Abstract
An experimental study was performed to investigate the effect of subcritical carbon dioxide (CO2) adsorption on mechanical properties of shales with different coring directions. Uniaxial compressive strength (UCS) tests were conducted on shale samples with different CO2 adsorption time at a pressure of 7 MPa and a temperature of 40 °C. The crack propagation and the failure mechanism of shale samples were recorded by using acoustic emission (AE) sensors together with ARAMIS technology. According to the results, samples with parallel and normal bedding angles present reductions of 26.7% and 3.0% in UCS, 30.7% and 36.7% in Young’s modulus after 10 days’ adsorption of CO2, and 30.3% and 18.4% in UCS, 13.8% and 22.6% in Young’s modulus after 20 days’ adsorption of CO2. Samples with a normal bedding angle presented higher brittleness index than that with a parallel bedding angle. The strain distributions show that longer CO2 adsorption will cause higher axial strains and lateral strains. The AE results show that samples with a parallel angle have higher AE energy release than the samples with a normal angle. Finally, samples with longer CO2 adsorption times present higher cumulative AE energy release.
Highlights
With the spread of fracking techniques, shale gas production has increased rapidly in the past ten years
Due to the relatively lower total organic carbon (TOC) and the constraints from the framework rocks, the strain from the shrinkage/swelling of the organic matter due to gas desorption/adsorption is expected to be much lower in the shale formations as observed by the geo-mechanical communities [23,24,25,26]
The results show that the decrease of water content will reduce the total released acoustic emission (AE) energy and the adsorption of CO2 will enhance the AE
Summary
With the spread of fracking techniques, shale gas production has increased rapidly in the past ten years. Due to the relatively lower total organic carbon (TOC) (usually less than 10% in shale compared with about 50%–60% in coal) and the constraints from the framework rocks, the strain from the shrinkage/swelling of the organic matter due to gas desorption/adsorption is expected to be much lower in the shale formations as observed by the geo-mechanical communities [23,24,25,26]. Choi and Song [27] stated that the adsorption of CO2 and water in shales leaded to significant decrease of compressive strength. With a gas saturation pressure of 1Mpa for 3 days, the testing results showed that the reduction of uniaxial compressive strength (UCS) and Young’s modulus of the coal were around 13% and 26%, respectively. Demonstrated that compressive strength and mechanical properties of shale decreased upon water adsorption.
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