Reservoir Formation Damage Episode 1 Part 6 docx

Cation Exchange Capacity CEC The total amount of ions anions and cations that are present at theclay surface and exchangeable with the ions in an aqueous solution incontact with the clay

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an integrated application of various techniques, such as polarized lightmicroscopy, X-ray diffraction, and SEM-EDS analyses, are required(Braun and Boles, 1992) Hayatdavoudi (1999) shows the typical X-raydiffraction patterns of the bulk and the smaller than 4 micron size clayfractions present in a core sample.

X-Ray CT Scanning (XRCT)

X-Ray CT (computer-assisted tomography) scanning is a nondestructivetechnique, which provides a detailed, two- and three-dimensional exami-nation of unconsolidated and consolidated core samples during the flow

of fluids, such as drilling muds, through core plugs and determines suchdata like the atomic number, porosity, bulk density, and fluid satura-tions (Amaefule et al., 1988; Unalmiser and Funk, 1998) This techniquehas been adapted from the field of medical radiology (Wellington andVinegar, 1987)

As depicted by Hicks Jr (1996), either an X-ray source is rotatedaround a stationary core sample or the core sample is rotated while theX-ray source is kept stationary The intensity of the X-rays passingthrough the sample is measured at various angles across different crosssections of the core and used to reconstruct the special features of theporous material The operating principle is Beer's law, which relatesthe intensity of the X-ray, through the linear attenuation coefficient, tothe physical properties of materials and different fluid phases in thesample (Wellington and Vinegar, 1987; Hicks Jr., 1996) A schematic of

a typical X-ray scanning apparatus is shown by Coles et al (1998) Theimage patterns can be constructed using the linear attenuation coefficientmeasured for sequential cross-sectional slides along the core sample asshown by Wellington and Vinegar (1987) These allow for reconstruction

of vertical and horizontal, cross-sectional images, such as shown byWellington and Vinegar (1987) Three-dimensional images can be recon-structed from the slice images as illustrated by Coles et al (1998).Tremblay et al (1998) show the cross-sectional and longitudinal images

of a typical wormhole, perceived as a high permeability channel, growing

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in a sand-pack Such images provide valuable insight and understanding

of the alteration of porous rock by various processes

X-Ray Fluoroscopy (XRF)

The X-Ray fluoroscopy technique is used for determining the drillingmud invasion profiles in unconsolidated and consolidated core samplesand it is especially convenient for testing unconsolidated, sleeved coresamples (Amaefule et al., 1988) Amaefule et al (1988) show a typicalX-ray fluoroscopic image

Scanning Electron Microscope (SEM)

The rock and fluid interactions causing formation damage is a result

of direct contact of the pore filling and pore lining minerals present

in the pore space of petroleum-bearing formations The mineralogicalanalysis, abundance, size, and topology and morphology of these mineralscan be observed by means of the scanning electron microscopy (SEM)(Kersey, 1986; Amaefule et al., 1988) Braun and Boles (1992) cautionthat, although the SEM can provide qualitative and quantitative chemicalanalyses, it should be combined with other techniques, such as thepolarized light microscopy (PLM) and the X-ray diffraction (XRD) tocharacterize the crystalline and noncrystalline phases, because amorphousmaterials do not have distinct morphological properties An energydispersive spectroscopy (EDS) attachment can be used during SEManalysis to determine the iron-bearing minerals (Amaefule et al., 1988).Various specific implementations of the SEM are evolving For example,the environmental SEM has been used to visualize the modification ofthe pore structure by the retention of deposits in porous media (Ali andBarrufet, 1995) The cryo-scanning electron microscopy has been used

to visualize the distribution of fluids in regard to the shape and spatialdistribution of the grains and clays in the pore space (Durand andRosenberg, 1998) The SEM has also been used for investigation of thereservoir-rock wettability and its alteration (Robin and Cuiec, 1998;Durand and Rosenberg, 1998)

The SEM operates based on the detection and analysis of the radiationsemitted by a sample when a beam of high energy electrons is focused

on the sample (Ali and Barrufet, 1995) It allows for determination ofvarious properties of the sample, including its composition and topography(Ali and Barrufet, 1995)

Typical SEM photomicrographs are shown by Amaefule et al (1988).The environmental SEM images shown by Ali and Barrufet (1995)illustrate the modification of the pore structure by polymer retention in

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porous media As can be seen by these examples, the SEM can providevery illuminating insight into the alteration of the characteristics of theporous structure and its pore filling and pore lining substances.

