Water Film or Water Bridge? Influence of Self-Generated Electric Field on Coexisting Patterns of Water and Methane in Clay Nanopores

Water always occurs in gas shales, especially during the treatment of shale gas hydraulic fracturing. In sharp contrast to the prevailing view that water film is ubiquitous in shale formations, we observed an unusual phenomenon that water bridge instead of water film dominates in some illite and kaolinite slit pores when we are investigating the coexisting pattern of water and methane inside shale nanopores using molecular dynamics simulations. The network orientation structure and hydrogen bond of water molecules are analyzed, and the results indicate that appearance of water bridge is attributed to the strong internal, self-generated electric field induced by surface charge contrast between different pore surfaces. Four factors can significantly influence this self-generated electric field strength: pore surface chemistry, mineral type, pore shape, and pore size. When the pore size is within several nanometers, a small charge difference could induce strong electric field and change the structural properties of water clusters. The water film or water bridge inside shale nanopores alters the hydraulic diameter of the pore and the fluid flow pattern. These findings may provide a better and microscopic insight of the water-gas flow behavior and the electric field inside clay nanopores.