Quantitative study of different patterns of microscale residual water and their effect on gas permeability through digital core analysis.

Residual water significantly influences the permeability and recovery of low-permeability reservoirs. In this study, we evaluated the effect of different residual water clusters on gas permeability in low-permeability sandstone by using the X-ray computed tomography technique, digital core analysis and different numerical methods (i.e., the finite difference method (FD), lattice Boltzmann method (LBM), and Kozeny-Carman method (KC)). The residual water distribution patterns are classified into water film, isolated water and branch water by the sphericity index and Euler number, and these two parameters can respectively characterize the shape and the connectivity of the residual water clusters. The water film significantly influences the gas permeability, while the influence of branch water and isolated water on permeability is observed to be insignificant. The difference in gas permeability values influenced by branch water and isolated water is minimal. The FD and KC equation express the average properties of the study space, which will result in the permeability to be insensitive to the space structure affected by the different residual water clusters. The LBM is appropriate to simulate the microscopic flow with complex boundary conditions, and is recommended to evaluate the permeability affected by the different residual water clusters in microscale, because of its sensitivity to the change of space structure. [ABSTRACT FROM AUTHOR]