Realistic Numerical Models for Fractured Reservoirs

Abstract

Summary Characterization and forecasting in fractured reservoirs is one of the most challenging topics in the oil and gas industry. Managing such reservoirs requires construction of representative reservoir models that can handle both fracture and matrix systems, and their interaction, correctly. Data integration from all disciplines is required for construction of these reservoir models. Commercial numerical models capable of handling flow in fractured reservoirs have been present in the industry for quite some time. However, the correct application of those simulators for representative reservoir models is not easy. This paper will discuss parametric ways to improve construction of representative reservoir models. Discussion will focus on transition from single-porosity representation to dual-porosity models and pseudoization of capillary pressure process employed in between. A fractured reservoir undergoing a waterflood is simulated with a single-porosity formulation, where the matrix blocks are subdivided into core-plug-size gridblocks and the fractures are subdivided into even smaller blocks. The results from this fine grid are considered the "solution" to the displacement in the fractured reservoir. The grid is coarsened and the effects of scale-up are observed. Pseudos are developed for a coarse grid so that the results match the solution. The reservoir is then simulated with a dual-porosity simulator. The sensitivity of the dual-porosity results to the refinement of the numerical grid is studied. A comparison is made between the solution and the dual-porosity simulator results. Several of the advanced features found in commercial dual-porosity models are tested to see how well they improve the comparison. A scale-up pseudoization is determined that allows the dual-porosity simulation to match the solution. A larger sector model is then simulated with the pseudocapillary pressure to investigate the robustness of the scale-up pseudoization. Recommendations are provided that describe a procedure for using the dual-porosity model to simulate displacements in a fractured reservoir in a more accurate way. The scale-up process discussed in this paper is just the first step of a two-step procedure. The next step is to scale-up these curves to a field-scale gridblock (i.e., 100×100×20ft or m). If one uses nested matrix blocks for this first step of the scale-up process: the numerical computation problem is much more difficult and expensive and the next step scale-up calculation is confusing and difficult. Thus, this process is preferred in the final analysis.