Experimental Investigations into CO₂ Injection Associated Fracture Behaviour in Shale Caprocks

Leakage of CO2 through fractures is a risk for the secure storage of CO2. Fracture closure, stress state and fracture geometry will control CO2 leakage rates in storage reservoirs where the seal has been compromised by fractures. This study investigates an acid induced fracture closure remediation technique using a series of laboratory acid injection experiments in shale caprocks. The test aims to reduce the permeability of fractures through induced fracture closure in order to demonstrate the technique as a remedial measure to reduce leakage. In the tests viscous acid is injected through a range of fractured shale caprock samples under confining stress. Preliminary tests on a single sample of shale caprock show that a significant reduction in fracture permeability is achieved using acid injection across a range of confining stresses. CT scans of the fracture show the fracture closure, and appear to suggest that dissolution of asperities on the fracture face may promote fracture closure. Ongoing work will involve testing the technique in a wide range of caprock samples with different mineralogy, artificial fracture surfaces and with gaseous CO2 and CO2 rich brines included in the injected fluids to determine any reduction in the effectiveness of the remediation technique, more extensive analysis of the CT imagery will also be carried out.