Thin Section Petrography (TSP)

The thin section petrography technique can be used to examine thethin sections of core samples to determine the texture, sorting, fabric,and porosity of the primary, secondary, and fracture types, as well asthe location and relative abundance of the detrital and authigenic clayminerals and the disposition of matrix minerals, cementing materials, andthe porous structure (Kersey, 1986; Amaefule et al., 1988) Amaefule et

al (1988) show the examples of typical thin section photomicrographs

Petrographic Image Analysis (PIA)

As stated by Rink and Schopper (1977), "The physical properties ofsedimentary rocks strongly depend on the geometrical structure of theirpore space Thus, a geometrical analysis of the pore structure can providevaluable information in formation evaluation." The petrographic imageanalysis (PIA) technique analyzes the photographs of the cuttings, thinsections, or slabs of reservoir core samples using high-speed imageanalysis systems to infer for important petrophysical properties, includingtextural parameters, grain size and distribution, topography, directionaldependency of textural features, pore body and pore throat sizes, porosity,permeability, capillary pressure, and formation factor (Amaefule et al.,1988; Rink and Schopper, 1997; Oyno et al., 1998)

The images of the rock surfaces can be obtained by photographing onpaper using standard cameras or digital video cameras attached to amicroscope, but computer-aided digital storage and analysis of imagesprovide many advantages (Oyno et al., 1998) Saner et al (1996) showtypical thin section photomicrographs of typical carbonate lithofacies Thephotographs shown by Ehrlich et al (1997) indicate the packing flaws

in typical sandstone samples Coskun and Wardlaw (1996) show the porelsize spectra and binary images of five pore types of some North Seasandstones Such images can be analyzed by various techniques to deter-mine the textural attributes and to derive the petrophysical characteristics

of the petroleum-bearing formation (Rink and Schopper, 1977; Ehrlich

et al., 1997; Coskun and Wardlaw, 1993, 1996; loannidis et al., 1996)

Polarized Light Microscopy (PLM)

The polarized light microscopy (PLM) technique can be utilized foreffectively detecting amorphous substances in porous media because,

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being optically isotropic, amorphous substances can be distinguished fromthe majority of the crystalline matter, except for the optically isotropichalides (Braun and Boles, 1982) The polarized light microscopy is based

on distinguishing between various substances by the difference in theirrefractive indices Braun and Boles (1982) recommend supporting thePLM method by at least another method, such as the scanning electronmicroscopy combined with the energy dispersive X-ray spectrometry(SEM-EDS) and the X-ray diffraction (XRD) method

Nuclear Magnetic Resonance Spectroscopy (NMR)

The nuclear magnetic resonance spectropy is a nondestructive nique, which measures the spin-lattice and spin-spin relaxation times bymeans of the radio-frequency resonance of protons in a magnetic field

tech-to infer for the petrophysical parameters, including porosity, permeability,and free and bound fluids using specially derived correlations (Unalmiserand Funk, 1998; Rueslatten et al., 1998) Because fines mobilization,migration, and retention in porous media causes porosity variation,the NMR can also be used for examination of core plugs during finesinvasion For example, Fordham et al (1993) examined the invasion ofclay particles within natural sedimentary rocks by injection of suspension

of clay particles using the NMR imaging technique Fordham et al (1993)show that the proton spin-lattice relaxation time profiles measured atdifferent times indeed indicate the effect of clay fines invasion into coreplugs This information can be used to determine the penetration depth

of the clay fines and the effect of fines invasion to permeability Xiao et

al (1999) state that:

The NMR (nuclear magnetic resonance) techniques, namely NMRI(nuclear magnetic resonance imaging) and NMRR (nuclear magneticresonance relaxation), can support the observations obtained withthe return permeability tests, helping in the identification andcomprehension of the formation damage mechanisms caused bysolids and filtrate invasion in the pores of a reservoir rock

However, the NMR techniques are expensive and time consuming, andbetter suited for in depth studies (Xiao et al., 1999) Xiao et al (1999)show typical NMR images and relaxation time curves on invasion of

a typical bentonite/mixed metal hydroxide (MMH)/sized carbonatemud system into a core plug The core plug images provided visual

inspections for the core initially saturated with a 3% NH 4 Cl brine, then

contaminated by mud invasion, and finally back flushed with brine formud removal, respectively

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Acoustic Techniques (AT)

The acoustic techniques facilitate acoustic-velocity signatures andcorrelations of the acoustic properties of rocks to construct acousticvelocity tomograms to image the rock damage by deformation, such aselastic and dilatant deformations, pore collapse, and normal consolidationprocesses (Scott et al., 1998) Scott et al (1998) describe the acousticvelocity behaviors during compaction of reservoir rock samples Scott et

al (1998) show a schematic of a confined-indentation experiment usedand the acoustic velocity tomograms obtained by the indentation tests

Cation Exchange Capacity (CEC)

The total amount of ions (anions and cations) that are present at theclay surface and exchangeable with the ions in an aqueous solution incontact with the clay surface, is referred to as the ion-exchange capacity

