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Small-angle Neutron Scattering (SANS) Characterization of Clay- and Carbonate-rich Shale at Elevated Pressures
journal contribution
posted on 2020-06-29, 23:51 authored by Chelsea W. Neil, Rex P. Hjelm, Marilyn E. Hawley, Erik B. Watkins, Cody Cockreham, Di Wu, Yimin Mao, Timothy B. Fischer, M. Rebecca Stokes, Hongwu XuUnconventional oil and gas from shale
formations have emerged as
some of the fastest growing energy resources in the United States,
providing both cleaner energy to consumers and reducing the nation’s
reliance on energy imports. To properly harness these important natural
resources, the nanopore structure of associated shales must be fully
understood, particularly under hydraulic fracturing conditions, where
they are exposed to both overburden compressive and hydrostatic fluid
pressures. The current study uses small-angle neutron scattering (SANS)
to characterize pore structure, including porosity, pore accessibility,
and pore size distribution, in the 1–100 nm regime at elevated
pressures for mineralogically distinct clay- and carbonate-rich shales
from the Permian Basin. Unlike typical porosity measurement techniques,
SANS is uniquely capable of characterizing both open and closed porosity,
allowing measurement of how pore accessibility changes with pressure
and determination of the size range of accessible versus inaccessible
pores. The porosity of the clay-rich shale was 7.7%, compared to 0.51%
for the carbonate-rich shale. However, only 2.6% of the nanopores
in the carbonate-rich shale were inaccessible to water at 8 kPSI (55.1
MPa) compared to 7.8% for the clay-rich shale. Further analyses indicated
that the closed pores fall within distinct size ranges, likely corresponding
with the chemical nature of the pore host material. These results
provide valuable insight into the effects of shale petrophysical properties
on hydrocarbon extraction from unconventional reservoirs.