Oil & Gas Geology ›› 2025, Vol. 46 ›› Issue (3): 983-994.doi: 10.11743/ogg20250319

• Methods and Technologies • Previous Articles     Next Articles

Degradation of the mechanical properties of medium-rank coals under supercritical CO2 soaking and its mechanisms

Huanpeng CHI1,2(), Caiqin BI1,2(), Dongping WAN3, Chengxiang WEI4, Chengzheng CAI5, Shouceng TIAN4, Fanshi ZHENG6, Tianyu WANG4   

  1. 1.Oil and Gas Resources Survey，China Geological Survey，Beijing 100083，China
    2.Key Laboratory of Unconventional Oil and Gas Geology，China Geological Survey，Beijing 100083，China
    3.Xinjiang Yaxin Coalbed Methane Investment and Development Co. ，Urumqi，Xinjiang 830009，China
    4.State Key Laboratory of Petroleum Resources and Engineering，China University of Petroleum （Beijing），Beijing 102249，China
    5.State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China
    6.Liaoning Province Geology and Minerals Group Energy Co. ，Ltd. ，Shenyang，Liaoning 100011，China
  • Received:2025-01-24 Revised:2025-03-26 Online:2025-06-30 Published:2025-06-26
  • Contact: Caiqin BI E-mail:chp2121@126.com;bicaiqin@mail.cgs.gov.cn

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Abstract:

Exploring the mechanical properties of coals subjected to supercritical carbon dioxide （SC-CO2） soaking holds great significance for enhanced hydraulic fracturing performance of deep coalbed methane （CBM） reservoirs and CO2 geological sequestration. Using tri-axial compression tests， mineralogical analysis， and porosity and permeability measurements， we systematically reveal the SC-CO2 soaking-induced degradation mechanisms of the macroscopic mechanical properties of medium-rank coals under varying confining pressures. The results indicate that SC-CO2 soaking leads to more pronounced initial compaction and residual failure characteristics in the stress-strain curves of coals， accompanied by a significant reduction in peak stress. This can effectively diminish the strength enhancement of coals caused by confining pressure. After reaching peak strength， coals experience rapid instability and failure， with a higher confining pressure associated with more rapid failure. SC-CO2 soaking induces more significant degradation of the compressive strength of coals compared to their elastic modulus. With an increase in the confining pressure， both the compressive strength and elastic modulus of coals trend upward before and after SC-CO2 soaking； however， the degrees of their degradation gradually decrease. Under low confining pressure， SC-CO2 soaking induces particularly prominent degradation of the mechanical properties of coals. Regarding the degradation mechanisms， SC-CO2 undergoes physicochemical reactions with mineral components in coals， leading to the dissolution of carbonate minerals. This process alters the pore structure and promotes the formation and propagation of micropores. As a result， both the porosity and permeability of coals increase significantly， accompanied by the breakdown of the coals’ surface structures. Compared to active water soaking， SC-CO2 soaking causes more severe degradation of coals. Under the same confining pressure， the degrees of degradation of coals’ compressive strength and elastic modulus caused by SC-CO2 soaking increase by 7.7 % to 36.7 % and 1.7 % to 19.4 %， respectively， compared to those induced by active water soaking. SC-CO2 soaking degrades the mechanical properties of coals primarily through chemical dissolution， while active water soaking induces degradation mainly via physical dispersion. This difference is due to the greater permeability and chemical reactivity of SC-CO2. It has been demonstrated that the chemical and physical synergistic degradation mechanism of SC-CO2 can significantly weaken the strength of coals while enhancing their permeability. The application of SC-CO2 soaking to the well involved in deep CBM tests in the Jixi Basin， Heilongjiang Province， has successfully reduced the fracturing pressure by 6 MPa and enabled a stable CBM production exceeding 1000 m3/d. Therefore， the degradation of coal properties induced by SC-CO2 soaking plays a significant role in promoting deep CBM production via volume fracturing.

Key words: mechanical property, permeability, pore structure, supercritical CO2 （SC-CO2）, deep coalbed methane （CBM）, Jixi Basin

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