a standard 3000 m (10,000 ft) longwall, 3-30 gob wells may be necessary for effective degasification To manage higher methane emissions during the initial stages of the longwall caving operation, a greater density of wells is often implemented at the beginning of the process.
At their Virginia operation in the U.S., CNX Gas typically drill 6-7 gob holes in the first 305 m (1000 ft) of the panel and continue on a 150 m (500 ft) spacing [CNX, 2007]
Jim Walter Resources in Alabama, typically drill 5-6 gob holes per 3,660 m (12,000 ft) longwall panel [JWR, 2008]
Research from the U.S Bureau of Mines has shown that gas production from wells situated in the tension zone along the margin of longwall panel 5 is 77 percent higher than that from wells drilled in the compression zone along the traditional centerline Operators typically choose the first gob well's location based on this tension zone advantage and analyze its production data to inform the placement of additional wells.
5 When a coal seam is mined, the overlying strata subside into the void left behind forming the gob (Exhibit 20)
Maximum subsidence is observed along the centerline of the gob, where the gob material experiences compression In contrast, the edges of the gob have their overlying strata partially supported by unmined rock, resulting in a "stretched" condition that creates tension This tension is believed to increase fracture permeability and enhance gas production, as noted by Diamond et al (1994).
Vertical gob wells are pre-drilled to a depth of 6-28 meters (20-90 feet) above the working coal seam, typically cased and cemented just above the highest coal seam or gas-bearing layer that may release gas during longwall mining The lower section of the well may remain uncased as an open hole or be fitted with slotted casing to ensure borehole stability while facilitating gas flow.
The slotted liner is suspended from the bottom of the casing rather than being cemented, which allows for flexibility in gob well completions across various coal basins in the U.S These completions are influenced by factors such as depth, expected gas and water flows, and the geomechanical properties of the overlying strata.
Exhibit 22: Profile of a typical U.S vertical gob well
Effective gob well completion requires careful consideration of several key factors, including the vertical placement of the well within the gob, maintaining well integrity and productivity post-undermining, ensuring connectivity with the fracture zone for optimal gas flow, and isolating shallow, water-bearing strata through proper well-casing cementation Significant water inflow may be unavoidable and can increase after mining due to enhanced conductivity in fractured strata, which ultimately hinders gas production.
The West Elk Mine in Colorado, operated by Mountain Coal Company, features a borehole measuring 311 mm (12 1/4 inch) in diameter, drilled to a depth of 610 m (2000 ft) After casing the hole with 244 mm (9 5/8 inch) steel from the surface, an additional 222 mm (8 3/4 inch) diameter hole was drilled for 91 m (300 ft), stopping 8 m (25 feet) above the targeted seam To preserve borehole integrity, a 178 mm (7 inch) slotted steel casing was installed, left uncemented to facilitate gas flow from the gob and surrounding fracture zone into the wellbore.
In the Pittsburgh coalbed of Pennsylvania, USA, gob wells have been drilled to depths of 9-14 m (30-46 ft) above the coal seam, where the average overburden depths range from 152-274 m (500-890 ft) These wells are completed with 178 mm (7 inch) casing and feature 61 m (200 ft) of slotted pipe, as noted by Karacan et al (2007).
Gob wells 311 mm (12 1/4 inch) in diameter have been drilled as deep as 915 m (3000 ft) in Virginia, USA, [Atlas Copco, 2007]
Non-U.S coal operators often struggle with gob well degasification due to ineffective completion practices Wells that are primarily completed in an "open hole" design and extend into the rubble zone risk encountering water issues or shearing after undermining, which can shorten their productive lifespan To enhance performance, it is crucial to properly case the well above the gob area to isolate surface water-bearing zones, thus preventing water accumulation Additionally, using slotted casing to protect the borehole in the gob area can significantly reduce the risk of shearing and extend the well's production life.
