Experimental Evaluation of Lactic Acid for Matrix Acidizing of Carbonates

Abstract

Abstract To improve efficiency of standard HCl stimulation treatments, many alternative acid systems have been developed to mitigate corrosion, increase wormhole efficiency, and divert fluids for better acid coverage. However, these alternative systems come at a price compared to HCl which is cheaper and sufficient in most applications. Lactic acid is an organic acid that is less corrosive and has reduced reactivity compared to HCl. The advantage and application of lactic acid has not been studied extensively like other alternative acids. To evaluate lactic acid as a viable alternative acid system, we conducted a series of linear core flood matrix acidizing experiments using 40 weight percent lactic acid at two temperatures over a range of injection rates. The goal was to characterize the wormholes created by lactic acid and identify the appropriate condition that lactic acid can outperform HCl. Core flood tests were also conducted at high temperature and lower concentrations to observe change in behavior and performance. Lactic acid performance was analyzed by comparing pore volumes to breakthrough (PVbt) with previous HCl experiments. Lactic acid was found to be more efficient than HCl at injection rates below optimum for 40 %. At the lower temperature of 150 °F, lactic acid maintained similar PVbt over the range of injection rates. Wormhole geometry from CT imagery and pressure response data was studied to identify unique characteristics or behavior lactic acid may have. CT images of lactic acid generated wormholes reveal a geometry versus injection rate relationship that is contrary to the conventional understanding of wormhole growth patterns. Images show extensive branching in most low injection rate tests. The results are characterized as appearing to have self-diverting behavior. Pressure differential data across the core shows pressure drop increases in nearly every experiment. For the lower concentration tests, pressure differential increase was not observed for 10 % and 20 % and only occurred at 30 % lactic acid. Precipitation occurred during and immediately after most high temperature experiments and after the 30 % test which is suspected to be plugging permeable channels. Further investigation may provide more insight into the unique capabilities of a simple yet overlooked acid system.