Microemulsion Surfactant Improved Performance and Cost of Single-Phase Low Viscosity Acid Systems

A significant portion of global oil and gas reserves are found in carbonate reservoirs, primarily composed of calcite and dolomite. To enhance production from these reservoirs, acid stimulation technologies are used to create conductive fractures or wormholes. Recently, we reported the use of single-phase low viscosity acid systems (LVAS), which is a mixture of concentrated organic and inorganic acids. However, the main challenge associated with LVAS is its higher cost when compared to emulsified acid. A microemulsion surfactant, designed to improve flowback characteristics, achieved a desired retardation effect on acid and rock reaction rates. The properties, such as surface tension, stability on dilution and emulsion testing of the acidizing fluid containing acid retarder, were determined at a concentration of 2 gpt. To evaluate the performance of the acid, static dissolution testing was performed followed by corrosion evaluation at high temperature and finally characterization of acid propagation in core samples using a coreflood setup. Finally, CT scans of the acidized cores were undertaken to analyze wormhole propagation. Static dissolution experiments were performed using calcite cores with acid formulations containing different ratios of organic acid to hydrochloric acid (HCl), both with and without the retarder. The corrosion rate was measured using T-95 metal coupons. Core flow tests were performed at 300°F, comparing the original and modified LVAS formulations to ascertain the volume of acid needed to achieve breakthrough. The acid retarder (FBE-R) was developed from a blend of ester or terpene solvents, alcohols, and surfactants. The modified formulation-maintained characteristics such as reaction rate with calcite, acid propagation and acceptable corrosion performance with tubing metallurgies. The addition of acid retarder (FBE-R) to the modified LVAS acid system (80/20 of 28 wt.% HCl / 70 wt.% MSA) enhanced its performance while reducing the volume fraction of the organic acid and also maintaining a corrosion rate below 0.05 lb./ft2. Acid penetration rate into the core achieved similar volume of acid to breakthrough compared to the LVAS acid system. In addition, the reduction of 60 % (from 400 gpt to 160 gpt) in the required volumes of MSA would result in significant cost reduction in the acid stimulation cost. The utilization of the new microemulsion surfactant, in conjunction with the LVAS acid system, has led to a significant reduction in the volume of MSA, decreasing the amount needed from 400 gpt to around 160 gpt. Such economic efficiency will support the wide implementation of the LVAS acid in the field to stimulate oil and gas wells drilled in carbonate reservoirs.