3.1.3 Gob gas production and quality
As with all gob degasification techniques, the methane quality and quantity produced from vertical gob wells vary depending upon many factors including:
Site-specific geological and reservoir characteristics,
Degasification and ventilation practices at the mine
Intrusion of mine ventilation air is common because of connectivity in the gob between the borehole and ventilation system Typical U.S gob well capture efficiencies 6 are in the 30
According to the USEPA (1999), methane capture efficiencies for gob wells can vary significantly, typically ranging around 70%, influenced by geological conditions and the number of wells in a panel However, some operators utilizing vertical gob wells in optimal geological and reservoir environments report capture efficiencies reaching as high as 80%.
Gas flow rates from the gob to the well are influenced by the permeability of the fracture zone, the natural pressure difference due to low-density methane gas ascending in the air, and the suction generated by surface vacuum pumps (exhausters).
Vertical gob wellhead operators have achieved remarkable gas production increases and reduced methane emissions in mine ventilation systems by applying minimal vacuum pressures Notably, some operators have experienced a three-fold increase in production rates by utilizing a suction pressure of just 6.9 kPa (1 psi) at gob wellheads.
Vertical gob well performance data shows that well productivity is closely tied to the dynamic formation of the gob and the amount of coal extracted In numerous U.S operations, the production rates of gob gas are influenced by the advancement rate of the longwall face Operators have observed that increasing longwall face productivity can lead to a two- to three-fold rise in gob gas production rates.
Cross-measure techniques
The cross-measure technique for longwall gob degasification is widely used in Europe and Russia, particularly in longwall mining operations involving steeply dipping coal seams deeper than 610 meters (2000 feet) While several U.S mines have successfully tested cross-measure boreholes, the preference for vertical gob holes due to their ease of use and cost-effectiveness has limited the adoption of cross-measure systems in the U.S However, in deeper, gassier mines or where vertical gob well placement is unfeasible, cross-measure systems may become a more appealing option for operators.
At the West Elk Mine in Colorado, the Mountain Coal Company conducted a comparison of methane drainage effectiveness between cross-measure (CM) holes and surface gob wells This study was crucial in a heavily faulted area where even upgraded ventilation systems struggled to maintain safe methane levels, leading to production slowdowns After an initial phase of drilling both types of boreholes, it was determined that gob wells significantly outperformed CM boreholes in terms of methane drainage efficiency.
CM holes were more prone to be affected by air ingress from the mine ventilation system decreasing the amount of methane captured by the system
Gob wells were able to drain up to 10 times the amount of methane per hole compared to CM holes at a cost approximately that of 5 times a CM hole
Gob wells proved to be more effective across a broader area than CM holes, leading to a significant and immediate reduction in gas concentrations near the longwall face This improvement resulted in a notable increase in mining production levels.
Cross-measure boreholes are strategically drilled at various angles into the roof or floor of gateroad entries ahead of the longwall face to effectively pre-drain over- and under-lying strata and capture gas from the gob area after the longwall has advanced Typically located in the return entry, these boreholes may also be installed in both intake and return entries under highly gassy conditions The specific angle, length, and spacing of these boreholes are tailored to site-specific factors, including the width of the longwall panel, depth below the surface, thickness of the mined seam, geomechanical properties of surrounding strata, and the limitations of available drilling equipment.
In retreating longwall mining, maintaining the integrity of cross-measure boreholes and gas gathering pipelines is challenging due to the collapse of gateroad entries adjacent to the gob during the retreat process In contrast, single entry retreat mining, commonly practiced outside the U.S., involves drilling boreholes from an additional gateroad developed along one side of the panel This approach not only facilitates access to the degasification boreholes but also creates a protective environment for the gas gathering system.
