Critical Gas Saturation and Relative Permeability for Pressure Depletion Experiments

The critical gas saturation marks the transition from mainly producing oil to mainly producing gas in oil fields that fall during their lifetime below the bubble point. Because of the associated significant technical and economic consequences there is large interest in determining the critical gas saturation in a reliable way. Experimentally measured critical gas saturations range from under 1% to about 50% which gives a very large uncertainty range. The wide range of critical gas saturations observed arises from a complex interaction between phase behavior, pressure decline rate and gas bubble nucleation kinetics. More importantly, critical gas saturations below 20% are below the percolation threshold which means that the underlying process leading to gas mobility is ultimately not understood. By conducting flow experiments with a model oil-gas system imaged at pore level by X-ray computed micro-tomography we aim to shed more light on the pore level processes. We find that pressure depletion experiments lead to critical gas saturations consistent with 3D percolation thresholds which does not explain gas mobility below 20% observed in many laboratory studies. When injecting pure gas near the bubble point we do observe gas mobility below the percolation threshold. The Peclet number < 1 suggests that diffusion leading to compositional gradients coupled with phase behavior effects may lead to the gas mobility below permanent pathways, i.e. below percolation threshold. These findings suggest that there is a far more complex interplay between flow, diffusive transport and compositional gradients than previously considered. In addition, this methodology can provide at the same time relative permeability computed by lattice Boltzmann simulations based on the imaged fluid distribution which in traditional core flooding experiments is very difficult to obtain.