Reactive-Dissolution Modeling and Experimental Comparison of Wormhole Formation in Carbonates with Gelled and Emulsified Acids

Polymer-based (gelled or in-situ gelled) and emulsified acids have been used for matrix acidization of carbonate reservoirs for several years. Gelled and emulsified acids are typically used for acidization of high-temperature carbonate reservoirs because of their lower reaction rate as compared with nongelled/emulsified acids, resulting in deeper penetration of acid, whereas in-situ gelled acid is used for acid diversion. Literature review indicates that several laboratory-scale experimental studies have been performed to analyze the effect of acid gelation and emulsion on carbonate acidization as compared with nongelled/emulsified acids. However, there are very few modeling or quantitative theoretical studies regarding carbonate acidization with gelled and emulsified acids that can be tested at laboratory or field scale. More specifically, a theoretical analysis of the effect of transport and rheological properties (i.e., shear-thinning behavior) of gelled and emulsified acids on the acidization process is not available in the literature. Therefore, the primary objective of this study is to analyze the effect of transport and rheological properties of gelled and emulsified acids on carbonate acidization in three dimensions, which can help in terms of design of gelled-and emulsified-acid properties to achieve lower leakoff rate and deeper penetration of wormholes. The authors present 3D numerical simulations of carbonate acidization with hydrochloric acid (HCl), gelled acid, and emulsified acid by use of a two-scale-continuum model. By use of this model, the effect of transport and rheological properties of these non-Newtonian acids on the acidization curve and dissolution pattern is analyzed and compared with the available laboratory-scale experimental data. It has been observed from the numerical simulations that a lower amount of acid is necessary to breakthrough, and thinner wormholes are formed for both gelled and emulsified acids compared with neat HCl. Additionally, acidization remains in the optimum dissolution regime for a large variation in terms of acid-injection rate for both gelled and emulsified acids compared with neat HCl. Finally, the authors develop a wormholing criterion for acids, the rheological behavior of which can be described by the power law. This criterion can be used to estimate the optimum injection rate for vuggy and nonvuggy carbonates.