Effects of Temperature and Water on Mechanical Properties, Energy Dissipation, and Microstructure of Argillaceous Sandstone under Static and Dynamic Loads

RMT-150B rock mechanics and split Hopkinson pressure bar (SHPB) devices were adopted to investigate the physical and mechanical properties, energy dissipation, and failure modes of argillaceous sandstone after different high temperatures under air-dried and saturation states. In addition, SEM and EDS tests were conducted to investigate its microstructure characteristics. Results showed that both the P-wave velocity and density of argillaceous sandstone specimen decreased with the increase of high temperature, while its porosity increased. Compared with static stress-strain curves, there was no obvious compaction stage for dynamic stress-strain curves, and the decrease rate of dynamic curves after peak strain was obviously slow compared with static curves. Both the static and dynamic strengths of argillaceous sandstone specimens decreased with increasing temperature, and the critical temperature point for the strength of argillaceous sandstone was 400°C. At the same temperature, the specific energy absorption under air-dried state was generally smaller compared with that under saturated state. Both the strain rate and temperature showed significant effect on the failure mode. After 100∼1000°C heat treatment, the granular crystals of the clastic structure gradually became larger, and both the number and average size of the original pores decreased, resulting in the deterioration of mechanical properties of argillaceous sandstone specimen.

CO2-enriched brine injection’s impact on mechanical properties of a sandstone specimen

CO2 capture and geological sequestration is one of the most practical and efficient methods of mitigating anthropogenic CO2 emissions. Due to the uncertainties associated with CO2 injection into deep saline reservoirs, the interaction between the host rock and the injected CO2 needs to be better understood as it can lead to considerable pore-structure changes. The geochemical reactions, especially mineral dissolution, can compromise the mechanical properties of the reservoir rock, which consequently threatens the reservoir stability and integrity. Therefore, it is crucial to capture the variation of mechanical properties of the reservoir rock upon CO2 injection. In this study the variation of elastic properties (e.g. Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio) of a brine-saturated sandstone specimen upon injecting CO2-enriched brine is investigated. The elastic properties of the specimen were initially characterized through multi-stage elastic (MSE) test before injecting the CO2-enriched brine. Then, the synthetic brine solution was enriched with CO2 and injected into the brine saturated sandstone specimen. The mechanical test results revealed that a significant mechanical weakening occurred upon injecting CO2-enriched brine into the sandstone specimen. This mechanical degradation can be attributed to the dissolution of calcite and clay minerals. The results from this study indicated that the mechanical deterioration of reservoir rock during CO2 injection should be considered through the entire CO2 sequestration process (i.e. site selection, injection operation, and post-injection monitoring).

Study of Testing Method for Dynamic Initiation Toughness of Sandstone under Blasting Loading

In this paper, an internal central single-cracked disk (ICSCD) specimen was proposed for the study of dynamic fracture initiation toughness of sandstone under blasting loading. The ICSCD specimen had a diameter of 400 mm sandstone disc with a 60 mm long crack. Blasting tests were conducted by using the ICSCD specimens. The blasting strain-time curve was obtained from the radial strain gauges placed around the blast hole. The fracture initiation time was determined by circumferential strain gauges placed around the crack tip. The stress history on the blast hole of the sandstone specimen was then derived from measured strain curve through the Laplace transform. The numerical solutions were further obtained by the numerical inversion method. A numerical model was established using the finite element software ANSYS. The type I dynamic stress intensity factor curves of sandstone under blasting loading were derived by the mutual interaction integration method. The results showed that (1) the ICSCD specimen can be used to measure dynamic initiation fracture toughness of rocks; (2) the stress on the blast hole wall can be obtained by the Laplace numerical inversion method; (3) the dynamic initiation fracture toughness of the ICSCD sandstone specimen can be calculated by the experimental-numerical method with a maximum error of only 7%.