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

Abstract
A three-dimensional 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 (2-D).
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, 4 prevailing types of displacements
emerge during each water flood and 4 different types emerge during each gas flood. When
suppressing chains longer than 2 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 2-D micromodels. The latter type of
experiments are particularly attractive, since lengths and types of displacement chains can be
observed and counted directly.