Water Liberating/Sealing effects on shale gas Extraction: A fully coupled multidomain and multiphysics model

Whether the gas in kerogen is liberated/sealed by water, is a crucial issue for successful shale gas reservoir exploitation. Although this issue has been recognized, no tools are developed to evaluate the impact of this gas liberation/sealing on gas production under in-situ conditions. In this work, a concept named kerogen threshold differential pressure (KTP) is proposed to describe the mass transport influenced by the wettability at the heterogeneous kerogen interface with external/connate water. KTP is defined as a critical pore pressure difference between inside and outside of kerogen pores. It corresponds to the energy barrier needed to remove the liquid film at the kerogen surface so that the gas in kerogen can flow out. Based on this definition, whether kerogen supplies gas to its surroundings or not is controlled by an on/off mechanism which is incorporated into a fully coupled, multidomain, and multiphysics model to simulate gas extraction from stimulated shale reservoirs. Gas flow in kerogen, gas–water-two-phase flowback in inorganic matrix and fractures, shale deformation, and gas sorption are considered in the model. The proposed model is verified against a set of gas production data from the field and the simulation results published by a previous study. Furthermore, a sensitivity analysis is performed to investigate the effects of important influencing factors on gas and water production. The results suggest four main findings: (1) Shale gas production behaviour is dependent on the combination of gas-supplying capabilities of the different components in different domains of shale reservoir. (2) Water influences shale gas extraction in two aspects including two-phase flow and on/off gas supplying mechanism. The former aspect affects the gas flow in inorganic matrix and fractures, while the latter aspect controls the gas liberation from kerogen and thus, the sustainability of gas extraction. (3) The shapes of the gas/water relative permeability curves of different components of shale reservoir are influenced by formation damage and stimulation operation. They mainly affect early-period water production. (4) Gas recovery is enhanced by decreasing KTP, which can be achieved by using appropriate surfactants and/or water-free fracking fluids to alleviate/eliminate formation damage.