Can network modelling predict two-phase flow functions?

Over the last two decades there has been an increase in activity on the pore-scale modelling of multiphase flow in porous media. Excellent progress has been made in many areas of pore scale modelling, particularly in (i) the representation of the rock itself and (ii) in our description of the pore-scale displacement physics (in model pore geometries). 3D voxelised images of actual rocks can be generated either numerically (e.g. from 2D thin sections) or from micro CT imaging. A simplified network involving more idealised nodes and bonds can then be extracted from this numerical rock model and this can be used in modelling pore scale displacement processes. Much progress has also been made in understanding these pore scale processes (i.e. piston-like displacement, snap-off events, layer formation/collapse, pore-body filling/draining). These processes can be mathematically modelled accurately for pores of non-uniform wettability, if the geometry of the pore is sufficiently simple. In fact, in recent years these various pore-level processes in mixed and fractionally wet systems have been classified as “events” in an entire capillary dominated “phase space” which can be defined in a thermodynamically
consistent manner. Advances in our understanding and ability to compute several two-(and three-) phase properties a priori have been impressive and the entire flooding cycle of primary drainage (PD), ageing/wetting change, and imbibition can be simulated.

In this paper, we review the successes of pore-scale network modelling and explain how it can be of great use in understanding and explaining many phenomena in flow through porous media. However, we also critically examine the issue of how predictive network modelling is in practice. Indeed, one of our conclusions on pore scale modelling in mixed wet systems is that we cannot predict two-phase functions reliably in “blind” tests. Interestingly, we make this statement not because we do not understand the pore-scale physics of the process, but because we do understand the physics. It is hoped that our comments will stimulate a more critical debate on the role of pore-scale modelling and its use in core analysis.