This study deals with the application of pore-scale network modelling techniques to depressurisation in heavy oil systems. A 3-phase simulator has been developed to account for the fundamental steps of such a depressurisation process, from the nucleation of embryonic bubbles, to their growth by solute diffusion and expansion, to the final stages of coalescence, migration, and production. The model is utilised to examine the impact of a variety of different physico-chemical properties — including bubble density and gas-oil diffusion coefficient — and different depletion strategies upon critical gas saturation and 3-phase relative permeability. Results pertaining to heavy oils are also compared to those from light oil depressurisation simulations and a number of important differences are highlighted.

The first sensitivity analyses heavy oil depressurisation under conditions of instantaneous and progressive nucleation (IN and PN). In the first scenario, the evolution of gas is characterised by embryonic bubbles that are nucleated randomly soon after the bubble-point pressure is reached. In the case of PN, a stochastic algorithm has been developed from related experimental observations. This models the progressive nucleation of additional bubbles as a function of local supersaturation (characterised by pore-scale variations in dissolved gas concentration). It is shown here that IN and PN observations are not necessarily contradictory — the model reconciles the two phenomena and shows how each relates to the underlying experimental parameters.

The vast majority of depressurisation studies reported in the literature to date have tended to focus upon light oils and binary systems. However, the rather different characteristics of heavy oils — high interfacial tension, low gas-oil ratio, high oil density — suggest that such studies may not be particularly representative of heavy oil depressurisation. In order to investigate this issue, a second set of sensitivity analyses is performed, whereby the network model is used to compare critical gas saturations and relative permeabilities arising from the depressurisation of light and heavy oils. It is found that substantial differences can arise in the two systems — in particular, the high interfacial tension of heavy oils is found to lead to a more compact, capillary-dominated pattern of gas evolution than that observed in light oils. As a consequence, improved recoveries (with higher critical gas saturations) are predicted for some heavy oil systems.

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