Leakage Processes in Damaged Shale: In Situ Measurements of Permeability, CO2-Sorption Behavior and Acoustic Properties
Abstract
Caprock integrity is one of the chief concerns in the successful development of a CO2 storage site. In this chapter, we provide an overview of the permeability of fractured shale, the potential for mitigation of CO2 leakage by sorption to shale, and the detection by acoustic methods of CO2 infiltration into shale. Although significant concerns have been raised about the potential for induced seismicity to damage caprock, relatively little is known about the permeability of the damaged shale. We present a summary of recent experimental work that shows profound differences in permeability of up to three orders of magnitude between brittle and ductile fracture permeability. In the ductile regime, it is possible that shale caprock could accommodate deformation without a significant loss of CO2 from the storage reservoir. In cases where CO2 does migrate through damaged shale caprock, CO2 sorption onto shale mineralogy may have a mitigating impact. Measured total storage capacities range from 1 to 45 kg-CO2/tonne-shale. Once CO2 is in the caprock, changes in the acoustic properties of CO2-saturated shale that are predicted by Gassmann fluid substitution calculations show a significant reduction of the bulk modulus of CO2-saturated shale.
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