Quantitative characterization of pore connectivity using NMR and MIP: A case study of the Wangyinpu and Guanyintang shales in the Xiuwu basin, Southern China

Pore connectivity is one of the most important characteristics of shale reservoirs because it significantly impacts the effective pore space, the fluid migration, and the gas production. In this work, pore connectivity and its primary controlling factors were investigated using a combination of field emission-scanning electron microscopy (FE-SEM), focused ion beam-scanning electron microscopy (FIB-SEM), mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR). The results show that using the difference between NMR and MIP is a reliable method to characterize pore connectivity. NMR pore size distribution (PSD) curves converted from T2 spectra, and MIP PSD curves were observed to have consistent shapes. The amplitude of NMR PSD curves is higher than that of MIP PSD curves for S-group pores (<200 nm), while the relationship is opposite for L-group pores (200 nm-10 μm), which may be due to the permeability of shale. A low permeability allows a smaller amount of mercury to intrude into the small pores. Based on experimental results, the pores of 8-20 nm make a contribution of 5%-11% to pore connectivity, whereas the pores of 200-700 nm are mainly interparticle pores and microfissures, contributing from 38% to 72% of pore connectivity. Stratification and pore morphology in the Lower Cambrian Wangyinpu and Guanyintang shales in the Xiuwu Basin are the two critical influencing factors of pore connectivity. The pore connectivity of well-laminated shale is higher than that of less-laminated shale. The laminated structures are usually composed of argillaceous and siliceous lamina, which tend to give rise to fissures during hydrocarbon generation or under confining stress. As a result, the pores around the microfissures are more likely to be communicating. Shales with the structure of uniformly distributed organic and inorganic minerals have the best pore connectivity. Both the interparticle pores and microfissures between organic matter and inorganic minerals or between inorganic minerals can effectively connect organic pore networks and greatly improve the pore connectivity.