Coupled Reservoir Simulation Applied to the Management of Production Induced Stress-Sensitivity

Reliable data input into the reservoir simulator of production-induced changes in matrix and fault permeabilities is essential for the successful planning and management of oil and gas reservoirs. Withdrawing and injecting fluid from/to the reservoir causes a change in pore pressure, which in turn, changes the three dimensional effective stress state. The stress path followed by the reservoir controls the evolution of the effective stress state, and with it the changes in deviatoric stresses which cause reservoir rock deformation and permeability changes. In order to allow for these effects, a new type of reservoir simulator - the stress-sensitive simulator - is required. This paper presents the results of a stress-sensitive simulation using a nonlinear finite element geomechanics model coupled to a finite difference fluid flow simulator, applied to a typical North Sea reservoir. This reveals the effects of dynamic changes in permeability and the reactivation of faults on the reservoir performance. In the 3D coupled field-sized reservoir model, the permeability of intact rock is stress sensitive; faults are represented by joint elements with a permeability which is strain sensitive. The stress path is recorded and the hysteresis effect is considered in an interface program to modify the permeability according to the current and previous stress states. The results show that the evolution of the stress state, captured as a stress path, significantly affects the oil production profile. The pressure recovery caused by injection may also be diminished by hysteresis. The results also show the spatial and temporal changes in stress state, which are useful data for the analysis and exploitation of production induced anisotropy in permeability, with particular reference to the location of by-passed oil.