Experiment on Hydraulic Fracturing in Rock and Induced Earthquake

Experiment on rock hydraulic fracturing strength under different confining pressures was conducted on a series of test specimens with various pre-cracks prepared from 7 types of rock. Combining the data of an actual reservoir-induced earthquake with the experimental results of the contemporary tectonic stress field according to the theory of rock strength and the principle and method of rock fracture mechanics, the authors tentatively investigated the earthquakes induced by pore-water pressure in rock and obtained the initial results as follows: （1） One type of induced earthquake may occur in the case of larger tectonic stress on such weak planes that strike in similar orientation of principle tectonic compressional stress in the shallows of the rock mass; the pore-water pressure σp may generate tensile fracture on them and induce small earthquakes; （2） Two types of induced earthquake may occur in the case of larger tectonic stress, i.e.,① on such weakness planes that strike in similar orientation of principle tectonic compressional stress, σ1, in the shallows of the rockmass, the pore-water pressure, σp, may generate tensile fracture on them and induce small earthquakes; ② When the tectonic stress approximates the shear strength of the fracture, the pore-water pressure σp may reduce the normal stress, σn, on the fracture face causing failure of the originally stable fracture, producing gliding fracture and thus inducing an earthquake. σp may also increase the fracture depth, leading to an induced earthquake with the magnitude larger than the previous potential magnitude; （3） There is a depth limit for each type of rock mass, and no induced earthquake will occur beyond this limit.

Hydraulic Fracturing, Cumulative Development and Earthquakes in the Peace River Region of British Columbia, Canada

Unconventional petroleum development involving large volume fluid injection into horizontal well bores, referred to as hydraulic fracturing (HF, or fracking), began in the Montney Trend of northeast British Columbia, Canada, in 2005, quickly initiating earthquakes. Earthquake frequency increased substantially in the Montney by 2008, in relation to the number of wells fracked and the volume of injected frack water. A spatiotemporal filter was used to associate earthquakes with HF wells. A total of 439 earthquakes (M 1.0 - 4.6 (NRCAN catalogue) during 2013-2019 have close association with HF activity, of which 77% are associated with three operators. Fifteen percent of HF wells in the Montney are associated with these earthquakes, while 1.7% of HF wells are associated with M ≥ 3.0 earthquakes. There are strong linear relationships between the maximum earthquake magnitude each year and the annual volume of injected frack fluid. M ≥ 3.0 earthquakes are associated with large cumulative frack water volumes for antecedent time periods of 1 - 3 years, often with fluid injection by multiple operators. Eighty-seven percent of the Montney M ≥ 3.0 earthquakes have associated HF triggering events, but a few are sufficiently distant to be ambiguous. Distances from the induced earthquake epicentres indicate a variety of causal mechanisms are involved. It is concluded that ~60% - 70% of M ≥ 3.0 earthquakes are induced by hydraulic fracturing. HF-induced earthquakes can be considered in part related to the cumulative development density from multiple proximal operators and cumulative antecedent fluid injection over periods ranging from a few months to a few years. It is probable that induced earthquakes of M > 5 will occur in the future. There are significant public safety and infrastructure risks associated with future HF-induced earthquakes in the Peace River area. To carry out HF operations effectively and safely, potentially destructive earthquakes must be avoided or mitigated. The Traffic Light Protocol mitigation system used in British Columbia appears unlikely to prevent large magnitude earthquakes. Risk avoidance therefore becomes important and could include the establishment of frack-free zones proximal to populations and critical infrastructure.

Numerical simulation of fault activity owing to hydraulic fracturing

We built a three-dimensional model to simulate the disturbance of the stress field near the reverse fault in Zhaziao, Leyi Township owing to hydraulic fracturing. The pore pressure, and shear and normal stresses during fracturing are analyzed in detail. Input rock mechanics parameters are taken from laboratory test data of shale samples from the study area. The simulation results suggest that after 16 hours of fluid injection, the pore-pressure variation can activate the reverse fault, i.e., we observe reverse slip, and the shear stress and displacement on the fault plane increase with time. The biggest stress–strain change occurs after one hour of fluid injection and the yield point appears about 0.5 h after injection. To observe the stress evolution in each section, the normal displacement on the boundary is constrained and the fault plane is set as nonpermeable. Thus, the sliding is limited and the shear displacement is only in the scale of millimeters, and the calculated magnitude of the induced earthquakes is between Mw-3.5 and Mw-0.2. The simulation results suggest that fluid water injection results in inhomogeneous fracturing. The main ruptured areas are around the injection positions, whereas the extent of rupturing and cracks in other areas are relatively small. Nevertheless, nonnegligible fault activation is recorded. Sensitivity analysis of the key parameters suggests that the pore pressure is most sensitive to the maximum unbalanced force and the internal friction angle strongly affects the fault slip. Finally, the comparison between the effective normal stress and the maximum and minimum principal stresses on the fault plane explains the fault instability, i.e., the Mohr circle moves towards the left with decreasing radius reduces and intersects the critical slip envelope, and causes the fault to slip.

