Systematic Evaluation of Alternative Acid System for High-Temperature Carbonate Formation

Alternative acid systems for carbonate acidizing have been developed to overcome some weaknesses of conventionally used hydrochloric acid. Such systems are of particular interest for the reservoirs with high temperature because of high reactivity and corrosiveness of HCl at temperatures above 200°F. The main advantages of the evaluated alternative acid include better wormhole efficiency at lower injection rates and less corrosiveness to protect tubulars and surface equipment. In a high-temperature environment, the performance of alternative acids needs to be evaluated for the feasibility of being an effective stimulation fluid. Even though there are plenty of core flood data in the literature for carbonate rocks, there are fewer studies/publications that provide acid performance at high temperatures. In this paper, we present an experimental study of an alternative acid over a wide range of temperatures up to 300°F. The goal is to confirm the applicability and identify the optimal conditions for field operations. We conducted core flood tests using the alternative acid to generate wormhole efficiency curves with different interstitial velocities. The optimal condition was determined from the experimental results for each temperature tested. The temperature for testing ranged from 125°F to 300°F. The tested acid system created a preferred wormhole structure with a dominant wormhole for all test conditions. This indicates that the alternative acid system can be used as a matrix acidizing fluid for carbonate reservoirs over a broad range of temperatures up to 300°F. As observed by HCl core flood tests, wormhole efficiency for the alternative acid also decreases with increasing temperature when compared with its performance at low temperature, but it showed reasonable results of wormholing, while HCl experiments was not practical at 300°F. The challenge of stimulating high-temperature formations can be addressed when the acid system and injection condition are designed correctly. The data presented in this paper add new information for high-temperature acid core flood testing, and provide useful information for matrix acidizing design using alternative acid systems.