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

Abstract

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 °C) and characterized over a neutron scattering vector (Q) range of 0.02 < Q < 50 Å–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 < ∼0.09 Å–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 ∼3.5 nm) and all of the micropores because they are effectively closed to CO2.

Monitoring the CO2 enhanced oil recovery process at the nanoscale: an in situ neutron scattering study

“…This finding is also in agreement with recent neutron experiments in shale that also showed that some of the smaller mesopores and all of the micropores are effectively closed to CO 2 . 30,31 The total neutron scattering profiles can also reveal the structural details of bulk liquids, despite the lack of long-range order, by evaluating the total structure factor comprising the intermolecular structure peak (representing the most probable distance between nearest-neighbour molecules) and the intramolecular oscillations (within the molecule) arising at higher Q values. In addition, when a fluid is confined in the pores, the structure factor is the superposition of the matrix-matrix (limestone), the fluid-fluid (CO 2 ), and also of the matrix-fluid interactions (between the limestone matrix and the CO 2 ).…”

“…In addition, the results have elucidated that the confined CO 2 is in a densified state having liquid-like properties. 30,31 It is noteworthy that strong densification of confined CO 2 has also been observed in microporous carbon 32–35 and in ordered mesoporous carbon (CMK-3) samples. 27…”

“…In addition, the results have elucidated that the confined CO 2 is in a densified state having liquid-like properties. 30,31 It is noteworthy that strong densification of confined CO 2 has also been observed in microporous carbon [32][33][34][35] and in ordered mesoporous carbon (CMK-3) samples. 27 Complementary to total neutron scattering that measures interatomic distances, small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) probe the structures at larger length scales, varying from 1 nm to a few hundred nanometers (SANS) and up to about 10 mm (USANS).…”

“…This finding is also interesting because it suggests that during anthropogenic CO 2 sequestration, the vast number of micropores in shale will be unavailable for storage because they are all inaccessible or closed to CO 2 . 30,31 In the present study, we attempt to establish a new methodology to monitor the CO 2 -EOR process in real time by neutron scattering. In particular, the oil recovery process is explored at the nanoscale by utilising total neutron scattering by in situ injection of supercritical CO 2 fluid into a limestone sample loaded with deuterated n-decane.…”

“…This finding is also interesting because it suggests that during anthropogenic CO 2 sequestration, the vast number of micropores in shale will be unavailable for storage because they are all inaccessible or closed to CO 2 . 30,31…”