New insights from shale gas production at the microscopic scale

Until recently, natural gas encountered in tight shales, which provided the source and seal of the gas, was considered uneconomical to produce. Although unconventional formations may be as porous as other reservoir rocks, their exceedingly small pore sizes and low permeability render them resistant to gas movement. Considering their importance to gas transport, we outline the characteristics of shale rocks, the mechanisms of Fickian and Knudsen diffusion as well as Klinkenberg's permeability. Given the challenges in unlocking natural gas from tight formations, various techniques such as the generation of artificial fractures and the introduction of pressurised fluids are detailed. To identify the parameters which govern natural gas production, we propose a computational porous rock model inspired from an actual image of a shale formation. The solution of the conservation of mass, momentum and energy equations appear to adequately capture the physics of gas transport at the microscopic level. Permitting the comparison between numerical and analytical gas velocity results, the validation framework we developed, demonstrates good agreement of numerical with theoretical findings. Gas pressure and velocity results point to the importance of pore throats, shale permeability and pressure maintenance in dislodging gas from the shale formations.