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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.
Designed & Presented by Mr ĐỖ QUANG KHÁNH, HCMUT 03/2014 Đỗ Quang Khánh – HoChiMinh City University of Technology Email: dqkhanh@hcmut.edu.vn or doquangkhanh@yahoo.com 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 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 CARBONATE ACIDIZING DESIGN o Introduction o Wormholes o Select of acid o Select of treatment volume o Select of injection rate o Select of treatment type o Select of diversion technique Introduction to stimulation of carbonate formations o Acidizing of carbonate formations is fundamentally different from the acidising of clastic formations due to their differing physical nature and chemistry: (i) Carbonates consist of very fine grains exhibiting a vugular or fracture porosity rather than the intergranular porosity shown by sandstones (ii) Carbonates react much more rapidly with hydrochloric acid than sandstones, for the same formation temperature Also, the use of mud acid is prohibited due to the limited solubility of calcium fluoride o Carbonates are normally found as massive deposits of chalk, limestone or dolomite Their constituent particles are much smaller than the typical sand grains found in clastic formations They will have undergone large porosity and permeability reductions during burial and diagenesis Although they are often pure (>95% wt carbonate), they can also include iron minerals, clays and silicaceous materials giving them a very variable composition o The many possible diagenetic processes can lead to formations with similar chemical compositions having a strength that varies from very strong to behaving similar to toothpaste Strong and weak layers can be present a small distance apart This complicates the planning of well completion - and stimulation - procedures Carbonate acidizing is a more difficult process to predict the outcome than sandstone acidizing because, despite the chemistry of the process being much simpler than that of sandstone acidizing, the physics is more complex In carbonates, surface reaction rates are very high, so mass transfer often limits the overall reaction rate, leading to highly non‐uniform dissolution patterns A few large channels, called wormholes, are created, such as shown in the figure following, caused by the non‐uniform dissolution of limestone by HCl in a linear core flood (Hoefner and Fogler, 1988.) Wormholes Wormholes: o “Wormholes”,created by dissolution of the rock uniformly (Hoefner and Fogler, 1988.), consist of a main channel from which many highly branched structures are formed o The structure of these wormhole patterns will depend on many factors, including (but not limited to) • flow geometry, • injection rate, • reaction kinetics, and • mass transfer rates Since wormholes are much larger than the pores in nonvugular carbonates, the pressure drop through the region penetrated by wormholes will be insignificant Thus, in matrix acidizing, knowledge of the depth of penetration of wormholes allows a prediction of the effect of acidizing on the skin effect Wormholing is also very significant in acid fracturing, as it will increase fluid loss rates, limiting the penetration of acid down the fracture Wormholes Wormhole Formation and Growth: o Wormholes form in a dissolution process due to – More acid tends to flow through the larger pores – Since reaction rate is fast almost all acid entering both small & large pores will react causing a dissolution of pore wall – Since larger pores receive more acid they tend enlarge at a rate higher than the rate at which smaller pores – This nonlinear process eventually produce a wormhole o This nonlinear process leading to wormhole formation will occur if the reactions are, – transfer limited ( fast reaction rates compared to mass transfer) – mixed kinetics, ( i.e the mass transfer and surface reaction rate are similar in size.) The injected acid does not dissolve the rock uniformly, instead it forms “wormholes” Wormholes o The number and extent of the wormholes depend on: (i) the carbonate formation’s reactivity (high reaction rates promote few, long wormholes); (ii) the acid leak-off rate into the matrix (controlled by formation permeability, acid and formation fluid viscosities and the injection pressure overbalance); (iii) The presence of higher permeability streaks, fractures, vugs etc will determine the preferred direction of wormhole growth At a very low injection rate, the inlet face of the rock will be slowly consumed as acid diffuses to the surface, – This type of dissolution will not occur in a practical acidizing situation in limestone – However, it is of interest as the limiting case as flow rate approaches zero With increasing flow rate, a dominant wormhole (or a few) forms and propagates into the porous medium At relatively low injection rates there will be little branching, and only one or a few large wormholes will be formed, called diffusion‐limited wormholing Select of acid o Hydrochloric acid is used to: (i) bypass drilling or completion damage by dissolving the rock matrix; (ii) widen natural fractures or secondary porosity so as to improve fluid conductivityto the wellbore (iii) increase the effective wellbore radius by wormhole formation o Dolomite reacts much more slowly with Hydrochloric Acid