Fracture acidizing

Fracture acidizing treatments have been generally confined to carbonate formations, including vugular and naturally fractured chalks, limestones, and dolomites. Acid fracturing treatments of carbonates are conducted to serve one or both of two purposes  

Acid fracturing is an alternative to matrix acidizing, as well as an alternative to hydraubc fracturing. The objectives of the fracturing applications are the 

With hydraulic fracturing, fracture conductivity is maintained by propping open a fracture created with a solid material, such as sand, bauxite, ceramic, and certain lighter-weight materials. When used in hydraulic fracturing, these materials are referred to as proppants. 

The dissolution patterns show the asperities. They form a conductive channel when the two halves are placed together—as long as the open channel can be supported under closure at the contact points. Fracture acidizing is discussed in part III. 

In carbonate acidizing, the acids commonly used are as follows  

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The drill pipe itself can be used for formation evaluation (Drill Stem Testing— DST), well stimulation (fracturing, acidizing), and fishing operations. 

In the third type of acidizing, fracture acidizing, acid is injected above the parting or fracture pressure of the formation. The acid reacts with the minerals on the exposed fracture face in a process called etching. With sufficient etching, the fracture does not reseal when normal well production or injection operations are resumed. 

Organic polymers have been used to increase the viscosity of acids. The primary application is in fracture acidizing. Binary and ternary acrylamide copolymers are the most commonly used chemicals for this application. Many of these polymers degrade rapidly in strong acids at temperatures >130 F development of more stable polymers suitable for high temperatures is desirable. Recently developed polymers for this application include acrylamide copolymers with ... 

Acids were an early exception to the no water rule. It was recognized that aqueous solutions of acids would inhibit swelling of clays and shales as well as dissolve any acid-soluble minerals contained in a formation. By 1933 commercial well stimulation with hydrochloric acid was of great interest. A whole separate methodology and treatment chemistry has since evolved around acidizing and fracture acidizing(54). Water emulsions, mainly emulsified acids, and gelled acids thickened with polymeric additives were applied early in the history of well treatment. 

Engelhardt, F., Kuhlein, K., Riegel, U., von Halasz, S., Dawson, J.C., and Reed, A.R. "Water Soluble Copolymers for Use in Fracture Acidizing of Wells," US Patent 4,500,437(1985). Holcomb, D.L., Jr. "A New Concept in Hydrochloric-Hydrofluoric Acid Mixtures for Acidizing Low Permeability Sandstone Formations," 1974 Proc. Annu. Southwest. Pet. Short Course, Lubbock, 41 8. 

Broaddus, G.C. "Preflush Concepts in Fracture Acidizing," 1975 Proc. Annu. Southwest Pet. Short Course, 91 97. 

Coulter, A.W., Alderman, E.N., Cloud, J.E., and Crowe, C.W. "Mathematical Model Simulates Actual Well Conditions in Fracture Acidizing Treatment Design," SPE paper 5004, 1974 SPE Annual Fall Meeting of AIME, Houston. 

Anderson, M.S. Fredrickson, S.E. "Dynamic Etching Tests Aid Fracture Acidizing Treatment Design," SPE/DOE paper 16452, 1987 SPE/DOE Low Permeability Reservoirs Sumposium, Denver. 

Bailey, D.E. and Wickham, J.F. "Sand Fracturing vs Fracture Acidizing," SPE paper 12898, 1984 Rocky Mountain Regional Meeting, Casper, May 21-23. 

A foam, aqueous or non-aqueous, that is injected into a petroleum reservoir to improve the productivity of oil- or gas-producing wells. Some mechanisms of action for foam stimulation fluids include fracturing, acidizing to increase permeability, and diversion of flow. 

Fracture acidizing is a technique to increase the productivity of oil wells. Here acid is injected at high pressures to fracture the rock and form a channel that extends out from the well bore. As the acid flows through the channel it etches the sides of the channel to make it larger and thus less resistant to the flow of oil. Eterive an equation for the concentration profile of acid and the channel width as a function of distance from the well bore. 

At each well, three samples were taken whenever possible. A 500-ml sample, acidified with HNO3, was taken for chemical analysis and 125-ml samples were taken for S and 5D analyses (untreated) and for 5 analysis (added to 5 ml of saturated SrCl2 and 10 ml of 5N NaOH). In addition, as much information as possible was obtained on production rates of oil/gas and water, well manipulation (fracturing, acidizing, etc.), well treatment, scaling problems, and previous water analyses. Well scale was obtained wherever possible. 

In matrix acidizing, the acid treatment is injected at matrix pressures or below formation fracturing pressure. In fracture acidizing, all (or at least a signiflcant portion) of the acid treatment is intentionally pumped above formation fracturing pressure. 

The formic-acetic acid blends are used to a lesser extent. They have been successfully applied in high-temperature fracture acidizing apphcations, where slower reaction and greater depth of live add penetration into the fracture are required. 

Acid fracturing is a treatment in which the fracturing and etching fluids are acid. The fracture is created with a viscous acid system, for example, which also etches the walls of the fracture dining the injection process. These days, either term, fracture acidizing or acid fracturing, may be used to describe the process of creating an acid-etched fracture in a carbonate formation. 

Treatments using fracture acidizing of carbonates are conducted for the purpose of either bypassing formation damage or stimulating undamaged formations. 

As mentioned in chapter 2, fracture acidizing is an alternative to hydraulic (propped) fracturing in carbonates. With fracture acidizing, fracture conductivity is achieved by using acid to etch the walls of the created fracture. With hydraulic fracturing, fracture conductivity is achieved by filling the fracture with solid proppant to hold it open. 

Fracture acidizing can be used to bjqjass formation damage in a carbonate formation. For damage b)qjass, a long fracture may not be needed. The purpose would be similar to that of a frac-and-pack procedure in a sandstone formation. In that case, a propped fracture with length and conductivity suffident to effectively extend the wellbore radius beyond a damage zone is all that is needed— not an extensive, propped fracture. 

In a carbonate formation, stimulation of the formation itself realistically can be achieved only with fracture stimulation methods. The fracture stimulation method can be either hydraulic fracturing or fracture acidizing (acid fracturing). To stimulate an undamaged formation, an extensive conductive flow path deep into the formation must be created. This can be accomplished only through fracturing. 

Although fracture length and conductivity are difficult to predict, fracture acidizing, like hydraulic fracturing, can substantially improve productivity. Moreover, in certain cases, fracture acidizing can increase field... 

Hydrochloric acid. The most common field solution for matrix and fracture acidizing application is 15% HCl. Concentrations may vary between 3 % (for tubing-cleaning application) and 35%, but they are typically between 15% and 28%. Acid solutions containing greater than 15% HCl are called... 

In natural fractures, the ratio of surface area to acid volume is much less, and deeper treatment is possible. In fracture acidizing, the ratio of rock surface area to acid volume is even lower. Very deep stimulation in fracturing applications is therefore possible, espedally with slower-reacting, low-leak-off add systems. 

In fracture acidizing, response depends on final fracture length and conductivity. Length depends on live acid penetration distance. Conductivity depends on the etching pattern (e.g., see figs. 10-1 and 10-2) imparted on the fracture walls by acid and its resulting flow-channel stability. 

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