Possible link between long-term and short-term water injections and earthquakes in salt mine and shale gas site in Changning,south Sichuan Basin, China

Late at night on 17 June 2019,a magnitude 6.0 earthquake struck Shuanghe Town and its surrounding area in Changning County,Sichuan,China,becoming the largest earthquake recorded within the southern Sichuan Basin.A series of earthquakes with magnitudes up to 5.6 occurred during a short period after the mainshock,and we thus refer to these earthquakes as the Changning M6 earthquake sequence(or swarm).The mainshock was located very close to a salt mine,into which for^3 decades fresh water had been extensively injected through several wells at a depth of 2.7–3 km.It was also near(within^15 km)the epicenter of the 18 December 2018 M5.7 Xingwen earthquake,which is thought to have been induced by shale gas hydraulic fracturing(HF),prompting questions about the possible involvement of industrial activities in the M6 sequence.Following previous studies,this paper focuses on the relationship between injection and seismicity in the Shuanghe salt field and its adjacent Shangluo shale gas block.Except for a period of serious water loss after the start of cross-well injection in 2005–2006,the frequency of earthquakes shows a slightly increasing tendency.Overall,there is a good correlation between the event rate in the Shuanghe area and the loss of injected water.More than 400 M≥3 earthquakes,including 40 M≥4 and 5 M≥5 events,had been observed by the end of August 2019.Meanwhile,in the Shangluo area,seismicity has increased during drilling and HF operations(mostly in vertical wells)since about 2009,and dramatically since the end of 2014,coincident with the start of systematic HF in the area.The event rate shows a progressively increasing background with some fluctuations,paralleling the increase in HF operations.More than 700 M≥3 earthquakes,including 10 M≥4 and 3 M≥5 in spatially and temporally clustered seismic events,are correlated closely with active fracturing platforms.Well-resolved centroid moment tensor results for M≥4 earthquakes were shown to occur at very shallow depths around shale formations with active HF,in agreement with some of the clusters,which occurred within the coverage area of temporary or new permanent monitoring stations and thus have been precisely located.After the Xingwen M5.7 earthquake,seismic activity in the salt well area increased significantly.The Xingwen earthquake may have created a unidirectional rupture to the NNW,with an end point close to the NW-trending fault of the Shuanghe earthquake.Thus,a fault in the Changning anticline might have terminated the fault rupture of the Xingwen earthquake,possibly giving the Xingwen earthquake a role in promoting the Changning M6 event.

水力压裂延迟活化断层的多场耦合数值模拟研究——以加拿大福克斯溪地区诱发地震为例

加拿大西部盆地的福克斯溪(Fox Creek)页岩气开采区自压裂开采以来,地震频度急剧增加,引发工业界和科学界的广泛关注。目前一些典型诱发地震案例的断层活化动力学机制尚未完全厘清。本文以2014年初福克斯溪地区Duvernay地层附近发生的地震群及构造为研究对象,开展地下断层受流体扰动而活化的多场耦合数值模拟研究。文中重点讨论的1号平台附近发生的诱发地震主要沿着两个较为清晰的发震断层发生,最大震级M_(W)3.9事件发生于压裂井下方约1 km处的结晶基底内,并具有典型的延迟触发特点。本文就上述断层的滞后活化现象开展数值模拟研究。首先,基于PKN裂缝扩展模型计算并验证注入流体的应力扰动输入项,根据地震数据识别出断层的具体位置,结合地层和构造信息建立二维地质模型;然后,耦合固体力学、流体渗流定律和断层活化理论搭建多孔弹性介质内的断层活化数值仿真模型;最后,采用有限元方法数值模拟水力压裂活化断层的全过程,通过计算库仑应力改变量(ΔCFS)的值来观测断层活化前后的流固耦合场和应力应变场的演化特征。结果表明,经过5天注水和15天扩散的流体作用,西断层附近的ΔCFS持续增加,验证西断层延迟活化的主要成因是流体逐渐扩散累积到结晶基底并改变了其应力状态。此外,模拟结果表明东断层的存在使得西断层更容易活化。断层位错产生的正ΔCFS区域与诱发地震发生位置高度吻合。本文的数值模拟研究重现了水力压裂活化断层的物理过程,相关机制的正演分析若能事先开展,将可能为地震危害性预测提供科学依据。

