Capillary Pressure Considerations in Numerical Reservoir Simulation Studies-Conclusion Maps

Abstract

Abstract Capillary pressure has been typically used for reservoir simulation model initialization where the initial fluids saturations are distributed all over the model. Different transition zones are set to avoid abrupt change from the minimum to the maximum saturation of any reservoir fluid, and consequently it directly affects the volumes of original fluids in place. After the step of initialization, the effect of capillary pressure on reservoir performance may not be fully understood for different drive mechanisms. In non-fractured reservoirs numerical simulation studies, capillary pressure is sometimes completely ignored during model construction and validation. On the other hand, in fractured reservoirs simulation studies, zero capillary pressure in fractures is usually used without a clear understanding of how this parameter affects the simulation accuracy. The aim of this work is to show the effect of both oil-water and gas-oil capillary pressures on the numerical simulation of reservoirs producing under natural depletion, water injection, and gas injection recovery processes over a wide range of reservoirs and production parameters. Capillary pressure effects are studied for both fractured and non-fractured reservoirs. For fractured reservoirs, both matrix and fracture capillary pressures effects were investigated when capillary pressure imbibition is the only drive mechanism and when both capillary pressure imbibition and gravity drainage recovery mechanisms are active together. To quantify this effect, simulation results of large and small transition zones were compared with models of zero transition zones and three difference indicators were calculated for each run. Hundreds of runs were made to quantify these effects in a variety of reservoir heterogeneity levels. For both fractured and non-fractured reservoirs, it was found that capillary pressure effects are different for different recovery processes. While capillary pressure is usually important for initialization, it may not be as important during flow calculations in every situation. In general capillary pressure is more important in depletion runs than in most displacement runs. It was found that no clear correlation existed between capillary pressure effects and the level of reservoir heterogeneity. Also, fracture gas-oil capillary pressure is important under certain conditions for fractured reservoirs. A field model, based on real data, was also used to confirm the results obtained from hypothetical models and validate the study conclusions. Finally, guidelines were developed to show when capillary pressure was important and when it can be ignored. This work is to complete the research done and published in Shams et al. (2013) and Shams (2014). Additional runs were performed to validate the results. The results and study conclusions are derived in shape of conclusion maps to be easier to use.