Exhibit 23: Cross-measure boreholes developed from a second entry for longwall gob gas recovery for retreating operations
Cross-measure boreholes, with diameters ranging from 50 to 100 mm (2-4 inches), are drilled at angles between 20 to 50 degrees from horizontal Research from the U.S and Europe shows that boreholes drilled at steeper vertical angles generally have a longer productive lifespan and yield purer methane However, it's crucial to recognize that each gob has an optimal drilling angle, and drilling beyond this angle can negatively affect the borehole's performance.
Research by the U.S Bureau of Mines indicates that up to 75% of gob gas is released from newly fractured strata and the stress relaxation zone behind the longwall face In European mines, cross measure boreholes are strategically angled toward and above the longwall face to effectively intercept this gas zone This orientation is crucial for single-entry gateroads used in retreat mining, as it helps maximize production within the limited lifespan of the borehole Once the longwall face moves past the wellhead, access to the borehole is lost, leading to the well being typically shut-in Boreholes are generally inclined at angles between 15 to 30 degrees relative to the longwall axis.
To create a consistent low-pressure zone over the gob using the cross-measure system, it is essential to space boreholes so that their influence zones slightly overlap If boreholes are too distant, accumulated gases may migrate toward the nearest mine entry, while excessively close spacing can lead to the unwanted influx of mine ventilation air into the gob, compromising the quality of the recovered gas.
Borehole spacing typically ranges from 25 to 60 meters (80 to 200 feet) and is influenced by the suction pressure at the wellhead and the permeability of the gob To optimize gas capture, spacing may be reduced at the beginning and end of new coal panels, where tension zones exhibit increased gas flows due to higher fracturing and permeability compared to other areas of the panel.
USBM tests on cross-measure systems along return gate-roads reveal that a borehole's horizontal projection over the longwall rib does not require extensive length Specifically, a 30-meter horizontal projection achieved capture efficiencies of 71 percent, suggesting that even shorter boreholes could be equally effective Thakur (2006) cites borehole lengths ranging from 18 to 152 meters (60 to 500 feet).
Horizontal and vertical placement considerations
The "collar location," where a borehole is drilled into the mine roof or floor, is essential for optimal borehole performance and gas recovery quality Typically positioned near existing pillars or roof supports, this strategic placement helps reduce fracturing as the longwall face advances To enhance connectivity with the gob, limit mine air inflow into the degasification system, and prevent borehole closure, a steel or plastic standpipe, reaching up to 10 meters (33 feet) in length, is often inserted and sealed in the initial borehole.
3.2.2 Recovered gas quality and production
The degasification system must work in harmony with the ventilation system and mining operations, necessitating effective management that incorporates measurement tools, monitoring, controls, and clear communication to enhance the efficiency of all three functions.
Exhibit 25: Cross-measure borehole wellhead configuration with monitoring provisions
Modern wellhead configurations facilitate the measurement of gas quality, flow rate, and pressure, ensuring effective monitoring practices These practices are essential for maintaining gas quality above the mine's limiting value.
25 shows low-cost provisions for suction control, pressure and flow monitoring, and water separation for a cross-measure wellhead
Cross-measure boreholes are linked to a gas collection manifold that typically operates under suction created by a vacuum pump This negative pressure is essential for regulating the gas flow rate from the boreholes Ensuring the integrity of the standpipe is vital for both the productivity of the boreholes and the quality of the recovered gas, as inadequate seals can lead to the intrusion of mine ventilation air due to the system's suction.
Methane capture efficiencies using the cross-measure borehole technique range from 20% to 70% [McPherson, 1993] This method often results in lower gas purities due to the number of boreholes drilled and their connectivity with the ventilation system Typically, an individual borehole produces a flow of 815 m³/day (28 Mcfd), although deeper boreholes can occasionally yield over 4000 m³/day (141 Mcfd) [Thakur, 2006].
Superjacent techniques
Superjacent techniques create low pressure zones in the strata above and sometimes below the rubble zone of the gob, facilitating the migration of gob gas away from mine workings These low pressure zones can be established through the use of existing mined galleries, the construction of new roadways, or by directionally drilling multiple boreholes.