长阳页岩气试验井压裂微震检测及特征

基于三峡地震台网2017年7—8月记录到的长阳地区发生的震群资料,通过PhaseNet进行震相识别、REAL震相关联、HypoInverse绝对定位、HypoDD相对定位等定位流程,构建了长阳页岩气试验井压裂震群的高分辨率地震目录。震群重定位结果表明:地震序列整体上沿着地表由南向北呈带状展布;在长阳页岩气试验井水力压裂前后,长阳地区的地震活动性非常低,但在试验井开始压裂期间,整体地震活动性猛增,最大震级可达ML3.4,但震群活动对周边区域地震活动性的影响较小;该震群成因可能为页岩气开采试验井水力压裂导致天阳坪断裂活化产生的。

干热岩地应力测量评价方法与前沿挑战

可靠的地应力数据能够指导开发干热岩资源。本文针对干热岩深部储层地应力调查存在的困难,按照基于钻孔、岩芯和地质的三类调查手段,首先总结提出了适用于干热岩地应力测量与评价的方法,并指出了其基本原理、测试仪器、应用场景与存在的优缺点。然后,以我国青海共和干热岩试采工程为例,提出了干热岩地应力测量与评价模式,给出了储层地应力特征的主要认识,并验证其与工程开发结果的一致性。最后,总结了干热岩地应力测量方法的认识,指出了干热岩开发中与地应力相关的两大前沿科学问题,即储层裂缝控制与诱发地震,并对其发展趋势与挑战进行了展望。

水力压裂诱发地震研究进展

随着近年来页岩气水力压裂技术的普及,该技术诱发的地震活动也引起了全世界的广泛关注。四川盆地作为开采中国页岩气的主要区域之一,自2015年水力压裂技术在盆地内全面推进,地震活动频度和震级均呈增长趋势。本文通过收集和分析全球水力压裂诱发的地震,尤其是四川盆地区域的地震,从地震活动的现象和特征、可能诱发机制、地震监测、风险减轻、灾害预测等方面进行了较为全面的阐述,对全面认识水力压裂诱发地震具有一定的参考价值。

花岗岩型干热岩储层信息获取方法及控震压裂技术介绍

干热岩型地热作为一种大储量的清洁能源,在日趋严峻的环保形势下,正在逐渐影响着世界能源格局,并成为学术界、政府和企业关注的焦点。开发花岗岩型干热岩需要建立增强型地热系统(EGS),其核心是向储层钻井并压裂形成一定规模的裂缝网络,构建注入井和生产井的循环回路来提取热能发电。从瑞士Basel EGS工程、韩国Pohang EGS工程等花岗岩型干热岩EGS压裂工程案例可以发现,诱发地震已成为制约花岗岩型干热岩开发的关键因素,其原因在于未明确大尺寸高倾角结构面的具体位置,无法预测压裂液在压裂过程中的流动方向,导致压裂液进入此类结构面造成结构面滑移从而诱发地震。本文将针对此问题通过查明干热岩所在储层的信息对开发花岗岩型干热岩控震压裂人工热储建造方法进行介绍。

岩石水压致裂和诱发地震的实验研究

用7种岩石各制备了几类不同预裂纹的系列试件,并在不同围压下进行水压致裂强度实验。结合典型的水库诱发地震的实际资料和现今构造应力场的实验结果,以及应用岩石强度理论和岩石断裂力学的一些原理、方法,进行岩石孔隙水压诱发地震的探讨。初步结果为:(1)若岩体内构造应力很小,一定大小的孔隙水压力σP可直接使岩体内的薄弱面致裂并发生小地震。(2)若构造应力较大,存在两类诱发地震的可能:①对岩体浅部一些走向与构造应力的主压应力σ1方向相近的薄弱面,σP可促使其发生张性破裂并诱发小地震。②当构造应力接近于断裂的抗剪强度时,因σP降低了断裂面上的正应力σn,使原处于稳定状态的断裂失稳,发生滑移破裂并诱发出地震。σP导致断裂的破裂深度增大,使诱发地震的震级大于原潜在的地震震级。(3)各种岩体均存在着一个极限深度,此深度后不再有诱发地震。

