A pore-scale modelling approach to the interpretation of heavy oil pressure depletion experiments

Pressure depletion (or solution gas drive) is a well-known drive mechanism for hydrocarbon recovery. Such a recovery strategy seeks to harness the energy obtained from the liberation and expansion of dissolved gas from the resident aqueous and oleic phases within the host porous medium. This energy can then be utilised to displace oil towards the well-bore region for subsequent recovery.Here, a pore-scale numerical model is developed and used to investigate the underlying dynamic processes that characterize pressure depletion of a heavy oil within a porous medium. This is achieved by direct modelling of experimental gas and oil production profiles under different depletion rates.Progressive nucleation is implemented in the simulator, whereby bubbles nucleate from sites of increased “nucleation potential”. Gas evolution is then simulated at the pore-scale via the implementation of local models for inter-pore diffusion, capillary-controlled gas expansion, buoyancy-driven migration and oil shrinkage.This generic numerical simulator is subsequently anchored to a specific porous medium using adjuvant petrophysical data (absolute permeability, porosity and mercury porosimetry data) and the experimental oil and gas saturation histories are successfully predicted at two different depletion rates. The corresponding relative permeability curves are also presented from the model.Finally, detailed analysis of the simulation data also allows us to investigate the widely-held belief that recovery efficiency is directly linked to depressurisation rate — the higher the depletion rate, the larger the number of bubbles nucleated, and the higher the observed hydrocarbon recovery. We show here that recovery is seen to depend not only upon the depletion rate and bubble density, but also upon the lengths of diffusion pathways, local supersaturation gradients, and gas cluster topology — all of which are related to the underlying connectivity of the pore system.