Effect of Pore Structure and Capillary Number on Gas-Water Flow Patterns in Carbonate Rocks

Rock pore structure and capillary number determine gas-water flow through limestone. Limestone is of key importance for natural gas production and gas storage; however, there is still limited direct evidence on the precise influence of the pore space morphology (vugs, fractures, and homogeneous pore matrix) and capillary number. Here, we thus studied gas-water flow patterns in various limestones via in-situ X-ray microtomography combined with numerical flow simulations. Pore structure heterogeneity significantly affected the fluid migration path. Gas flowed rapidly through large pores and vugs but flowed slowly through microfractures. In contrast, water flowed through microfractures and small pores but did not enter large pores at low capillary number conditions. Water flow simulations [performed for different capillary numbers directly on microcomputed tomography (mu-CT) images] demonstrated that snap-off and dead-end corners controlled the distribution of residual gas, consistent with the experiments. In addition, the simulations showed that less residual gas distributed around dead-end corners at a low capillary number, and a pail of the residual gas can be displaced by increasing viscous forces. Moreover, a power-low relationship between gas cluster volume and surface area was observed, and the gas cluster size distribution could also be fitted with a power-law correlation. In all types of tested limestones, the power-law exponents (p approximate to 0.77, tau approximate to 0.86) were lower than that predicted by simple percolation theory (which predicts p approximate to 1, tau approximate to 2.189). There was evidence that a simple percolation model was unlikely to provide reliable predictions in homogeneous porous media, and we further extended the application scope of this conclusion to heterogeneous porous media. This work therefore provides fundamental data and improves fundamental understanding of gas-water flow through limestones and aids in the further advancements of improved hydrocarbon recovery and gas storage in limestone reservoirs.