Invasion percolation and secondary migration: experiments and simulations

Experiments have been carried out to study the displacement of wetting fluids by immiscible non-wetting fluids in quasi-two-dimensional and three-dimensional granular porous media. These experiments included a systematic investigation of the effects of gravity acting on the density difference between the two fluids. The simple invasion percolation model provides a surprisingly realistic simulation of the slow fluid fluid displacement process in the absence of gravity, and a simple extension of the model can be used to simulate the most important features of gravity stabilized and destabilized fluid-fluid displacement processes. The dimensionless Bond number Bo (the ratio between buoyancy forces and capillary forces on the pore scale) can be used to compare experiments and simulations carried out using different (geometrically similar) porous media, different fluid-fluid interfacial tensions and different fluid densities. The complex patterns generated by gravity stabilized and gravity destabilized fluid fluid displacement processes can be described in terms of a pattern of blobs of size xi that have a fractal structure on length scales l in the range c less than or equal to l less than or equal to xi, where c is the characteristic porous medium grain size. The blob size xi is related to the Bond number by the simple scaling relationship xi similar to Bo(-v/(1+v)), which was first derived by Wilkinson (1984, 1986) for gravity-stabilized displacement. Here, v is the percolation theory correlation length exponent (v=4/3 in two-dimensional systems and v approximate to 0.88 in three-dimensional systems). The experiments and simulations have been extended to include fluid-fluid displacement in fracture apertures and the effects of how of the wetting fluid under the influence of a hydraulic potential gradient. These experimental and simulation results have important implications for our understanding of secondary migration. They indicate that the residual hydrocarbon saturation in the enormous volume of porous sedimentary rock (carrier rocks) between the hydrocarbon source and the reservoir can be very low, thus allowing significant quantities of oil and gas to reach the reservoir. Simulations have been carried out to explore the effects of heterogeneities on gravity destabilized fluid-fluid displacement processes and fluid fluid displacement in fracture apertures. However, the structure of the carrier rocks is highly dynamic on the time scales over which secondary migration takes place (of the order of 10(8) years, in many cases). A better understanding of the pore structure of the carrier rocks and its dynamics on long time scales is needed to more accurately model secondary migration. (C) 2000 Elsevier Science Ltd. All rights reserved.