Matrix Acidizing of Carbonate Rocks Using New Mixtures of HCl/Methanesulfonic Acid

Abstract

Abstract Successful matrix acidizing of carbonate reservoirs depends on the selection of optimal stimulation fluids. Because of the rapid reaction rate and corrosive nature of HCl in downhole conditions, other alternatives are much in demand. Organic acids, particularly methanesulfonic acid (MSA), offer a viable alternative to HCl in terms of being less reactive as well as less corrosive and environmentally benign. However, MSA is expensive. To reduce the cost, this study proposes to use blend of HCl and MSA for carbonate stimulation, while enhancing the properties of HCl. Coreflood studies were performed and the results were compared to those obtained by equivalent concentrations of the individual acids. Three different ratios of HCl and MSA were used to conduct coreflood experiments on 6-in. long Indiana Limestone cores at 250°F. The volume of acid required to reach breakthrough was recorded, and the cores were analyzed using CT scans. Wormhole structures were identified, and their tortuousities were determined. The effluent samples were analyzed for pH, calcium concentration, and unconsumed acid concentration. Coreflood studies indicated that 5:5 wt% HCl:MSA blend was the most suitable candidate for matrix acidizing among the three blends tested (2.5:7.5 and 7.5:2.5 wt% HCl:MSA being the other two blends investigated). At the optimum injection rate of 7.5 cm3/min, both 2.5:7.5 and 5:5 wt% HCl:MSA mixture required lesser pore volumes (PVs) of acid to reach breakthrough, compared to their individual acid controls. A single, straight, and dominant wormhole was observed with no branching and less tortuousity in both the cases. The control experiments with equivalent concentrations of HCl and MSA required higher PVs of acid to reach breakthrough with branching during wormhole propagation. Calcium ion dissolution was least for the 5:5 wt% mixture among the three blends tested. Higher unconsumed acid concentration was noted in case of 5:5 wt% compared to 2.5:7.5 wt% blend, thus promising greater penetration depth with the same PV of acid. On the other hand, the wormhole formed by the acid blend of 7.5:2.5 wt% HCl:MSA required almost the same PV of acid to reach breakthrough as its corresponding HCl control, and it was more enlarged and tortuous than its corresponding MSA control. 5:5 wt% HCl:MSA blend creates deeper wormholes and retards the HCl reaction with the rock matrix. Major advantages rendered by the new acid mixture include: (1) deeper wormholes that will ultimately result in enhanced well productivity, and (2) cost effectiveness in carbonate stimulation compared to standard systems currently used in the market.