地球物理学报  2021, Vol. 64 Issue (11): 3817-3836   PDF    
全球干热岩资源开发诱发地震活动和灾害风险管控
尹欣欣1,2, 蒋长胜1, 翟鸿宇1, 张延保1, 姜丛1, 来贵娟1, 祝爱玉1, 尹凤玲1     
1. 中国地震局地球物理研究所, 北京 100081;
2. 甘肃省地震局, 兰州 730000
摘要:干热岩地热资源作为一种绿色可再生的新型能源,其开发利用已成为当前世界各国尤其是发达国家能源战略的重要组成部分.但由于干热岩位于地壳浅部3~10 km,在采用增强型地热系统(EGS)等通用开发方式过程中,伴随着地壳应力状态的扰动,部分开采项目发生较大震级的诱发地震事件,甚至造成明显灾害、引起社会问题,亟待实现科学利用和风险管控.鉴于此问题在平衡能源开发战略和社会安全领域的重要性和关键性,本文梳理了全球干热岩开采诱发地震的总体情况、典型案例,整理了在成因和机理研究、地震灾害风险管控和缓解等方面的研究进展.综合分析结果表明,已有EGS项目案例中诱发地震震级超过3.0的达到31.2%、主要与断层活化有关,最大的诱发地震可发生在注水压裂、关井后、循环生产等各阶段.干热岩开采诱发地震有多种成因,已有案例多为多种成因共同作用,其中的关井后的尾随效应是目前重大难点.目前世界各国已开展了广泛的诱发地震机理研究并探索多种减灾措施,认为累积注水体积、注水速率与最大诱发震级之间不存在普适性的定标关系,前瞻性预测需要采用"一井一策"的方式.在缓解诱发地震灾害风险上,普遍采用科学的流体注入策略、对注采策略进行验证校准、持续开展地震活动监测等系列措施.此外,对储层的临界应力状态和应力时空演化的量化描述、地热储层内的先存断层与裂缝的探测识别、可有效管控地震发生的流体注入策略等,是当前干热岩资源开发减轻地震风险的主要技术难点,而利用地热储层实时感知信息技术、采用新的注入热交换载体、发展前瞻性的地震预测方法是该领域目前重点关注的技术方向.根据我国的干热岩资源开发和减轻地震灾害风险的实际情况,亟待建立开采场地安全性和灾害风险评价、多学科的地震监测网络和分析技术、地震灾害风险管控红绿灯系统等技术体系,并加强关井后的尾随现象、多场耦合等科学问题的基础研究.
关键词: 深层地热资源      干热岩      增强型地热系统      诱发地震      减轻地震灾害风险     
Review of induced seismicity and disaster risk control in dry hot rock resource development worldwide
YIN XinXin1,2, JIANG ChangSheng1, ZHAI HongYu1, ZHANG YanBao1, JIANG Cong1, LAI GuiJuan1, ZHU AiYu1, YIN FengLing1     
1. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China;
2. Gansu Earthquake Agency, Lanzhou 730000, China
Abstract: The hot dry rock geothermal resource is a new type of green and renewable energy. Its exploitation and utilization have become an important part of energy strategy in the world, especially in developed countries. However, as the hot dry rock is located at 3~10 km depth in the shallow crust, in the process of enhanced geothermal system (EGS), along with the disturbance of crustal stress state, some exploitation projects have induced sizable earthquakes, even causing significant disasters and social problems. In view of the importance of this problem in balancing energy exploitation strategy and social security, we summarize the global general situation and typical cases of induced earthquakes by hot dry rock exploitation, and sorts out the research progress in aspects of causes and mechanisms, earthquake disaster risk management and control technology. The comprehensive analysis results show that induced earthquakes with a maximum magnitude over 3.0 among current EGS project cases reach 31.2%, which are mainly related to fault activation. The largest induced earthquake can occur in various stages such as hydrofracking, injection well shut-in, and cyclic production. There are multiple causes for the induced earthquakes, and most of the current cases are interactions of multiple causes, among which the tailing effect after well shut-in is a major difficulty. At present, a variety of disaster reduction measures have been explored all over the world. There is no universal calibration relationship between the cumulative water injection volume, water injection rate, and the maximum magnitude of induced earthquakes. The forward prediction needs to adopt the measure of "one well, one strategy". In order to mitigate the risk of disaster by induced earthquakes, a series of measures are generally adopted, such as scientific fluid injection strategy, verification and calibration of injection production strategy, and continuous seismic activity monitoring. In addition, the quantitative description of critical stress state and spatiotemporal evolution of stress, detection and identification of pre-existing faults and fractures in the geothermal reservoir, and fluid injection strategy that can effectively control the occurrence of earthquakes are the main technical difficulties in the development of dry hot rock resources to reduce seismic risk. However, using real-time sensing information technology of geothermal reservoir, adopting new injection heat exchange carrier, and developing prospect are also discussed. At present, the key technology direction in this field is to improve the accuracy of earthquake prediction. According to the actual situation of dry hot rock resources development and earthquake disaster risk reduction in China, it is urgent to establish the technical system of mining site safety and disaster risk assessment, multi-disciplinary seismic monitoring network and analysis technology, earthquake disaster risk management, and control traffic light system, and strengthen the basic research of scientific problems such as tailing phenomenon after shut-in and multi-field coupling.
Keywords: Deep geothermal resources    Hot dry rock    Enhanced geothermal system    Induced seismicity    Earthquake disaster mitigation    
0 引言

地热能是一种清洁的可再生能源,在地球上广泛分布、可稳定持续供应.在世界能源协会2000年发布的《能源和持续性的挑战》报告中,地热能处于各种可再生能源的首要之位(廖志杰等, 2015).按照理论计算,地壳上部10 km的地热能储量高达1.3×1027 J,以2012年全球能源消耗量约6.0×1020 J为参考,这些地热能储量可以供应全球约2.17百万年使用(Lu, 2018).开采深度在数千米之内的深部地热能,主要是地壳3~10 km深度的不含水或少量含水的干热岩地热能,占全球地热资源的90%.保守估计全球蕴含的干热岩资源储量相当于全球所有石油、天然气和煤炭储藏能量的30倍(许天福等, 2012; 陆川和王贵玲, 2015).由于干热岩埋藏深、温度高,已成为世界各国地热能开发的重点.欧洲、北美、澳洲、亚洲的诸多国家相继开启了国家级的开采计划,例如美国在2015年开启的“地热能前沿观测站研究计划”(FORGE)(张森琦等, 2019)等等.经过发达国家40多年研究改进,干热岩开发的流体循环换热以及压裂造储等关键技术目前获得了较大成功,例如德法两国交界处的舒尔茨(Soultz)增强型地热系统(EGS)项目,目前已达到了兆瓦级的发电能力(Breede et al., 2013).中国是最早利用地热能的国家之一,拥有丰富的干热岩资源,但开发利用仍处于起步阶段(Zhu et al., 2015).

