Evaluation of Enhanced Retarded Acid Penetration in Acid Fracturing Using a Novel Acid Fracture Conductivity Measurement

The success of acid fracturing relies on propagation of live acid a sufficient distance along the created fracture to achieve the desired fracture conductivity. We evaluated a single-phase retarded acid system and compared it with standard hydrochloric acid (HCl) to determine its capability to maintain live acid far down the created fracture. An emulsified acid test was also conducted to provide a comparative reference for alternative acid systems. Using a newly developed acid fracture conductivity experimental procedure, we measured the created acid fracture conductivity for a wide range of treatment conditions. The experimental work utilized 4-inch diameter Indiana limestone cores, each 20 inches in length and sawed in half longitudinally. The permeability of the rock samples is below 2 mD. Shims were placed between the core halves to maintain a 1/16 inch wide fracture with a height of 2 inches. Different acid systems were injected at varying flow velocities and temperatures (up to 250 °F) for prescribed durations, and fracture conductivity was measured at a range of closure stresses. Effluent samples were analyzed for calcium ion concentration using inductively coupled plasma (ICP) to quantify rock dissolution, while computed tomography (CT) scans captured etched fracture morphology before and after acid injection. To measure fracture conductivity under closure stress, we flowed water through the acidized fracture while applying a range of pressures to the Hassler sleeve surrounding the core sample. The performance of the proposed retarded acid system was benchmarked against traditional HCl-based systems of identical concentration (28 weight%) with corrosion inhibitors. From the calcium effluent data and the measured profile of dissolution as a function of position along the fracture, the differences in the two acid systems were quantified. The results of this work demonstrate that the retarded acid system achieves longer etched fractures compared to conventional acids, indicating preferred acid transport and rock dissolution capabilities. These findings provide new insight into the interaction of temperature, flow rate, and acid type on etching efficiency and fracture conductivity in carbonate formations. They contribute to optimized acid fracturing designs tailored for challenging high-temperature reservoirs, enhancing treatment efficiency and reservoir productivity.