The Effect of Spent Acid on Carbonate Rock Wettability

Abstract

Abstract This is a study aimed at improving the efficiency of the matrix acidizing process, a well stimulation procedure applied to thousands of wells annually around the world. In these treatments, aqueous solutions of strong acid are injected into the formation. The injected water, which is referred to as spent acid after it reacts with the formation, will penetrate deeper into the formation beyond the stimulated zone. The spent acid needs to be easily removed so that the permeability to oil or gas can be restored. The time required for the cleanup of the spent acid from the formation depends on the formation permeability as well as the wettability and relative permeability characteristics. An investigation of the possible changes in the wettability of the carbonate rock due to the invasion of spent acid and as a result of the additives added to the acid treatment showed that the carbonate rock by itself is relatively water wet and spent HCl does not change wettability of the rock. Experiments were further repeated with Zonly-8740, as a surface tension reducer additive and showed that Zonly-8740 changes wettability to gas wetting. A future plan is underway to investigate the effect of various additives and to study the long term effect on the wettability at reservoir conditions.