(1. College of Mining Engineering，Taiyuan University of Technology，Taiyuan，Shanxi 030024，China；2. Key Laboratory of In-situ Property-improving Mining of Ministry of Education，Taiyuan University of Technology，Taiyuan，Shanxi 030024，China；3. College of Safety and Emergency Management Engineering，Taiyuan University of Technology，Taiyuan，Shanxi 030024，China)
Abstract：The self-developed “TCHFSM-I” large-scale true triaxial hydraulic fracturing seepage simulation device was used to study the crack initiation and propagation in coal-rock blocks under different stress conditions，and the hydraulic pressure and acoustic emission event evolutions were also analyzed. The results show that，when the difference of the axial stress and the minimum horizontal principal stress was equal or greater than 6 MPa，the hydraulic cracks can extend into coal across the interface，otherwise extend along the interface. The hydraulic pressure and acoustic emission response characteristics can well reflect the behaviors of hydraulic fractures across the interface. When the hydraulic cracks extend into the coal，the hydraulic pressure shows a significant secondary rise，and the cumulative number of the acoustic emission events increases significantly at a rate of 51.4%，most of which occur in coal. When the hydraulic cracks expand along the interface，the cumulative number of the acoustic emission events increases just by 6% and there is no secondary rise in fluid injection pressure. A crack expansion model crossing the interface，considering the angle between the crack and the interface，the interfacial friction and the stress state，was established，which can well describe and predict the crack expansion behavior crossing interfaces in hydraulic fractures. The research results in this paper can provide a technical guidance for efficient exploitation of coalbed methane.
[1] REN J H，ZHANG L，REN S R，et al. Multi-branched horizontal wells for coalbed methane production：Field performance and well structure analysis[J]. International Journal of Coal Geology，2014，131：52–64.
[2] 侯振坤，杨春和，王 磊，等. 大尺寸真三轴页岩水平井水力压裂物理模拟试验与裂缝延伸规律分析[J]. 岩土力学，2016，37(2)：407–414.(HOU Zhenkun，YANG Chunhe，WANG Lei，et al. Hydraulic fracture propagation of shale horizontal well by large-scale true triaxial physical simulation test[J]. Rock and Soil Mechanics，2016，37(2)：407–414.(in Chinese))
[3] 马 耕，张 帆，刘 晓，等. 地应力对破裂压力和水力裂缝影响的试验研究[J]. 岩土力学，2016，37(增2)：216–222.(MA Geng，ZHANG Fan，LIU Xiao，et al. Experimental study of impact of crustal stress on fracture pressure and hydraulic fracture[J]. Rock and Soil Mechanics，2016，37(Supp.2)：216–222.(in Chinese))
[4] 杨 潇，张广清，刘志斌，等. 压裂过程中水力裂缝动态宽度实验研究[J]. 岩石力学与工程学报，2017，36(9)：2 232–2 237.(YANG Xiao，ZHANG Guangqing，LIU Zhibin，et al. Experimental research on the variation of fracture width in hydraulic fracturing process[J]. Chinese Journal of Rock Mechanics and Engineering，2017，36(9)：2 232–2 237.(in Chinese))
[5] 卢义玉，廖 引，汤积仁，等. 页岩超临界CO2压裂起裂压力与裂缝形态试验研究[J]. 煤炭学报，2018，43(1)：175–180.(LU Yiyu，LIAO Yin，TANG Jiren，et al. Experimental study on fracture initiation pressure and morphology in shale using supercritical CO2 fracturing[J]. Journal of China Coal Society，2018，43(1)：175–180.(in Chinese))
[6] ZHANG X，LU Y，TANG J，et al. Experimental study on fracture initiation and propagation in shale using supercritical carbon dioxide fracturing[J]. Fuel，2017，190：370–378.
[7] CAI C，HUANG Z，LI G，et al. Feasibility of reservoir fracturing stimulation with liquid nitrogen jet[J]. Journal of Petroleum Science and Engineering，2016，144：59–65.
[8] ZHOU Z L，ZHANG G Q，DONG H R，et al. Creating a network of hydraulic fractures by cyclic pumping[J]. International Journal of Rock Mechanics and Mining Sciences，2017，97：52–63.
[9] PATEL S M，SONDERGELD C H，RAI C S. Laboratory studies of hydraulic fracturing by cyclic injection[J]. International Journal of Rock Mechanics and Mining Sciences，2017，95：8–15.
