Controls on matrix permeability of shale samples from Longmaxi and Niutitang formations, China

• An indicator is proposed to take the effect of pore structure on matrix permeability into account.
• A model is developed to evaluate the permeability-porosity relationship for Longmaxi and Niutitang shale in China.
• The low permeability and relatively high porosity accounts for the low production in Niutitang shale.
• Matrix permeability is approximately the plug permeability at the closing pressure for Longmaxi and Niutitang shale.

The Lower Silurian Longmaxi shale and the Lower Cambrian Niutitang shale are typical marine shales in China, promising for shale gas production, whereas the gas production from these two formations vary considerably because of the different flow properties determined by geology and lithology conditions. Matrix permeability, compared with fracture permeability, is an important factor controlling the long-term gas production from shale gas reservoirs. However, matrix permeability is difficult to measure at field due to the constrained measuring conditions. In this study, the Pressure-decay method was applied to measure the matrix permeability of the relatively high permeability Longmaxi and low permeability Niutitang shales. Moreover, controls of geochemical and geological factors on the matrix permeability were investigated for these two gas shales, differing in buried depth, porosity, Total Organic Carbon (TOC) contents and thermal maturity. Furthermore, the effects of dominant controlling factors on the matrix fluid conductivity were analyzed. Results show that matrix permeability increases with TOC content at different rates for Longmaxi and Niutitang shales. The difference in pore structure and pore-size distribution between the two shales is the main reason for the different matrix permeability and gas production. In addition, the development of organic nano pores with the partial fill-in of the minerals in the Niutitang shale was observed through the Scanning Electron Microscope (SEM), accounting for the contradiction of high porosity and low matrix permeability in the Niutitang shale. The precise determination of intermediate parameter in the Pressure-decay method is the key control for the accuracy of matrix permeability measurement and our study improves the understanding of the importance of pore-size distribution on flow properties in the matrix of shale gas reservoirs.