Simulation of Block-to-Block Processes in Naturally Fractured Reservoirs

Summary Simulation of fractured reservoirs with the dual-porosity/dual-permeability approach involves discretization of the solution domain into two collocated continua called the matrix and the fracture. The original idealized model assumes that the matrix acts essentially as a source or sink to the fracture, which is the primary conduit for fluid flow. In multiphase flow situations, this idealization was found to be inadequate. Enhancements are needed to represent the local matrix/fracture and matrix/matrix drainage and imbibition processes. Attempts to represent these processes include the gravity-segregated model, the subdomain model, the pseudofunction method, and the dual-permeability model. This work examines the mechanisms involved in gas/oil gravity drainage in terms of the block-to-block process. Current methods for treating this problem are reviewed to identify deficiencies. A new approach is proposed in which these mechanisms can be represented properly in the field-scale simulation of these reservoirs. The method involves the calculation of pseudo capillary potentials, which in an average sense (on a computational block basis) give the correct flow behaviors. These pseudos can be calculated a priori if a vertical equilibrium (VE) assumption can be made about the fluid distribution in the matrix blocks. When the VE assumption is not valid, the pseudos can be determined from fine-grid simulations.