河北高阳低凸起深部热储层回灌注水诱发断层失稳危险性探讨

Study on fault-slip potential induced by water injection in the deep thermal reservoir of the Gaoyang low uplift,Hebei Province

近年来,注水诱发断层活化引发地震已成为深部地热资源安全开采面临的突出地质安全问题。河北高阳低凸起深部岩溶热储丰富,区内分布多条隐伏断层,研究旨在探明未来大规模开发高阳低凸起深部地热资源是否会诱发区内隐伏断层失稳破坏。研究基于华北地区综合地应力场,利用莫尔-库伦准则计算热储区及邻区主要隐伏断层的初始稳定状态;选用Hsieh and Bredehoeft水文模型计算代表性地热井回灌注水10~40年产生的超孔隙水压力值;在初始稳定状态的基础上叠加该孔隙水压扰动,采用断层滑动失稳概率评价方法,分析长期回灌注水对断层稳定性的影响;讨论断层走向与最大水平主应力夹角变化对断层滑动失稳风险的作用规律。结果表明:高阳低凸起及邻区代表性地热井在170 m3/h注水速率条件下,持续注水40年产生的超孔隙水压力值最大不超过11 MPa,该扰动以注水井为中心向四周呈幂函数规律递减,其影响范围不超过8 km;持续回灌注水对高阳低凸起代表性地热井附近2 km范围内隐伏断层的稳定性影响显著,部分断层滑动失稳概率超过85%,具有极高的活化风险;距离注水井2 km范围内不同走向的断层,在单井回灌注水50年的作用下,滑动失稳概率随断层走向与最大水平主应力夹角的减小而迅速增加。文章研究方法及相关成果可为国内外深部地热资源安全开发利用提供地质科学支撑。

Recently,water injection-induced earthquakes caused by faulting instability have become a prominent geological safety issue for safely exploiting deep geothermal resources.This study investigates whether the future large-scale development of deep geothermal resources in the Gaoyang uplift will destabilize buried faults.There has a great amount of karst thermal reservoir in the Gaoyang low uplift,Hebei province.To find out whether the largescale deep geothermal exploitation in the future will induce faults distributed in and around Gaoyang geothermal reservoir to become unstable,firstly,we calculate the initial stable state of the main buried faults based on Mohr-Coulomb criteria using the comprehensive in-situ stress field of North China;then,under Hsieh and Bredehoeft hydrological model,we calculate the possible excess pore pressure caused by water injection for 10~40 years at representative geothermal wells;subsequently,combing this perturbation with the initial stable state,we obtain the fault slip potential of the main buried faults from 2022 to 2062 based on a probabilistic approach;ultimately,we discuss the impact on the changes of fault slip potential due to varying angles between the maximum horizontal principal stress and the fault orientation.The main conclusions of this work can be drawn as follows.With the injection rate of 170 m3/h,the maximum excess pore pressure caused by a single geothermal well does not surpass 11 MPa,and it obeys a power decrease distribution with increasing distance from the center of the injection well;its influence scope is no more than 8 km.Continuous water injection strongly changes the stability of those buried faults distributed 2 km within the geothermal wells,and fault-slip potentials of some segmental faults even exceed 85%,corresponding to high unstable risk.Under 50 years of water injection at an injection well,the fault-slip potential of faults with different strikes within 2 km from the injection well increases rapidly with the declining angle among its orientation and the regional maximum horizontal principal stress.This paper’s study methodology and associated findings can offer geoscientific justification for the safe exploration and exploitation of deep geothermal resources domestically and internationally.This study can provide a method reference for the location of injection wells and the selection of faults in different orientations in geothermal fields at home and abroad,thus promoting the safe and efficient development and utilization of geothermal resources.