Application of a Local Grid Refinement Protocol in Highly Faulted Reservoir Architectures
Authors: Kilic A. , Ertekin T. 
Published: March 2010

A three-dimensional, three-phase black oil simulator is used to model the flow regimes and flow geometries around sealing/non-sealing flow barriers in hydrocarbon reservoirs with homogeneous property distribution. A local grid refinement technique is utilized in these numerical simulation models to accurately capture the flow regimes and flow geometries.

Previous work examined a local grid refinement algorithm that was applied to study flow around impermeable barriers, water coning problems, and waterflood front-tracking in horizontal wells. In this paper, we extend the application of the algorithm to account for inclined faults.

A system of faults in a large reservoir is considered in all of the cases studied. Sections of the reservoir are solved individually in the presence of geological discontinuities. The results from the static local grid refinement technique indicate a much higher level of accuracy over the results generated using a coarse grid approximation around faulted zones of the reservoir. This level of accuracy is characterized by comparing the results generated by a completely fine grid and/or by a conventional refinement protocol.

The proposed model is capable of providing a better representation of faulted reservoir architectures using significantly less CPU time. The proposed model can also be used in pressure transient analysis of wells located near non-sealing fault structures.

Introduction

In most reservoir simulation studies, the scale of the base grid system is relatively coarse. For more accurate solutions, coarse grid blocks are subdivided into smaller grid blocks, resulting in a fine grid system. In a local grid refinement technique, only certain course blocks, representing specific regions of the area, are subdivided into smaller grid blocks. Local grid refinement applications can be grouped into three general categories: conventional grid refinement, static local grid refinement, and dynamic local grid refinement. In conventional grid refinement, certain regions in a coarse system are subdivided and grid lines are extended to the external boundaries of the reservoir. Static local grid refinement is the same as conventional grid refinement, but the grid lines are not extended to the boundaries of the reservoir. Finally, dynamic local grid refinement finds locally refined areas in the coarse system changing during the simulation. These grid systems are presented in Figure 1.

Refinement of the coarse grid system near the faulted areas gives better approximation to the flow model under consideration. Therefore, in this study, only faulted areas and the immediate vicinity of the wells are locally refined. Use of the local grid refinement technique reduces the computational work significantly and results in almost perfect agreement with the fine and conventional grid system's solutions. In order to capture the flow geometries around inclined faults, 3D versions of black oil simulators and single-phase oil and gas models are used. In nature, most of the faults are inclined. Therefore, whether a numerical model can capture the physics and flow regimes around these inclined impermeable barriers will have a critical effect on the accuracy of the results.

The local grid refinement technique developed by Biterge and Ertekin(1) is used in this study.