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