Accessible to Total Nanopore Structure and Complexity in Cambay Shales, India: An Implication on Storage and Transport of Hydrocarbon
Authors: Bal Abinash , Misra Santanu , Sen Debasis 
Published: September 2022

ABSTRACT:
Accessible to inaccessible nanopore structure, distribution and complexity controls the storage, transport, and recovery of gas-shale. Fluid- (low-pressure N2 and CO2 adsorption) and radiation-based (small-angle scattering) methods used to characterize the connected and isolated nanopores in the early Tertiary sequence (1043-2573 m) of Cambay basin, India. The obtained continuous pore size distribution was deconvoluted into12 pore families. The mean width (w) of the micropore and mesopore families are w~0.50, 0.61, 0.77, 1.11 nm, and ~4.85, 8.0, 10.5, 14.5, 17.0, 30.0 nm, respectively. The multivariate partial least square regression model estimated the dependency of various dependent pore parameters (e.g., deconvoluted pore families, micro-, meso-, total pore volume, specific surface area – SSA, etc.) on intrinsic, independent variables (e.g., quartz, Fe-bearing minerals, clay and total organic carbon - TOC) of the shale samples. Our result suggests, TOC strongly enhances the storage capacity by increasing SSA and pore volumes (micro-, meso-, total pore volume), which eventually enables higher sorption and free gas storage capacity. Increased clay content increases the inaccessible pores, whereas TOC increases the accessible pore volumes. Families from micropore region (w~0.50, 0.61, 0.77, 1.11 nm) show positive dependency with the TOC, while the Families with w~ 10.5.14.5,17.0 nm show negative dependency with TOC.

1. INTRODUCTION
Research on nanoscale pores hosting abundant oil and gas in shale have become a global focus for unconventional oil and gas exploration (Song et al., 2017). The estimation of gas in place, ease of extraction, and overall reservoir characterization (rock composition, pore structure characterization, permeability, and strength) before, during, and after the extraction are essential for successful planning and implementation of the shale-gas project (Loucks et al., 2009; Ross & Bustin, 2009). In these lines of studies, pore structure characterization (geometry, shape, distribution, and the interconnected relationship between the pores and throats) is one of the fundamental tasks for estimating oil and gas extraction potential in shale-gas reservoir rocks, which generally have low porosity and ultralow-permeability (Aguilera, 2016; Gan et al., 1972; Gasparik et al., 2014; Javadpour, 2009; Kuila, 2013). Pore size, another important parameter in shale varies from millimeters in fractures to nanometers in individual pores. Shale consists of a wide range of nanopores; the International Union of Pure and Applied Chemistry (IUPAC) subdivided the entire spectrum of pore diameters into micropores (&lt; 2 nm), mesopores (2-50 nm), and macropores (&gt; 50 nm). In a typical shale-reservoir natural gasses exist in free form in the open space of pores; along with this, a significant volume of gas remains as an adsorbed state on the surface-matrix/pores, and also a small amount as a dissolved gas in organic matters (Chalmers & Bustin, 2008; Ross & Bustin, 2008). The estimation of transport and recovery requires understanding the accessibility of nanopores structure, distribution, and detailed nanopore-structural framework in a shale-gas reservoir at different intrinsic properties of shale.