Physicochemical factors impacting CO2 sequestration in depleted shale formations: The case of the Utica shale

摘要

Fractured shale formations could serve as an attractive target formation for geologic carbon sequestration once they have been depleted of hydrocarbons. The low intrinsic permeability of the shale matrix could reduce the CO2 leakage potential, the kerogen in the shale would provide a matrix within which the CO2 can be permanently sorbed, and the infrastructure in place at gas production sites could all be leveraged to minimize costs. Here, a modeling framework previously developed by the authors to estimate the sequestration capacity of shale formations is extended to better capture the physicochemical realities associated with injecting CO2 into fractured shale formations. The model uses CH4 production data to fit key parameters about the formation and applies those to a unipore diffusion model to characterize the controlling gas transport processes. A number of parameters, including the gas diffusion coefficient, the ratio of adsorbed gas to free phase gas, water saturation and gas adsorption isotherms are considered and their effect on modeling estimates is explored. The model is found to be most sensitive to the ratio of adsorbed gas to the total gas which includes both adsorbed and free phase gas. The equilibrium adsorption parameters of CH4 and CO2 also have significant influence largely because published estimates for these parameters vary considerably. The effect of pore collapse following production was explored in terms of its effect on characteristic diffusion length. The results indicate that increasing this characteristics length by an order of five would triple the time it takes to complete the injection of CO2 into the formation. Similarly, an increase in water content in the formation or in the ratio of free CH4 to sorbed CH4 would decrease the sequestration potential of the formation. Based on this improved constitutive understanding of the modeling inputs and the estimates, the CO2 sequestration capacity of the Utica Shale was calculated and the results were compared with those from Marcellus Shale. The differences could be understood in terms of the distinct petrophysical properties of those two shale formations. This analysis provides recommendations about experimental directions that could be very useful for improving the accuracy of sequestration capacity models.