Numerical analysis of two-phase acidizing in fractured carbonate rocks

Abstract

A three-dimensional two-phase acidizing model is developed to simulate the acid treatment in fractured carbonate rocks based on a unified pipe-network method. A sequential implicit time technique and an adaptive time step method are used to precisely capture the acidizing process. The Corey model is used to characterize the acid displacement in the water/oil system of the rock matrix, while the Brooks and Corey model is used for the two-phase fluid flow in fractures. The stability and reliability of the current numerical method are evaluated and confirmed by performing numerical verification and convergence analysis with different grid densities. A sensitivity analysis with respect to initial water saturation shows improved acidizing efficiency using the optimized injection rate with a low value of initial water saturation. The effects of fractures, wettability, and porosity heterogeneity on wormhole propagation in fractured carbonate rocks are also investigated. Fractures with dip angles greater than 45° maintain a high acidizing efficiency. More acid is required to abtain acid breakthrough for a fracture-dominant system. However, the injected acid pore volumes to breakthrough (PVBT) decrease for a matrix-dominant system. Higher acidizing efficiency can be achieved if the rock matrix and fractures are water-wet for a fractured carbonate rock. The PVBT in the matrix-dominant system drops markedly when porosity heterogeneity magnitude (dφm) increases from 0.025 to 0.125, and grows when dφm increases from 0.125 to 0.175 due to the generation of multiple wormhole branches. These results provide insights for field-acidizing treatment: Maintaining a high level of initial oil saturation can help increase the acidizing efficiency. A water-wet environment is preferred for acidizing fractured carbonate rocks. Higher pore volumes of acid are required before acid breakthrough in a fracture-dominant system.