Pore-scale network modeling study of gravitational effects during solution gas drive: results from macrocscale simulations

Although experimental work for solution gas drive processes is routinely carried out and interpreted for the purpose of defining critical gas saturations and relative permeability data, developing a thorough understanding of the results to facilitate confident application to the field is a hard task. Unfortunately, existing macroscopic models are unable to reproduce or take into account several different features of solution gas drive experiments. For example: (i) the impact of outlet boundary conditions upon the formation of saturation gradients, and (ii) transitions between different flow regimes (disconnected immobile gas, disconnected mobile gas, continuous gas flow) that are characterized by different pore-scale dynamics. These features can be considered using pore-scale modeling techniques. However, such microscopic applications, even in the few cases where they are refined enough to include viscous or gravitational forces, have perhaps their greatest limitation in the number of pores that can be simulated. This consequently makes it virtually impossible to simulate flow regimes that are not capillary dominated at an appropriate physical length scale.

In this work a pore scale network modeling approach is presented which is capable of reproducing gravitational effects during oil depressurization through simulation in samples of macro-scale height. The models used comprise several hundred thousand pore elements. The primary objective is to fully simulate the physical height of routinely used laboratory samples, in this way reproducing the real scale and pressure dependent balance of forces: this allows us to show how the ratio of gravity to capillarity changes in relation to the rates of depletion (bubble densities), the rock, the fluid properties and the scale of the sample, and how this contributes to affect relative permeabilities and critical gas saturations. In particular it is shown that relative permeabilities can be predicted according to the particular flow regime operating during a given experiment (dispersed and/or continuous). Moreover we show that flow is largely determined by the size and density of gas clusters, whether originating from nucleation or from break-up of larger structures during migration. Furthermore, the ways in which different saturation gradients can develop over the length of a sample and the effects of the outlet boundary conditions upon such gradients are explained. The results are compared to available experimental data.