(IEC) of the clay minerals and it is measured in meq/100 g (Kleven and

Alstad, 1996) The total ion-exchange capacity is therefore equal to the

sum of the cation-exchange capacity (CEC} and the anion-exchange capacity (AEC):

IEC = CEC + AEC (6-1)

During reservoir exploitation, when brines of different composition thanthe reservoir brines enter the reservoir formation, an ion-exchange processmay occur, activating various processes leading to formation damage (seeChapter 13) In the literature, more emphasis has been given to themeasurement of the cation-exchange capacity, because it is the primaryculprit, responsible for water sensitivity of clayey formations (Hill andMilburn, 1956; Thomas, 1976; Huff, 1987; Muecke, 1979; Khilar andFogler, 1983, 1987)

The mechanisms, by which aqueous ions interact with the clay mineralspresent in petroleum-bearing rock, have been the subject of many studies.Kleven and Alstad (1996) identified two different mechanisms: (1) latticesubstitutions and (2) surface edge reactions The first mechanism involvesthe ion-exchange within the lattice structure itself, by substitution of A/3+for 574+, Mg2+ for A/3+, as well as other ions to a lesser degree, and does

not depend on the ionic strength and pH of the aqueous solution (Kleven

and Alstad, 1996)

The second mechanism involves the reactions of the functional groupspresent along the edges of the silica-alumina units and it is affected by

the ionic strength and pH of the aqueous solution (Kleven and Alstad,

1996) The relative contributions of these mechanisms vary by the claymineral types It appears that montmorillonite and illite primarily undergo

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lattice substitutions, and surface edge reactions are dominant for kaoliniteand chlorite (Kleven and Alstad, 1996) Expansion of swelling clays, such

as montmorillonite, increases their surface area of exposure and, therefore,their cation-exchange capacity (Kleven and Alstad, 1996) Theoreticaldescription of the ion-exchange reactions between the aqueous phase andthe sedimentary formation minerals is very complicated because of various

effects, including ion composition, pH, and temperature (Kleven and

Alstad, 1996)

The methods used for measurement of the ion-exchange capacity vary

by the reported studies For example, Kleven and Alstad (1996) measured

the CEC of clays using Ca 2+ brines without the presence of NaCl and measured the AEC using SO%~ brines Rhodes and Brown (1994) point out the CEC measurement of clays by commonly used methods, such as

the ammonium ion and methylene blue dye adsorption methods, haveinherent shortcomings, leading to inaccurate results Therefore, Rhodes

and Brown (1994) have used the adsorption of the colored Co(H 2 O) ion, which yields a very stable hydrated Co(If) complex Rhodes and Brown (1994) have determined that the CECs of four different Na + -

montmorillonites measured by three different adsorption methods differappreciably The methylene blue adsorption method generates significantlydifferent results from the cobalt and ammonium ion adsorption methods,which agree with each other within acceptable tolerance

Because the ion-exchange reactions in petroleum-bearing rock are usuallytreated as equilibrium reactions for practical purposes, ion-exchange isothermsrelating the absorbed and the aqueous phase ion contents in equilibriumconditions are desirable For example, Kleven and Alstad (1996) deter-mined the cation-exchange isotherms shown in Figures 6—4, 6-5, and 6-6,respectively, for single cation-exchange reactions involving

SOl ~^ d • When more than one ions are present in the system, some

are preferentially more strongly adsorbed than the others depending on

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Calcium ions in solution, meq/L

Figure 6-4 Calcium-sodium ion-exchange isotherms (circles = kaolinite,

squares = montmorillonite, open figures = 20°C, and closed figures = 70°C)

(Reprinted from Journal of Petroleum Science and Engineering, Vol 15,

14

12

S 10

Barium Ions In solution, meq/L

Figure 6-5 Barium-sodium ion-exchange isotherms (circles = kaolinite,

squares = montmorillonite, open figures = 20°C, and closed figures = 70°C)

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12

10

0 10 20 30 40 50 Calcium and barium ions in solution, meq/L

Figure 6-6 Calcium (open figures) and barium (closed figures) ion-exchange

isotherms at 70°C (circles = kaolinite and squares = montmorillonite)

0,8

Figure 6-7 Sulfate-chloride ion-exchange isotherms at low sulfate

concentrations (circles = kaolinite, squares = montmorillonite, open figures

= 20°C, and closed figures = 70°C) (Reprinted from Journal of Petroleum

Science and Engineering, Vol 15, Kleven, R., and Alstad, J., "Interaction of

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the affinities of the clay minerals for different ions This phenomenon isreferred to as the selectivity Kleven and Alstad (1996) have determined

that the kaolinite and montmorillonite clays prefer Ba2+ over Ca2+, as

indicated by the normalized cation-exchange isotherms shown in theirFigure 6-8 Similarly, their Figure 6-9 showing the normalized anion-

exchange isoterms indicate that the kaolinite clay prefers 5O|~ over Cl~.

