C7 matrix acidizing

o Matrix stimulation is a technique in which a solvent is injected into the formation to dissolve some ofthe materials present and hence recover or increase the permeability in the nearwellbore region.Such treatments are called “matrix” treatments because the solvent is injected at pressures belowthe parting pressure of the formation so that fractures are not created. The objective is to greatlyenhance or recover the permeability near the wellbore, rather than affect a large portion of thereservoir.o The most common matrix stimulation treatment is acidizing, in which an acidic solution is injected todissolve minerals in the formation. However, other solvents are also used. The next most commonfluids onversare organic solvents aimed at dissolving waxes, paraffins, asphaltenes or other organicdamaging materials.o (Matrix) acidizing is a nearwellbore treatment, with all the acid reacting withino about 1 ft of the wellbore in sandstone formations ando a few inches to perhaps as much as 10 ft from the wellbore in carbonates.

Đỗ Quang Khánh – HoChiMinh City University of Technology Email: dqkhanh@hcmut.edu.vn or doquangkhanh@yahoo.com

Designed & Presented by

Mr ĐỖ QUANG KHÁNH, HCMUT

Content & Agenda

 Introduction

 Used acidic solutions in matrix acidizing

 Basic properties of acid-mineral interactions

 Candidate selection

 Treatment type selection

 Sandstone acidizing design-typical acidizing formulations for sandstone formations

 Carbonate acidizing design

Ref:

 Reservoir Stimulation, 3e – Economides & Nolte

 Petroleum Production Systems - Economides et al., 1994

 Production Operations: Well Completions, Workover, and Stimulation -Thomas O Allen,

Alan P Roberts,1984

Introduction

o Matrix stimulation is a technique in which a solvent is injected into the formation to dissolve some of the materials present and hence recover or increase the permeability in the near-wellbore region Such treatments are called “matrix” treatments because the solvent is injected at pressures below the parting pressure of the formation so that fractures are not created The objective is to greatly enhance or recover the permeability near the wellbore, rather than affect a large portion of the reservoir

o The most common matrix stimulation treatment is acidizing, in which an acidic solution is injected to dissolve minerals in the formation However, other solvents are also used The next most common fluids onversare organic solvents aimed at dissolving waxes, paraffins, asphaltenes or other organic damaging materials

o (Matrix) acidizing is a near-wellbore treatment, with all the acid reacting within

o about 1 ft of the wellbore in sandstone formations and

o a few inches to perhaps as much as 10 ft from the wellbore in carbonates

Introduction

o Matrix acidizing can significantly enhance the productivity of a well when near-wellbore formation damage is present and, conversely, is of limited benefit in an undamaged well

o The goal of a matrix acidizing treatment is to reduce the non-mechanical skin effect to near zero

o Main applications for matrix acidizing:

o only when a well has a high skin factor that cannot be attributed to partial penetration, perforation efficiency or other mechanical aspects of the completion

o in highly productive wells, the productivity improvement of about 20% that is possible with matrix stimulation of an undamaged well may be economic

o in naturally fractured or highly vugular carbonate reservoirs, live acid may penetrate to a sufficient distance to yield a productivity enhancement greater than that normally expected from a true matrix treatment

o High permeability formation with damage

o Formations not suitable for fracturing

o Water/Gas Cap near oil zone

o Mechanical treating limitations

o To Supplement Fracturing

Introduction

o An ideal matrix treatment restores the permeability in the near-wellbore region to a value at least as high

as the original undamaged permeability; it accomplishes this over the entire completed interval and it leaves the formation in the treated region with high relative permeability to the oil and/or gas phase

o Designing a treatment should strive to achieve this ideal at the lowest possible cost, which requires consideration of the many physical and chemical interactions taking place between the injected fluids and the reservoir minerals and fluids

 mass transfer of acid molecules to the mineral surface and subsequent reaction at the surface

 changing pore structure

 precipitation of reaction products

 acid fluid–reservoir fluid interactions

 variations in reservoir permeability or the distribution of damage

USED ACIDIC SOLUTION IN MATRIX ACIDIZING

o Hydrochloric Acid (HCl)

o Organic Acids

o Acetic acid (CH3COOH)

o Formic acid (HCOOH)

o Mud Acid (HCl/HF)

Hydrochloric Acid (HCl)

Organic Acids

Mud Acid

Mud Acid

Mud Acid

o The clays, micas, etc dissolved by mud acid undergo a series of reactions resulting in precipation of silica gel (Si(OH 4) – a hydrated form of silica

