Macro- and Microscopic Deformation Mode of Deep Coal Rock

Global coalbed methane (CBM) resources are estimated at 260 × 1012 m3, with 74 countries hosting CBM deposits. Over 61.9% of these resources are in coal seams deeper than 1500 m. Increasing extraction depths lead to complex geological conditions and significant differences in mechanical properties between midshallow and deep coal. Drilling in deep seams faces challenges, with horizontal coal seams prone to wellbore collapse, leading to complex drilling conditions. To address these issues, this study used microfocus CT triaxial testing on No. 8 coal and coal gangue samples from the Daning–Jixian block, investigating mechanical properties under varying confining pressures and coring angles. In addition, we analyzed horizontal and vertical crack propagation patterns and failure mechanisms of coal samples before and after damage to No. 8 coal and coal gangue samples under four different coring angles. The results reveal three stress–strain stages (elastic, plastic, failure) and show significant effects of the confining pressure and coring angle on mechanical properties. Coal gangue exhibited 10–20 MPa higher compressive strength and 2–6 GPa higher elastic modulus than No. 8 coal under the same conditions. At coring angles of 30–60°, both materials showed reduced compressive strength and increased collapse risk. After damage, new cracks in No. 8 coal primarily formed at matrix–mineral junctions, while coal gangue showed fewer, simpler cracks along weak structures like bedding planes. The macro- and microscopic mechanical properties obtained in this study provide valuable insights into the stability of wellbores in deep coalbed methane formations. These findings serve as a foundation for optimizing drilling engineering designs, thereby mitigating the risks of downhole collapse and spalling.