The zone is sealed and subjected to vacuum pressure
Superjacent drainage systems offer the benefit of being set up away from active mining operations, allowing for the strategic placement of boreholes ahead of the mining face in both advancing and retreating longwall systems This method of gob degasification is particularly useful in mines that are unable to utilize surface-drilled gob wells or manage gob gas emissions effectively with these wells alone Additionally, superjacent techniques provide a cost-effective alternative for both implementation and operation compared to cross-measure boreholes.
Advanced drainage gallery techniques, utilized in deeper and gassier mining operations across Eastern Europe, Russia, and China, were pioneered in the late 1940s in highly gassy coal seams of French and German mines These roadways are constructed ahead of mining activities in both overlying and underlying strata to enhance safety and efficiency in gas management.
In mining techniques, a gallery is created 20-35 meters (65-120 ft) above the coal seam, which can help reduce development costs if it is constructed within the coal seam itself This gallery is sealed and linked to a high vacuum gas collection system, establishing a low-pressure area that facilitates the migration of gob gas To enhance gas migration, small diameter, short boreholes can be drilled into the adjacent gassy strata from the gallery.
Utilizing pre-existing galleries enhances the economic feasibility of methane capture techniques, with operators reporting efficiencies of up to 80 percent, and some studies indicating efficiencies as high as 90 percent (Liu and Bai, 1997) Thakur (2006) notes that methane flow rates from superjacent galleries typically average between 28 to 40 Mcmd (1 to 1.4 MMcfd).
7 Although the term “superjacent” literally means “lying above”, drainage galleries and boreholes developed under
Exhibit 26: A sealed superjacent gallery with drainage boreholes
Exhibit 27 shows two superjacent gob drainage techniques used in eastern European mines
Exhibit 27: Degasification of gob areas using the superjacent method in Eastern Europe
In the last twenty years, advanced in-mine directional drilling techniques have been implemented in countries such as Japan, China, Australia, Germany, and the U.S These methods involve strategically placing boreholes above or below mining seams prior to longwall operations, utilizing cutting-edge drilling equipment typically reserved for methane drainage or exploration.
In-mine gob boreholes, 76-152 mm (3-6 inches) in diameter, are drilled into the strata overlying or underlying un-mined panels, to lengths up to 1,000 m (3,280 ft) as previously shown in Exhibit 13
Ideally, overlying boreholes should be positioned taking into account as many of the following factors as possible:
In or below the lowest producing source seam,
To intersect the fracture zone above and below the rubble zone after the gob forms,
Over the tension zones near the edges of the panel,
To effectively harness gob gas migration patterns influenced by the mine's ventilation system and the gob's geometry, it is essential to recognize that gob gas tends to accumulate on the low-pressure side of the gob and at higher elevations.
When drilling, it's essential to manage water accumulation effectively by either sloping upwards from the collar to enable drainage back to the wellhead for separation or, upon reaching the target horizon, drilling downgrade to facilitate water drainage down the borehole and into the gob.
To remain intact following undermining and produce gob gas over the entire length of the borehole
The ideal vertical positioning of a borehole is usually established through trial and error, necessitating careful monitoring of gas flow and quality, longwall face production, and vacuum control at the well heads.
Superjacent directionally drilled boreholes have several advantages over the cross-measure method, namely:
The boreholes can be developed in advance of mining, away from mining activity for either advancing or retreating longwall systems,
Fewer, longer boreholes can produce an effective low pressure zone over the gob,
Strategic placement may allow borehole collars to remain intact (protected from the effects of local stress redistribution) and allow boreholes to remain productive after longwall mining is completed
The system may be more effective and less costly to implement and easier to operate than a system of cross-measure boreholes
Implementing horizontal gob boreholes is more cost-effective compared to systems using galleries, especially when the galleries are designed specifically for degasification and are mined in rock or unprofitable coal seams.