Figure 6-8 also shows that the selectivity is also influenced by theswelling properties of clays It is apparent that the affinity of divalent

cations (such as Ca2+) over monovalent cations (such as Na+) is much higher

for kaolinite (nonswelling clay) than montmorillonite (swelling clay).Petroleum-bearing formations contain various metal oxides, includ-

ing Fe2O3, Fe3O4, MnO2, and SiO2 Tamura et al (1999) propose a

hydroxylation mechanism that the exposure of metal oxides to aqueoussolutions causes water to neutralize the strongly base lattice oxide ions

to transform them to hydroxide ions, according to

(6-5)Hence, the ion-exchange capacity of the metal oxides can be measured

by determining the hydroxyl site densities on metal oxides by various

Extractions of calcium ions In solution at equilibrium

Figure 6-8 Normalized calcium-sodium ion-exchange isotherms (circles =

kaolinite, squares = montmorillonite, open figures = 20°C, and closed figures

= 70°C) (Reprinted from Journal of Petroleum Science and Engineering, Vol.

15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth andSulphate Ions with Clay Minerals and Sedimentary Rocks," pp 181-200,

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0,2 0,4 0.6 0,8 1

Eq fractions of sulphate Ions In solution

Figure 6-9 Normalized sulfate-chloride ion-exchange isotherms (circles =

kaolinite, squares = montmorillonite, open figures = 20°C, and closed figures

= 70°C) (Reprinted from Journal of Petroleum Science and Engineering, Vol.

15, Kleven, R., and Alstad, J., "Interaction of Alkali, Alkaline-Earth andSulphate Ions with Clay Minerals and Sedimentary Rocks," pp 181-200,

methods, including reactions with Grignard reagents, acid-base exchange reactions, dehydration by heating, infra-red (IR) spectroscopy,tritium exchange by hydroxyl, and crystallographic calculations (Tamura

ion-et al., 1999) Figure 6-10 by Tamura ion-et al (1999) shows a typical

isotherm for OH~ ion for hematite Figure 6-11 by Arcia and Civan

(1992) show that the cation-exchange capacity of the cores extracted fromreservoirs may vary significantly by the clay content

5 (Zeta)-Potential

When an electrolytic solution flows through the capillary paths inporous media, an electrostatic potential difference is generated along theflow path because of the relative difference of the anion and cation fluxes.Because the mobility of the ions is affected by the surface charge, thispotential difference, called the zeta-potential, can be used as a measure

of the surface charge (Sharma, 1985) The zeta-potential can be measured

by various methods, including potentiometric titration, electrophoresis, andstreaming potential

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[OH1 free /moldm' 3

Figure 6-10 Hydroxyl-hematite ion-exchange isotherm indicating the amount

of hydroxyl ion consumed per unit surface area of hematite vs the hydroxylion concentration in solution (after Tamura et al., 1999; reprinted by permis-sion of the authors and Academic Press)

The Helmholtz-Smoluchowski equation of the zeta-potential for ular porous media is given by Johnson (1999) as:

gran-r 4gran-rcuX dU

based on the cylindrical capillary bundle of tubes model In Eq 6-6, £denotes the zeta-potential of the capillary surface, |i is the viscosity, (££0)

is the permittivity, (dU I dp) is the streaming potential pressure gradient, U

is the streaming potential, p is pressure, A and L are the cross-sectional area and length of porous media, respectively, (j) is the porosity, and R is the

electrical resistance Figures 6-12 and 6-13 by Johnson (1999) show the

dependency of the zeta-potential on the ionic strength and pH of the aqueous

solution, obtained by the electrophoresis and streaming potential methods

Wettability

Wettability of the pore surface is one of the important factors ing the distribution and transport of various fluid phases and thereforethe extent of formation damage in petroleum-bearing formations Because

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Figure 6-11 Cation exchange capacity of the various Ceuta field core

by permission of the Canadian Institute of Mining, Metallurgy and Petroleum)

the wettability of rocks is altered by the rock and fluid interactions andvariations of the reservoir fluid conditions, prediction of its effects onformation damage is a highly complicated issue Although mineral mattersforming the reservoir rocks are generally water-wet, deposition of heavyorganic matter, such as asphaltenes and paraffines, over a long reservoirlifetime may render them mixed-wet or oil-wet, depending on the compo-sition of the oil and reservoir conditions Wettability may be expressed

by various means, including the Amott and USBM indices (See Chapter4.) During reservoir exploitation, wettability may vary by various reasons.For example, Figure 6-14 by Burchfield and Bryant (1988) is an evidence

of the alteration of the wettability of a water-wet berea sandstone to astronger water-wet state in contact with microbial solutions Madden andStrycker (1988) determined that the wettability of the Berea sandstonesaturated with oils vary by their asphaltene and polar components content

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