目前在碳氢化合物提取和天然气储存作业、页岩气开采、深层地热能开发、采矿作业、二氧化碳封存、水库蓄水等多种工业活动中,均可观测到诱发地震活动(Mcgarr et al., 2002).尽管这些工业开采活动的诱发地震在开采原理上分为重力地震(例如采矿诱发地震等)、重复注水和水力压裂诱发地震、加载地震(Doglioni, 2018),但总体上主要是改变了浅层地壳的应力状态而引起的(Grigoli et al., 2017).目前对这些工业活动诱发地震的预测和风险管控的难度极大(Petersen et al., 2016),全球多数国家对此类工业活动已实施严格的监管计划(Kettlety et al., 2020).干热岩开采目前在全球范围内尚未形成完全的商业化,其中一个重要因素就是受到诱发地震的影响(Trifu, 2002).干热岩储层建造过程中引发了较大规模的诱发地震并造成灾害、引发系列的公共安全问题,一些开采项目甚至因此而终止,例如瑞士巴塞尔(Basel)干热岩项目在注水期间引发了四个3级以上的地震、造成建筑物破坏,迫使该项目终止、导致巨额投资失败、引发大量法律纠纷(Häring et al., 2008).2017年韩国浦项(Pohang)的干热岩开采项目,引发了附近断层上的MW5.4地震并造成严重经济损失,韩国政府被迫中止该项目的运行(Kim et al., 2018).诱发地震还引起了系列的社会问题,公众甚至会在未造成建筑物结构性破坏情况下也会向保险公司提出索赔要求,例如法国苏尔苏斯发(Soultz-sous-Forêts)的干热岩开采项目(Majer et al., 2007),严重影响了深部地热资源的顺利开发.

干热岩开采的多个生产阶段都能观测到诱发地震(Rathnaweera et al., 2020),包括:增产阶段的地热工作流体初始注入、从地热储层中抽出工作流体、抽热后重新注入工作流体、闭井后(Okamoto et al., 2018)等等.围绕干热岩开采诱发地震的机理研究、风险管控技术、行业监管和社会治理等多环节问题,已成为国际上的研究热点.鉴于诱发地震的减灾研究在实现包括干热岩开采等绿色新型能源战略顺利实施中的关键影响,本文收集整理了全球干热岩开采和诱发地震活动的数据,并对目前国际上干热岩开采诱发地震的总体情况、成因机理、采取的减灾措施等的研究进展进行了分析,以期通过上述梳理,为推进相关基础研究、发展关键技术和装备、解决监管和社会治理问题等,提供科学参考.

1 干热岩开采与诱发地震活动 1.1 全球干热岩开采与诱发地震活动总体情况

适合商业开发的干热岩储层主要是花岗岩或其他结晶基底岩石,深度约为5~6 km时温度应在150 ℃至500 ℃范围内(Potter et al., 1974).目前全球的干热岩开采主要采用增强型地热系统(EGS)的方式进行,主要是通过人工在渗透率较低的干热岩中建立热储,通过冷水注入再回抽,带出干热岩中的热能(郭盼等, 2020),利用抽出的热水进行热电交换(Massachusetts Institute of Technology, 2007; 许天福等, 2018),图 1为一般干热岩开采站点示意图. EGS的主要步骤包括:资源勘探与评估、开采/回注井钻探、储层建立、注入井和生产井循环采热、电厂运行、储层维护,实现的主要技术环节包括干热岩资源的靶区定位、储层建造或改造、微震示踪、能源转换等等.

图 1 干热岩开采站点简单示意图.据Olasolo等(2016)改绘 Fig. 1 Simple schematic diagram of hot dry rock mining site. Modified from Olasolo et al. (2016)

作为EGS核心环节的储层建造,在实现技术上包括水力压裂法、爆炸法、热应力法和化学刺激法等(Luo et al., 2018),其中的水力压裂法是目前国际上人工热储建造的主流技术.水力压裂法源于油气行业、目前已较为成熟.其原理是在干热岩体中至少钻两口井并分段封隔,通过地面注入高压流体产生裂缝从而实现两井的连通,形成的裂隙网络就构成了人工地热储层.由于水力压裂造缝往往受地应力控制,实际开发场地的深部岩石构造较为复杂,压裂过程中遇到的不可控因素多,因而无法准确控制裂隙的延伸方向以达到事前预期效果.为此一般会使用微震监测示踪技术和数值模拟的方法来实时监测地热储层的裂隙结构和走向,进而评估人工热储的规模、推断地应力特征(Maurer et al., 2015; Cladouhos et al., 2016).

目前全球大部分EGS项目是商业目的,但仍有部分以科学实验为目的,例如日本的肘折(Hijiori)和雄胜(Ogachi)的EGS项目(Tenma et al., 2008; Lu, 2018).其中,1990年实施的日本雄胜EGS项目尝试在井深1000 m、井底温度230 ℃的注入井分别在不同深度实施压裂,在生产井OGC-2进行压裂来提高回收率,以及对两口井进行二次压裂、在储层系统中补充第三口井等科学实验.随着水力压裂技术的成熟,EGS的成功率逐渐提高,例如法国舒尔茨(Soultz)、澳大利亚的哈瓦那罗(Habanero)和巴拉那那(Paralana),德国的印希姆(Insheim)和兰道(Landau),美国的纽伯里(Newberry)、沙漠峰(Desert Peak)和盖伊瑟斯(Geysers)等EGS项目.表 1列出了可从公开发表的出版物中收集的全球已开展干热岩开采的79个项目.这些开采项目主要分布在欧洲、北美、澳洲以及部分亚洲地区,共涉及21个国家和地区.

