Pore structure characterization of organic-rich Niutitang shale from China: Small angle neutron scattering (SANS) study

Pore structure can greatly impact the gas storage and flow mechanisms in shale reservoirs. Pore characteristics, including the pore (body/throat) size distribution (PSD), porosity, specific surface area, tortuosity, and connectivity, from four Niutitang shale samples were investigated with small-angle neutron scattering (SANS), mercury injection capillary pressure (MICP), and helium porosimetry to complement earlier gas (N2, CO2) sorption and petrographic analyses. In particular, the SANS data provides information on the total porosity, including both the closed and open pores. The large-Q (scattering vector) “flat” background of the SANS data was used to estimate the volumes of pores with diameters of < 5 nm. By evaluating the differences between porosity values determined from multiple complementary methods, the closed fraction of the pores in the Niutitang Shale was derived. Combining these pore-size distribution analyses (body/throat) and previous field emission-scanning electron microscopy observations, it was concluded that the closed pores were rarely present in illite but were mainly within organic matter. Moreover, the fraction of closed pores has a negative correlations with the matrix permeability and geometrical tortuosity. Overall, an improved understanding of the fraction of closed pores will lead to a better understanding of gas storage and production.