New Insights into Fracture Evolution in Rocks Relevant to the Geological Carbon Sequestration from In Situ Synchrotron X-ray Microtomography

Abstract:

Fractured rocks rich in carbonates can be found as candidates for both reservoirs and seals of importance to GCS; however, the mechanisms involved in the evolution of the fractures are still not fully understood despite its importance for both reservoir rocks, where CO₂-induced variations in aperture can significantly change injectivity, as well as seals, where feedbacks between dissolution and flow could lead to either leakage or self-sealing behavior.

Two samples, a dolomite and a carbonate rich shale, were analyzed in situ using dynamic synchrotron X-ray microtomography (SXR-μCT), flowing CO₂-saturated water along a cylindrical sample (~3/8” diameter, ~1” height) with a single fracture along the flow direction. Experiments lasted about one week each, with a constant flow of 5 μl/min under 1400 psi pore pressure and 1700 psi confining pressure. XR tomographic scans were taken at different time steps to monitor the evolution of the fracture characteristics.

A comparison of the two systems will be presented. In both samples significant dissolution occurs during the experiment, with the development of wormholes clearly visible from segmented fracture aperture maps. Both samples develop a weathered zone on the fracture surface, but the microstructure and the extent of this weathered layer is markedly different. Effluent chemistry analysis and SEM/EDS also help to reveal some time-dependent processes (e.g. increase in the dolomite dissolution rate with time). In both the experiments the crack surfaces becomes enriched in less soluble minerals and some migration of fines is visible as well. The mobilization of clays in the shale sample is unexpectedly small. With the extent of the dissolution allowed by our experiments, no noticeable closing of the fracture due to the confining pressure is visible, and the contact area evolution is barely noticeable.

Peclet and Dahmköhler numbers for this experiments seem to suggest that the self-sealing behavior observed in prior experiments with carbonate-rich shales is not universal. A very large amount of dissolution would be needed to obtain a significant closure of the fracture. Moreover the evolution of fractures during dissolution is a complicated system where several different processes (which can also be time dependent) need to be taken into account.