Abstract:

Deep discrete fractured reservoirs show large variations in porous media type—including both discrete and continuous media—and fracture scale, and the porous media are pressure sensitive and prone to deformation under high pressures. Although the existing numerical simulation methods for oil reservoirs take into account discrete media, they do not consider the impact of fracture deformation and therefore are not applicable for deep, discrete fractured reservoirs. To address this problem we developed a new simulation method using a multi-scale fluid-rock coupling model. First, a subscale model for rock matrix, microfractures, small/medium-sized fractures, and discrete fractures was proposed, and a multi-scale fluid-rock coupling model for discrete fracture network was established. Second, mathematical models for fluid-rock coupling at different fracture scales were created. Pressure sensitivity and fluid-rock coupling were treated separately in consideration of fluid flow and stress sensitivity characteristics in fractures of different scales, while stress-sensitive microfractures and rock matrix were treated together in the pressure sensitivity calculation model to establish a compression coefficient calculation method to describe fluid flow in the fractures. As small/medium-sized fractures are easy to close during oil/gas production, it was proposed to establish a calculation model for fracture opening and closing. Furthermore, as discrete fractures control the direction and scale of fluid flow and are easy to fill, a fully coupled mathematical model was established to treat discrete fractures using the fluid-rock coupling model. Third, finite volume and finite element numerical simulation models were established where fluid flow/ volume were discretized using finite flow/volume methods, and the linear equations were solved by using a fully implicit numerical solution method with relatively good convergence. Finally, the new method was verified by comparing theoretical models with actual reservoir models, which show a good agreement between the numerical calculation results using a two-dimensional consolidation model and the theoretical solution with consideration for pressure sensitivity and fluid-rock coupling. This novel, high accuracy simulation method addresses the issues—such as high-pressure, high-stress, and large variations in fracture scale—associated with deep, discrete fractured reservoirs, and is highly practical and can be used to provide technical support for the efficient development of such reservoirs.

Key words: deep reservoir, fractured reservoir, fluid-solid coupling, numerical simulation, finite element, finite volume