表 1 全球EGS项目和诱发地震信息表 Table 1 Information table on global EGS projects and induced seismicity

诱发地震是干热岩开采尤其是采用水力压裂技术的EGS项目顺利实施的重要决定因素,在EGS的人工热储建造过程和关井后常常引起诱发地震事件.其中部分诱发地震危害较大,例如韩国浦项(Pohang)以及瑞士巴塞尔(Basel)的EGS项目诱发的地震事件.如果EGS站点邻近居民区,较小的地震事件也会造成一定的社会影响(Bommer et al., 2006). 根据表 1统计的79个干热岩开采项目可查的数据表明,其中有33个项目的最大诱发地震震级超过2.0级(不区分震级标度),占全部可查最大震级的48个项目的68.8%;最大诱发地震震级超过3.0级(不区分震级标度)的占比则超过31.2%,其中最大震级事件为韩国浦项(Pohang)2017年发生的MW5.5地震.图 2给出了与表 1相对应的干热岩开采项目和最大诱发地震的空间分布.

图 2 全球干热岩开采项目及其最大诱发地震的空间分布.其中最大诱发地震的震级 由圆圈表示相对大小,并仅标出了表 1中可查的最大震级数据 Fig. 2 Distribution of the hot dry rock mining sites and their related biggest induced earthquakes worldwide. The biggest induced earthquakes are marked as circles, the size of which indicates the relative magnitude, and only the data available in Table 1 are given here
1.2 干热岩开采诱发地震案例

本文按照注水压裂期间发生最大诱发地震、未发生有感地震,以及关井后和循环生产阶段发生最大地震等情况,分别介绍表 1中列出的部分案例.

(1) 注水压裂期间发生较大诱发地震的案例.瑞士巴塞尔(Basel)EGS项目在2006年12月8日开始压裂,采用在16小时内逐渐提高的流量(最高55 L/s)和井口压力(最高29.6 MPa)的方式注水压裂,期间引起数千次微地震、最大震级为ML2.6,而在压裂停止后的2小时即发生震级最大的ML3.4地震(Häring et al., 2008).瑞士圣加仑(St.Gallen)EGS项目的GT-1井在2013年7月14日开始的注水压裂实验中,2小时内以最高54 L/s的注入速率注入175 m3水(Wolfgramm et al., 2015),此后分两次注入145 m3稀盐酸、700 m3钻井泥浆,微震活动随着注入明显增加,在20日先后发生ML2.1地震以及最大的ML3.5(MW3.3)地震(Diehl et al., 2017).法国舒尔茨(Soultz)EGS项目的3口井中,2号井(GPK3)在注水压裂期间更容易产生较大震级事件,地震活动的b值达到与天然构造地震活动相近的0.94,被认为这些地震与GPK3相交的主要断裂带的活化有关.3号井(GPK4)在采用30~45 L/s注水速率、井口峰值压力18.5 MPa情况下共注入21500 m3液体,记录到128起事件、震级ML1.0~2.7,且在关井期间未发生ML2.0以上事件(Baria et al., 2004).德国巴特乌拉赫(Bad Urach)EGS项目采用35~50 L/s的注入速率和12.3 ~34 MPa的井口压力进行注水压裂,期间发生的地震分布在井口附近500 m范围内、震级范围为MW-0.6~1.8(Tenzer et al., 2004).澳大利亚库珀盆地(Cooper Basin)EGS项目在水力压裂期间,在其中的Habanero地热田以最大48 L/s注入速率注入20000 m3液体,期间检测到60000多次ML1.6~MW3.7事件(Baisch et al., 2006, 2015),被认为Habanero地热田存在较大规模的逆冲断层、被流体注入而活化(Holl and Barton, 2015).澳大利亚巴拉那那(Paralana)EGS项目在2011年7月10—15日的注水压裂期间,以27 L/s的注入速率和62 MPa的井口压力注入了总计3.1×106 m3的水,期间发生3个MW2.4地震和1个MW2.5事件,这些地震被认为与井场存在的右旋逆断层有关(Albaric et al., 2014).美国科索(Coso)EGS项目在2004年8—9月对34A-9井注水压裂期间,在注入速率和井口压力最高时发生了最大的2.8级地震(Julian et al., 2010).日本雄胜EGS项目在注水压裂过程中,采用注入速率40 m3/h、注入峰值压力20 MPa注入总计10140 m3水,期间记录到1553次微地震事件,地震空间上沿地层岩石天然裂隙方向延伸,最大震级2.0级(Kaieda et al., 2010).

(2) 注水压裂期间未发生有感地震的案例.德国兰道(Landau)EGS项目的两口井均存在过井断层,在2005—2006年期间的注水压裂,尽管采用了190 L/s的注入速率和13.5 MPa的井口压力,但未引发有感地震(Evans et al., 2012).德国舍纳贝克(groß Schönebeck)EGS项目的1号井(GrSk3/90)在2003年压裂期间,采用80 L/s的速率注入了约10000 m3的水、井底压力超过最小水平应力Shmin约5 MPa,未发生有感地震(Evans et al., 2012).2号井(Gt GrSk4 /05)采用150 L/s的注入速率和59 MPa的井口压力向火山岩注入了13000 m3水(Evans et al., 2012),仅记录到70个震级在ML-1.9~-1.1范围的地震事件.澳大利亚库珀盆地(Cooper Basin)的Jolokia地热田,在水力压裂期间只发生了ML<1.6的地震,Baisch等(2015)认为主要是不存在大规模断层的原因.日本肘折(Hijiori)EGS项目对HDR-1井采用了短期内大流量注入的策略,在12小时内按照1.0、2.0和4.0 m3/min的注入速率注入2115 m3水(Kaieda et al., 2010),期间记录到107个微震事件,最大震级仅为0.3级(Sasaki and Kaieda, 2000).

(3) 关井后发生最大诱发地震的案例.韩国浦项(Pohang) EGS项目在2016年开始实施了4次水力压裂,注入总量为12800 m3、注入速率1.00~46.83 L/s,每次压裂都产生一系列地震、最大事件为MW3.2,但最大的MW5.5发生在压裂关井后的2个月(Yeo et al., 2020).目前认为水力压裂施工激活了井场附近断层,但具体成因上分别有高压注水直接注入到临界断层上(Kim et al., 2018)、孔隙压力增加和地震相互作用共同导致断层弱化(Yeo et al., 2020)等不同认识.美国盖瑟尔斯(Geysers)EGS项目的开发始于1969年,地震活动随着注水压裂迅速增加,发生了包括1982年的4.6级地震、2006年的3次4.0级以上地震(Majer et al., 2007). 其中的P32井在2011年10月6日开始注水压裂,采用了先进行24小时的60~65 L/s高注入速率、此后55天内降至25 L/s的方式,在注入结束11个月后,P32注入井周围记录了约3000次微震事件,震级在0.4到3级之间,研究认为与区域构造活动有关(Jeanne et al., 2014).

