Anisotropic Pore Structure of Marcellus Shale under Uniaxial Compression: A Small-Angle Neutron Scattering Study

Abstract

Shale gas becomes an alternative energy resource compared to coal and crude oil due to its less carbon emission. The production of shale gas has rapidly grown up in the US for a decade due to mature well completion technologies including horizontal drilling combined with hydraulic fracturing stimulation. Shale, as a source rock, has a complex pore structure with low porosity and ultralow permeability. Despite extensive studies in anisotropic features of transport and strength in macroscale, limited studies have been devoted to characterizing anisotropic nanopore structure of shale in microscale. In this study, small-angle neutron scattering (SANS) has been conducted to characterize the anisotropic pore structure of Marcellus shale under uniaxial stress condition. Two thin sections cut parallel and perpendicular to the bedding were prepared, where the fresh shale bulk was collected from an outcrop of Marcellus shale formation in Frankstown in Pennsylvania. Chemical compositions show that the shale is organic rich (8.5 wt.% of organic matter) and has 75 wt.% of quartz. For the shale sample cut parallel to the bedding, the obtained 2D scattering profile, showing a circular shape, was radially averaged to the 1D scattering profile. Whereas, for the shale sample cut perpendicular to the bedding, an elliptical 2D scattering pattern was sectorial averaged to 1D scattering profiles along the long and short axes of the ellipse. Pore volume distribution, porosity, and specific surface area (SSA) were estimated based on the model-fitting of the 1D scattering profiles. For the vertical cut sample, pore properties along the long axis (bedding direction) are all higher than that for the short axis (direction normal to the bedding). The porosity of the horizontal cut sample is reasonably similar to that for the short axis of the vertical cut sample. However, the horizontal cut sample has the highest SSA. Degree of anisotropy was estimated based on several annular 1D scattering profiles in the 2D scattering pattern. The results show that the degree of anisotropy for the horizontal cut sample is small, which is close to isotropy. Whereas, the degree of anisotropy increases with increasing pore size for the vertical cut shale sample. Under uniaxial compression condition, porosity and SSA increase with increasing uniaxial stress from 0 to ~700 bar for the shale cut perpendicular to the bedding. While porosity and SSA first decrease and then increase with increasing uniaxial stress from 0 to ~500 bar for the shale cut parallel to the bedding. Degree of anisotropy has variations with increasing uniaxial stress for both the vertical and horizontal cut shale samples. In conclusion, anisotropy is inherent in the nanopore structure of Marcellus shale. Uniaxial compression has different effects on the nanopore structure of the shale samples cut parallel and perpendicular to the bedding.

AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019