The effects of CO₂-water-rock interactions on the mechanical properties of shale are essential for estimating the possibility of sequestrating CO₂ in shale reservoirs. In this study, uniaxial compressive strength (UCS) tests together with an acoustic emission (AE) system and SEM and EDS analysis were performed to investigate the mechanical properties and microstructural changes of black shales with different saturation times (10 days, 20 days and 30 days) in water dissoluted with gaseous/super-critical CO₂. According to the experimental results, the values of UCS, Young’s modulus and brittleness index decrease gradually with increasing saturation time in water with gaseous/super-critical CO₂. Compared to samples without saturation, 30-day saturation causes reductions of 56.43% in UCS and 54.21% in Young’s modulus for gaseous saturated samples, and 66.05% in UCS and 56.32% in Young’s modulus for super-critical saturated samples, respectively. The brittleness index also decreases drastically from 84.3% for samples without saturation to 50.9% for samples saturated in water with gaseous CO₂, to 47.9% for samples saturated in water with super-critical carbon dioxide (SC-CO₂). SC-CO₂ causes a greater reduction of shale’s mechanical properties. The crack propagation results obtained from the AE system show that longer saturation time produces higher peak cumulative AE energy. SEM images show that many pores occur when shale samples are saturated in water with gaseous/super-critical CO₂. The EDS results show that CO₂-water-rock interactions increase the percentages of C and Fe and decrease the percentages of Al and K on the surface of saturated samples when compared to samples without saturation.

Jet pump is a type of cavitation reactor with great potential because of strong shear flow. In the present paper, experiments were carried out to investigate the cavitation characteristics of jet pump cavitation reactors (JPCRs) with different throat lengths, throat types and diffuser angles. Cavitation images and sound pressure signals in water corresponding to the hydraulic parameters are introduced to judge the aggressive intensity of cavitation in JPCRs. The flow ratios varying from the maximum limited value to -1 were measured for all JPCRs. It suggests that throat structure plays a more important role in the cavitation and flow characteristics of JPCR when compared with diffuser structure. Specifically, convergent throat results in large bubble density in the diffuser while divergent throat results in choke in the throat compared to the original JPCR. And cavitation bubble density in throat increases with increasing throat length. With the decrease of the flow ratio (q > 0), sound pressure level (SPL) decreases from the maximum to the minimum and then increases again. As the flow ratio decreases further (q < 0), SPL keeps on increasing first and then decreases, finally it takes a turn and increases to a stable level. Further study on actual SPL induced by cavitation in JPCR indicates that small diffuser angle, divergent and long throat enhance the aggressive intensity of cavitation. This result is of great significance to the design of JPCR.

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.