Development of Acid Fracturing Model for Naturally Fractured Reservoirs

A successful application of the acid fracturing stimulation in naturally fractured carbonate reservoirs is challenged by an increased acid leakoff into natural fractures intersecting a hydraulically induced (main) fracture. This may limit the propagation of the main fracture resulting in conductivity reduction. At the same time, the etching of the natural fractures by the leakoff acid may increase the overall conductivity of the fracture system resulting in enhanced well productivity. The effect of the natural fractures on the efficiency of acid fracturing stimulation treatment is not fully understood. A model was developed to evaluate the acid fracturing performance in naturally fractured carbonate reservoirs. The model simulates an acid fracturing stimulation and production in a vertical well located in the center of a rectangular reservoir. The well contains a bi-wing vertical hydraulically induced fracture intersected by symmetric transverse natural fractures. The model simulates the acid flow and reaction in both the hydraulically induced fracture and the natural fractures. The acid leakoff is simulated by modeling the flow through the porous media. The model estimates the conductivity of the main fracture and the natural fractures based on the final fracture width achieved during the acid injection. Finally, the well productivity is calculated by simulating the flow through the porous media, including the main fracture and the natural fractures. Simulation results showed that the presence of natural fractures may enhance the post-stimulation productivity compared to the reservoirs without natural fractures. It was found that the natural fracture length and spacing had a significant effect on the stimulation efficiency. The well productivity decreased with increasing natural fracture length. Also, for very closely spaced and very widely spaced natural fractures the post-stimulation productivity in a naturally fractured reservoir was lower than the productivity in a reservoir without natural fractures. The well productivity could be enhanced by increasing the injection rate. In high permeability formations the performance was improved by increasing the total volume of acid injected in order to compensate for a high acid leakoff rate