(4) 循环生产阶段发生较大诱发地震的案例.德国兰道(Landau)EGS项目2008年2月—2009年11月的循环生产阶段,注水速率65~70 L/s、井口压力3.0~6.0 MPa,期间发生6次ML1.6~1.9地震以及1次ML2.7地震(Evans et al., 2012).英国罗斯马诺卫斯(Rosemanowes)EGS项目3号井在1985年8月—1989年12月的循环生产阶段,在井口压力为11.1 MPa、注入速率33 L/s时(低于循环生产阶段的峰值注入速率38 L/s,也低于注入压裂阶段的峰值注入速率260 L/s),1987年7月发生的ML2.0地震被认为激活了井口附近的小尺度断层(Evans et al., 2012).瑞典菲亚巴卡(Fjällbacka)EGS项目在1989年的循环生产阶段,采用注入速率1.8 L/s和3.0~5.2 MPa井口压力进行了40天循环生产试验,漏液率达到50%,期间在井口400 m范围内记录到几百次微震事件,含一次有感地震(Eliasson et al., 1990).德国安达赫治(Unterhaching) EGS项目在循环生产阶段,以注入速率120 L/s和井底压力超过地层压力2.5 MPa的参数下,记录到注入井口附近1 km范围内发生了11次0.7~2.4级地震(Kraft et al., 2009),被认为与注入井穿过的断层活动有关(Wolfgramm et al., 2007).

综合上述分析可见,最大的诱发地震未必发生在注水压裂阶段,其还可在关井后、循环生产等各阶段发生.而3级以上的破坏性地震主要与过井断层或井场周围断层的活化有关,且并不与高注入速率、井底压力是否超过最小水平应力等因素有简单的必然关联.

2 干热岩开采诱发地震的成因机理 2.1 注入时诱发地震的发生机理

干热岩开采诱发地震的基本成因机理,总体上被认为与储层岩石的应力状态改变和断层激活有关(Zang and Stephansson, 2010),并要求断层上的应力达到破坏强度的临界值(Kim et al., 2018).但与其他新型工业活动诱发地震不同的是,干热岩开采诱发地震涉及到固-液-热-化学多场耦合过程,不能单一的用某种机制来解释(Rawal and Ghassemi, 2014).由于诱发地震的高度复杂性,其具体的物理演化过程与形成机理的基本理论解释仍在探索中.

干热岩开采诱发地震的成因较为复杂,涉及多种物理过程:(1)孔隙压力扩散诱发地震.流体注入储层后,随着孔隙压力的增加,预先存在的断层摩擦阻力随之减小、引发地震.已有诸多研究表明,当断层接近临界状态时,孔隙压力的微小增加会通过降低有效正应力而导致断层破坏(Majer et al., 2007; Rathnaweera et al., 2020);(2)温度变化诱发地震.一种观点认为,注入的较冷的流体与高温岩石相互作用,引起热弹性应变并造成裂缝表面的收缩,裂缝的轻微张开会减少静摩擦,并触发沿区域应力场中已经接近破坏的裂缝滑动(Rawal and Ghassemi, 2014).此外沿储层岩石非均质断层的热应力调制,也被认为显著影响注入诱发地震活动的发生(Norbeck et al., 2018),也即注入流体冷却岩石并增加孔隙压力,由此形成热弹性和孔隙弹性应力的变化并诱发地震活动(Johnson, 2017; Yu et al., 2018);(3)流体注入和回采导致的体积变化诱发地震.当从储层回抽或注入流体时储层岩石可能会压实或受压,这些体积变化会引起局部应力扰动.当局部应力已经接近破坏状态时,可能导致储层内部或周围发生地震.体积变化引发地震或岩爆等事件更常见于深部矿井的矿物开采(Majer et al., 2007).此外,流体注入诱发地震活动还包括流体直接进入已有断层引起的膨胀压力扰动(Eaton and Igonin, 2018; Zang et al., 2019);(4)裂隙表面的化学性质变化诱发地震.外部流体注入地层可能会导致裂隙表面的地球化学性质变化,从而改变这些表面上的摩擦系数,而此类情况下摩擦系数减少容易发生微震、摩擦系数增大则容易发生较大震级事件(Pennington et al., 1986);(5)地震之间相互作用诱发地震.诱发地震的静态和动态应力变化本身可能会引发其他地震,在某些情况下这些应力变化还会抑制进一步的地震活动(Catalli et al., 2013).

干热岩开采诱发地震可能是多种机制共同作用的结果.图 3给出了干热岩开采EGS项目的部分诱发地震发生机理示意图,包括对上述物理过程(1)—(3)的描述,以及对与储层直接相连的断层(图中标号1)和与储层相隔一定距离断层(图中标号2)两种情况.图 3a为注水压裂阶段,由于储层被注入液体填满,造成孔隙压力增加,同时也使岩体的体积变化引起孔隙弹性应力变化,因此断层1和2均受到挤压的应力作用.其中,孔隙弹性应力变化的大小取决于压力变化、岩体的弹性性质和承受压力的岩体的几何形状(Segall and Fitzgerald, 1998; Soltanzadeh and Hawkes, 2008). 图 3b为回采阶段,此时由于冷水的注入和回收导致了干热岩储层的温度下降、岩石体积缩小,进而对断层1和2产生了拉张的应力作用,即热弹性应力.与其他注入诱发地震不同(废水注入等),热弹性应力是干热岩开采独有的地震诱发机制.图 3cd分别为与图 3ab对应的摩尔应力圆(Buijze et al., 2019).

