Mechanical characteristics and gas flow laws of drilled coal under different true triaxial stress paths
LIU Yubing1，2，WANG Enyuan1，2，ZHAO Dong1，2，ZHANG Li1，2
(1. Key Laboratory of Gas and Fire Control for Coal Mines，Ministry of Education，China University of Mining and Technology，Xuzhou，Jiangsu 221116，China；2. School of Safety Engineering，China University of Mining and Technology，
Xuzhou，Jiangsu 221116，China)
Abstract：To study the mechanical properties and gas flow laws during the progressive failure of the drilled coal，the multi-functional true triaxial fluid-solid coupling test system was used to carry out true triaxial tests. The strength and failure characteristics，and permeability evolution laws of the drilled coal under different true triaxial stress paths were obtained. The results show that：(1) There are differences in the drilled coal strength for different borehole directions，among which the strength perpendicular to the bedding plane direction is the largest，followed by the strength perpendicular to the butt cleat direction，and the smallest perpendicular to face cleat direction. Both the linear Mogi-Coulomb strength criterion and the Nadai strength criterion are well fitted with the strength data of the drilled coal under different true triaxial stress paths. The Nadai strength criterion has a better fitting effect when the pre-existing cleats are obvious. (2) The permeability of the drilled coal at different deformation stages，when subjected to the true triaxial loading stress path perpendicular to cleat directions，is significantly smaller than that measured under the true triaxial loading-unloading stress paths. (3) When perpendicular to the bedding plane，the macro-fractures around the borehole are mainly tension fractures. When perpendicular to the butt cleat，there are two types of shear and tension macro-fractures generated around the borehole. When perpendicular to the face cleat，the macro-fractures around the borehole were mainly shear cracks. Besides，the number of macro-fractures，especially those accompanied by macro-fractures for drilled coal tested under the stress path of increasing maximum principal stress and decreasing intermediate and minimum principal stresses increased significantly.
[1] 谢和平. 深部岩体力学与开采理论研究进展[J]. 煤炭学报，2019，44(5)：1 283–1 305.(XIE Heping. Research review of the state key research development program of China：Deep rock mechanics and mining theory[J]. Journal of China Coal Society，2019，44(5)：1 283–1 305.(in Chinese))
[2] 袁 亮. 深部采动响应与灾害防控研究进展[J]. 煤炭学报，2021，46(3)：716–725.(YUAN Liang. Research progress of mining response and disaster prevention and control in deep coal mines[J]. Journal of China Coal Society，2021，46(3)：716–725.(in Chinese))
[3] 尹光志，鲁 俊，李 星，等. 真三轴应力条件下钻孔围岩稳定性及塑性区特性[J]. 煤炭学报，2018，43(10)：2 709–2 717.(YIN Guangzhi，LU Jun，LI Xing，et al. Stability and plastic zone characteristics of surrounding rock under true triaxial stress conditions[J]. Journal of China Coal Society，2018，43(10)：2 709–2 717.(in Chinese))
[4] 马念杰，李 季，赵希栋，等. 深部煤与瓦斯共采中的优质瓦斯通道及其构建方法[J]. 煤炭学报，2015，40(4)：742–748.(MA Nianjie，LI Ji，ZHAO Xidong，et al. High quality gas channel and its construction method applied to coal and gas simultaneous extraction in deep mining[J]. Journal of China Coal Society，2015，40(4)：742–748.(in Chinese))
[5] ZHANG J，AI C，LI Y W，et al. Energy-based brittleness index and acoustic emission characteristics of anisotropic coal under triaxial stress condition[J]. Rock Mechanics and Rock Engineering，2018，51(11)：3 343–3 360.
[6] YIN G Z，JIANG C B，WANG J Q，et al. Geomechanical and flow properties of coal from loading axial stress and unloading confining pressure tests[J]. International Journal of Rock Mechanics and Mining Sciences，2015，76：155–161.
[7] XUE Y，RANJITH P G，GAO F，et al. Mechanical behaviour and permeability evolution of gas-containing coal from unloading confining pressure tests[J]. Journal of Natural Gas Science and Engineering，2017，40：336–346.