The volume of gob gas emissions from longwall panels dictates the number of necessary boreholes, with at least three boreholes recommended per panel for effective gas capture and redundancy against potential borehole failures Long boreholes, exceeding 500 meters (1640 feet) in length and 100 mm (4 inches) in diameter, can achieve a high vacuum of 100 mm Hg, enabling the recovery of approximately 15 Mcmd (530 Mcfd) of gob gas Additionally, short deviated boreholes can be drilled from the main borehole to enhance the reduced pressure zone over the gob.
To establish a continuous low-pressure zone over the gob, it is essential for the influence zones of boreholes to overlap slightly, similar to cross-measure boreholes If the boreholes are inadequately sized and spaced too far apart, there is a risk that gob gases will migrate towards the mine entry.
Over-designed and closely spaced boreholes can lead to the unwanted migration of mine ventilation air into the gob Opting for fewer, larger-diameter boreholes (150 mm or 6 inches) spaced farther apart can enhance gas recovery while reducing pressure losses compared to smaller, closely spaced holes This approach not only minimizes drilling and wellhead connections but also decreases leakage, ultimately improving gas production rates and quality While the higher costs associated with drilling larger-diameter holes and potentially requiring different equipment must be considered, the resulting benefits in gas production and quality can justify the investment.
Recovered gas quality and production
The efficiency of gob gas drainage and the purity of gas in superjacent systems are influenced by various factors, including geological and reservoir conditions, the orientation of galleries and boreholes, their size and spacing, structural integrity, suction control, water accumulation, and the effectiveness of mine ventilation.
Superjacent borehole drilling is technically more complicated than in-seam drilling, and this is reflected in higher drilling costs of $100-130 per meter ($30-40 per foot)
REI Drilling, Inc conducted test studies in mines across the U.S., Japan, China, and Germany, achieving average borehole production rates between 8,300 mcm/d and 15,000 mcm/d (293-530 Mcfd) from boreholes measuring 500-800 m (1640-2624 ft) in length The quality of gob gas varied from 35% to 90%, influenced by longwall face advance rates, necessitating continuous monitoring and vacuum control Additionally, REI Drilling explored the impact of larger 150 mm (6 in.) diameter boreholes for enhanced gob gas recovery in highly gassy environments, implemented perforated steel casing to bolster borehole integrity, and developed parabolic boreholes to effectively mimic surface-drilled angled gob wells.
Gas Gathering and Collection
Underground gas collection systems
Underground gob gas collection systems are typically more difficult to control and maintain than surface systems because of mining activity and the complex subsurface environment
Gas collected from underground degasification boreholes comes to the surface via a network of pipes fitted with safety devices, water separators, monitors and controls, and vacuum pumps (Exhibit 28)
Exhibit 28: Layout of a horizontal borehole methane drainage system showing both in-mine and surface facilities 8
In-mine methane drainage boreholes normally connect to a collection line via flexible hoses
Collection lines are either suspended or laid on the mine floor (Exhibit 29), and transport
8 Source: Hartman et al., 1997 Copyright 1997, John Wiley & Sons, Inc Reprinted with permission of John Wiley
& Sons, Inc has implemented a methane drainage system that connects to a main gas line leading to a vertical collection well, which can either be freestanding or attached to the lining of an exhaust shaft According to U.S regulations, it is essential that methane drainage pipes are installed in return airways, remain visible along their entire length, are not submerged at any point, and undergo pressure testing during installation to ensure safety and compliance.
Air leakage in a negative pressure gas collection system compromises both gas quality and overall system performance Minimizing leaks at pipe joints and fittings reduces methane dilution, enhances system suction pressures, and ultimately increases the quality and volume of gas collected at the surface.
Pipelines are typically made of steel or high-density polyethylene (HDPE), with steel being favored for its mechanical strength, particularly in underground to surface connections However, HDPE is often chosen for its ease of handling and non-corrosive properties.
