The use of reservoir simulation coupled with geomechanics has been increasing in recent years as its utility in modeling physical phenomena such as compaction, subsidence, induced fracturing, enhancement of natural fractures and/or fault activation, and steam-assisted gravity drainage (SAGD) recovery has become apparent. Among different methods investigated by researchers, the iterative explicit method appears to be the preferred method for field-scale simulation.
This method is a loose coupled approach between a reservoir simulator and a geomechanical simulator. At user-defined steps, the fluid pressures are transmitted to the geomechanical tool, which computes the actual stresses and reports the modifications of porosities and permeabilities back to the reservoir simulator.
This paper presents a new iterative scheme that allows any reservoir simulator to be coupled with any nonlinear finite-element-method (FEM) package for the stress analysis without any limitation on the functionality of either simulator. The convergence of this new scheme is discussed, and results are presented for three cases described below.
The first case is a validation case used by other SPE papers. The second case is a synthetic model of a highly compacting reservoir sensitive to water saturation. The third case is a full-field reservoir model.
The importance of geomechanics in problems such as wellbore stability, hydraulic fracturing, and subsidence is well known. In recent years, there has been growing awareness of the importance of the link between fluid flow and geomechanics in the management of stress-sensitive reservoirs (Chen and Teufel 2001; Gutierrez et al. 1994, 1995; Gutierrez and Lewis 1998; Osorio et al. 1999; Settari and Mourits 1998; Somerville and Smart 2000; Stone et al. 2000; Tran et al. 2002). New needs for coupled simulations appear, such as assessing the integrity of the overburden for heavy-oil recovery using thermal mechanisms (e.g., SAGD technique) or for acid-gas injection. Standard reservoir simulation of compaction drive accounts for nonlinear porosity changes determined from uniaxial-strain tests on cores. In many cases, laboratory-derived compressibility must be adjusted to match the contribution of compaction to total hydrocarbon recovery. Geomechanical effects such as stress arching and nonunique stress path are among the causes of discrepancy between laboratory-derived and field compressibility factors. If compressibility varies linearly with the mean reservoir pressure, then predictive reservoir modeling can be achieved without coupling between stress and flow. However, geomechanical effects are rarely linear, for a number of reasons. These include load variations because of modification of pressure, temperature, and saturation; change of the mechanism of production; and progressive activation of faults, and fractures that affect mechanisms such as stress arching and a nonlinear stress path. Unlike standard compaction-drive simulation, there is no simple linear method to account for the effects of stress on permeability, especially for fractured systems, in which the changes of permeability might be directional, localized, and strongly nonlinear.
There are several ways to achieve the coupling between flow and stress (Charlier et al. 2002; Samier et al. 2006; Yale 2002; Chen and Teufel 2000; Koutsabeloulis and Hope 1998; Lewis and Ghafouri 1997; Settari and Walters 1999; Mainguy and Longuemare 2002; Dean et al. 2006; Gutierrez and Lewis 1998; Thomas et al. 2002). The most rigorous coupling is achieved with fully coupled simulators, which not only solve the flow and the mechanical equations simultaneously but also allow for anisotropy and nonlinearity of the rock constitutive model. The feasibility and accuracy of such simulators, as far as complex and large-scale reservoir systems are concerned, have yet to be proved. Partial coupling on the other hand consists of linking a flow simulator with a stress simulator, allowing a good compromise between feasibility and accuracy. A one-way link from flow to stress simulator is often used for subsidence forecasts. However, to solve the compaction-drive problem, one-way coupling is not sufficient. To ensure the compatibility of pore-volume calculations from the flow and the stress simulators, iterations must be performed within each stress-analysis step before proceeding to the next stress step with or without permeability changes.