图 3 干热岩开采EGS项目诱发地震发生机理示意图. 图中E为杨氏模量,σ为应力,υ为泊松比,T为温度,τ为剪应力,P为孔隙应力.根据Buijze等(2019)改绘 Fig. 3 The mechanism diagram of earthquake induced by EGS project of hot dry rock mining. In the figure, E is young′s modulus, σ is stress, υ is Poisson′s ratio, T is temperature, τ is shear stress, P is pore stress. Modified from Buijze et al., 2019

在机理和理论研究上,干热岩开采诱发地震的认识经历了长期过程.最早试图解释干热岩开采诱发地震的机理始自20世纪70年代.Kisslinger和Cherry(1970)利用Biot增量应变理论描述了横波沿断层传播时垂直于剪切破坏面的瞬态拉应力的变化.如果拉应力大于其临界值,则通过减小断层上的正应力来实现断层弱化,从而引发断层失稳并产生地震.目前正在探索的解释干热岩开采诱发地震发生机制的基本理论包括:

(1) 摩尔-库仑理论和库仑应力变化理论.Kisslinger(1976)用有效应力理论和摩尔-库仑理论解释了地热流体注入诱发地震活动性的机理.对多孔介质,在主应力不变的情况下,当孔隙压力增大时,摩尔圆趋向于向左移动,当摩尔圆与破坏包络线相切时导致剪切破坏.但由于流体注入地热储层引起化学矿物学改变,上述摩尔-库仑理论并未解释预先存在的断层中的泥状物质可能出现明显弱化的情况. Blöcher等(2018)通过耦合滑移倾向分析和摩尔-库仑理论,研究了地下应力状态变化对预存断层滑移倾向的影响.利用真三轴破坏准则和断层滑移趋势研究,证明了摩尔-库仑理论不一定是注入诱发地震分析中的最终破坏准则,并得到可通过垂直于断层的单位矢量的三维余弦值来确定断层方向.Li等(2018)基于Nur和Booker(1972)的经典触发机制和摩尔-库仑理论,引入库仑应力变化对有效应力变化进行了修正,并认为预先存在断层和裂缝的破坏是通过有效正应力的局部减小、剪应力的增加或两者同时引发.

(2) 临界压力理论(CPT)和速率-状态依赖性摩擦本构关系(RST).CPT和RST分别由Shapiro(2015)Dieterich(1994)发展,目前已被用于评估干热岩开采诱发地震活动(Wenzel, 2017).CPT假定局部的地震密度与流体注入速率的关系,仅考虑有效法向应力的临界变化.RST相比CPT更为复杂,它解释了孔隙压力变化和诱发地震活动的时间延迟,且同时考虑了正应力和剪切应力的变化,并描述了断层活动行为(Wenzel, 2017).

(3) 自电场理论也是研究干热岩开采诱发地震机理的重要理论(Bogoslovsky and Ogilvy, 1970),诱发地震区域通常会伴生自电场异常现象.相比热电势和扩散势,干热岩压裂注入流体的流动势是自电场异常最重要的来源(Jardani et al., 2008),因此除理解诱发地震机理外,自电场异常也被用来监测深部地热储层的水文参数(Murakami et al., 2001).Troiano等(2017)基于水-热耦合的多物理模型和基于数值计算的非饱和水热输运模型,解释了自电场现象是一种地震后事件,并可利用自电场异常评估诱发地震风险.

2.2 注入后诱发地震的发生机理

干热岩开采诱发地震存在一类特殊现象,就是在关井后仍可发生地震,这被称为“尾随效应”(Segall and Lu, 2015).关于尾随效应的解释目前仍不成熟,且发现孔隙弹性效应、注入策略和热机械过程均可导致尾随效应的发生.

孔隙压力扩散被认为可用于解释尾随效应.Parotidis等(2004)从孔隙压力扩散与地震活动关系角度,解释了关井后的诱发地震发生机制,但仅考虑了储层的线性孔隙弹性响应,而未考虑非线性和完全耦合的孔隙弹性应力,因此在物理过程描述上存在明显缺陷(Goebel and Brodsky, 2018).Segall和Lu(2015)认为孔隙弹性效应可能是尾随效应的主因.Baisch等(2010)通过对大型断层上的水力超压进行数值模拟,阐明了注入后诱发地震活动的机理.研究表明,关井后局部水压升高导致了孔隙压力扩散并使断层接近临界应力状态,少量的应力变化会导致大面积的超临界状态,增加关后地震发生的可能性.Mukuhira等(2017)研究表明,关井后的孔隙压力分布更倾向于覆盖和破坏大范围的裂隙或断层,如果大部分裂隙或断层已经达到临界应力状态,当再分布的孔隙压力局部超过临界孔隙压力时,可能导致局部剪切滑移、引发大地震事件.

关井后注入井附近断层的正常闭合被认为是尾随效应的重要机制.这种机制认为,流体注入期间储层内的裂隙或断层的张开度增加,当流体注入终止时,由于井口压力的快速下降,裂隙或断层开始关闭并作为流体屏障阻止了流体回流、促进流体朝更远区域流动.当加压流体扰动了处于临界应力状态的裂缝时,容易发生尾随效应(Ucar et al., 2017). McClure和Horne(2011)使用剪切引起的孔体积膨胀解释了尾随效应,也有研究认为剪切滑动受断层剪切扩张的强烈影响(Dang et al., 2019).此外,Zang等(2013)认为尾随效应与注入速率的突变有关,Zhuang等(2016, 2017)的室内实验以及Zang等(2017)的野外现场实验为这一观点提供了证据.

干热岩储层的其他物理过程也被认为可解释尾随效应.De Simone等(2017)认为在尾随效应研究中应将发生在靠近注入井的裂缝中的平流和发生在远处稀疏裂缝区域的弥散性分开考虑,在数值模拟研究中需要考虑流体力学、热力学和滑移引起的应力变化三个主要耦合过程.热力过程也可能会影响注入后的地震活动,在流体注入过程中热对流是传热的主导过程,但当流体注入停止时对流消失,随后热传导控制缓慢冷却过程,从而在关井后长期保持热应力,而热应力反过来又促进了剪切滑移的触发、扰动了应力场并通过降低水力梯度荷载影响断层的稳定性(Deichmann et al., 2014).

