SHALE-CO2 REACTIVITY: IMPLICATIONS FOR CAPROCK INTEGRITY OF CARBON STORAGE RESERVOIRS

Over the past decade, Canada has globally accounted for 5 Mtpa (million tonnes per annum) of CO₂ storage and it is estimated to have a total storage capacity of approximately 404 Gt (10⁹ tonnes) (Global CCS Institute, 2021). To reach this storage capacity, Canada needs to explore, identify, and develop large new carbon storage reservoirs. One aspect of the successful underground carbon storage is containment, meaning the existence of a proper caprock with low permeability, intact structural integrity, and high capillary entry pressure over CO₂ storage reservoirs.

The current study presents the results of reactivity experiments of CO₂ with multiple shales, such as the Duvernay, Upper and Lower Bakken, and Deadwood formations from the Western Canada Sedimentary Basin (WCSB). The main objective of this study is to better understand the seal integrity of these formations as caprock for potential CO₂ storage sites in the basin. This study particularly centers on the mineralogical transformations and microstructural changes occurring in the samples after exposure to dry supercritical CO₂ (scCO₂) and brine- saturated scCO₂ at 17 MPa and 95°C.

Backscattered Electron Diffraction (BSED) images and Raman Spectroscopy analyses acquired from the first phase of the study (reaction with dry scCO₂) showed no significant reactivity between dry CO₂ and the shale samples implying minimal changes to porosity and permeability. The second phase, in which samples were exposed to NaCl brine-saturated scCO₂, however, revealed noticeable increase in porosity and permeability in the samples. This occurred primarily due to the dissolution of carbonate minerals and etching and pitting of the shale matrix.

Estimated porosity from BSED images shows an increase in porosity (from 50% up to several times the original porosity), with the largest increase occurring in calcareous shales such as the Duvernay and Deadwood formations. Bakken shale also shows signs of increase in porosity through both mineral as well as organic matter dissolution. Although the total porosity remains low for the shales even after the increase due to CO₂ exposure, the increasing trend is not favorable for the long-term CO₂ storage leak prevention.

References:

Global CCS Institute, 2021. Global status of CCS 2021, CCS accelerating to net zero, 43 p.