Steel pipes are joined by threaded connections, or by gasketed, flanged connections, and both types corrode and leak over time, particularly if frequent pipeline moves are necessary
Exhibit 29: HDPE gas collection piping
HDPE pipe sections are lightweight, non-corrosive, and can be seamlessly fused, significantly minimizing air leakage in pipeline systems This ease of handling translates to lower installation and maintenance costs compared to traditional steel pipes Additionally, HDPE systems can enhance gas quality recovery by up to 50% compared to flanged steel pipe networks, making them a superior choice for efficient gas management.
Steel pipe High‐density polyethylene pipe
Connections can corrode and leak over time
Heavy and difficult to move
Non‐corrosive ‐resistant to H2S, does not rust
Less mechanical strength than steel
Lighter and easier to handle than steel, reducing installation and maintenance costs
Some concern about static electricity issues
Connections can be fused together minimizing leaks
Exhibit 30: Summary of gas collection pipe properties
Safety devices are essential for protecting pipeline infrastructure from leaks during ruptures Operators commonly install automatically activated safety shut-off valves at each borehole and at regular intervals along the pipeline This setup helps to sectionalize the system and reduces methane release into the mine ventilation system in the event of a pipeline breach The valves are typically activated pneumatically or electrically through methane sensors in the airway, pressure sensors, or, more frequently in the US, protective monitoring tubing devices.
Water traps, also known as separation devices, are strategically installed at low points in methane drainage networks to prevent the buildup of water, such as condensate or formation water, which can hinder gas production Large separators are positioned at wellheads and the bottoms of vertical collection wells These devices work by causing a sudden expansion of the drained methane, which reduces its velocity and allows any entrained water to settle out.
Exhibit 31: Separation system at the base of a vertical collection well
A gas collection system effectively monitors pressure, flow rate, and gas constituent concentration, utilizing valves that can be activated manually or remotely based on sensor readings Maintaining optimal negative pressure at the wellhead is crucial for gas production and quality, as high suction pressures can introduce mine ventilation air, while low suction may hinder production and elevate methane emissions Strategic placement of control valves, wellhead monitoring, and careful design of the vacuum pump and gathering system are essential for effective pressure control in the drainage system Each drainage borehole has specific pressure responses, and frequent monitoring at critical underground junctions enhances system performance and alerts operators to increased demands, ultimately leading to improved system efficiency and higher gas recovery quality.
Three common types of extractor pump systems used for creating negative line pressures in degasification pipe networks are water seal extractors, centrifugal blower/exhausters, and rotating pumps Water seal extractors are particularly favored for underground installations due to their safety advantages, as they generate a vacuum without significantly raising gas temperatures and operate without direct contact between stationary and moving components.
Surface gas collection systems
At the surface, gas is collected from vertical frac wells, surface-drilled horizontal wells, gob wells and centralized vacuum stations, which collect the gas produced by in-mine boreholes
Ideally, all CMM collected at the surface would be used commercially Depending on produced gas quality and volumes, CMM can be used for a number of purposes:
Fed into to a natural gas pipeline,
Used to power electricity generators for the mine or local region,
Used as a an energy source – co-firing in boilers, district heating, coal drying, use as a vehicle fuel, and manufacturing or industrial uses such as ammonia production
Many global CMM drainage projects find commercial use of CMM to be neither technically nor economically feasible, leading to the venting of drained gas directly into the atmosphere through an exhauster or well head blower To mitigate the environmental impact of this direct venting, one effective solution is to incinerate the vented methane using a controlled flare system Although burning methane produces carbon dioxide—a greenhouse gas—its global warming potential is significantly lower, as carbon dioxide is 23 times less potent than methane While CMM flaring has been successfully implemented in the U.K and Australia, it has yet to be widely accepted in the U.S coal mining industry.