3 诱发地震减灾措施 3.1 风险评估监控措施

采用科学合理的地震风险管控措施是干热岩开采过程中减轻地震灾害威胁的重要途径,也是采取缓解措施的重要基础.目前世界各国干热岩开采风险管控的措施包括如下环节:

(1) 开采前的地震风险评估和生产许可.概率地震危险评估(PSHA)是开采前进行场地地震安全性评价的重要方法,一般是评估注入诱发地震的特征,包括超出预期的地震发生率和断层位移量(McGuire, 2004).由于PSHA是用来预测构造地震灾害的,因此难以涵盖干热岩增产和生产阶段,并且忽略了排量和流体注入量的影响(Hakimhashemi et al., 2014),这也决定了开采前的地震危险性和风险评估仅仅是风险管控的一般性参考.干热岩开采前的地震风险评估在部分国家已被制度化,例如荷兰自2003年起,在《采矿法》中规定包括干热岩的开采活动需要事先进行地震风险评估、拟定缓解措施的说明,并作为申请生产许可的必要条件(Muntendam-Bos et al., 2015; Wiemer et al., 2017).这些评估还需要给出预期最大震级、监测计划并经能源监管部门批准,如果地震震级或影响超过计划或批准范围,能源监管部门就可以干预(Van Eck et al., 2006).

(2) 建立诱发地震监测台网.实时运行的地震监测台网是干热岩开采诱发地震风险管控的重要基础设施,在干热岩开采区建立合理的地震监测台网、实现对诱发地震的高精度有效监测目前已成为几乎所有干热岩开采项目的通行做法.例如在芬兰埃斯波(Espoo)的地热井中使用了24台钻孔地震仪网络来监测诱发的地震事件,这成功地避免了MW2.0及以上诱发地震的成核作用(Kwiatek et al., 2019).由于在实时的风险管控红绿灯系统(TLS)中,常常采用震级、峰值地面速度(PGV)作为风险管控分级的输入参数,因此地震监测台网一般同时采用速度型地震计、强震动仪等分别组网,并采用地表浅井、深井阵列等多种方式提高诱发地震检测识别和参数测定的精度与可靠性.为确保诱发地震监测台网可达到减灾目的,一些国家专门制定了相应的地震监测法规(Van Eck et al., 2006; Grigoli et al., 2017).

(3) 实时风险管控技术系统.借助实时的、分级的地震风险管控技术系统将可极大提高干热岩开采诱发地震风险管控科学性和实际效率.目前常采用的此类系统是红绿灯系统(TLS),也即采用震级、PGV或地震风险评估值等阈值,以及开采区社会公众的风险可接受程度来确定风险等级(绿、黄、橙、红等),根据诱发地震实际发生情况或前瞻性的向前预测来及时调整压裂增产或返排、流体抽取等施工措施(Häring et al., 2008; Ellsworth, 2013; Bosman et al., 2016; Baisch et al., 2019; Ader et al., 2020).

3.2 诱发地震的前瞻性预测

在破坏性的较大的诱发地震发生前实现前瞻性的预测,是减轻干热岩开采诱发地震风险的重要途径.前瞻性的预测在开采前的场地风险评估、压裂增产和开采过程中的TLS运行都需要进行,但鉴于其重要性,这里单独描述研究进展.相比天然地震极高的预测难度(Foulger et al., 2018),包括干热岩开采在内的流体注入诱发地震活动的预测难度明显降低,这是由于地热储层的流体注入过程是可控的、也可更多地获取施工参数和井下物理化学状态.目前得到发展的干热岩开采诱发地震前瞻性预测主要集中在对最大震级的预测,已发展的主要方法:

(1) 统计预测方法.统计预测方法只需要记录储层刺激之前和期间的地震事件目录(Schoenball et al., 2012; Luginbuhl et al., 2018),其中包括发生时间、震级和事件地点(Langenbruch et al., 2011).早期的统计预测方法研究中,Shapiro等(2010)以及Dinske和Shapiro(2013)利用震级-频度分布曲线预测后续阶段的地震,但该方法假设地热储层的孔隙压力恒定不变或增加,在关井阶段不具有可参考性.Barth等(2013)提出的统计预测模型被认为可有效评估关井后注入诱发地震的发生概率.Shapiro等(2011)发展了评估注入点的地震构造状态的发震指数方法,并认为发震指数随时间变化大致稳定(Dinske and Shapiro, 2013).该方法对诱发地震的发生概率预测是通过建立了高于震级阈值(M)的地震事件预测数量(NM)、G-R关系的斜率(b值)、累积流体注入量(Q)和发震指数(Σ)之间的关系来实现,并要求b值和发震指数Σ随时间保持不变预测才有效.McGarr(2014)假设诱发地震是由于流体注入体积变化造成的断层有效法向应力变化,局部诱发地震事件被限制在流体注入区,给出了最大震级与净注入体积(ΔV)之间的线性关系,可用来估计诱发地震的地震矩上限.但由于忽略了注入流体在储层内外扩散的方式或位置等流体力学细节,该方法的适用性受到了限制(Van Der Elst et al., 2016; Li et al., 2018).

(2) 物理预测方法.基于物理的预测方法模拟流体注入引起的储层物理变化,并间接使用记录的地震目录进行模型校准(Cloetingh et al., 2010).这些方法考虑了地下特征,包括空间和时间的应力变化、断层破坏准则和破裂动力学、热传导和对流等.Galis等(2017)利用破裂物理学来解释破裂在应力扰动区域之外的传播,区分出已记录的破裂和逃逸型破裂,并建立了量化的物理模型来估计由孔隙压力扰动引起的最大破裂尺度.该模型考虑了包括孔隙压力、摩擦参数和应力状态对破裂传播的作用,并将估计的逃逸型破裂最大震级表示为与动摩擦系数μd、背景应力降Δτ0、储层岩石体积模量K、储层厚度h的简化表达式.

同样由于干热岩开采过程涉及到固-液-热-化学等多场耦合问题,进行诱发地震的前瞻性预测难度较大.为说明EGS项目前瞻性预测的技术选择难点,本文利用表 1的震例分析了诱发震级与累积注水体积和注入速率的关系,只选择了表 1里有明确的MLMW震级标度、最大震级大于1级的19个EGS项目和诱发地震案例,结果如图 4所示.对图 4ab的分析表明,在全球EGS项目之间的横向对比上,累积注水体积、注入速率两个参数与诱发最大震级之间均不存在明显的简单的线性关系,但对于注入速率如果不考虑Rosemanowes、Bad Urach和Desert Peak三个EGS项目,可发现最大震级与注入速率之间存在正比增加的趋势、3级以上地震更倾向于出现在注入速率接近50 L/s时.图 4c给出了韩国浦项(Pohang)EGS项目各注入阶段的累积注水体积与序列最大震级的关系,与目前对前瞻性预测科学认识相一致的是,累积注水体积能够很好地反映最大震级的预期水平,尽管MW5.5地震的发生机制仍存在争议.图 4d给出了瑞士巴塞尔(Basel)和韩国浦项(Pohang)EGS项目各注入阶段的注水速率与序列最大震级的关系,展示了两者的完全不相关性.总结来看可获得如下认识:(1)在全球EGS项目的横向比较上,累积注水体积、注水速率与最大诱发震级之间不存在统一的相依关系,可能是由每个项目自身的场地地质条件和地震活动水平的巨大差异造成,但注水速率控制在50 L/s以内有利于限制3.0级以上诱发地震的发生;(2)对具体的EGS项目而言,目前广泛采用的利用累积注水体积预估诱发最大震级的技术方法仍有适用性,而注入速率则未展现这种普适作用.

