Effect of shale matrix heterogeneity on gas transport during production: A microscopic investigation

Shale gas is playing increasingly important roles in transforming the global energy markets due to the huge demand for cleaner energy in the future. To extract shale gas at the commercial rate, formations must be hydraulically fractured to provide more contact area between the wellbores and reservoirs. Although the properties of hydraulic fractures are the key to gas production in the early stage, it is the matrix properties that determine the long-term production performance of the wells. However, understanding the gas transport behaviours in shale matrix is still a challenge at present. In this paper, to analyze shale gas flow at the microscale, an improved numerical model is developed, which is able to simultaneously capture the mechanical deformation of organic matter (OM) and inorganic matter (iOM), gas flow in OM and iOM, and fluid and solid interaction between the OM and iOM. The modelling results show that due to the coexistence of OM and iOM, the gas transport and shale matrix deformation behaviours exhibit regional differences. Specifically, during the gas production process, pore pressure in OM declines slower, gas density and Knudsen number in OM own larger values, and the apparent permeability and volumetric strain of OM are smaller. Therefore, taking matrix heterogeneity into consideration is necessary for accurately describing shale gas flow. In addition, numerical simulations are carried out to investigate the effect of a multitude of parameters on the modelling results. These parameters include nanopore compressibility, nanopore radius, matrix porosity, Langmuir volume and Langmuir pressure, and total organic carbon (TOC). The sensitivity of apparent permeability evolution in OM and iOM on each parameter is reported and discussed. The obtained results provide new insights for understanding gas transport in shale reservoirs from a microscopic perspective.