Field Evaluation of Viscous Pads In Acidizing Carbonate Rock

An acidizing technique has been developed for treating carbonate reservoirs that will give deep penetrating fractures with high flow capacity. Production results from these treatments have been excellent. The technique uses the principle of hydraulic fracturing with a thickened fluid. Instead of using a proppant in the fluid, acid is used to produce flow capacity by controlled etching of the fracture system. Overall treatment success has been good. New and old producers with varying productive capacity have been treated. New wells have performed at least equal to and in most cases better than conventionally treated offset wells. Some old wells have performed exceptionally well; others indicated no favorable increase in production. The advent of complex fracturing fluids led to the idea of using a viscous pad to control leak-off of acid for more fracture height and width. These factors, it was theorized, would give greater acid penetration. The method was first tried in a well where conventional treatments had been unsuccessful. Results, which were good, led to laboratory work to determine possible reasons of success. This work indicated that the fingering of thin acid through a viscous fluid was the key to the treatment. Figures 1, 2 and 3 show laboratory models illustrating what occurs in this treatment. An analytical procedure for treatment design followed this investigation. Figure 4 is a typical computer-designed treatment.