Reduced methane recovery at high pressure due to methane trapping in shale nanopores

Abstract

By 2050, shale gas production is expected to exceed three-quarters of total US natural gas production. However, current unconventional hydrocarbon gas recovery rates are only around 20%. Maximizing production of this natural resource thus necessitates improved understanding of the fundamental mechanisms underlying hydrocarbon retention within the nanoporous shale matrix. In this study, we integrated molecular simulation with high-pressure small-angle neutron scattering (SANS), an experimental technique uniquely capable of characterizing methane behavior in situ within shale nanopores at elevated pressures. Samples were created using Marcellus shale, a gas-generative formation comprising the largest natural gas field in the United States. Our results demonstrate that, contrary to the conventional wisdom that elevated drawdown pressure increases methane recovery, a higher peak pressure led to the trapping of dense, liquid-like methane in sub-2 nm radius nanopores, which comprise more than 90% of the measured nanopore volume, due to irreversible deformation of the kerogen matrix. These findings have critical implications for pressure management strategies to maximize hydrocarbon recovery, as well as broad implications for fluid behavior under confinement.

Technical Aspects of the Pore Structure in Shale Measured by Small-Angle and Ultrasmall-Angle Neutron Scattering: A Mini Review

“…There are some researchers who adopted a different approach to analyze the neutron scattering data. In these studies, SANS/USANS data were fitted by the following equation: ,,,, This approach excluded the power law scattering portion from the scattering data for the derivation of the pore size distribution. There are also a few reports that only used I ( q ) = bq –α + bkg to extract D s = 6 – α from SANS data of shale samples. , …”

“…A pressure cell that can mimic reservoir temperature and pressure conditions is desirable. Pressure studies relevant to hydraulic fracturing , or CO 2 sequestration ,, are particularly interesting. However, application of pressure to shale may introduce uncertainties in data interpretation because it can compress or swell the shale matrix, cause capillary condensation, , and lead to clustering of methane molecules and deformation of the kerogen matrix .…”

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

“…The intensity of SANS and USANS scattering, which effectively takes place on the porerock matrix interface, is controlled by the geometry and content of the pore space [13]. The contrast matching technique takes advantage of the possibility of modifying the scattering intensity, I(Q), by forcing liquids or pressurized gases into the pore space, thereby changing the scattering contrast between the rock matrix and the pore content, where (∆ρ) = ρ m − ρ p is the difference between the scattering length density (SLD) of the rock matrix, ρ m , and the SLD of the pore content, ρ p [3,12,[23][24][25][26][27][28]. The variable Q = (4π sinθ)/λ is the scattering vector, where 2θ is the scattering angle and λ is the neutron wavelength; for small scattering angles, Q is simply a re-scaled measure of the angular deviation of the neutron beam transmitted through the rock slice [4].…”

“…value of SSA (D = 0.4 nm) accessible to methane is of the order of 10 4 cm 2 /cm 3 -at least one order of magnitude smaller than the inaccessible SSA (with the exception of sample McAtee-2798). After pressure cycling, some calculated SSA values become very small or negative (such as sample Mar_7084); this is an artifact caused by neglecting the densification of methane confined in nanopores (further discussed in Section 4.5), as well as from the possible entrapment of molecular CD4 in kerogen, which is forced by the elevated pressure of methane [28]. Extrapolated SSA results for D = 0.4 nm for the New Albany Shale samples and the Mar_7084 sample are compiled in Figure 6.…”

“…It is evident that for most of the samples th value of SSA (D = 0.4 nm) accessible to methane is of the order of 10 4 cm 2 /cm 3 -at leas one order of magnitude smaller than the inaccessible SSA (with the exception of sampl McAtee-2798). After pressure cycling, some calculated SSA values become very small o negative (such as sample Mar_7084); this is an artifact caused by neglecting th densification of methane confined in nanopores (further discussed in Section 4.5), as we as from the possible entrapment of molecular CD4 in kerogen, which is forced by th elevated pressure of methane [28]. Figure 7 shows the apparent fraction of accessible pores, F a , for mid-mature sample 2 (McAtee-2798) before and after pressure cycling, calculated according to Equation ( 4) and plotted versus the pore diameter ("apparent" means that data pertain to the in-bedding pore diameter only).…”