Lactic acid is an organic acid that is less corrosive and less reactive than HCl. The benefits and applications of lactic acid have not been as extensively studied as other alternative acids.
To evaluate the performance of lactic acid as a viable alternative acid system, a series of linear coreflood matrix acidizing experiments were conducted using Indiana limestone samples at two temperatures and different injection rates using a 40% lactic acid solution.
The objectives were to characterize the wormholes generated by lactic acid and to determine the appropriate conditions under which lactic acid performs better than HCl.
Coreflood tests were also performed at high temperatures and lower concentrations to observe its behavior and performance changes.
The performance of lactic acid was analyzed by comparing the pore volume at breakthrough (PVbt) with results from previous hydrochloric acid experiments. Since hydrochloric acid is less efficient when injected at less than optimal conditions, lactic acid was found to be more efficient than hydrochloric acid at 40% mass fraction and injection rates less than optimal. At 150°F, lactic acid maintained similar PVbt over a range of injection rates.
Wormhole geometry was investigated using computed tomography ( CT ) images and pressure response data to identify unique properties or behaviors of lactic acid. CT images showed that the relationship between wormhole geometry produced by lactic acid and injection rate was contrary to traditional understanding of wormhole growth patterns. Images showed that the wormholes were heavily branched in most of the low injection rate tests. These results suggest that lactic acid appears to have self-steering behavior.
At 240°F, precipitation was evident after lactic acid reacted with calcium carbonate. Pressure differential data across the core showed an increase in pressure drop in almost every experiment. No increase in pressure differential was observed when lower concentrations of lactic acid were used. Precipitation occurs during and immediately after most high-temperature experiments and can block permeation pathways at concentrations above 30% by mass.
The study concluded that lactic acid has an advantage in wormhole generation efficiency compared to hydrochloric acid at 150°F (relatively low temperature compared to 240°F). Combined with its low corrosiveness to wellbore tubing and surface equipment, lactic acid can be an excellent chemical system for carbonate acidizing. For high-temperature formations, lower concentrations of lactic acid can be used as long as the wormhole generation efficiency is not affected. It is recommended that the appropriate lactic acid concentration be determined through laboratory testing for each specific reservoir to ensure the efficiency of the stimulation measure, avoid the generation of harmful precipitation, and save operating costs.