CO2‐Induced alterations due to thermal maturation in shale: Implications for CO2 utilization and storage
Authors: Onwumelu Chioma , Kolawole Oladoyin , Bouchakour Imene , Ozotta Ogochukwu , Nordeng Stephan , Alamooti Moones 
Published: September 2023

Shales have low to ultra‐low porosity and permeability, which makes them an attractive candidate for CO2 utilization during CO2‐enhanced oil recovery (CO2‐EOR) or for geologic CO2 storage (GCS). Shale are source rocks, and thus, there is a continuous induced diagenetic process that can alter their properties as they reaches maturity at greater in situ temperature. However, there are significant knowledge gaps in the possibility of CO2 utilization during this diagenetic process (thermal maturation) to achieve long‐term CO2 storage. This experimental study investigates the potential for CO2 utilization in shale due to induced thermal maturation at in situ conditions, and the implications of pre‐maturation CO2 injection in shale for GCS and CO2‐EOR. Here, we used subsurface hydrocarbon‐rich Bakken and Green River shales exposed to CO2 for a specific period. This is followed by inducing the unexposed and CO2‐exposed shales to thermal maturity. Subsequently, we evaluated the total organic carbon (TOC), liberated hydrocarbons (S2), and the mineralogical and mechanical properties of the mature and CO2‐exposed mature shales. We further assessed the implications of CO2 utilization and storage in thermally matured Bakken and Green River shales for long‐term storage or CO2‐EOR. The results indicate that if CO2 is injected into shales before attaining maturity, higher hydrocarbon production and more significant mechanical weakness can be expected when they attain maturity in Bakken shales (+30% liberated hydrocarbons; −31% Young's modulus; −34% hardness) and Green Rivers shales (+8% liberated hydrocarbons; −40% Young's modulus; −30% hardness), and this is relative to Bakken and Green River shales without CO2 injection before attaining thermal maturity. Further, CO2‐exposed mature Bakken and Green River shales can alter the minerals in shales with the dissolution of dolomite and precipitation of calcite, which promotes mineral trapping and achieve a lower TOC (Bakken shale = −24%; Green River shale = −26%), and this is relative to Bakken and Green River shales without CO2 injection before attaining maturity. Analyses of the results suggest that the application of this proposed CO2 injection and utilization in immature shales could access more excellent CO2‐storage reservoirs in Bakken and Green River shales without waiting for a more extended period for the shales to become viable and mature, which is the case with the present GCS and CO2‐EOR operations in shale reservoirs globally. Also, our proposed pre‐maturation CO2 injection could rejuvenate mature shales for increased hydrocarbon production through CO2‐EOR, yield a greater sealing efficiency, and mitigate leakage risks for long‐term CO2 storage. The results from this study provide novel insights that can advance CO2 utilization for future GCS and/or CO2‐EOR in immature Bakken and Green River shales while at the same time providing an immediate and viable option for storage by depleting atmospheric CO2 to meet the global net‐zero by 2050. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.