A Predictor for Accelerated Coupled Rock Mechanics and Reservoir Simulation

The impact of the stress field on reservoir fluid flow and production can be significant for many kinds of reservoirs, and hence coupled Rock Mechanics and Reservoir Simulation has been seeing a growing popularity. A much used scheme is iterative coupling, where compaction is computed at each stress step by iteratively updating cell pore volumes in the reservoir simulator by values calculated from strain in the stress simulator. <br>Although the procedure works satisfactory, it may be slow, as often many iterations are needed. Further, the pore volume corrections will only be performed at selected stress time steps, such that pressure and compaction in the flow simulator are not continuous in time. Many reported schemes assume specific poro-elasto-plastic models, as e.g. linear elastic, and also require modification of code.<br>It is well known that compaction is a function of strain, while reservoir simulators use fluid pressure, the only compaction energy available. On this background few if any coupled procedures utilize the compaction vs. fluid relationship at all. <br>In this paper we show that the relationship can nevertheless be used as basis for constructing a predictor for the actual stress / strain computations, which leads to significant speed-up. Many of the features of the predictor can be determined from the first stress time step only, and for later stress steps it can be improved with small effort. The scheme is valid irrespective of the poro-elasto-plastic model, and is based on information exchange, so no simulator code modification is necessary. <br>The compaction state is primarily dependent on the materials, boundary conditions, and the production process, with the geometry dependency as the governing. The predictor is constructed by modifying compaction vs. fluid pressure to take account of geometry variation. A good predictor will result in an improved pressure field as computed by the reservoir simulator, hence providing the stress simulator with a better pseudo-initialiser, such that it converges quicker, and in the pore volume iteration scheme fewer if any iterations are required. <br>In total we have experienced a reduction in total computer time of more than 90% in some cases, and as a bonus the fluid pressure field is continuous in time.<br>