图 4 诱发地震震级与累积注水体积和注入速率的关系 Fig. 4 The statistical relationship between the magnitude of induced seismicity and cumulative injection volume and injection rate
3.3 缓解风险措施

除了及时对诱发地震灾害风险进行监测、评估,甚至前瞻性预测外,对出现了不同风险等级警告后,还应采取具体的缓解风险的措施.目前已有的实践与认识包括:

(1) 采用科学的流体注入策略.与天然地震不同,干热岩开采可通过控制流体注入速率(排量)、注入总量、井口压力等工程措施实现减轻诱发地震灾害风险.当红绿灯系统(TLS)出现风险等级最高的红色警告时,一般会采用直接停止注入作业和关井的方式(Porter et al., 2019),并在地震活动恢复至背景水平后,经过能源监管部门批准方可复工(Braun et al., 2020),例如芬兰赫尔辛基阿尔托大学(Aalto University)城市校区干热岩开发项目.当出现橙色警告时,会采用减少流体注入量等调整注入参数等方式(Ader et al., 2020).TLS作为实现地震灾害风险管控的核心技术系统,是衔接监测系统、风险调查系统、前瞻性预测系统、监管与决策系统的关键.此外,在具体的流体注入策略和技术实现上,包括周期性软压裂(CSS)管控措施近年来也得到应用尝试,这是采用了疲劳水力压裂概念(Zang et al., 2013),通过控制压力和流量、交替进行加压和减压,来实现对裂隙边缘应力的控制、力图将可能发生的大震级事件转化为大量的微震事件(Yoon et al., 2015).目前,CSS的措施已在韩国浦项EGS站点进行了尝试(Hofmann et al., 2018).除了以上方式,还存在如运用侧支管概念,将注入流体产生的压力分配在注入井的两个独立端之间等技术尝试(Breede et al., 2013).

(2) 及时对注采策略进行验证校准.当钻井完成后,应制定对诱发地震危险性和灾害风险模型的验证和校准策略,包括对低发生率和高灾害后果事件的充分考虑.能源开采监管机构也需要设定明确的验收标准、明确能源开采运营企业在整个项目周期内的风险控制目标(Wiemer et al., 2017).对压裂增产阶段的注采参数和工程措施、诱发地震潜在危险性和风险评估模型等,在各阶段应及时进行专门的测试和校准,确认是否达到了预期效果.例如,利用施工早期阶段观测的诱发地震强地面运动数据,及时校准预先设置的地震动预测模型(GMPE),以及根据诱发地震的发生率和震级来验证灾害风险评估中的假设是否合理等等(Zang et al., 2013).在具体的干热岩施工过程中,稳妥的方式是采用分阶段实施的方式,施工与注采策略验证校准交替进行,这可能是缓解诱发地震灾害风险的重要途径.

(3) 持续开展地震活动监测和缓解风险措施.受到诱发地震机理复杂性影响,干热岩开采区在关井停产多年后仍可观测到诱发地震活动,例如既有瑞士圣加仑地区干热岩开采区生产测试后地震活动立即停止(Diehl et al., 2017),也有瑞士巴塞尔地区开采活动结束11年后地震活动仍处于较高水平的情况(Kraft et al., 2016).这在客观上需要对开采区进行长期的、持续的地震活动监测,并保持诱发地震风险缓解措施长期运行有效.对此类复杂情况除持续监测外,Rathnaweera等(2020)还建议应以完全透明的方式对社会公众在内的各方提供诱发地震活动发生实况,以及未来预期地震活动等信息.此外,从长期角度看,完全消除干热岩开采诱发地震几乎不可能,这需要对整体的和长期的风险与开采商业收益之间平衡好,例如采用“风险-成本-效益”分析模式来保障在高度不确定性下进行复杂决策的科学性(Fischhoff, 2015).

4 总结和讨论 4.1 主要认识

本文整理了可获得数据信息的全球79个干热岩开发项目的注水与相关诱发地震数据,系统总结了干热岩开采诱发地震的总体情况、成因机理、减灾措施等领域目前的国内外研究动态,分析了各领域存在的问题.对国际上关于干热岩开采诱发地震研究现状的分析表明:

(1) 在全球干热岩开采尤其是利用EGS技术建储的项目中,诱发地震最大震级超过2.0级的占68.8%、超过3.0级的达到31.2%,其中最大的诱发地震事件达到MW5.5.鉴于诱发地震活动的普遍性,减轻地震灾害风险成为干热岩地热资源开发不可回避的重要事项.最大震级的诱发地震可发生在注水压裂、关井后、循环生产等各阶段,破坏性地震的发生主要与过井断层或井场周围断层的活化有关.

(2) 干热岩开采诱发地震的成因机理,由于涉及到固-液-热-化学等多场耦合过程,包括孔隙压力扩散、温度变化、流体注入和回采导致的体积变化、裂隙表面的化学性质变化、地震之间相互作用等多种因素,都可能是诱发地震的成因.通过对已有机理认识的震例分析表明,这些震例均为多因素和多机理共同作用.目前对流体注入时的诱发地震发生机理,已发展了摩尔-库仑和库仑应力变化准则、临界压力和速率-状态依赖性摩擦本构关系、自电场等理论框架.关井后仍发生诱发地震的尾随效应,是目前减轻干热岩开采诱发地震灾害风险的重大难点、尚无完全成熟技术参照,科学上正在积极探索利用孔隙压力扩散、断层闭合、热力过程变化等机理解释研究.