考虑断裂破碎带的丹江口水库诱发地震成因数值模拟分析

根据丹江口库区的地质构造特征,考虑库区已有的断裂破碎带和岩石力学参数随深度的变化,运用ABAQUS软件建立丹江口库区三维有限元模型,模拟计算其构造应力场和水压应力场,分析两场的共同作用,在此基础上探讨丹江口水库提高水位后的诱发地震成因。结果表明：一期蓄水引起的水压应力场和位移场是诱发库岸外围一系列微震的主要因素;二期蓄水引起水压应力和位移的增加,会在库岸外围—厚坡、九重、彭桥、仓房、田川等地再次诱发一系列微震,在宋湾—关防滩—旧淅川、上寺—黑桃园、林茂山—丹江口—青山港和紫山凹—旧均县等地新诱发一系列小震,震源深度在5~11km范围内;蓄水后的构造应力场和蓄水前的基本相同,由水体静压力引起的应力变化很小,水体荷载产生的应力直接诱发较大震级的地震是不可能的。

从岩石水力学观点探讨水库诱发地震机理

水库诱发地震机理的研究目前还停留在定性的层面上,没有进行系统的理论推导和计算。本文从岩石水力学观点出发推导了水体荷载、孔隙水压力和水岩物理化学作用诱发地震的数学模型,并以丹江口水库为例进行了计算,得出了一些很有价值的规律,为水库诱发地震机理研究提供了理论指导。

干热岩压裂建造人工热储发展现状及建议

能源是经济社会长期稳定发展的有力保障。同时,我国也进入了生态文明建设的关键时期。为实现此目标,亟需建设清洁、低碳、高效、多元的现代能源体系。干热岩型地热作为一种新兴的环境友好型资源,有望推进能源结构转型。开发干热岩需要建立增强型地热系统,其核心是向储层钻井并压裂形成一定规模的裂缝网络,构建井间循环回路来提取热能发电。20世纪70年代以来,多个发达国家先后进行干热岩资源开发尝试,然而受到人工热储建造和诱发地震防控等关键技术的限制,成功运行的EGS工程屈指可数。近些年来,干热岩资源的优越性和规模化开发可行性进一步得到国际社会的认可,投入建设的EGS数量总体上不断增加。水力压裂是建造人工热储的核心技术手段之一,水力裂缝的形态直接决定了换热体积和取热效率。本文在分析国内外典型EGS压裂案例的基础上,总结了干热岩水力压裂的工艺特点。此外,结合几种较为流行的理论模型和我国首例EGS工程——共和盆地恰卜恰干热岩试采工程的实际情况,简要阐述了干热岩压裂与诱发地震之间的关系。最后从压裂工艺、智能化开发、微地震监测矩张量反演等方面为干热岩水力压裂向更深层次发展提出建议。

水力压裂诱发地震活动中的b值时空异质性及其应用

诱发地震及其灾害风险对非常规油气资源开发和废水回注等新型工业活动的顺利实施存在严重威胁,并早已引发社会各界及相关科研人员的广泛关注,由此也产生了许多需要进一步研究且亟待解决的科学问题。震级-频度分布(FMD)是描述地震活动的重要内容,对诱发地震的b值开展系统研究有助于揭示区域应力累积状态、地下构造特征以及诱发地震危险性。文中系统梳理了近十年来国内外工业开采水力压裂诱发地震b值的数值大小、时空异质性、物理机制解释、受影响因素、应用现状等相关的研究进展;归纳了前人发现的可能影响b值大小的因素,包括原位应力场、断层几何参数、断层成熟程度、震源深度等客观因素,以及注液体积、注液速率等由具体施工情况引起的孔隙压力大小变化的主观因素;总结了前人提出的包括孔隙压力控制论、原位差应力大小控制论、最大剪应力方向控制论、地质条件非均匀性决定论等多种可能的物理机制。厘清其中已取得的共识性认识、争议科学问题后,我们认为诱发地震b值或许可以成为判断储层改造效果、储层应力状态以及诱发地震灾害风险防治的入手点之一。文中对水力压裂诱发地震b值特征的总结分析可为从事新型能源开发、地震安全监管和科学研究领域的企业、管理人员及科学研究人员提供科学参考。

对龙羊峡水库发生诱发地震可能性的探讨

本文用龙羊峡水电站的水压致裂应力测量结果分析了该水库发生诱发地震的可能性。结果表明,该水库库区浅层的应力特点为:S_H>Sh>Sv,具备发生诱发地震的应力条件,但浅部和深部的应力状态可能不同。这给由浅部应力测值推测可能发生的诱发地震的震级的上限带来困难。

