Quantification of CO₂ Uptake Capacity in Source Rock Shales for Geologic Carbon Sequestration: A ¹³C Nuclear Magnetic Resonance Spectroscopy Method

Geologic carbon sequestration (GCS) is an accepted technology to reduce anthropogenic carbon dioxide (CO2) from the atmosphere. To determine the suitability of underground rocks for GCS, the prospective ultimate CO2 storage capacity of a unit volume of an intact rock sample must be accurately estimated. Unconventional source rock shale reservoirs, which are currently the most abundant and exploited reservoir types, are being field assessed for GCS based on their physical and chemical structures and their unique capacity to store the injected CO2. Gas storage capacity estimation in these low permeability organic-rich rocks is essential and has been practiced using pulverized rock samples and traditional adsorption methods. The reliability and accuracy of these classical methods are compromised first by the destruction of the intact source rock sample and second by the unsuitability to mimic in situ reservoir rock conditions. A new method was developed using 13C nuclear magnetic resonance (NMR) spectroscopy to measure the ultimate storage volume of CO2 in preserved structurally intact organic-rich source rocks. The total volume of CO2 storage in these extremely low permeability rocks gives the ultimate CO2 uptake capacity under the set conditions and is termed absolute gas sorption (AGS). Using the NMR properties, the chemical shift and the transverse relaxation time of CO2 in interstitial and bulk states allow us to separate CO2 inside and outside the bulk shale rock. The NMR methods have been shown to accurately measure AGS of CO2 on the preserved intact organic-rich source rock plugs containing in situ original fluids. These new estimates in CO2 storage volumes measured show the feasibility and practicality of using organic-rich shale reservoirs as a potential low-cost field operation worldwide for GCS.