Development of Acid Fracturing Model for Naturally Fractured Reservoirs

Abstract

Abstract During an acid fracturing treatment in a carbonate reservoir, acid is injected into the formation creating hydraulic fractures and opening existing natural fractures. As the acid flows into natural fractures intersecting hydraulic fractures (main fractures), it etches the walls of the natural fracture, which increases the natural fractures' width and generates conductivity. On the other hand, because of the existence of natural fractures, the acid volume in the main fracture reduces, resulting in less conductivity for the main fracture. Existing acid fracturing models estimate the fracture conductivity by assuming the acid flows and reacts in the hydraulic fractures only. In order to accurately predict the performance of acid fracturing in naturally fractured carbonate reservoirs, the acid etching of natural fractures should be taken into account when calculating the overall fracture conductivity. A model is developed to predict the acid fracturing performance in naturally fractured reservoirs. The model assumes that the main fracture is intersected by transverse symmetric natural fractures. The model simulates the acid transport, acid-rock reaction, fracture width increase due to etching of fracture walls, and acid leakoff through natural fractures. The model also assumes that the flow into natural fractures and the leakoff are pressure dependent and are changing with time. The conductivity calculation is based on the previously developed correlation that takes into account the small scale rock heterogeneities. The conductivity of natural fractures was found to be a strong function of the leakoff Peclet number, which depends on the velocity of acid flow into natural fractures from an intersecting hydraulic fracture. The effect of natural fractures' geometry on leakoff Peclet number and created fracture conductivity was investigated. The results show that both length and dynamic width, as well as the number of natural fractures, play a significant role in defining the leakoff rate and the conductivity of the hydraulic fracture and the natural fractures. It was also found that the position of the natural fractures along the hydraulic fracture length affects the etching of the natural fractures and the resultant conductivity. The model will enable the prediction of acid fracture conductivity for naturally fractured reservoirs and improve the feasibility of acid fracturing applications for these type of formations.