Polymer-based gelled (or in-situ gelled) and emulsified acids have been used for matrix acidization of high-temperature carbonate reservoirs for several years. Gelled and emulsified acids are typically used to help reduce the leakoff rate from the wormhole walls as compared to non-gelled 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 to non-gelled acids. However, there are very few modeling or quantitative theoretical studies regarding carbonate acidization with gelled and emulsified acids that can be tested in laboratory or field scale. More specifically, an analysis of the 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 dimension (3-D), which can help in terms of design of gelled and emulsified acids properties to achieve lower leakoff rate and deeper penetration of wormholes.
The authors present 3-D numerical simulations of carbonate acidization with HCl, gelled, and emulsified using a two-scale continuum (TSC) model. Using the model, the effect of transport and rheological properties of these non-Newtonian acids on the acidization curve and dissolution pattern is analyzed and compared to the available laboratory scale experimental data. From the numerical simulations, it has been observed that a lower amount of acid is necessary to breakthrough, and thinner wormholes are formed for both gelled and emulsified acids compared to neat hydrochloric acid (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 to neat HCl. Finally, the authors develop a wormholing criterion for acids whose rheological behavior can be described by the power-law. This criterion can be used to estimate the optimum injection rate for vuggy and nonvuggy carbonates.