Numerical Simulation of Reactive Flow in Fractured Vuggy Carbonate Reservoirs Considering Hydro-Mechanical-Chemical Coupling Effects

Fractured vuggy carbonate reservoirs are critically important, contributing significantly to hydrocarbon reserves and production. The presence of fractures and vugs distinctly influences fluid flow and transport within carbonate rocks, differentiating fractured vuggy carbonate reservoirs from most other geological formations. Apart from matrix carbonate rocks, isolated fractured vuggy carbonate reservoirs are still the targets for acid stimulation due to the limited contribution of isolated fractures and vugs to fluid flow capacities. This study is motivated to investigate the acid stimulation process in isolated fractured vuggy carbonate reservoirs. In this work, the classical two- scale continuum model has been extended to describe the transport and reactive dissolution processes within complex media comprising matrix, fractures, and vugs. The discrete fracture model and the Navier- Stokes equation are used to respectively characterize fluid transport in the fractures and vugs regions. Fluid interactions between different regions are governed by the extended Beavers- JosephSaffman (BJS) interface conditions. Dynamic boundary conditions are applied to describe the dissolution and deformation behaviors at the boundaries of vugs. In addition, Biot equations are utilized to specifically examine the mechanical responses within the poroelastic region during the acid stimulation process. A finite element model has been developed, incorporating an effective loosely coupled sequential iterative scheme for the numerical discretization and solution of the coupled hydrological- mechanical- chemical control equations. The simulation results show that the presence of fractures and vugs in carbonate formations does not perturb the equilibrium conditions necessary for wormhole formation, thereby preserving the dissolution patterns associated with a specific acid injection rate. Nevertheless, mechanical stress shows a significant influence on fracture closure behavior. The stress- induced alteration in the acid flow and dissolution structures necessitates an increased pore volume to breakthrough (PVBT) to attain comparable dissolution effects. The increment in acid breakthrough volume finally escalates both the operational costs and complexity.