Reduced methane recovery at high pressure due to methane trapping in shale nanopores
Integrated molecular simulations and small-angle neutron scattering suggest that liquid-like methane can be trapped in shale nanopores when exposed to higher peak pressures during shale gas extraction.
Unconventional shale reservoirs currently produce more than 60% of US natural gas, a number predicted to rise to 75% by 2050 . Despite this, gas extraction efficiencies from unconventional reservoirs are limited to around 20%, due in large part to limited recovery from the shale matrix during the late stage of well life (Figure 1) . Maximizing recovery at this late stage necessitates better understanding of hydrocarbon interactions in the shale matrix, where hydrocarbon flow through nanopores can be significantly different due to nano-confinement. These pores are difficult to interrogate and remain a major knowledge gap in understanding and optimizing unconventional hydrocarbon production.
In this study, our team has integrated molecular simulation with high-pressure small-angle neutron scattering (SANS) to examine methane behavior in shale nanopores during pressure cycling. Pressure management is a cheap but effective strategy that operators can employ to improve recovery, however they currently only have adhoc knowledge on optimizing production using this method. Due to the high penetrating ability of uncharged neutrons, SANS is uniquely capable of characterizing nanopores and hosted fluid behavior in situ within a pressure cell. We previously used this technique to compare the water accessibility to nanopores in clay- and carbonate-rich shales . SANS experiments were conducted at the NIST Center for Neutron Research (NCNR) .
Methane behavior was compared during two pressure cycles with peak pressures of 3000 psi and 6000 psi. Changes in scattering intensity indicate that more methane was removed from larger pores upon depressurization from the higher drawndown pressure of 6000 psi. This result is in line with conventional wisdom, and was modeled using molecular dynamics (MD) simulations. However, SANS also revealed unexpected methane behavior occurring in very small nanopores (< 2 nm). Increases in scattering intensity, indicative of methane cluster formation in these nanopores , occurred upon methane pressurization up to both peak pressures, and were reversible during drawdown from the 3000 psi pressure peak but irreversible from the 6000 psi pressure peak. This peculiar observation indicates that a higher peak pressure results in trapping of these dense methane clusters in sub-2 nm radius nanopores, which comprise more than 90% of the measured shale nanopore volume.
Combining our experimental observations with previously-modeled kerogen mechanical behavior at high pressures , we propose the mechanism depicted in Figure 2. Methane incorporation into kerogen results in swelling of the kerogen pore matrix. Up to 3,000 psi, this swelling is reversible due to the mechanical flexibility of relatively ductile kerogen, and we call this the “elastic” regime. However, continued methane infiltration beyond this pressure causes permanent kerogen deformation, dubbed the “plastic” regime, irreversibly trapping methane clusters in the kerogen pore space. These insights enrich our understanding of confined fluid behavior during hydraulic fracturing, and can help optimize operational parameters that maximize hydrocarbon recovery, particularly at later operation times when recovery is controlled by matrix processes.
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