Accessibility of Pores to Methane in New Albany Shale Samples of Varying Maturity Determined Using SANS and USANS

Abstract

The accessibility of pores to methane has been investigated in Devonian New Albany Shale Formation early-mature (Ro = 0.50%) to post-mature (Ro = 1.40%) samples. A Marcellus Shale Formation sample was included to expand the maturation range to Ro 2.50%. These are organic matter-rich rocks with total organic carbon (TOC) values of 3.4 to 14.4% and porosity values of 2.19 to 6.88%. Contrast matching small-angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) techniques were used to generate porosity-related data before and after pressure cycling under hydrostatic (in a vacuum and at 500 bar of deuterated methane) and uniaxial stress (0 to ca. 350 bar) conditions. Our results showed that the accessible porosity was small for the samples studied, ranging from zero to 2.9%. No correlation between the accessible porosity and TOC or mineralogical composition was revealed, and the most likely explanation for porosity variation was related to the thermal transformation of organic matter and hydrocarbon generation. Pressure caused improvements in accessible porosity for most samples, except the oil window sample (Ro = 0.84%). Our data show that densification of methane occurs in nanopores, generally starting at diameters smaller than 20 nm, and that the distribution of methane density is affected by pressure cycling.

Multiscale micro-architecture of pore space in rocks: size, shape, deformation and accessibility determined by small-angle neutron scattering (SANS)

“…This development made it possible to separately characterise the total porosity, specific surface area and pore size distribution for porous space that is accessible or inaccessible to penetrating fluids. Such results are of particular interest for tight unconventional gas reservoirs, where the accessible porosity is concentrated mainly in the nanopores and often constitutes a small fraction of total porosity [ 23 , 54 , 55 ].…”

Microstructure Characterization Techniques for Shale Reservoirs: A Review

“…It is believed that high burial depth/maturity reduces the pore connectivity of the shale and increases the content of closed pores. The continuously increasing burial depth compresses the space between mesopores and macropores (Blach et al, 2021), thereby preserving the law of micropore volume. This also means that SAS can effectively characterize the evolution of shale closed pores.…”