Abstract
Pore connectivity is crucial for shale gas production. However, the three-dimensional (3D) characteristics and distribution of pore networks and, more fundamentally, the underlying role of different pore types on pore connectivity in shales are inadequately understood. By comparing the 3D pore connectivity derived from direct microstructural imaging of pores filled with Wood’s metal at a pressure corresponding to the finest accessible pore throat in the resolution ranges that may be achieved by X-ray micro-CT and SEM, it is possible to evaluate pore connectivity of different types of shales. The pore connectivity of three shales including a mixed mudstone, siliceous shale, and argillaceous shale from the Silurian Longmaxi Formations is investigated via combined broad ion beam (BIB) polishing, and SEM and X-ray micro-CT imaging after Wood’s metal injection at a pressure up to 380 MPa. The three shales show significant differences in pore connectivity. The mixed mudstone shows excellent pore connectivity in the matrix; the siliceous shale shows an overall poor connectivity with only a small amount of OM (organic matter) pores immediately adjacent to microfractures displaying interconnectivity, while the pores in the argillaceous shale, dominated by plate-like clay pores, are largely not interconnected.
Highlights
Shales are sedimentary rocks containing both organic and inorganic constituents with microstructures exhibiting a high degree of complexity and heterogeneity
The recent development of atomic force microscopy-based infrared spectroscopy (AFM-IR) was employed to examine the molecular structure of organic matters to provide an insight into the chemical diversities of an inertinite composition (Jubb et al, 2020)
The total organic carbon (TOC) contents of the three studied shales are in the range of 0.96–6.11 wt% with helium porosities ranging from 4.63 to 7.31% (Table 1)
Summary
Shales are sedimentary rocks containing both organic and inorganic constituents with microstructures exhibiting a high degree of complexity and heterogeneity. MICP may significantly underestimate the porosity of shales because of incomplete mercury intrusion into fine pores, which are smaller than 3 nm Both conformance and compression effects lead to an overestimation of porosity and affect the pore throat size distribution (Comisky et al, 2011; Kuila and Prasad, 2013; Peng et al, 2017). The water immersion porosimetry (WIP) technique was developed to measure the total porosity of shale samples for its simple and rapid advantage, and the density and porosity obtained are quite precise and highly reproducible (Kuila et al, 2014) This technique may overestimate porosity in samples with significant amounts of water-soluble minerals, and it is not recommended for smectite-rich shales or shales with high illite–smectite expandability
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