Abstract
Pore connectivity of lacustrine shales was inadequately documented in previous papers. In this work, lacustrine shales from the lower Cretaceous Shahezi Formation in the Changling Fault Depression (CFD) were investigated using field emission scanning electron microscopy (FE-SEM), mercury intrusion capillary pressure (MICP), low pressure gas (CO2 and N2) sorption (LPGA) and spontaneous fluid imbibition (SFI) experiments. The results show that pores observed from FE-SEM images are primarily interparticle (interP) pores in clay minerals and organic matter (OM) pores. The dominant pore width obtained from LPGA and MICP data is in the range of 0.3–0.7 nm and 3–20 nm. The slopes of n-decane and deionized (DI) water SFI are in the range of 0.34–0.55 and 0.22–0.38, respectively, suggesting a mixed wetting nature and better-connected hydrophobic pores than hydrophilic pores in the Shahezi shales. Low pore connectivity is identified by the dominant nano-size pore widths (0.3–20 nm), low DI water SFI slopes (around 0.25), high geometric tortuosity (4.75–8.89) and effective tortuosity (1212–6122). Pore connectivity follows the order of calcareous shale > argillaceous shale > siliceous shale. The connected pores of Shahezi shales is mainly affected by the high abundance and coexistence of OM pores and clay, carbonate minerals host pores.
Highlights
Organic-shales and coals are proposed to contain complex pore systems with various pore types, shapes and wide pore size distribution (PSD) ranges [1,2,3,4,5,6,7]
Quartz and clay mineral contents are dominant in these samples, accounting for over 70 wt % in the Shahezi shale samples
DI water mayand preferentially be imbibed into the hydrophilic pores, while n-decane may be primarily imbibed into the hydrophobic pores [24]
Summary
Organic-shales and coals are proposed to contain complex pore systems with various pore types, shapes and wide pore size distribution (PSD) ranges [1,2,3,4,5,6,7]. Pore structure characterization is the key issue of shale gas resource and coalbed methane assessment [14,15,16,17,18,19,20,21]. The pore characteristics (types, shapes, geometries, PSDs and evolution mechanisms) of gas shales were extensively studied around the world in previous studies by several researchers [1,3,13,14,21,22,23,24,25,26,27,28]. Field emission scanning electron microscopy (FE-SEM) [1,7,29], small angle neutron scattering (SANS) [2,24], low pressure gas adsorption (LPGA) and mercury intrusion capillary pressure (MICP) [2,3,9] were used to characterize the pore structure in shales. Pore connectivity significantly affects the gas flow distance in the shale matrix, and Minerals 2019, 9, 198; doi:10.3390/min9030198 www.mdpi.com/journal/minerals
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