BASIS PROPERTIES OF ACID-MINERAL INTERACTIONS

o Acid-mineral reaction stoichiometry

o Acid-mineral reaction kinetics

o Precipitation of reaction products

Acid-mineral reaction stoichiometry

 Chemistry

o Minerals present in sandstone pores include

o Montmorillonite (Bentonite) , Kaolinite, Calcite, Dolomite, Siderite, Quartz, Albite (Sodium Feldspar), Orthoclase, and others…

o Mineral origin from invasion of drill, cementing or completion fluids, or host materials occurring naturally

in the rock formation

o Most commonly used acids to dissolve these minerals: hydrochloric acid (HCl) and hydrofluoric acid (HF)

o Silicate materials (such as clays and feldspars) in sandstone pores normally removed using mixtures of HCl and HF acid

o Carbonate materials usually addressed by HCl alone

Acid-mineral reaction stoichiometry

 Primary chemical reactions in acidizing

Acid-mineral reaction stoichiometry

 The stoichiometry of chemical reaction: the number of moles of each species involved in the reation

 Dissolving power of acids: the amount of mineral that can be consumed by a given amount of acid on a mass or volume basis

Acid-mineral reaction stoichiometry

 Ex: Calculate the Gravimetric and Volumetric dissolving power of 15% Hydrochloric acid with Calcium carbonate with ρ acid = 0.0654 lb/ft3 , ρ CaCO3= 0.17 lb/ft3

 Sol: • β 15 = 0.15 x 1 x 100.1 / (2 x(36.5)) = 0.206 lb m CaCO 3 / lb m 15 wt% HCl solution

• X 15 = 0.206 x 0.0654 / 0.17= 0.08 cuft CaCO 3 / cuft 15 wt% HCl solution

Acid-mineral reaction stoichiometry

 Ex: A sandstone formation with a porosity of 0.2 contains 5-vol% albite (sodium feldspar) What is the minimum volume of 3% HF solution required to dissolve all the albite a distance of 6 in beyond a 6-in diameter wellbore?

Acid-mineral reaction kinetics

 Reaction rate

o The reaction between an acid and a mineral occurs when acid reaches the surface of the mineral by diffusion or convection from the bulk solution

o The overall rate of acid consumption or mineral dissolution depends on two distinct phenomena:

o the rate of transport of acid to the mineral surface by diffusion or convection, and

o the actual reaction rate on the mineral surface

o Usually, one of these processes is much slower than the other In this case,

o the fast process can be ignored,because it can be thought of as occurring in an insignificant amount of time compared with the slow process

 Acid reaction occurring in a system

Acid-mineral reaction kinetics

 Reaction kinetics

o Kinetics of a reaction is a description of the rate at which the chemical reaction takes place, once the reacting species have been brought into contact; driven by reactivity of the mineral per unit of surface area

o Generally:

- HCl / Carbonate reactions can be considered almost instantaneous, and limited by mass transport

- Most HF-Mineral reactions are slow compared to mass transport rates so overall acid consumption

limited by reaction rate

o Clays will react with HF approx 2 orders of magnitude faster than silica due to far greater surface area / volume

o Felspars will react with HF approx 1 order of magnitude faster than silica due to greater reactivity

Precipitation of reaction products

o A major concern in acidizing, particularly the acidizing of sandstones, is damage caused by the precipitation of acid-mineral reaction products In acidizing sandstones with HF, the formation of some precipitates is probably unavoidable However, the amount of damage they cause to the well productivity depends on the amount and location of the precipitates These factors can be controlled to some extent with proper job design

o The most common damaging precipitates that may occur in sandstone acidizing are :

o calcium fluoride (CaF 2 ),

o colloidal silica (Si(OH) 4 ),

o ferric hydroxide (Fe(OH) 3 ), and

o asphaltene sludges

o The tendency for precipitation reactions to occur in acidizing is predicted with comprehensive geochemical models of the chemical reactions between aqueous species and the host of minerals present

Precipitation of reaction products

o The most common type of geochemical model used to study sandstone acidizing is the local equilibrium model, such as described by Walsh et al (1982) and Faber et al (1994)

o assumes that all reactions are in local equilibrium; i.e., all reaction rates are infinitely fast

o a time-distance diagram for the injection of 11% HCl–4% HF into a formation containing calcite, kaolinite and quartz This plot shows regions where amorphous silica and aluminum fluoride will tend to precipitate

o A vertical line on the plot represents the mineral species present as a function of distance if all reactions are in local equilibrium

o By coupling this model with a model of the formation per meability response to both dissolution and precipitation, predictions of the productivity improvement expected from particular acid formulations may be obtained

Precipitation of reaction products

o Recently, Sevougian et al (1992) and Quinn (1994) presented a geochemical model that includes kinetics for both dissolution and precipitation reactions This model predicts

o less permeability damage than a local equilibrium model because the finite rate of the reactions allows displacing the pre cipitate farther from the wellbore

Candidate Selection

 Candidate Selection (Recognition) is the process of identifying and selecting wells for treatment which have the capacity for higher production and better economic return