[8] 王登科，于 充，魏建平，等. 基于LBM方法的裂隙煤岩应力–应变过程中渗流特性研究[J]. 岩石力学与工程学报，2020，39(4)：695–704.(WANG Dengke，YU Chong，WEI Jianping，et al. Research review of the state key research development program of China：Deep rock mechanics and mining theory[J]. Chinese Journal of Rock Mechanics and Engineering，2020，39(4)：695–704.(in Chinese))
[9] 王 刚，李胜鹏，刘义鑫，等. 真三轴下注水条件对煤体超声波传播特性影响规律的研究[J]. 岩石力学与工程学报，2022，41(11)：2 225–2 239.(WANG Gang，LI Shengpeng，LIU Yixin，et al. Analysis of water injection volume of coal and study of ultrasonic propagation characteristics under true triaxial stress[J]. Chinese Journal of Rock Mechanics and Engineering，2022，41(11)：2 225–2 239.(in Chinese))
[10] 李兆霖，王连国，姜崇扬，等. 基于实时CT扫描的岩石真三轴条件下三维破裂演化规律[J]. 煤炭学报，2021，46(3)：937–949.(LI Zhaolin，WANG Lianguo，JIANG Chongyang，et al. Three-dimensional fracture evolution patterns of rocks under true triaxial conditions based on real-time CT scanning[J]. Journal of China Coal Society，2021，46(3)：937–949.(in Chinese))
[11] FENG X T，ZHANG X W，KONG R，et al. A novel Mogi type true triaxial testing apparatus and its use to obtain complete stress-strain curves of hard rocks[J]. Rock Mechanics and Rock Engineering，2016，49(5)：1 649–1 662.
[12] 冯 帆，陈绍杰，王 琦，等. 真三轴卸载–动力扰动下自然与饱水砂岩破坏特性试验研究[J]. 岩石力学与工程学报，2022，41(11)：2 240–2 253.(FENG Fan，CHEN Shaojie，WANG Qi，el al. Experimental study on failure characteristics of natural and saturated sandstone under true triaxial unloading and dynamic disturbance condition[J]. Chinese Journal of Rock Mechanics and Engineering，2022，41(11)：2 240–2 253.(in Chinese))
[13] 尹光志，刘玉冰，李铭辉，等. 真三轴加卸载应力路径对原煤力学特性及渗透率影响[J]. 煤炭学报，2018，43(1)：131–136.(YIN Guangzhi，LIU Yubing，LI Minghui，et al. Influence of true triaxial loading-unloading stress paths on mechanical property and permeability of coal[J]. Journal of China Coal Society，2018，43(1)：131–136.(in Chinese))
[14] LIU Y B，YIN G Z，LI M H，et al. Mechanical properties and failure behavior of dry and water-saturated anisotropic coal under true-triaxial loading conditions[J]. Rock Mechanics and Rock Engineering，2020，53(11)：4 799–4 818.
[15] 段敏克，蒋长宝，郭现伟，等. 真三轴循环载荷作用下煤岩力学及损伤特征试验研究[J]. 岩石力学与工程学报，2021，40(6)：1 110–1 118.(DUAN Minke，JIANG Changbao，GUO Xianwei，et al. Experimental study on mechanical and damage characteristics of coal under cyclic true triaxial loading[J]. Chinese Journal of Rock Mechanics and Engineering，2021，40(6)：1 110–1 118.(in Chinese))
[16] 刘 超，靖洪文，蔚立元，等. 高压流体注入对煤岩变形和破裂特性的影响[J].煤炭学报，2022，47(5)：2 027–2 040.(LIU Chao，JING Hongwen，YU Liyuan，et al. Effect of high-pressure fluid injection on deformation and fracture characteristics of coal[J]. Journal of China Coal Society，2022，47(5)：2 027–2 040.(in Chinese))
[17] 张晨阳，潘俊锋，夏永学，等. 真三轴加卸载条件下组合煤岩冲击破坏特征研究[J]. 岩石力学与工程学报，2020，39(8)：1 522–1 533. (ZHANG Chenyang，PAN Junfeng，XIA Yongxue，et al. Research on impact failure characteristics of coal-rock combination bodies under true triaxial loading and unloading conditions[J]. Chinese Journal of Rock Mechanics and Engineering，2020，39(8)：1 522–1 533.(in Chinese))
[18] ZHANG C L，XU J，PENG S J，et al. Dynamic evolution of coal reservoir parameters in CBM extraction by parallel boreholes along coal seam[J]. Transport in Porous Media，2018，124(2)：325–343.
[19] GUO H，TODHUNTER C，QU Q D，et al. Longwall horizontal gas drainage through goaf pressure control[J]. International Journal of Coal Geology，2015，150–151：276–286.
[20] LIN H S，OH J，CANBULAT I，et al. Experimental study on borehole size effect and prediction of breakout initiation stress[J]. International Journal of Rock Mechanics and Mining Sciences，2021，142：104762.
[21] WANG S F，LI X B，DU K，et al. Experimental study of the triaxial strength properties of hollow cylindrical granite specimens under coupled external and internal confining stresses[J]. Rock Mechanics and Rock Engineering，2018，51(7)：2 015–2 031.
[22] LU J，YIN G Z，ZHANG D M，et al. Mechanical properties and failure mode of sandstone specimen with a prefabricated borehole under true triaxial stress condition[J]. Geomechanics for Energy and the Environment，2021，25：100207.
[23] YOUNESSI A，RASOULI V，WU B. Sand production simulation under true-triaxial stress conditions[J]. International Journal of Rock Mechanics and Mining Sciences，2013，61：130–140.
