Impacts of Mineralogical Variation on CO2 Behavior in Small Pores from Producing Intervals of the Marcellus Shale: Results from Neutron Scattering

The Near and InterMediate Range Order Diffractometer (NIMROD) was used to examine the potential impact of shale mineralogy on CO2 behavior within micropores. Two samples with varying mineral compositions were obtained from producing intervals in the dry gas window in the Middle Devonian Marcellus Shale. One of the samples contained relatively high amounts of quartz and clay and low carbonate, the other contained relatively equal amounts of quartz, carbonate, and clay. The samples were probed with CO2 at subcritical pressures (20-50 bar) and temperature (22 degrees C) and characterized over a neutron scattering vector (Q) range of 0.02 < Q < 50 angstrom(-1). This Q range provides information from the atomistic length-scale up to pore radii of 10 nm. Mineralogy variations between the samples did not affect scattering ratios over the entire Q range accessible with the NIMROD. Q values for the minimum scattering ratios of both samples at similar pressures are remarkably similar, particularly for Q < similar to 0.09 angstrom(-1), and maximum scattering ratios are similar in both samples suggesting that mineral pores are so uncommon in the pore sizes examined that they cannot be resolved due to the overwhelming amounts of organic pores in these samples. Overall, these findings suggest that mineralogical variations have little effect on CO2 behavior within organic matter-hosted shale micropores at high thermal maturities and they lend support to the assertion that CO2 cannot be stored in the vast surface areas of micropores in organic material in shale formations. In addition, CO2 enhanced oil recovery (EOR) is unlikely to displace petroleum from some of the smaller mesopores (2.5 to similar to 3.5 nm) and all of the micropores because they are effectively closed to CO2 .