An Efficient Retarded Acid System for Deep and High Temperature Carbonate Reservoirs

Deep carbonate reservoirs have larger reserves, and due to their deeper burial, the average temperature reaches about 140 ~ 180℃. In the process of acid fracturing of deep carbonate reservoirs, the performance of acid fluid directly affects the final simulation performance. The development of high temperature resistant acid system suitable for deep carbonate rocks has always been a difficult problem and challenge in the petroleum industry. However, most of the existing acid systems suffer from poor retardation, high corrosion rates, poor permeability, and instability under high temperature conditions. In this study, we optimized a high temperature resistant retarded acid system suitable for acid fracturing of deep carbonate reservoirs, with a high temperature resistance of 180 °C. Surfactants that can enhance the oil and gas imbibition ability are also added to the acid system. The acid-rock reaction and dynamic corrosion rates under high temperature conditions were quantitatively characterized. The relationship between acid viscosity and temperature was measured by rheometer, and the conductivity of acid etched rock slab was tested. The experimental results showed that the viscosity of the retarded acid system was 14 ~ 22 mPa·s at a temperature of 20 °C, and decreased to less than 1.2 mPa·s as the temperature increased to 180 °C. However, even if the viscosity of the acid system is very low, the acid rock reaction rate of this retarded acid system is only 1.58 × 10–5 gmol/cm2 s at 180 °C, which is about 10 times lower than the existing acid system. The dynamic corrosion rate of this acid system is less than 45.33 g/(m2·h) at 180℃, which is better than the existing acid system. In addition, when the closure pressure is 75 MPa, the rock slab conductivity after acid etching can still be maintained at about 56.3 D·cm. The experimental results showed that the surface tension of the acid system was less than 55 mN/m, and the acid flowback rate reached 89% during the acid etching experiment. In this study, a retarded acid system with a high temperature resistance of 180 °C was optimized, and a surfactant that can enhance the imbibition ability was added to improve the flowback rete of the acid system, which provided guidance for the acid fracturing process of deep carbonate reservoirs.