ABSTRACT
The basic physics of propped fracturing that are described in the other chapters in this book all hold true for acid fracturing. The main difference is that in acid fracturing the chemical details are often quite complex and sometimes difžcult to understand. Reactive µuids (either strong acid such as hydrochloric [HCl] or a weak acid such as the organic acids acetic or formic) are injected at a rate/pressure sufžcient to open and propagate a hydraulic fracture. Acid µows along the fracture and reacts with the carbonate formation (limestone/dolomite) along the fracture face. Due to the inherent inhomogeneity of most rock, acid etching is not uniform, so rough, uneven faces are created. Thus, when pressure is released and the fracture “closes,” open µow paths back to the wellbore remain-providing (ideally) high fracture conductivity. When considering acid fracturing, there are several factors that help make the decision. Table 13.1 provides a summary of the decision factors, but overall the best candidates for acid fracturing are naturally fractured heterogeneous carbonated that have solubility <70%. The conductivity created by acid fracture occurs because of asperities that are created on the face of the fracture by the acidizing process. As shown in Figure 13.1, the conductivity (fracture width times fracture permeability or KfW) is created by proppant placed in the fracture, whereas in acid fracturing, the conductivity is generated by the acid-etching pattern. Because acid fracturing has no proppant, the created KfW is inžnite that makes acid fracturing particularly appealing in higher perm rock. Figure 13.2 compares the fold of increase that would be expected with a 100 ft acid frac versus several different length propped fractures with žnite conductivities. The main advantages of using an acid frac are as follows:
1. The treatments are more conservative in that there is a low risk of failing to complete the treatment.
