Pore-scale modelling of three-phase flow in mixed-wet porous media: multiple displacement chains

Abstract

A three-dimensional (3D) pore-scale network simulator is presented for modelling capillary-dominated three-phase flow in porous media where the wettability varies from pore to pore. The physics of weakly wetted systems has been included by allowing a pore to have any contact angle and, indeed, for each pore to have a different contact angle from a chosen distribution. An important complication arising from weakly wetting conditions is the absence of wetting films, which strongly reduces the continuity of the phases throughout the network. This reduction in phase continuity implies that during water-alternating-gas (WAG) injection processes a large number of phase clusters may form, that are disconnected from both inlet and outlet. Mobilisation of these clusters can happen through so-called multiple displacement chains, which involve a string of different phase clusters between inlet and outlet. We have explored the impact of these multiple displacement chains on WAG flow processes and the underlying three-phase flow mechanisms in a mixed-wet porous medium with the larger pores oil-wet. Assuming total absence of wetting films, we have varied the connectivity of the network (co-ordination number and dimension), the size of the network, the allowed maximum length of the displacement chains and the number of WAG cycles. The results are presented not only in terms of saturations and oil recovery, but also through statistics per flood of the length and type of displacement chains, the pore occupancy and through snapshots of the actual flood distributions (2D). From the simulations we conclude that for highly connected networks a steady state is reached after only a few WAG cycles, during which oil production ceases. However, in this state oil continues to be moved around within the network as a result of multiple displacement chains. The maximum allowed chain length has a substantial effect on the WAG saturation path, although the presence of longer chains during higher order WAG cycles is reduced for smaller networks and for networks with higher connectivity. For the investigated wettability state of the porous medium, four prevailing types of displacements emerge during each water flood and four different types emerge during each gas flood. When suppressing chains longer than two displacements in each flood one type disappears. Finally, we have listed some ways in which the predictions from the simulations may be tested experimentally both in core material and also in 2D micromodels. The latter type of experiments are particularly attractive, since lengths and types of displacement chains can be observed and counted directly.