[10] 梁天成，刘云志，付海峰，等. 多级循环泵注水力压裂模拟实验研究[J]. 岩土力学，2018，39(增1)：355–361.(LIANG Tiancheng，LIU Yunzhi，FU Haifeng，et al. Experimental study of hydraulic fracturing simulation for multistage circulating pump injection[J]. Rock and Soil Mechanics，2018，39(Supp.1)：355–361.(in Chinese))
[11] 张宏源，黄中伟，李根生，等. 煤岩径向井–脉动水力压裂裂缝扩展规律与声发射响应特征[J]. 石油学报，2018，39(4)：472–481.(ZHANG Hongyuan，HUANG Zhongwei，LI Gensheng，et al. Fracture propagation laws and acoustic emission response characteristics of coal radial well-pulse hydraulic fracturing[J]. Acta Petrolei Sinica，2018，39(4)：472–481.(in Chinese))
[12] 鲍先凯，杨东伟，段东明，等. 高压电脉冲水力压裂法煤层气增透的试验与数值模拟[J]. 岩石力学与工程学报，2017，36(10)：2 415–2 423.(BAO Xiankai，YANG Dongwei，DUAN Dongming，et al. The experiment and numerical simulation of penetration of coalbed methane upon hydraulic fracturing under high-voltage electric pulse[J]. Chinese Journal of Rock Mechanics and Engineering，2017，36(10)：2 415–2 423.(in Chinese))
[13] 吴晶晶，张绍和，孙平贺，等. 煤岩脉动水力压裂过程中声发射特征的试验研究[J]. 中南大学学报：自然科学版，2017，48(7)：1 866– 1 874.(WU Jingjing，ZHANG Shaohe，SUN Pinghe，et al. Experimental study on acoustic emission characteristics in coal seam pulse hydraulic fracturing[J]. Journal of Central South University：Science and Technology，2017，48(7)：1 866–1 874.(in Chinese))
[14] 张 群，葛春贵，李 炜，等. 碎软低渗煤层顶板水平井分段压裂煤层气高效抽采模式[J]. 煤炭学报，2018，43(1)：150–159. (ZHANG Qun，GE Chungui，LI Wei，et al. A new model and application of coalbed methane high efficiency production from broken soft and low permeable coal seam by roof strata-in horizontal well and staged hydraulic fracture[J]. Journal of China Coal Society，2018，43(1)：150–159.(in Chinese))
[15] LI D Q，ZHANG S C，ZHANG S A. Experimental and numerical simulation study on fracturing through interlayer to coal seam[J]. Journal of Natural Gas Science and Engineering，2014，21：386–396.
[16] TAN P，JIN Y，HAN K，et al. Vertical propagation behavior of hydraulic fractures in coal measure strata based on true triaxial experiment[J]. Journal of Petroleum Science and Engineering，2017，doi：10.1016/j. petrol.2017.08.076.
[17] OLSEN TN，BRENIZE G，FRENZEL T. Improvement processes for coalbed natural gas completion and stimulation[C]// SPE Annual Technical Conference and Exhibition. Denver，Colorado：[s. n.]，2003：5–8.
[18] OLSEN T N，BRATTON T R，DONALD A，et al. Application of indirect fracture for efficient stimulation of coalbed methane[C]// SPE Rocky Mountain Oil and Gas Technology Symposium. Denver，Colorado：[s. n.]，2007：16–18.
[19] 蔡 峰，刘泽功. 深部低透气性煤层上向穿层水力压裂强化增透技术[J]. 煤炭学报，2016，41(1)：113–119.(CAI Feng，LIU Zegong. Simulation and experimental research on upward cross-seams hydraulic fracturing in deep and low-permeability coal seam[J]. Journal of China Coal Society，2016，41(1)：113–119.(in Chinese))
[20] 武鹏飞，梁卫国，廉浩杰，等. 大尺寸煤岩组合体水力裂缝越界形成缝网机制及试验研究[J]. 煤炭学报，2018，43(5)：1 381–1 389. (WU Pengfei，LIANG Weiguo，LIAN Haojie，et al. Mechanism and experimental investigation of the formation of hydro-fracture system by fracturing through the interface of large-size coal-rock[J]. Journal of China Coal Society，2018，43(5)：1 381–1 389.(in Chinese))
[21] 程远方，徐太双，吴百烈，等. 煤岩水力压裂裂缝形态实验研究[J]. 天然气地球科技，2013，24(1)：134–137.(CHENG Yuanfang，XU Taishuang，WU Bailie，et al. Experimental study on the hydraulic fractures morphology of coal bed[J]. Natural Gas Geoscience，2013，24(1)：134–137.(in Chinese))
[22] 陈 勉，庞 飞，金 衍. 大尺寸真三轴水力压裂模拟与分析[J]. 岩石力学与工程学报，2000，19(增)：868–872.(CHEN Mian，PAN Fei，JIN Yan. Experiments and analysis on hydraulic fracturing by a large-scale triaxial simulator[J]. Chinese Journal of Rock Mechanics and Engineering，2000，19(Supp.)：868–872.(in Chinese))
[23] DEHGHAN A N，GOSHTASBI K，AHANGARI K，et al. The effect of natural fracture dip and strike on hydraulic fracture propagation[J]. International Journal of Rock Mechanics and Mining Sciences，2015，75：210–215.