The main components of a surface gas collection system comprise of the well head equipment, gathering pipelines, any necessary gas processing equipment and compressors
Gas is transported from individual wells through an in-field gathering system to a central processing facility, where it is treated and compressed to meet transmission pipeline specifications To ensure efficiency, the pipeline gathering system utilizes pipes of various diameters at different intervals.
Flowlines are small diameter, low pressure pipelines, typically made of high-density polyethylene, designed to transport gas or water from the wellhead to a larger pipeline that carries the fluid to a central treatment facility These flowlines generally range from 100 to 200 mm (4 to 8 inches) in diameter, although water flowlines can be as small as 50 mm (2 inches).
A trunkline, made of steel and featuring a larger diameter, serves as the primary pipeline in gas transportation As field development progresses, intermediate lines, often called gathering lines, connect the trunkline to flowlines, facilitating system expansion After processing and compression, a high-pressure transmission pipeline, operating at 4,480-8,620 kPa (650-1250 psi), efficiently transports gas from the project area to a designated market.
CMM is extracted from the wellbore under low pressure and is subsequently compressed to meet the necessary requirements for injection into a transmission pipeline The number of compression stages, typically three to four in U.S CBM/CMM projects, is determined by the required suction and discharge pressures for well production and gas transmission A common suction pressure ranges from 70-210 kPa (10-30 psi) for flowlines connecting well sites to treatment facilities Depending on engineering needs, compressors may be installed at individual well sites or centralized at a treatment facility.
Gas extracted from vertical frac wells, horizontal wells, and in-seam boreholes typically meets pipeline quality standards, containing over 90% methane and requiring minimal processing In contrast, gas from gob wells and cross-measure boreholes exhibits more variability in quality, ranging from 30-80% methane due to air dilution To enhance gas quality for pipeline injection, an integrated processing plant can be established to remove contaminants through a series of connected processes This treatment includes the removal of hydrogen sulfide, excess oxygen, carbon dioxide, water vapor, and nitrogen In the U.S., pipeline quality gas must adhere to strict specifications, including less than 0.2% oxygen, under 3% nitrogen, below 2% carbon dioxide, and a maximum of 112 kg/MMcm (7 lbs/MMcf) of water vapor, while maintaining a heating value exceeding 967 Btu/scf.
In 2000, Jim Walter Resources implemented a low-quality methane recovery plant at its Blue Creek Coal mines in Alabama, processing 230 Mcmd (8 MMcfd) of gas containing 60% methane, which results in the production of 115 Mcmd (4 MMcfd) for injection into a sales pipeline.
Summary
Benefits of CMM drainage for coal mines
Many benefits accrue from a methane drainage system An efficient methane drainage system can achieve the following:
Improve mine safety resulting from lower methane contents in the face, returns, gobs and bleeders;
Enhance coal productivity because of less frequent downtime or production slowdowns caused by high methane concentrations in the mine;
Decrease fan operating costs because of reduced ventilation air requirements for methane dilution;
Reduce shaft sizes and number of entries required in the mains;
Increase tonnage extracted from a fixed-size reserve as a result of shifts of tonnage from development sections to production sections;
Decrease dust concentrations and improve worker comfort through reduction of ventilation air velocities at the working face; and
Reduce mining problems caused by water
Each of these benefits is described below
Implementing a methane drainage system significantly enhances the safety of mining operations, leading to positive outcomes (Ely and Bethard, 1989) Mines with high methane levels face increased hazardous conditions, whereas those equipped with a methane drainage system experience improved safety measures and reduced risks.
The installation of methane drainage systems significantly boosts coal productivity, offering substantial economic benefits This advantage becomes particularly evident when considering that the average value of coal produced in a single longwall shift can reach approximately $100,000.
The average cost of a modern longwall setup is approximately $200,000, and excessive methane levels in mine workings can lead to significant production losses, costing between $200 to $400 per minute of downtime Research indicates that a single longwall can experience up to 11,000 minutes of downtime per month, often due to high methane concentrations, resulting in slowdowns or complete halts in production Therefore, implementing an effective methane drainage system can provide substantial economic benefits by minimizing these costly downtimes.
