Engineers involved in numerical reservoir simulation studies typically use capillary pressure for model initialization. However, the effects of capillary pressure on reservoir performance may not be fully understood for different drive mechanisms.
The aim of this work is to show the effect of both oil-water and gas-oil capillary pressures on reservoir performance for reservoirs producing under natural depletion, water injection, and gas injection recovery processes over a wide range of reservoir and production parameters. Both conventional and naturally fractured reservoirs are studied.
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.
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. Capillary pressure was more important in depletion runs than in most displacement runs. No clear correlation exists between capillary pressure effects and the level of reservoir heterogeneity. It was also found that fracture gas-oil capillary pressure effects increase in certain conditions for naturally fractured reservoirs. A real reservoir model 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.