Experimental Evaluation of Lactic Acid for Matrix Acidizing of Carbonates

Summary To improve the efficiency of standard hydrochloric acid (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 with HCl, which is cheaper and sufficient in most applications. Lactic acid is an organic acid that is less corrosive and has reduced reactivity compared with HCl. The advantages and application of lactic acid have not been studied extensively like other alternative acids. To evaluate lactic acid as a viable alternative acid system, we conducted a series of linear coreflood matrix acidizing experiments using Indiana limestone rock samples with 40 wt% 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. Coreflooding tests were also conducted at high temperatures and lower concentrations to observe a change in behavior and performance. Lactic acid performance was analyzed by comparing pore volumes (PVs) to breakthrough (PVbt) with previous HCl experiments. Because HCl exhibits low efficiency when injected below the optimal condition, lactic acid was found to be more efficient than HCl at injection rates below optimum for 40 wt%. At a temperature of 150°F, lactic acid maintained similar PVbt over the range of injection rates. Wormhole geometry from computed tomography (CT) imagery and pressure response data was studied to identify unique characteristics or behaviors of lactic acid. CT images of lactic acid-generated wormholes reveal a geometry vs. injection rate relationship that is contrary to the conventional understanding of wormhole growth patterns. The images show extensive branching in most low injection rate tests. The results are characterized as appearing to have self-diverting behavior. At an elevated temperature of 240°F, precipitation of lactic acid after reacting with calcium carbonate became visible. Pressure differential data across the core showed the pressure drop increased in nearly every experiment. When lower lactic acid concentrations were used, the pressure differential increase was not observed. Precipitation occurred during and immediately after most high-temperature experiments, and when the concentration is above 30 wt%, it is suspected to be plugging permeable channels. The study concludes that at a temperature of 150°F (relatively low compared with 240°F), lactic acid has advantages in wormhole efficiency when compared with HCl. Combined with less corrosiveness to the well tubulars and surface equipment, lactic acid can be a good chemical system for carbonate acidizing. For high-temperature formation, lower concentration lactic acid can be used as long as wormhole efficiency is not compromised. It is recommended that for each specific reservoir formation, the appropriate lactic acid concentration should be identified from laboratory tests to ensure the stimulation efficiency, avoid detrimental precipitation, and save operation costs.