Reduced Methane Recovery at High Pressure Due to Methane Trapping in Shale Nanopores
By: Chelsea W. Neil, Mohamed Mehana, Rex P. Hjelm, Marilyn E. Hawley, Erik B. Watkins, Yimin Mao, Hari Viswanathan, Qinjun Kang, Hongwu Xu (2020)
Shale gas production is projected to dominate the US natural gas landscape by 2050, accounting for over three-quarters of the total production. Despite its significant potential, the recovery rate of unconventional hydrocarbon gas currently hovers at a modest 20%. Addressing this gap necessitates a deeper grasp of the processes responsible for hydrocarbon retention in the minuscule pores of shale. To shed light on this, we adopted a dual approach: molecular simulation paired with high-pressure small-angle neutron scattering (SANS), a unique method adept at portraying methane's activity within shale nanopores at heightened pressures. Using samples from the Marcellus shale — the US's largest natural gas field — our study unearthed a surprising finding. Contrary to the prevailing belief that increasing the drawdown pressure bolsters methane recovery, we found that an uptick in peak pressure actually causes denser, liquid-like methane to get trapped in nanopores smaller than 2 nm in radius. This unexpected outcome is attributed to irreversible changes in the kerogen matrix structure. Our discoveries not only reshape our understanding of methane behavior but also suggest a need to reconsider pressure management techniques to optimize hydrocarbon extraction.