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

Tomasz Blach,Yeping Ji,Maria Mastalerz,Phung Vu,Liliana De Campo,Andrzej P Radlinski,Klaus Regenauer-Lieb,Elliot P Gilbert

doi:10.3390/en14248438

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.

Highlights

Relating porosity to permeability in organic-matter-rich shales and determining how organic matter connectivity and accessibility to hydrocarbons influence reservoir properties are important aspects of shale studies that can help predict hydrocarbon producibility from unconventional shale reservoirs [1,2,3]
The experimental structural data needed for the development of statistically relevant permeability models could be partly provided by X-ray tomography, including specific information pertaining to fractures [8,9])
The present study investigates the accessibility of pores to methane in five New Albany Shale samples of varying maturity: (1) Ro 0.50%—SDH-308 from Harrison County, Indiana; (2) Ro 0.70%—McAtee-2798 from Pike County, Indiana; (3) Ro 0.84%—Gibson-3997 from Gibson County, Indiana; (4) Ro 1.04%—Hardin-IL2 from Hardin County, Illinois, and (5) Ro 1.40%—Hardin-IL3 from Hardin County, Illinois

Summary

Introduction

Relating porosity to permeability in organic-matter-rich shales and determining how organic matter connectivity and accessibility to hydrocarbons influence reservoir properties are important aspects of shale studies that can help predict hydrocarbon producibility from unconventional shale reservoirs [1,2,3]. This is a complex issue due to the intricate multiscale geometry of the pore space, which is characterized by a very wide distribution of pore sizes (from nm to cm [4]) and the presence of fractures. That study suggested that maturity variation influenced pore volumes throughout the entire pore size range detected by MICP; the correlation was strongest for pores smaller than 100 nm, and the lower-maturity shales hosted significantly larger pore volumes than the higher maturity samples

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Results