页岩气开采诱发地震的主要机理与影响因素

Main Mechanism and Influencing Factors of Earthquakes Induced by Hydraulic Fracturing for Shale Gas Exploitation

随着水力压裂技术的发展与应用,各地区页岩气开采区的地震活动显著增强,且中等以上地震明显增多,严重影响工业和人类活动,为确保安全、绿色的页岩气开采,避免或减少破坏性地震活动,研究诱发地震机理和影响因素具有重要意义。为此总结美国、加拿大和我国典型页岩气开采区地震活动特征,并结合断层力学与莫尔-库仑破坏准则,较为系统地分析了目前对水力压裂技术诱发地震机制的主要认识,以及诱发地震的影响因素。研究结果表明,基于莫尔-库仑准则可以在宏观上解释注入式诱发地震活动,断层面摩擦系数、正应力、剪切应力和孔隙压力的变化都可能影响诱发地震活动的发生;在断层与诱发地震相对关系方面,地震活动有3种诱发机制,包括孔隙压力作用下的断层活化、孔隙弹性效应导致的断层活化、无震滑动引起的断层活化;诱发地震活动不仅与流体注入参数有关,还取决于区域断层孕震情况和应力状态等条件。目前由于影响水力压裂作用下断层剪切破裂起始及扩展的因素尚不完整,同时也缺乏有力的试验验证,有必要开展水力压裂试验工作,模拟页岩气开采过程中流体加载和应力边界等条件,进一步确定断层剪切破裂的驱动机制和关键影响因素。

With the development and application of hydraulic fracturing technology,the seismic activity in shale gas mining areas in various regions has obviously increased especially those above moderate level,seriously affecting industrial and human activities.In order to ensure safe and green shale gas exploitation and avoid or reduce destructive seismic activity,it is of great significance to study the mechanism and influencing factors of hydraulic fracturing induced earthquakes.By summarizing the seismic activity characteristics of typical shale gas mining areas in the United States,Canada and China,and combining fault mechanics and Coulomb-Mohr failure criteria,this paper systematically sorts out the main mechanisms and the influencing factors of hydraulic fracturing induced earthquakes.The conclusion and analysis show that injection-induced seismicity can be explained macroscopically based on Coulomb-Mohr criterion,and the changes of friction coefficient of a fault,normal stress,shear stress and pore pressure in reservoir area can affect the occurrence of induced seismicity.From the relative relationship between faults and induced earthquakes,there are three possible mechanisms:the first is the fault activation caused by pore pressure change,the second is the fault activation caused by pore elasticity effect,and the third is the fault activation caused by aseismic slip.Induced seismicity is not only related to fluid injection parameters,but also depends on conditions such as seismogenic background and stress state of regional faults.At present,due to the incomplete factors affecting the initiation and propagation of fault shear fracture under hydraulic fracturing,and the lack of strong experimental verification,it is necessary to carry out hydraulic fracturing experiments to simulate the conditions such as fluid loading and stress boundary during shale gas production,and further determine the driving mechanism and key influencing factors of fault shear fracture.