Combined effect of capillary and viscous forces on waterflood displacement efficiency in finely laminated porous media

Recent work on fluid flow upscaling in clastic facies has demonstrated the importance of both the small scale lamination structure and the local viscous/capillary force balance. This paper presents a pore-scale analysis of unsteady-state imbibition and drainage processes, which attempts to elucidate the interaction between capillary and viscous forces during waterfloods in such finely laminated porous media. Network simulation studies have been performed to evaluate their combined effect upon 'residual' oil trapping and microscopic displacement efficiency. The three main purposes of this paper are (i) to demonstrate these small scale structure/force balance mechanisms from first principles (i.e. from microscopic modelling at the pore scale); (ii) to examine sensitivities of the laminar capillary trapping mechanisms to various factors; (iii) to comment on aspects of the pore to core upscaling of these effects in the light of the recently proposed 'geopseudo' methodology. Results demonstrate how small-scale lamination can greatly affect the larger-scale dynamics of a flood, especially where the principal flow direction is across the lamina as in cross-bedded systems. Sensitivity studies have been undertaken to examine the effects of lamina orientation and matrix wettability: a range of lamina structures has been considered under both water-wet and oil-wet conditions. The importance of scaling these effects in the light of the underlying geological structure is discussed. In the course of developing the network simulator for this study, some developments are presented on the modelling of the imbibition process.