The influence of transient evaporation and geometric characteristics on shale gas transmission in inorganic nanopores

Abstract

Transient evaporation characteristics of water film in inorganic nanopores play an important role in the shale gas storage and transmission. Meanwhile, Inorganic nanopores in shale minerals exhibit complex geometric characteristics. In this work, TESCAN MAIA3 is used to observe the pore shape and size of inorganic nanopores. The pore shapes of rectangular or wedge-shaped are frequently occurred in inorganic nanopores. Water distribution characteristic influenced by surface wettability and disjoining pressure in slit-like nanopores is then analyzed in detail. We divide the water film attached to the surface of the inorganic nanopores into two layers: the high-viscosity water film and the bulk water film. The Moving Mesh Method is used to simulate the evaporation of water film and a mass flow prediction model synthetically considering the pore shape, evaporation, water film slip and layering is established in a single inorganic nanopore. Based on this model, the influence of four different parameters including pressure gradient, water-methane diffusivity coefficient, pore size and wedging coefficient on the shale gas production are investigated. The results show that the presence of water film and its evaporation have significant influence on the production of shale gas. This phenomenon can be interpreted by the increase of pore size caused by evaporation. However, the water film slip and layering have little effect on the shale gas flow rate. With the increase of pressure gradient, the mass flow rate increases significantly. The water-methane diffusion has a considerable influence on the time to reach the steady state. Lower wedging coefficient and larger pore size contribute to higher mass flow rate. Meanwhile, as the wedging coefficient increases from 1.0 to 1.8, the contribution of evaporation increases from 39.34% to 45.33%. Generally, the effective migration space of shale gas directly determines the amount of gas production.