工业开采诱发地震的应力降研究现状和主要科学议题

诱发地震及其灾害风险已成为非常规油气资源开发和废水回注等新型工业活动顺利实施的重要威胁。应力降是反映地震破裂前后断层上平均应力差的物理量,对诱发地震中应力降的系统研究有助于揭示诱发地震的震源特征、高频地震动特征、构造背景和地震危险性。本文系统地总结了近20年来工业开采诱发地震中应力降的相关研究进展,分别从计算所用理论方法、获得的平均应力降数值、影响应力降数值的主客观因素、围绕应力降的重要科学讨论等方面做了归纳分析。介绍了直接进行谱拟合、经验格林函数(EGF)、谱叠加和广义反演3种适用性的计算方法,通过案例考察了工业开采诱发地震的应力降平均值,分析影响应力降测定数值的主客观因素,并总结应力降空间分布依赖性、与震源机制类型相依性、自相似性、深度依赖性等4个主要科学问题。同时,阐述了诱发地震应力降的测定可靠性仍未突破、主要科学问题存在广泛争议、在新型工业开采活动中具有重要应用潜力等现状。本文的总结分析可为从事新型能源开发、地震安全监管和科学研究领域的企业、管理人员和科学研究人员提供参考。

全球油气开采诱发地震的研究现状与对策

人工诱发地震现象已经有很久的历史.水库蓄水、采矿、地热开发、从地下提取液体或气体,或将液体注入地球内部都可能诱发地震.大量地震监测数据与科学分析结果显示:美国俄克拉何马州的地震剧增主要与页岩油气开采的废水回注量相关;加拿大阿尔伯塔省的地震剧增主要与页岩油气开采水力压裂的工作量相关;而荷兰罗宁根天然气田的传统天然气开采也同样诱发了较强的地震活动.在中国四川盆地的页岩油气开发区域,地震活动近几年也大幅度增强,但目前监测与科研工作较少,对某些地震成因尚有争议.目前研究诱发地震问题已成为学术界与工业界的一门专业学科.推断诱发地震,除了分析时空分布与工业活动的相关性之外,本文综述了该领域基于地震学、地质动力学、构造地质学的多种分析方法.如何在油气开采过程中减少诱发地震的灾害影响成为当前相关各界极为关注的科研问题,本文介绍了多个国家或地区建立的控制诱发地震的管理系统、基于地震大数据的诱发地震概率预测方法,以及基于地球物理与地质信息的综合诱发地震风险评估方法,并对我国控制诱发地震问题提出建设性意见.

流体-地质力学耦合建模表征水力压裂诱发地震:以加拿大Fox Creek地区为例

随着水力压裂技术在页岩气开发中的广泛应用,加拿大西部盆地的诱发地震活动显著增加.目前对于诱发地震的综合表征方法还不成熟.本文采用一种综合地质、岩石力学及流体力学的研究方法,对Fox Creek地区2015年2月8日发生的M 3.0诱发地震事件进行了综合表征.首先,利用高分辨率三维反射地震资料,采用蚂蚁体追踪技术识别潜在断层.其次,利用测井曲线和压裂施工数据等资料定量求取岩石力学及地应力参数,建立三维地质力学模型,明确水力压裂缝的空间扩展规律.最后,建立流体-地质力学耦合模型,计算水力压裂过程中断层附近的孔隙压力及局部应力变化,利用摩尔-库仑破裂准则判定断层激活的时间与空间位置,揭示本次诱发地震事件的触发机制并提出风险控制对策.结果表明,三条由Precambrian基底向上延伸至Duvernay地层的近垂直断层在水平井压裂过程中被激活.由于水平井的部分压裂缝与断层沟通,注入流体沿断层的高渗透破裂带向下迅速扩散,在基底位置激活断层并诱发M 3.0地震事件.其中孔隙压力增加是本例中断层活化的主要因素.现场措施表明,增大压裂水平井与已知断层之间的距离被可以有效地降低地震风险.因此在进行水平井钻井及压裂作业之前,明确地下断层的分布位置至关重要.

水力压裂法在页岩气开发应用时的诱震分析

页岩气是一种开采于泥页岩及其夹层中的天然气资源,我国的页岩气可采储量较大。为了能够有效提高页岩气的开采率,水力压裂法通常被应用到实际开采作业中。然而,水力压裂法也会带来诸如诱发地震等各种问题,故对于水力压裂法诱发地震的可能性进行了分析,以期提高水压致裂法开采页岩气的效率与可靠性,并提出新的技术展望。

页岩气开采诱发地震的主要机理与影响因素

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。