Experimental study on tensile strength and acoustic emission characteristics of shale exposure to supercritical CO₂

ABSTRACT

The effect of supercritical CO₂ on the mechanical properties of shale is a long-term process. To study the influence of supercritical CO₂ on the tensile strength and acoustic emission characteristics of shale, the Brazilian splitting experiment equipped with a real-time monitoring of acoustic emission were conducted on the shale collected from Longmaxi formation in Sichuan Basin before and after supercritical CO₂ treatment at different exposure times (0 day, 15 days, and 30 days) and a constant pressure (8 MPa) and temperature (40°C). The results show that the tensile strength of shale decreases and the failure deformation increases after supercritical CO₂ treatment. The peak acoustic emission energy generated by the treated shale specimens in the process of Brazilian splitting experiment was significantly lower than that of the untreated, indicating that supercritical CO₂ deteriorated the failure strength of the shale. The percentage of C element content has no obvious change, and the percentage of H element content decreased after supercritical CO₂ treatment, which indicates that the supercritical CO₂ fluid has negligible influence on organic matter in shale, while the water content of shale reduced. After supercritical CO₂ treatment, the pore structure of shale measured by Poremaster-33 shows an increasing trend, the most probable pore size is shifted from 100–150 μm to 50–100 μm, and many new micro-pores (<20 μm) are generated in the shale. In addition, the formed CO₂-water weak acid system in the process of supercritical CO₂ treatment dissolves the carbonate and silicate in the shale measured by SEM/EDS. The main reasons for the decrease of tensile strength of shale after supercritical CO₂ treatment can be attributed to the increased pore structure/volume and the dissolution of carbonate and silicate in the shale. The research results can provide reference for supercritical CO₂ fluid exploitation of shale gas.

KEYWORDS:

• Supercritical CO₂
• shale
• tensile strength
• pore
• mineral dissolution

Additional information

Funding

The research presented here was funded by the National Program on Key Basic Research Project of China (973 Program) [2014CB239204], the Project supported by the National Natural Science Foundation of China [51574049, 51774060]. The funder of the two project is Yongdong Jiang.

Notes on contributors

Chao Qin

Chao Qin is a doctor, whose main research field is the development and utilization of unconventional natural gas.

Yongdong Jiang

Yongdong Jiang is a professor, whose main research fields are the development and utilization of coalbed methane and unconventional natural gas.

Zhipeng Kang

Zhipeng Kang is  a master, whose main research field is the development and utilization of unconventional natural gas.

Xiao Song

Xiao Song  is a master, whose main research field is the development and utilization of unconventional natural gas.

Hao Liu

Hao Liu  is a doctor, whose main research field is the development and utilization of unconventional natural gas.