Acid Fracture Design Optimization in Naturally Fractured CarbonateReservoirs

Carbonate formations, which are candidates for acid fracturing, are usually naturally fractured. Theexistence of fracture networks has important impacts on fracture design outcomes. Previous studies haveinvestigated the interaction between induced and natural fractures. However, these studies have seldomconsidered the impact of reactive fluid systems. In this work, a model was developed to explain aciddistribution in hydraulic and intersecting natural fractures. This model was then used to optimize acidfracture design parameters based on the goal of maximizing productivity. A fracture propagation model coupled with acid transport, reaction, and heat transfer was employed todetermine the acid etched-width distribution and stimulated reservoir area. The outcome of the coupledmodel was the acid dissolution and conductivity distribution of a natural fracture network intersecting ahydraulic fracture. Then, a productivity model was utilized to evaluate the performance of the acid fracturedwell. A parametric study was also conducted to understand the impact of natural fractures on the optimumdesign conditions. Different natural fracture intensities (i.e., spacing) were investigated at different reservoirpermeability. It was observed that the existence of natural fractures significantly altered acid placement inthe reservoir. The result was well productivity quite different from what was seen in cases with no naturalfractures. Also, the optimum design conditions (e.g., injection rate) differed based on the natural fractures'characteristics and reservoir properties. It was found that the existence of natural fractures significantlyreduced the productivity of fractured wells. The model developed here was used to explain the complex interactions of acid fractures in naturallyfractured carbonate formations. The effects of natural fractures on acid placement and optimum designconditions can now be estimated. Such information, which has rarely been considered, is imperative forbetter stimulation design in this type of formation.