CO2-Induced alterations due to thermal maturation in shale: Implications for CO2 utilization and storage

Abstract

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.