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 (N-2, 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.