A comparative study of gas transport in dry and moisturized shale matrix considering organic matter volume fraction and water distribution characteristics

Understanding gas transport in shale matrix is of great significance for reservoir evaluation and gas well productivity. Up to now, numerous transport models were developed based on the hypothesis of homogeneous confined cylindrical nanopores by coupling with multiple mechanisms. In this work, a new transport model is proposed by coupling different transport mechanisms employing the volume fraction of organic matter (OM) instead of the total organic carbon content (TOC). The signature that the OM density is generally lower than the bulk matrix is also considered. The porosity of OM and inorganic matter (iOM) is determined by rock pyrolysis analysis, respectively. Water distribution in OM and iOM pores in the form of water clusters and adsorbed water films is quantified by water adsorption experiments. Gas transport model in moisturized shale matrix pores is then established considering the difference in water distribution. Meanwhile, both of the proposed models are analytical solutions with the hypothesis that the OM and iOM are arranged parallelly. The impacts of different factors on gas transport capacity are analyzed and discussed. Results indicate that the apparent permeability of the shale matrix decreases with the decline of pore radius. For the same pore diameter, the gas transport capacity of OM pores is much greater than that of iOM pores. The apparent permeability decreases with the increasing OM fraction. The irreducible water within the shale matrix can reduce the gas flow capacity considerably, and the apparent permeability is more sensitive to the change of irreducible water saturation at low pressure comparing with that at high pressure. This study sheds fundamental light on the gas transport distinctions in dry and moisturized shale matrix, which provides insights into the development of water-bearing shale gas reservoirs.