Room-and-pillar operations can benefit economically from addressing production interruptions caused by methane emissions in work areas As the productivity of continuous miners continues to increase, the associated costs of downtime can become significant.
$50 - $100 per minute of downtime averted by a well-designed gas drainage system
Numerous studies have highlighted the costs related to ventilating high-methane mines, with Aul and Ray (1991) demonstrating that implementing a methane drainage system can cut ventilation requirements by nearly half, leading to substantial savings in ventilation power costs These savings vary based on factors such as mine size, ventilation strategies, electrical power prices, and the volume of air conserved within the ventilation network Wang (1997) confirmed that significant reductions in ventilation power costs are achievable, particularly when gas emissions during mining exceed 10 m³/tonne (400 ft³/ton) Additionally, Wang's research indicated that potential power cost savings are even greater in continuous mining operations compared to longwall mining.
Reduced development costs and increased reserves
Implementing a methane drainage system can greatly decrease mine ventilation needs, enabling the extraction of broader longwall panels This reduction in ventilation requirements may lead to smaller and fewer shafts and development openings linking the coal seam to the surface Additionally, extracting wider longwall panels minimizes the number of development entries within the mine.
Longwall mining can achieve coal recovery rates of 85-95% under optimal conditions, whereas development sections typically recover only about 50% of the coal The extraction costs for coal from development sections are generally higher on a per-ton basis compared to coal from longwall panels By increasing panel width and reducing the number of development entries, mining operations can enhance mineable coal reserves and decrease extraction costs per ton Previous studies have highlighted that these cost differences can be substantial.
Reduced dust problems and increased worker comfort
High air velocities in mining environments, particularly those exceeding 180 meters per minute (600 ft/min), can significantly reduce worker comfort and increase dust generation, making routine tasks more challenging In longwall sections, the downward air movement from the shearer can cause dust to create a "sand blasting" effect on workers' exposed skin, posing both discomfort and eye hazards Although the number of workers positioned downwind of the shearer is typically limited, the associated risks are considerable yet preventable.
Water presence in coalmine roof strata can lead to significant delays in underground mining operations, particularly in development sections, with variability based on geological conditions When high methane levels accompany water, implementing a methane drainage system can help mitigate these issues Notably, a study by Reese and Reilly (1997) highlighted a Pennsylvania longwall mine where gas drainage wells resulted in a 63% reduction in water downtimes and a 16% decrease in methane downtimes, showcasing the economic benefits of such systems.
Environmental benefits of CMM drainage
The primary environmental advantage of coal mine methane (CMM) drainage and utilization lies in its ability to significantly decrease methane emissions that contribute to climate change By capturing methane and either flaring it or using it as an energy source, the combustion process converts methane into carbon dioxide (CO2), a greenhouse gas that is twenty-three times less potent than methane, thereby reducing overall greenhouse gas emissions.
According to USEPA [2008 a], over 200 coal mine methane (CMM) projects are operational globally, effectively draining, capturing, and utilizing methane to decrease atmospheric emissions by more than 3.8 billion cubic meters (134.1 billion cubic feet) annually, which is equivalent to a reduction of 59.1 million metric tons of CO2 equivalent.
According to the USEPA (2008), only 35% of the methane released from the fifty gassiest mines in the United States is currently being utilized, with 36 of these mines lacking any methane drainage and utilization projects This results in an estimated annual release of 1.3 Bcm (46.5 Bcf) of methane into the atmosphere If recovery projects were implemented, with a recovery efficiency ranging from 20% to 60%, it is estimated that 264-791 MMcm/yr (9-28 Bcf/yr) of methane emissions could be avoided, equating to approximately 4-12 MTCO2e Additionally, there is significant potential for enhanced methane recovery at mines that already have operational recovery projects.