Exploring methane behavior in Marcellus Shale micropores via contrast matching neutron scattering

Petroleum in shale reservoirs is hosted in organic matter and mineral pores as well as in natural fractures and voids. For thermally mature plays, e.g., the Marcellus Shale, methane and other light alkane gases are thought to be primarily contained in organic matter pores with radii ≦50 nm. Thus, in order to understand natural gas occurrence, transport, storage, and recoverability within unconventional reservoirs at the dry-gas stage of thermal maturity, it is critical to characterize the associated organic matter porosity across length scales from 50 nm down to the angstrom level. We utilized wide Q-range neutron total scattering to characterize deuterated methane (CD₄) adsorption at 60ºC up to the zero average contrast (ZAC) pressure (~60 MPa) within two mineralogically different samples collected from the same producing interval from the Middle Devonian Marcellus Shale. The neutron scattering approach used here provides structural information from the inter-atomic regime up to a nominal pore radius of ~12.5 nm and, by reaching the CD₄ ZAC pressure (~60 MPa), it is possible to examine the distribution of open versus closed pores within this pore size range in the samples. Our results indicate that ~10% of the largest pores measured are closed to CD₄ for a quartz-rich sample whereas up to 25% of pores with a nominal radius of ~12.5 nm are inaccessible within a sample with an equivalent proportion of quartz, carbonate, and clay. As pore size decreases, accessibility also decreases; all pores with radii ~0.5 nm are effectively closed to CD₄ in both samples. Additionally, up to ~4.5× more CD₄ is adsorbed within the quartz-rich sample at 60 MPa and we see no evidence for densification of CD₄ within the shale pores. These findings suggest that, for shale samples within the dry-gas window, (i) nanometer-scale porosity is primarily located within organic matter, (ii) the amount of available nano-porosity can vary widely over meter scales, and (iii) mineralogy plays a secondary role in dictating methane behavior within these systems.