Estimation of critical gas saturation during pressure depletion in virgin and waterflooded reservoirs

An important issue in petroleum engineering is the prediction of gas produc tion during reservoir depletion - either following conventional waterfloodi ng operations or in the early stages of hydrocarbon production. The estimat ion of critical gas saturation for use in corresponding simulation studies is clearly a primary concern. To this end, a 3D, three-phase numerical pore -scale simulator has been developed that can be used to estimate critical g as saturations over a range of different lengthscales and for a wide range of fluid and rock properties. The model incorporates a great deal of the kn own physics observed in associated laboratory micromodel experiments, inclu ding embryonic nucleation, supersaturation effects, multiphase diffusion, b ubble growth/migration/fragmentation, oil shrinkage, and three-phase spread ing coefficients. The precise pore-scale mechanisms governing gas evolution have been found to be far more subtle than earlier models would suggest be cause of the large variation of gas/oil interfacial tension IFT with pressu re. This has a profound effect upon the migration of gas structures during depletion. In models pertaining to reservoir rock, the process of gas migra tion is consequently much slower than predictions from more simplistic mode ls would imply. This is the first time that bubble fragmentation and IFT va riations have been included in a model of gas evolution at the pore-scale a nd the implications for production forecasting are expected to be significa nt. In addition, novel scaling groups have been derived for a number of differe nt facies under both virgin and waterflooded conditions. One future applica tion of these groups would be to scale S-gc values obtained from high rate depressurization experiments to the low rate conditions more characteristic of held operations.