Optimization of acid fracturing for a tight carbonate reservoir

In this study, acid fracturing treatments were simulated for a tight limestone reservoir within a shale formation using FRACPRO software. The purpose was to investigate the optimum acid fracturing design that leads to a higher fracture etched length and width, and higher fracture conductivity. Moreover, the impact of the rock-acid contact time and whether to consider a post-flush or fluids flowback, on the acid fracturing outcomes were also investigated. A simple geological model was constructed which consists of different lithological layers. Different acid fracturing design scenarios were considered starting from a single stage of acid injection to multi-stage treatment. In multi-stage acid treatment, alternate acid-slickwater injection was considered. Plain HCl acids with different concentrations and other acids that are already included in FRACPRO database were used. The results showed that the acid loss during post-flush is among the main problems of the acid fracturing in tight carbonate shale reservoir. For the single stage of acid injection, it is recommended to flow back the acid after well shut-in instead of considering a post-flush stage. The multi-stage alternate acid slickwater injection reduces or even eliminated the acid loss. However, it is recommended to inject a slickwater before well shut-in to reduce the rock-acid contact time, thus reducing the formation damage. The results also showed that the created fracture etched width decreases and the fracture etched length increases as the fracturing stages increase. In this study, because of the low carbonate layer permeability and compressive strength, a two-stage alternate 28% HCl and slickwater injection with a post-flush stage is recommended.