Two problems for the comparison of fractured reservoir models are presented. The first problem is a simple single block example. The second problem is a more complicated cross-sectional example. The cross-sectional example has been developed to simulate depletion, gas injection and water injection cases.
In selecting the problems for the present Comparative Solution Project, various aspects of the physics of multiphase flow in fractured porous media have been considered. The question of fracture capillary pressure and its influence on reservoir performance has been addressed by including cases with zero and non-zero gas-oil capillary pressure in the fractures.
Ten organizations participated in the Sixth Comparative Solution Project. Most of the current techniques in dual-porosity simulation are represented in the results of the participants.
In recent years, there has been an increased interest in the simulation of naturally fractured petroleum reservoirs. For the present SPE Comparative Solution Project, reservoir test problems have been designed to illustrate various aspects of the physics of multiphase flow in fractured reservoirs as well as the modelling techniques to account for capillary and gravity forces. The approach to the solution of the problems has been limited to dual-porosity models.
An important element in simulating a fractured reservoir using a dual-porosity technique is the proper calculation of the fluids exchange between the matrix blocks and the fractures. In the conventional approach, the transfer term for a particular phase is directly related to the shape factor, σ, fluid mobility and potential difference between the matrix and fracture. Shape factors have been developed based on first-order finite-difference approximations and by matching fine grid multiphase simulations of matrix-fracture flow.
In most dual-porosity models, the matrix block heights are assumed to be at the same depth as the corresponding fracture blocks, and therefore, gravity has no explicit effect on the fluid exchange between the matrix and the fracture. Pseudo capillary pressures have been used to account for the effect of gravity. The gravity segregation concept has also been used to compute the fluid levels in the matrix and fractures to account for the gravity contribution.