than chalk or limestone - the optimum reaction rate is achieved with a concentration of 28% wt HCl acid for all dolomite reservoirs o 15% wt HCl is used with the other carbonate formation types o The amount of rock dissolved by the acid is determined by: constant = {volume acid * concentration acid * reaction stochiometry}; depends on the units employed e.g 1m3 of 4% wt HCl will dissolve 206kg of limestone with a volume of 0.073m3 assuming a porosity of 5% vol The corresponding amounts dissolved when dolomite is treated are some 7.5% smaller Choices limited by corrosion realities Weak acids are suggested for perforation fluid and cleanup, and strong acids are recommended for other treatments All theorical models of wormhole propagation predict deeper penetration for higher acid strengths, so a high concentration of acid is always preferable for wormholes Recommended acid type and strength for carbonate acidizing (McLeod, 1984) Select of acid volume o The acid volume can be calculated with two methods: Daccord’s wormhole propagation model and The volumetric model The former is usually optimistic, whereas the latter is more realistic (Economides et al 1994) The volume injected will directly affect the wormhole length, in turn affecting post treatment skin Assuming infinite conductivity in wormholes that have penetrated past any damage radius, post treatment skin can be calculated from: S = ln(rac / rw) Ex: What would be the resultant skin from a treatment developing 1ft wormhole penetration in a 0.3ft radius wellbore? Solution: S = ‐ln(1.3/0.3) = ‐1.4 Note that unlike sandstone stimulation , we are developing negative skin Select of acid volume o Daccord’s Wormhole Progression Model Select of acid volume o Volumetric Wormhole Model Select of acid volume o Example: A 28 wt% HCl is needed to propagate wormholes ft from a 0.328‐ft radius wellbore in a limestone formation (specific gra vity 2.71) with a porosity of 0.15 The designed injection rate is 0.1 bbl/min‐ft, the diffusion coefficient is 10‐9 m2 /sec, and the density of the 28% HCl is 1.14 SG In linear core floods, 1.5 pore volume is needed for wormhole breakthrough at the end of the core Calculate the acid volume requirement using (a) Daccord’s model, and (b) the volumetric model Sol: Select of injection rate o The maximum injection rate and pressure for carbonate acidizing can be calculated the same way as that for sandstone acidizing Models of wormhole propagation predict that wormhole velocity increases with injection rate to the power of 1⁄2 to Therefore, the maximum injection rate is preferable o However, this approach may require more acid volume If the acid volume is constrained, a slower injection rate may be preferable If a sufficient acid volume is available, the max injection rate is recommended for limestone formations A lower injection rate may be preferable for dolomites This allows the temperature of the acid entering the formation to increase, and thus, the reaction rate increases – At a sufficiently high temperature, the dolomite‐HCl reaction may become diffusion limited, leading to much faster wormhole propagation; that is, at an elevated temperature, dolomite will behave more and more like limestone • By increasing the reaction rate, acid will penetrate farther into the formation Select of injection rate • For a wormhole to form, the initial pore radius must be large enough to allow acid transport beyond the pore inlet • According to Daccord et al (1989, 1993), the Peclet number P e , which represents the ratio of axial flow to radial transport in the pores, is the dimensionless variable governing the transition between compact dissolution at low rates and wormholing at higher rates for a transport-limited reaction (i.e., calcite and high-temperature dolomite) Select of treatment type Matrix Treatments: Wormhole formation during matrix treatments improves the well inflow performance by providing a high conductivity channel at depth from the wellbore They are created using either a: (a) low rate, low volume, low acid concentration treatment (Typical values are 0.004m /min/m, 0.3m /m and 14% wt HCl for the injection rate, injection concentration and acid concentration respectively) The low rate and long contact time encourages wormhole formation and the bypassing of shallow formation damage This type of treatment is most suitable for short intervals (< 12m) (b) high(er) rate, large volume, high concentration treatment (Typical values are 0.025m /min/m, 1.6m /m and 14-28% wt HCl respectively) The larger acid volume compensates for the reduced wormhole formation caused by the use of the higher pump rate Ball sealers are more effective in high rate treatments - making them more suitable for treating longer perforated zones Both types of treatments have been applied with success - the preferred method probably depends on the local situation with regard to formation damage, presence of natural fractures & vugs etc Select of treatment type Acid Wash (or Soak) Type Treatments: Wormhole formation is undesirable if the treatment objective is to remove near well bore damage (e.g perforations plugged with drilling mud, cement etc.) present in a new completion or after a workover This is because forming the wormhole will consume a large part of the available acid Wormhole formation is avoided by keeping the injection rate very low (