超临界CO浸泡中阶煤岩力学性能劣化特性及其机制

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

迟焕鹏 毕彩芹 宛东平 魏呈祥 蔡承政 田守嶒 郑凡石 王天宇

Huanpeng CHI Caiqin BI Dongping WAN Chengxiang WEI Chengzheng CAI Shouceng TIAN Fanshi ZHENG Tianyu WANG

研究煤岩在超临界CO浸泡后的力学特性对于提高深层煤层气储层压裂效果及实现CO地质封存具有重要意义。通过三轴压缩实验、矿物分析以及孔渗测试，系统研究并揭示超临界CO浸泡对不同围压下中阶煤岩宏观力学性质的劣化机制。研究结果表明：① 经超临界CO浸泡后，煤岩应力-应变曲线呈现出更为显著的初始压密和残余破坏特征，峰值应力明显降低，可有效削弱围压对煤岩强度增强的效应。煤岩达到峰值强度后会迅速发生失稳、被破坏，围压越高，破坏越快。②超临界CO浸泡对煤岩抗压强度的劣化作用较弹性模量更为显著。随着围压的增大，煤岩在超临界CO浸泡前、后的抗压强度和弹性模量均呈增大趋势，但其劣化度逐渐降低。在低围压条件下，超临界CO浸泡对煤岩力学性质的劣化效应尤为突出。③超临界CO浸泡对煤岩力学性质的劣化机制是：超临界CO与煤岩中的矿物组分发生物理、化学反应，导致碳酸盐矿物被溶蚀，改变煤岩的孔隙结构，促进微观孔隙的发育和扩展，导致煤岩孔隙度和渗透率显著提升，并造成煤岩表面结构被破坏。④与活性水相比，超临界CO浸泡对煤岩的劣化作用更强；相同围压条件下，超临界CO对煤岩抗压强度和弹性模量的劣化度比活性水分别高7.7 % ~ 36.7 %和1.7 % ~ 19.4 %；超临界CO主要通过化学溶蚀作用劣化煤岩力学性质，活性水则以物理分散作用为主导，导致这种作用机制差异的原因在于超临界CO的渗透能力和化学反应活性更强。研究证实，超临界CO的化学-物理协同劣化机制可显著降低煤岩强度并提升其渗透性，在黑龙江鸡西盆地深层煤层气试验井应用中，成功降低破裂压力6 MPa，实现煤层气日稳产量达1 000 m以上。超临界CO浸泡对煤岩的劣化作用有助于深层煤层气体积压裂增产。

Exploring the mechanical properties of coals subjected to supercritical carbon dioxide （SC-CO） soaking holds great significance for enhanced hydraulic fracturing performance of deep coalbed methane （CBM） reservoirs and CO geological sequestration. Using tri-axial compression tests， mineralogical analysis， and porosity and permeability measurements， we systematically reveal the SC-CO soaking-induced degradation mechanisms of the macroscopic mechanical properties of medium-rank coals under varying confining pressures. The results indicate that SC-CO 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-CO 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-CO soaking； however， the degrees of their degradation gradually decrease. Under low confining pressure， SC-CO soaking induces particularly prominent degradation of the mechanical properties of coals. Regarding the degradation mechanisms， SC-CO 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-CO 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-CO soaking increase by 7.7 % to 36.7 % and 1.7 % to 19.4 %， respectively， compared to those induced by active water soaking. SC-CO 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-CO. It has been demonstrated that the chemical and physical synergistic degradation mechanism of SC-CO can significantly weaken the strength of coals while enhancing their permeability. The application of SC-CO 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 m/d. Therefore， the degradation of coal properties induced by SC-CO soaking plays a significant role in promoting deep CBM production via volume fracturing.

10.11743/ogg20250319