Use of Improved Gridding Technique in Coupled Geomechanics and Compositional Reservoir Flow Simulation

摘要

Coupled reservoir flow and geomechanics simulations are important in many compositional fluid flow processes like water- alternating-miscible gas injection and CO2 storage. The intention in these models is to understand the effect of stress and deformation consequent to changes in reservoir flow and fluid phase behavior. Safe CO2 storage in deep saline aquifers calls for evaluation of cap-rock and seal integrity via stress responses so that undesired leakage through the seals could be avoided. In the geomechanics calculations unknowns at a large number of finite element nodes need to be solved at each coupling step, which can lead to overwhelmingly large computation time for the coupled simulations. Recently an improved gridding technique was applied to model coupled geomechanics and reservoir flow in a thermal simulator. In this technique, a dual grid system consisting of separate grids for the reservoir flow and the geomechanics calculations is used. The exchange of information of the variables between reservoir and geomechanics simulators was accomplished in an iteratively coupled fashion. This method showed significant reduction in CPU time compared to single grid coupled simulation. This paper presents an extension of the above method wherein the geomechanics model is coupled with an equation-of-state compositional simulator in place of a thermal simulator. Use of the dual grid system is demonstrated through simulation models for two compositional processes. Comparison between a dual grid system and a single grid system is presented. The results show that there is substantial saving in computation time and memory requirement without significant loss in the quality of flow or geomechanical results. The dual grid system presented in the paper can be used to model compositional processes in stress-sensitive reservoirs to yield satisfactory results in considerably less time than would be possible through a conventional single grid system for coupled flow and deformation simulation.