Development of Acid Fracturing Model for Naturally Fractured Reservoirs

Summary During an acid fracturing treatment in a carbonate reservoir, acid is injected into the formation, thus creating hydraulic fractures and opening existing natural fractures. As the acid flows into natural fractures that intersect hydraulic fractures (main fractures), it etches the walls of the natural fractures, which then increases the natural fractures’ width and generates conductivity. On the other hand, because of the increased acid leakoff into natural fractures, the acid volume in the main fracture decreases, resulting in less conductivity for the main fracture. Existing acid fracturing models estimate the fracture conductivity by assuming that the acid flows and reacts in the hydraulic fractures only. 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 was developed to predict the acid fracturing performance in naturally fractured reservoirs. The model assumed that the main fracture was intersected by transverse symmetric natural fractures. The model simulated the acid transport, acid/rock reaction, fracture width increase resulting from etching of the fracture walls, and acid leakoff through natural fractures. The model also assumed that the flow (into natural fractures) and the leakoff were pressure–dependent and were changing with time. The conductivity calculation was based on the previously developed correlation that accounts for the heterogeneous nature of carbonate rock. The effect of the natural fractures’ geometry on leakoff and created fracture conductivity was investigated. The results showed that length and dynamic width, as well as the natural–fracture spacing, played 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 affected the etching of the natural fractures and the resultant conductivity. The aim of the model is to enable better prediction of the acid fracture conductivity for naturally fractured carbonate reservoirs and improve the feasibility of acid fracturing applications for this type of formation.