Carbonate fracture acidizing

APPENDIX C CONVENTIONAL CARBONATE FRACTURE ACIDIZING PROCEDURES 233... 

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 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. 

Most carbonate acid treatments are conducted to remove or bypass formation damage, either real or perceived. If damage is not present, fracturing with acid or with proppant should be considered. If damage is present and believed to be very severe or very deep, fracture acidizing and propped fracturing are the preferred options. 

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