[24] ZHOU J，CHEN M，JIN Y，et al. Analysis of fracture propagation behavior and fracture geometry using a tri-axial fracturing system in naturally fractured reservoirs[J]. International Journal of Rock Mechanics and Mining Sciences，2008，45：1 143–1 152.
[25] GU H，WENG J，MARK M，et al. Hydraulic fracture crossing natural fracture at Nonorthogonal Angles：a criterion and its validation[C]// SPE Hydraulic Fracturing Technology Conference. Woodlands，Texas，USA，[s.n.]，2012：1–11.
[26] 杨焦生，王一兵，李安启，等. 煤岩水力裂缝扩展规律试验研究[J].煤炭学报，2012，37(1)：73–77.(YANG Jiaosheng，WANG Yibing，LI Anqi，et al. Experimental study on propagation mechanism of complex hydraulic fracture in coal-bed[J]. Journal of China Coal Society，2012，37(1)：73–77.(in Chinese))
[27] PARAB N D，CHEN W W. Crack propagation through interfaces in a borosilicate glass and a glass ceramic[J]. International Journal of Applied Glass Science，2014，5(4)：353–362.
[28] FU W，AMES B C，BUNGER A P，et al. Impact of partially cemented and non-persistent natural fractures on hydraulic fracture propagation[J]. Rock Mechanics and Rock Engineering，2016，49(11)：4 519–4 526.
[29] 孟尚志，侯 冰，张 健，等. 煤系“三气”共采产层组压裂裂缝扩展物模试验研究[J]. 煤炭学报，2016，41(1)：221–227.(MENG Shangzhi，HOU Bing，ZHANG Jian，et al. Experimental research on hydraulic fracture propagation through mixed layers of shale，tight sand and coal seam[J]. Journal of China Coal Society，2016，41(1)：221–227.(in Chinese))
[30] PAGGI M，REINOSO J. Revisiting the problem of a crack impinging on an interface：A modeling framework for the interaction between the phase field approach for brittle fracture and the interface cohesive zone model[J]. Computer Methods in Applied Mechanics and Engineering.，2017，321：145–172.
[31] XU W，ZHAO J，RAHMAN S S，et al. A Comprehensive Model of a Hydraulic Fracture Interacting with a Natural Fracture：Analytical and Numerical Solution[J]. Rock Mechanics and Rock Engineering，2018，http: //DOI：org/10.1007/s00603–018–1608–9.
[32] KHOEI A R，VAHAB M，HIRMAND M. Modeling the interaction between fluid-driven fracture and natural fault using an enriched-FEM technique[J]. Rock Mechanics and Rock Engineering，2018，DOI： 10.1007/s10704–015–0051–0.
[33] SHI F，WANG X L，LIU C，et al. An XFEM-based method with reduction technique for modeling hydraulic fracture propagation in formations containing frictional natural fractures[J]. Engineering Fracture Mechanics，2017，173：64–90.
[34] ZHANG X，JEFFREY R G，THIERCELIN M. Deflection and propagation of fluid-driven fractures at frictional bedding interfaces：a numerical investigation[J]. Journal of Structural Geology，2007，29(3)：390–410.
[35] ZHANG X，THIERCELIN M J，JEFFREY R G. Effects of frictional geological discontinuities on hydraulic fracture propagation[C]// Society of Petroleum Engineers Hydraulic Fracturing Technology Conference. Texas，USA：[s. .n.]，2007：29–31.
[36] ZHANG X，JEFFREY R G. Reinitiation or termination of fluid driven fractures at frictional bedding interfaces[J]. Journal of Geophysical Research，2008，113(B8)：1–16.
[37] BEHNIA M，GOSHTASBI K，MARJI M F，et al. Numerical simulation of interaction between hydraulic and natural fractures in discontinuous media[J]. Acta Geotechnica，2015，10(4)：533–546.
[38] BLANTON T L. Propagation of hydraulically and dynamically induced fractures in naturally fractured reservoirs[C]// Proceedings of SPE Unconventional Gas Technology Symposium，Society of Petroleum Engineers. Louisville，Kentucky：[s. n.]，1986：1–15.
[39] WARPINSKI N R，TEUFEL L W. Influence of geologic discontinuities on hydraulic fracture propagation[J]. Journal of Petroleum Technology，1987，39(2)：209–220.