“Good Wells Make the Best Candidates for Well Stimulation”

1 It must have production potential impaired by damage

2 That damage must be treatable through acidizing

3 The NPV of treating with acid should be more attractive than other possible treatments

4 Must be sufficient capacity downstream of the sandface to accommodate expected incr production

 Candidate selection process

o • Review numerous wells

o • Review of well logs/records, reservoir characteristics and information on the completion/previous workovers

o • Map the productivity of each well

o • Establish reasonable upper production potential for fracturing and matrix stimulation techniques

o • Evaluate potential mechanical problems

o • Focus on wells with the highest reward and lowest risk

CANDIDATE SELECTION

 Typical screening criteria

o It’s all about the size of the prize

TREATMENT TYPE SELECTION

 The chosen chemical treatment fluid should be targeted at the particular type and location of the formation damage to be removed or treated

 The formation damage/impairment may be related to:

(i) drilling, completion or workover operations,

(ii) produced or (continually) injected fluids,

(iii) injected fluids during specific well operations e.g well killing

 Chemical treatment types

DESIGN CONSIDERATIONS

o In considering the many aspects of the matrix acidizing process, the focus is on the key design variables; to be useful, any model of the process must aid in optimizing the design

o The primary design considerations are:

• fluid selection—acid type, concentration and volume

• injection schedule—planned rate schedule and sequence of injected fluids

• acid coverage and diversion—special steps taken to improve acid contact with the formation

• real-time monitoring—methods to evaluate the acidizing process as it occurs

• additives—other chemicals included in the acid solution to enhance the process or to protect tubular goods

DESIGN CONSIDERATIONS

 A Typical Treatment Stages:

 Preflush: [usually 50 to 100 gal/ft of perforations is advisable]

– A fluid stage, normally hydrochloric acid HCl, pumped ahead of the main treating fluid

in a sandstone matrix‐stimulation treatment

– One of the purposes of a preflush is to displace formation brines that contain K, Na, Ca ions away from the wellbore, decreasing the possibility of crystallizing alkali‐fluosilicates that could plug the pores

–The other purpose of a preflush is to dissolve calcareous materials to minimize calcium fluoride [CaF2 ] precipitation, and to dissolve iron scale or rust to avoid the precipitation of the gelatinous, highly insoluble ferric hydroxide [Fe(OH)3 ]

 Main treating fluid: [usually 50 to 200 gal/ft of perforations is required]

– The mixture of hydrofluoric HF and hydrochloric HCl or organic acids formulated to address

formation damage

 Overflush:

– A specially prepared fluid used to displace matrix acid treatments away from the wellbore at the conclusion

of a stimulation treatment The overflush is typically formulated from a weak acid (5% HCl) solution or

Ammonium chloride [NH4Cl] brine to maintain a low pH environment in the near‐wellbore formation that

prevents the precipitation of reaction products as the treatment fluids are flowed back

 Displacement:

– Fluid volume required to push all overflush into the formation Can be brine/diesel/nitrogen

depending upon cost, expected contamination or reservoir pressure issues

SANDSTONE ACIDIZING DESIGN Typical acidizing formulations for sandstone formations

o Introduction

o Sandstone acidizing models

o Select of acid composition (type & strength (ie concentration))

o Select of treatment volume

o Select of injection rate

o Select of additives

o Select of treatment type

o Select of diversion technique

Introduction to Sandstone reservoirs

Sandstone acidizing models: Two-mineral model

Sandstone acidizing models: Two-acid, three-mineral model

Typical Sandstone Treatment Stages

Select of acid

o The chemistry of a mud acid treatment illustrating how the impairment, formation clays and inter-granular cements are removed by the mud acid and partially replaced by secondary reaction products However, there is an overall increase in porosity and permeability, leading to stimulation of the well

Select of acid

 Acid selection guidelines

Select of treatment volume

 Typical mud acid treatment volume guidelines

o Above volumes often adjusted to take into account:

(i) Field experience when treating wells in the same or similar fields Often between 50% and 100% of the volumes suggested in are used

(ii) Practical considerations such as logistics e.g

(a) how much acid can be delivered to the wellsite? or

(b) how large an acid the volume pumpable during daylight hours?

(iii) Economics (how much acid can we afford based on the expected gain in hydrocarbon production ?)

(iv) Laboratory core flow testing (acid volume required to increase the permeability (by a target amount) The core may be pre-treated to include damage to the core inlet face by the suspected form of formation damage

Select of treatment volume

 Ex:

Select of injection rate

 Determining Fracture Pressure: can be determined a number of ways:

 ISIP (Instantaneous Shut In Pressure)

 Step Rate Tests

 Pre-treatment breakdown (w/brine)

 Assumption of 0.7 psi/ft

 Injection Pressure