Dynamic Pore Network Simulator For Modelling Buoyancy-Driven Migration During Depressurisation of Heavy-Oil Systems
Authors: Ezeuko Cosmas Chigozie , McDougall Steven Robert , Bondino Igor , Hamon Gerald 
Published: October 2008

Abstract
A number of vertically-oriented heavy oil depletion experiments have been conducted in recent years in an attempt to investigate the impact of gravitational forces on gas evolution during solution gas drive. Although some experimental result indirectly suggest the occurrence of gas migration during these tests (especially at slow depletion rates), a major limitation of such an interpretation is the difficulty in visualising the process in reservoir rock samples. In contrast, experimental observations using transparent glass models have proved invaluable in this context and provide a sound physical basis for modelling gravitational gas migration in gas-oil systems. The experimental observations often exhibit somewhat contradictory trends however - some studies showing dispersed gas migration, whilst others describe fingered, channelised flow - and, to date, there appears to have been little systematic effort towards modelling the wide range of behaviours seen in or inferred from laboratory tests.
To this end, we present a new pore network simulator that is capable of modelling the time-dependent migration of growing gas structures. Multiple pore filling events are modelled dynamically with interface tracking allowing the full range of migratory behaviours to be reproduced, including braided migration and discontinuous dispersed flow. Simulation results are compared with experiments and are found to be in excellent agreement. Moreover, simulation results clearly show that a number of network and fluid parameters interact in a rather complex manner and as a consequence, the competition between capillarity and buoyancy produce different gas evolution patterns during pressure depletion. The implications of evolution regime on recovery from heavy oil systems undergoing depressurisation are extensively discussed.

Introduction
Improved recovery by depressurisation, primarily involving solution-gas drive and fluids of considerable density difference (gas/oil, gas/water), has previously been implemented at the field scale, and proved successful as evidenced by the increased hydrocarbon ultimate recovery, Gallagher et al., (1999). In addition to oil recovery by depressurisation, such buoyancy-driven, immiscible gas migration in porous media is central to a number of other processes, including gas injection to stimulate the biodegradation of volatile organic contaminants in saturated aquifers (gas sparging) and the study of CO2 migration for designing effective sequestration strategies.
In the context of hydrocarbon reservoir engineering, a number of depletion experiments have been carried out in the last decade using vertically orientated core samples in order to investigate the impact of gravitational forces on gas evolution during solution gas drive - both for light oil (Piccavet et al., 2006; Drummond et al., 2001; Naylor et al., 2000) and heavy oil (Tang et al., 2006; Bayon et al., 2002; Tang et al., 1999) systems. Although some experimental core results suggest the occurrence of buoyancy-driven gas migration (Piccavet et al., 2006), such behaviour can only be inferred - by means of heterogeneity in measured gas saturation profiles - and not observed explicitly. Experimental observations in transparent glass models and in bead/sand packs, however, have demonstrated observable migratory behaviour (Geistlinger et al., 2006;
McDougall and Mackay, 1998; Birovljev et al., 1995; Frette et al., 1992; Dumoré, 1970) and such observations can be used as a sound basis for model development. Generally, the temporal evolution of gas clusters during pressure depletion is a consequence of the interplay between gravity, viscous and capillary forces, which govern the displacement sequences and affect the overall gas topology.