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| Effects of CO2 state and anisotropy on the progressive failure characteristics of bituminous coal：an experimental study |
| SUN Zedong1，FENG Gan2，SONG Xuanmin1，MENG Tao1，3，ZHU Defu1，HUO Yuming1，WANG Zhonglun1 |
(1. Key Laboratory of In-situ Property Improving Mining of Ministry of Education，Taiyuan University of Technology，Taiyuan，Shanxi 030024，China；2. College of Water Resource and Hydropower，Sichuan University，Chengdu，Sichuan 610065，China；3. School of Chemical and Biological Engineering，Taiyuan University of Science and Technology，
Taiyuan，Shanxi 030024，China) |
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Abstract In order to explore the mechanical properties of the reservoirs during the CO2 displacement coalbed methane exploitation process，CO2 adsorption experiments under different pressures(4，6，8，and 12 MPa)，Brazilian disc splitting experiments，low temperature nitrogen adsorption experiments and fracture analysis were carried out on bituminous coal samples. The influence of CO2 state and anisotropy on mechanical response of bituminous coal to progressive failure was studied. The results show that the Brazilian splitting strength(st) and modulus(ET) of bituminous coal decrease first and then increase under the influence of CO2 adsorption. Under 8 MPa supercritical CO2 adsorption，st and ET decrease by 49.8% and 33.9% respectively. The order of st of three types of bituminous coal samples is divider type followed by arrester type and short converse type. With increasing the CO2 pressure，bituminous coal shows a transformation law of failure characteristics from sudden instability to gradual instability and to sudden instability. At the same time，the CO2 adsorption effect weakens the matrix and the bedding plane of bituminous coal，which makes the failure path of anisotropic bituminous coal more complicated with increasing the CO2 pressure. The failure path can be divided into single failure，multi-source failure and fragmentation failure tracks，which can be further divided into twelve sub categories. The above-obtained results are due to the differences of CO2 adsorption，organic matter extraction，dissolution and compression under different phase states and pressures as well as the corrosion of CO2 acid fluid. The specific surface area，the total pore volume and the pore size distribution uniformity of bituminous coal are changed by the interaction of CO2 and bituminous coal. The change of the micro structure is related to the macro mechanical properties，which affects the mechanical properties and failure characteristics of bituminous coal. The research results can provide certain reference value for reservoir reconstruction，stability evaluation and exploration of high-efficiency production technology in CO2 displacement coalbed methane development project.
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2022, Vol. 41 
