Percolation Pore Network Study on the Residue Gas Saturation of Dry Reservoir Rocks

Abstract:

We tried to model the effect of pore size heterogeneity and pore connectivity on the residue gas saturation for dry gas reservoir rocks. If we consider that snap-off does not exist and only piston displacement takes place in all pores with the same size during imbibition process, in the extreme case, the residue gas saturation will be equal to zero. Thus we can suppose that the residue gas saturation of dry rocks is mainly controlled by the pore size distribution. To verify the assumption, percolation pore networks (i.e., three-dimensional simple cubic (SC) and body-center cubic (BCC)) were used in the study. The connectivity and the pore size distribution in percolation pore network could be changed randomly. The concept of water phase connectivity z_w(i.e., water coordination number) and gas phase connectivity z_g (i.e., gas coordination number) was introduced here. z_w and z_g will change during simulation and can be estimated numerically from the results of simulations through gradually saturated networks by water. The Simulation results show that when z_g less than or equal to 1.5 during water quasi - static imbibition, the gas will be trapped in rock pores. Network simulation results also shows that the residue gas saturation S_rg follows a power law relationship (i.e.,S_rg∝σ_r^α, where σ_r is normalized standard deviation of the pore radius distribution, and exponent α is a function of coordination number). This indicates that the residue gas saturation has no explicit relationship with porosity and permeability as it should have in light of previous study, pore radius distribution is the principal factor in determining the residue gas saturation of dry reservoir rocks.