Carbonate Matrix Acidizing with Acetic Acid

Abstract

Abstract Conventional matrix acidizing in carbonate reservoirs uses hydrochloric acid to remove formation damage and stimulate well performance. Many experimental and theoretical studies in carbonate acidizing have confirmed the existence of an optimal acid injection rate at which major wormholes are formed, and the benefit from stimulation is maximized. This optimal rate depends on reservoir conditions, rock properties and chemical reaction rate. At high reservoir temperature, the optimal rate of hydrochloric acid is usually too high, and it sometimes is beyond the maximum injection allowed (the rate to avoid fracturing formation). In this case, weak acids, such as acetic acid, are alternative fluids to stimulate the wells. In our previous study, a theoretical model showed that under the same condition, the optimal injection rate for weak acids is relatively lower than the one for strong acids. This paper presents an experimental study of the wormholing process in carbonate acidizing with acetic acid. Carbonate rock samples were acidized, and the effectiveness of the process and the optimal injection rate were studied by measuring the acid volume needed to propagate wormholes through the cores and by making castings of the wormhole structures after acidizing. The experimental results from this study confirmed that the optimal injection rate of acetic acid is lower than the optimal rate for hydrochloric acid. It also showed from the castings that the radius of wormholes created by acetic acid at the optimal rate is larger than that created by hydrochloric acid at the optimal rate. We then demonstrate how these results can be used to determine the optimal acid system and injection schedule for field application.