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Experimental Investigation Into Fracture Closure in Caprocks in CO2 Storage Reservoirs


The risk of CO2 leakage through fractures is one of the primary risks for the secure containment of CO2 in geological storage formations. The potential for fracture closure with pore pressure reduction and sub-surface stress changes, and fracture geometry will control CO2 leakage rates in storage reservoirs where the seal has been compromised by induced fractures or reactivated pre-existing fractures. Modelling of fracturing formation and reactivation with injection induced stress changes will be important in preventing leakage from occuring, however if leakage does occur, understanding fracture closure and whether fracture closure can be increased or accelerated will be important for remediation efforts. In this study we investigate a speculative acid induced fracture closure remediation technique using a series of laboratory acid injection experiments in shale caprocks. In the tests viscous acid is injected through a range of fractured shale caprock samples under confining stress. The permeability of the fractured samples is measured before and after acid treatments using brine flow rates and pressure differentials, to determine how the acid treatment affects fracuture apertures and brine flow. Fracture aperture widths are characterised from CT sections taken across the core during flooding cycles. The fracture samples include naturally fractured samples and artificially induced fractures including samples with artificial regularly arranged asperities. Fracture roughness is also measured using photogrammetry before and after acid treatments. The caprocks currently collected for the study are a range of onshore UK shales including Kimmeridge Clay, Whitby Mudstone, and Accrington Mudstone from the Pennine Coal Measures. The range of lithologies allows correlation between observed fracture behaviour and mineralogical variation, for example clay content and cement type. The study focusses on the extent to which fracture closure can be enhanced, and the effectiveness of the technique in varying rock types. Future work will involve introducing CO2 rich brines and alternative acid formulations into the experimental programme, and a comparative assessment of the natural and artificially induced fractures (by correlating fracture roughness and permeability) to determine the effectiveness of acid treatment with variable confining stress in treated vs. non-treated samples.