[24] PAN Z，CONNELL L D. Modelling permeability for coal reservoirs：a review of analytical models and testing data[J]. International Journal of Coal Geology，2012，92：1–44.
[25] 王 涛，葛丽娜，赵洪宝，等. 非对称荷载下含中心孔洞煤岩变形损伤特性与本构模型研究[J]. 煤炭学报，2022，47(11)：4 040– 4 054.(WANG Tao，GE Lina，ZHAO Hongbao，et al. Deformation damage characteristics and constitutive model of raw coal and briquette with central hole under asymmetric loading[J]. Journal of China Coal Society，2022，47(11)：4 040–4 054.(in Chinese))
[26] NIU Y，ZHANG X，WANG E Y，et al. A new method of monitoring the stability of boreholes for methane drainage from coal seams[J]. Measurement，2020，154：107521.
[27] 李忠辉，王恩元，郑安琪，等. 煤层瓦斯抽采钻孔变形破坏测试技术及其应用研究[J]. 煤炭科学技术，2020，48(10)：37–44.(LI Zhonghui，WANG Enyuan，ZHENG Anqi，et al. Research on testing technology of deformation and failure of boreholes in coal seam gas drainage and its application[J]. Coal Science and Technology，2020，48(10)：37–44.(in Chinese))
[28] 陆菜平，窦林名，谢耀社，等. 煤样三轴围压钻孔损伤演化冲击实验模拟[J]. 煤炭学报，2004，29(6)：659–662.(LU Caiping，DOU Linming，XIE Yaoshe，et al. Experimental simulation of drilling impact of coal sample in the three-axle enclosed pressure[J]. Journal of China Coal Society，2004，29(6)：659–662.(in Chinese))
[29] LIU Y B，LI M H，YIN G Z，et al. Permeability evolution of anthracite coal considering true triaxial stress conditions and structural anisotropy[J]. Journal of Natural Gas Science and Engineering，2018，52：492–506.
[30] 尹光志，李铭辉，许 江，等.多功能真三轴流固耦合试验系统的研制与应用[J]. 岩石力学与工程学报，2015，34(12)：2 436–2 445. (YIN Guangzhi，LI Minghui，XU Jiang，et al. A new multi-functional true triaxial fluid-solid coupling experiment system and its application[J]. Chinese Journal of Rock Mechanics and Engineering，2015，34(12)：2 436–2 445.(in Chinese))
[31] FENG X T，HAIMSON B，LI X C，et al. ISRM suggested method：determining deformation and failure characteristics of rocks subjected to true triaxial compression[J]. Rock Mechanics and Rock Engineering，2019，52(6)：2 011–2 020.
[32] KANG H P，ZHANG X，SI L P，et al. In-situ stress measurements and stress distribution characteristics in underground coal mines in China[J]. Engineering Geology，2010，116(3/4)：333–345.
[33] 谢和平，周宏伟，刘建锋，等. 不同开采条件下采动力学行为研究[J]. 煤炭学报，2011，36(7)：1 067–1 074.(XIE Heping，ZHOU Hongwei，LIU Jianfeng，et al. Mining-induced mechanical behavior in coal seams under different mining layouts[J]. Journal of China Coal Society，2011，36(7)：1 067–1 074.(in Chinese))
[34] ZHANG D C，RANJITH P G，PERERA M S A. The brittleness indices used in rock mechanics and their application in shale hydraulic fracturing：A review[J]. Journal of Petroleum Science and Engineering，2016，143：158–170.
[35] ZHANG Y H，LEBEDEV M，AL-YASERI A，et al. Characterization of nanoscale rockmechanical properties and microstructures of a Chinese sub-bituminous coal[J]. Journal of Natural Gas Science and Engineering，2018，52：106–116.
[36] LIU Y B，YIN G Z，LI M H，et al. Anisotropic mechanical properties and the permeability evolution of cubic coal under true triaxial stress paths[J]. Rock Mechanics and Rock Engineering，2019，52(8)：2 505–2 521.
[37] CHEN H D，CHENG Y P，ZHOU H X，et al. Damage and permeability development in coal during unloading[J]. Rock Mechanics and Rock Engineering，2013，46(6)：1 377–1 390.
[38] NADAI A. Theory of flow and fracture of solids[J]. Journal of Applied Mechanics，1963，30(4)：640.
[39] MOGI K. Fracture and flow of rocks under high triaxial compression[J]. Journal of Geophysical Research Atmospheres，1971，76(5)：1 255–1269.
[40] AL-AJMI A，ZIMMERMAN R. Relation between the Mogi and the Coulomb failure criteria[J]. International Journal of Rock Mechanics and Mining Sciences，2005，42(3)：431–443.
[41] ZHANG S C，LI Y Y，SHEN B T，et al. Effective evaluation of pressure relief drilling for reducing rock bursts and its application in underground coal mines[J]. International Journal of Rock Mechanics and Mining Sciences，2019，114：7–16.
2023, Vol. 42 
