A Novel Emulsified Acid System for Stimulation of Very High-Temperature Carbonate Reservoirs

Because the world demand for energy is expected to continue growing, exploration is turning to deeper and high-temperature reservoirs. Such reservoirs include fields with high bottomhole static temperatures (BHSTs), such as the Ursa (250°F) and Thunder Horse (280°F) in Gulf of Mexico (GOM). Acid stimulation of such reservoirs at high temperature is a challenging task. Emulsified acid systems are expected to perform better in reservoirs with BHSTs ranging from 275 to 375°F compared to nonretarded acids and gelled acid systems. However, fluid stability and the inhibition of corrosion are major challenges to overcome for successful implementation of this technology. Emulsion instability and the corrosion rate are interrelated, and both increase with higher temperature. Also, fluid stability decreases as a result of corrosion of the metal surfaces. At the same time, an excessive addition of corrosion inhibitor destabilizes the fluid system. Hence, the proper selection and balance between the corrosion inhibitor and emulsifiers are required. Three different types of corrosion inhibitors were evaluated, and an emulsified system was designed with proper optimization of various ingredients, including corrosion in hibitor, an intensifier, and a cationic emulsifier. The system was tested for stability and corrosion loss with static corrosion test using P-110 coupons. After reviewing the literature, it is believed that this emulsified system is the only one to pass static corrosion tests at 275°F for 4 hr and remain stable at 300°F for 2 hr with 28% acid strength. This enables the acid stimulation of carbonate reservoirs having BHSTs up to 300°F while reducing the corrosion rate. As per the study, the effect of the intensifier was different to that found in plain acid, suggesting possible interactions of the additives with the emulsifier. Because fluid stability and the rate of corrosion are interrelated, they should be evaluated together, especially for designing emulsified acid systems for stimulation of very high-temperature carbonate reservoirs.