A New Leakoff Analysis Approach for Acid Fracturing in Naturally Fractured Carbonate Gas Reservoirs

Although most carbonate reservoirs are naturally fractured, no quantitative research has been conducted on the calculation of acid leakoff in naturally fractured carbonate reservoirs. In this paper, a comprehensive dual-porosity gas/acid flow model is developed to predict the acid leakoff in naturally fractured carbonate gas reservoirs. The model considers many factors, including the acid-rock reaction, the acid flow, and the fracture width variation caused by rock dissolution on the fractured surfaces. On this basis, a numerical simulator was designed for the model, verified through comparison with Eclipse, and applied to simulate an actual naturally fractured gas carbonate reservoir. The results show that the acid leakoff is completely different from that without considering acid-rock reaction on the fracture surfaces. The acid leakoff is controlled by viscosity and compressibility, and the leakoff rate increased first and then decreased. Meanwhile, the leakoff rate of the traditional non-reactive liquid decreased continuously. Moreover, the acid concentration increased the main natural fracture width and the acid leakoff, revealing the importance to maintain a high acid viscosity for acid fracturing. The main natural fractures (width>10 mu m) dominated the leakoff, and overshadowed the contribution of micro-fractures (width<5 mu m) to leakoff. Finally, the main natural fractures exerted a greater impact on acid leakoff than any other part of the reservoir. The research findings provide insights into the mechanism of acid leakoff in naturally fractured formation and offer an accurate method for calculating leakoff in naturally fractured carbonate gas reservoirs.