Effect of GOR on Gas Diffusivity in Reservoir Fluid Systems

Abstract Molecular diffusion plays a dominant role in various reservoir processes, especially in the absence of convective mixing. In general, gas diffusion in oils depend on several factors such as pressure, temperature, oil viscosity and gas-to-oil ratio (GOR). Out of these factors, GOR effects/live oil compositional changes on diffusivity is rare or not available in literature. The current work fills this gap and present the experimental observations on the effect of GOR on gas diffusivity in reservoir fluid systems. Synthetic live oils were created by combining stock tank oil (STO) and methane in various ratios. Constant composition Expansion (CCE) experiments were performed with these oils to obtain their bubble points and liquid-densities in relation to GOR. Methane diffusivity in these oils were obtained from pressure-decay tests at high temperature/pressure conditions. The diffusion and solubility parameters were estimated from pressure-decay data using the diffusion model and integral-based linear regression presented in the previous works ([1]-[2]). The experimental and modeling methodology are presented here in sufficient detail to allow readers to replicate and evaluate the results. In this work, we have experimentally investigated the effect of GOR on methane diffusivity in oils at high temperature/pressure conditions using pressure-decay tests. In particular, We present experimental data for bubble-points and liquid density of synthetic oils having various GOR-values. For the range of GOR's considered, these measurements show that the bubble-point pressure increases linearly with GOR.Late transient solution of the pressure-decay model was utilized to obtain diffusivity parameters by regressing against experimental data. It is found that as GOR-value increases (i.e. when oil becomes lighter), the diffusivity-value increases, which is in accordance with Stokes-Einstein relation.Most importantly, an empirical correlation is developed based on limited data set to describe the variation in diffusivity values with GOR. This can be very important when experimental data for the STO is available but not for the live oils. It can also be extremely useful in gas injection processes where amount of gas dissolved in the oil varies leading to variations in diffusivity as well.