New Insights for the Use of Lactic Acid in Carbonate Acidizing

Abstract Utilizing HCl on deep high-temperature formations is associated with corrosion and reactivity concerns. Organic acids are a common choice for conducting matrix acidizing on carbonate formations where the use of HCl is limited or unfavorable. An organic acid can stimulate as a standalone formulation, mixed with another type of organic acid, or mixed with HCl. One commonly used organic acid is Lactic acid, which has a dissociation constant similar to formic acid, which is approximately 10 times larger than acetic acid. The objective of this work is to show experimental results conducted using lactic acid for matrix acidizing. Solubility tests using a HPHT autoclave were done to evaluate acid-carbonate reaction and produced by-product salts. Coreflood tests were performed on Indiana limestone core plugs to mimic a matrix acidizing procedure and to investigate the amount of pore volumes required to breakthrough. These tests were monitored by using inductively coupled plasma (ICP), X-ray diffraction (XRD), and computed tomography (CT) scans to measure different ion concentrations in solution, to identify precipitated solids, and to evaluate dissolution patterns generated in the coreflood test, respectively. The reaction of lactic acid with carbonate rocks was associated with production of calcium lactate which found to be soluble above 150°F. Lactate-based precipitation was avoided by incorporating gluconic acid along with lactic acid. In addition, lactate-based precipitation was avoided by mixing low concentration lactic acid with HCl. Adding HCl to lactic acid solution allowed for the reduction of lactic acid concentration to a precipitation-free level where lactate ions would be at a minimum. Coreflood results established more understanding of lactic acid utilization in matrix acidizing in terms of minimum pore volumes to breakthrough and generated wormholes. The results highlighted the correlation between acid-core reactivity, injection rate, and dissolution pattern. Coreflood tests showed that the lactic acid blends at optimum injection rate penetrated tested core plugs with minimal acid pore volume without any face dissolution or salt precipitation on the plug faces. Coreflood tests showed that the lactic/gluconic acid blend can stimulate carbonate cores and generate an optimum wormhole. A lactic/HCl blend was more reactive than a formic/HCl or lactic/gluconic acid blends as more pore volumes were needed to breakthrough Indiana core plugs. Thereby, a higher injection rate is required to obtain an optimum pore volume to breakthrough despite the high reactivity. The detailed experimental work shown in this study shows major advantages that can be achieved by using different lactic acid-based blends. Among these advantages are favorable dissolution pattern due to lactic acid retardation, and less corrosion rates that can reduce allocated costs for maintenance and replacements.