[40] RENSHAW C E，POLLARD D D. An experimentally verified criterion for propagation across unbounded frictional interfaces in brittle，linear elastic materials[J]. International Journal of Rock Mechanics and Mining Sciences，1995，32(3)：237–249.
[41] 程 万，金 衍，陈 勉，等. 三维空间中水力裂缝穿透天然裂缝的判别准则[J]. 石油勘探与开发，2014，41(3)：336–340.(CHENG Wan，JIN Yan，CHEN Mian，et al. A criterion for identifying hydraulic fractures crossing natural fractures in 3D space[J]. Petroleum Exploration and Development，2014，41(3)：336–340.(in Chinese))
[42] 孙 博，薛世峰，周 博. 水利裂缝与天然裂隙相互作用与影响[J]. 科学技术与工程，2016，36(16)：1 671–1 815.(SUN Bo，XUE Shifeng，ZHOU Bo. The interaction between hydraulic fracture and natural fracture and influencing factors[J]. Science Technology and Engineering，2016，36(16)：1 671–1 815.(in Chinese))
[43] 程远方，常 鑫，孙元伟，等. 基于断裂力学的引言储层缝网延伸形态研究[J]. 天然气地球科技，2014，25(4)：603–611.(CHENG Yuanfang，CHANG Xin，SUN Yuanwei，et al. Research on fracture propagation pattern of shale reservoir based on fracture mechanics[J]. Natural Gas Geoscience，2014，25(4)：603–611.(in Chinese))
[44] 王 涛，柳占立，高 岳，等. 基于给定参数的水力裂缝与天然裂缝相互作用结果的预测准则[J]. 工程力学，2018，35(11)：216–222.(WANG Tao，LIU Zhanli，GAO Yue，et al. A prediction criterion for the interaction between hydraulic fracture and natural fractures based on given parameters[J]. Engineering mechanics，35(11)：216–222.(in Chinese))
[45] 衡 帅，杨春和，曾义金，等. 页岩水力压裂裂缝形态的试验研究[J]. 岩土工程学报，2014，36(7)：1 243–1 251.(HENG Shuai，YANG Chunhe，ZENG Yijin，et al. Experimental study on hydraulic fracture geometry of shale[J]. Chinese Journal of Geotechnical Engineering，2014，36(7)：1 243–1 251.(in Chinese))
[46] 侯 冰，陈 勉，谭 鹏，等. 页岩气藏缝网压裂物理模拟的声发射监测初探[J]. 中国石油大学学报：自然科学版，2015，39(1)：66–71. (HOU Bing，CHEN Mian，TAN Peng，et al. Monitoring of hydraulic fracture network by acoustic emission method in simulated tri-axial fracturing system of shale gas reservoirs[J]. Journal of China University of Petroleum：Natural Science，2015，39(1)：66–71.(in Chinese))
[47] LI N，ZHANG S，ZOU Y，et al. Experimental Analysis of hydraulic fracture growth and acoustic emission response in a layered formation[J]. Rock Mechanics and Rock Engineering，2018，51(4)：1 047–1 062.
[48] LI N，ZHANG S，ZOU Yi，et al. Acoustic Emission Response of Laboratory Hydraulic Fracturing in Layered Shale[J]. Rock Mechanics and Rock Engineering，2018，https：//doi.org/10.1007/s00603–018–1547–5.
[49] PATER C J D，BEUGELSDIJK L J L. Experiments and numerical simulation of hydraulic fracturing in naturally fractured rock[C]// Paper ARMA 780 Presented at the 40th U.S. Symposium on Rock Mechanics. Anchorage，Alaska. [S.l.]：[s.n.]，2005.
[50] KIM C M，ABASS H H. Hydraulic fracture initiation from horizontal wellbores：Laboratory Experiments[C]// Symposium on Rock Mechanics. Norman，Oklahoma，[s.n.]，1991.
[51] HANSON M E，ANDERSON G D，SHAFFER R J，et al. LLNL gas stimulation program[R]. Quarterly progress report，United States：N. p.，Web. 1980.
[52] 左建平，裴建良，刘建锋，等. 煤岩体破裂过程中声发射行为及时空演化机制[J]. 岩石力学与工程学报，2011，30(8)：1 564–1 570. (ZUO Jianping，PEI Jianliang，LIU Jianfeng，et al. Investigation on acoustic emission behavior and its time-space evolution mechanism in failure process of coal-rock combine body[J]. Chinese Journal of Rock Mechanics and Engineering，2011，30(8)：1 564–1 570.(in Chinese))
[53] BLANTON T L. An Experimental study of interaction between hydraulically induced and pre-existing fractures[C]// Proceedings of SPE Unconventional Gas Recovery Symposium，Society of Petroleum Engineers. [S.l.]：[s.n.]，1982：1–13.
2019, Vol. 38 
