Acid Placement: An Effective VES System to Stimulate High-Temperature Carbonate Formations

Abstract

AbstractThe gelling performance of VES acid systems dramatically weakens at high temperature. Therefore, these fluids are typically limited to placement in relatively low-temperature carbonate formations. This study was conducted to introduce a new VES system that can gel and maintain useful viscosity up to 300F.The new surfactant system is completely compatible with HCl, brine, and even high iron contamination. Rheological studies defined the operational temperature limit where viscosity was sufficient for the new VES system to be used successfully as diversion agent. It was found that initial acid concentration and the degree of acid neutralization were critical parameters affecting the new VES system diversion performance at elevated temperatures.The effects of four corrosion inhibitors were examined. One is recommended for use with this system because it minimizes negative effects on the operational temperature range. Also, it enhances the values of elastic modulus, which enhances the VES system's diversion performance.Coreflood studies using limestone and dolomite cores confirmed that the new VES system increased differential pressure sufficiently to achieve diversion. For limestone cores, the pressure drop increased by a factor of 10 during VES acid injection; for dolomite cores, the pressure drop increased by a factor of 100. The pressure drop changed in a cycling manner, where the crest and trough of each cycle increased with time. Cycling of the pressure drop indicates that the acid was able to change its direction inside the core. Coreflood testing also indicated that there is no need for a breaker, as 18% permeability enhancement was observed with CaCl2 brine flowback. This paper will discuss the results obtained and recommend the conditions under which the new system is most likely to be successful in the field.