Pore-scale simulation of hysteresis effects during repressurisation of gas-oil systems

Repressurisation of a hydrocarbon reservoir may occur either as a natural response to subsurface phenomena (for example, aquifer encroachment or fault re-activation) or as a consequence of a particular production strategy. In this paper we examine, for the first time to our knowledge, the pore scale mechanisms underlying the repressurisation of a gas-oil reservoir that has previously been exploited by solution gas drive. Our investigation, triggered by experimental observations in both glass micromodels and cores, utilizes a pore-scale network model: a modelling approach that is nowadays sufficiently mature to facilitate virtual experiments that are rapid, cheap and physically realistic. The new physics implemented here captures a wide range of phenomena related to multiphase repressurisation, including gas re-dissolution in oil by diffusion, bubble retraction, fragmentation and re-stabilisation under gravitational forces, and oil reimbibition. It is found that repressurisation is not simply the reverse of primary depletion, as the previously developed gas phase does not simply retract from the latest pores invaded during depressurization: rather, the process exhibits a large degree of hysteresis in both local supersaturation and gas phase distribution. We have studied the effects of several combinations of depressurization and repressurisation rates upon phase distribution and recovery, as the system is first depleted below bubble point and subsequently repressurised back to the starting pressure. Crucially, we find that a highly dispersed gas phase can persist once the original bubble point of the oil has been reached at the end of repressurisation. It is clearly shown that the repressurisation of an unequilibrated system essentially generates a high bubble density: this could theoretically be exploited by a secondary depletion to obtain improved recovery, as it is well known that the generation of large numbers of bubbles (generated by nucleation or, as we suggest here, induced by repressurisation) is highly beneficial for solution gas drive performance. Copyright 2010, Society of Petroleum Engineers.