(3) 在诱发地震减灾措施上,目前已尝试采用在开采前进行地震风险评估与生产许可、建立诱发地震监测台网、采用实时风险管控技术系统等措施,研发了前瞻性的统计预测和物理预测方法.由于全球EGS项目的场地地质条件和地震活动水平的巨大差异,累积注水体积、注水速率与最大诱发震级之间不存在普适性的定标关系,但注水速率 < 50 L/s有利于限制3.0级以上地震的发生,对具体的EGS项目应采用“一井一策”的前瞻性预测和风险管控方案.

(4) 在缓解诱发地震灾害风险上,学术界和工业界开展了面向限制地震发生的流体注入策略、及时对注采策略进行验证校准、持续性的地震活动监测和施加缓解措施等探索实践,但这些缓解诱发地震灾害风险的技术有效性仍有待更多的检验,总体上属于将现有科学知识用于最大限度缓解灾害风险的尝试阶段.

4.2 存在问题和技术趋势

综合来看,当前在干热岩开采活动中减轻诱发地震灾害风险仍面临一些难点问题,包括:(1)对地下应力状态的量化分析.这包括需要了解地下初始应力状态、由于流体注入而引起的应力时空演化,以及断层失稳的应力临界阈值.而为了理解孔隙压力耗散、热扩散以及化学矿物学和孔隙弹性变化等对预存断层的局部应力状态的影响,也必须获得量化的应力状态.但由于干热岩开采过程涉及到固-液-热-化学等多场耦合问题,应力状态的量化仍然是非常具有挑战性的难题.(2)断层系统复杂性及其影响.断层系统的复杂性限制了对流体注采诱发地震的预测能力.尽管可以定性地确认流体的注入改变了地热储层的应力场,大多数诱发地震也确实发生在储层内,但是对注入流体与断层系统之间相互作用的评估和解释仍难度极高(Yoon et al., 2014).对基岩的地热储层内的先存断层与裂缝的探测识别,目前在勘探地球物理学领域仍属于难点问题.而事实上,目前的破坏性诱发地震也主要是与盲断层的活化有关(Choi et al., 2019),储层以外的偏远断层系统也常控制着诱发地震的持续发生(Mukuhira et al., 2017).(3)科学合理的流体注入策略.尽管目前在红绿灯系统(TLS)中已经采用了多种模式的缓解地震灾害风险的流体注入策略,以及采用基于疲劳水力压裂(Zang et al., 2013)和多阶段水力压裂(Meier et al., 2015)理论的流体注入策略也得到尝试,但包括CCS措施并没有阻止韩国浦项(Pohang)MW5.5地震的发生.这使得持续地开展更广泛的流体注采参数对诱发地震活动影响研究,探索真正可有效减轻诱发地震灾害风险的注采工程措施,仍极为必要.

目前一些新技术的应用,为突破干热岩开采过程中减少诱发地震灾害风险的科学瓶颈提供了新的思路.例如,利用穿透式雷达和光纤传感器监测地热储层注采过程的研究(Barnwal et al., 2017),利用物联网(loT)和深度学习算法实现在真实温度压力条件下实时感知、测量和传输信息(Mohanty, 2017),利用深度神经网络和卷积神经网络等深度学习技术提升诱发地震事件预测的准确性(Wang et al., 2017),探索利用超临界二氧化碳作为注入的热交换载体技术,以及评估其对诱发地震的影响(Brown, 2000)等等.在前瞻性预测方向,从统计预测模型、物理预测模型以及借鉴两者优缺点的混合型模型(hybrid model)也在持续开发探索中.然而,系统性地解决干热岩开采诱发地震成因机理、减轻地震灾害风险的有效管控措施等问题,仍是长期且艰巨的过程.作为潜力巨大的绿色新型能源,干热岩开采活动未来也必将突破商业上完全成熟可行的各类问题,以安全、环境友好和可持续发展的方式开发利用.

4.3 对我国干热岩开采诱发地震风险防控的启示

我国干热岩储量丰富、开采潜力巨大.目前已在青海贵德-共和盆地、福建漳州、松辽盆地、四川康定、山东利津(许天福等, 2018)、海南北部、河北乐亭(齐晓飞等, 2020)等开展试采工作和规模化压裂.在EGS建储相关的诱发地震研究和风险管控技术体系建设上,相比北美、欧洲和澳洲等地区,我国总体起步较晚、尚未系统开展.鉴于当前全球在干热岩开采活动中减轻诱发地震灾害风险仍面临难点,为确保能源战略顺利实施和社会公众的地震安全保障,我国亟待开展相关研究、迎头赶上.

其中应采用的与开采施工活动相关的技术措施包括但不限于:(1)由于我国大陆地区尤其是西部地区的构造变形剧烈、地层差应力大,利用EGS技术的干热岩储层建造,面临诱发地震灾害的风险较大.应对开采场地进行地震安全性评价,井场应尽量远离具备发震能力的活断层、对已知断层开展滑移趋势分析,开展施工前的地震灾害风险评估;(2)在开采场区建立完善的地震监测系统,包括采用地面与深井相结合的高精度微震监测网络、覆盖周边有人居住区域和重大基础设施的强震动监测网络,以及电磁法等和重力观测网络,开展实时地震监测速报,测定震源机制和应力降等震源参数、监控注入流体运移状态、建立地震动衰减关系;(3)建立诱发地震灾害风险管控红绿灯系统,利用统计模型、物理模型和混合模型相结合的方式实施前瞻性的诱发地震危险状态分析,开展地震灾害风险情景构建和动态的地震风险评估,建立开采企业、能源监管机构、地震行业部门、地方政府联动的风险管控和应急处置体系.

此外,在基础研究和应用研究领域,应加强诱发地震机理的室内实验和理论研究,发展注采施工期间介入式的诱发地震控制、多模式的流体注入策略、诱发地震灾害风险缓解措施、考虑诱发地震灾害风险的区域地震区划等关键技术,加强关井后的尾随现象、地下应力状态量化分析、固-液-热-化学多场耦合等关键问题的科技攻关.

致谢  中国地震局地球物理研究所郑钰工程师、中国地质调查局水文地质环境地质调查中心吴海东博士与作者进行了有益讨论,张琰博士研究生为本文的文献检索提供了帮助.四位审稿人提出了建设性意见、对本文的质量提升帮助很大.在此一并表示感谢.
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