Hydraulic fracturing: a toxicological threat for groundwater and drinking-water?

This paper deals with the possible impact of hydraulic fracturing (fracking), employed in the exploitation of unconventional shale gas and tight gas reservoirs, on groundwater, which is the most important source of drinking-water in Germany and many other European countries. This assessment, which is part of an interdisciplinary study by a panel of neutral experts on the risks and environmental impact of hydraulic fracturing, is based mainly on data obtained from three ExxonMobil drilling sites in northern Germany. First, the basic technical aspects of fracking and its relevant water fluxes are explained. The type, purpose and fate of the constituents of the fracking fluids are discussed. The chemicals used in the fracking fluids are assessed with regard to their hazardous properties according to the Regulation (EC) No. 1272/2008 of the European Parliament and of the Council on the classification, labelling and packaging of substances and mixtures (CLP regulation) and the German “Water Hazard Classes”. Contamination of groundwater by ingredients of fracking fluids may occur from under ground or may result from above-ground accidents associated with the transport, storage and handling of hazardous substances used as additives in fracking fluids. The degree of groundwater contamination cannot be predicted in a general way. Therefore, different dilutions of the fracking fluid in groundwater are considered. It is shown that the concentrations of most ingredients resulting from a 1:10,000 up to 1:100,000 dilution of the fracking fluid in groundwater are below health-based reference values such as the limit values of the European Drinking Water Directive, the WHO Guideline Values for Drinking-water Quality, and other health-based guide values for drinking-water. Regarding the salinity of fracking fluids, a dilution of 1:1,000 is sufficient to reach concentrations which are acceptable for drinking-water. From the human-toxicological point of view, the constituents of flowback water are more problematic with respect to drinking-water produced from groundwater than those of the fracking fluids. The few reliable data which have become available, as well as hydrogeological considerations, point in the direction of considerable salt concentrations and toxic constituents, e.g., Hg, As, Pb, Zn, Cd, BTX, PAHs, or even radioactive elements. The identification and assessment of reaction products and metabolites, which are produced as a result of the fracking operation and the metabolic activity of microorganisms, are important topics for further research. The recommendations include the need for a better understanding of the environmental impact of fracking operations, especially with regard to the development of sustainable rules for planning, permission, performance and management of fracking, and for the monitoring of groundwater quality around fracked drilling sites.     

Hydraulic fracturing in unconventional gas reservoirs: risks in the geological system, part 2

Hydraulic fracturing is a method used for the production of unconventional gas resources. Huge amounts of so-called fracturing fluid (10,000–20,000 m³) are injected into a gas reservoir to create fractures in solid rock formations, upon which mobilised methane fills the pore space and the fracturing fluid is withdrawn. Hydraulic fracturing may pose a threat to groundwater resources if fracturing fluid or brine can migrate through fault zones into shallow aquifers. Diffuse methane emissions from the gas reservoir may not only contaminate shallow groundwater aquifers, but also escape into the atmosphere where methane acts as a greenhouse gas. The working group “Risks in the Geological System” as part of ExxonMobil’s hydrofracking dialogue and information dissemination processes was tasked with the assessment of possible hazards posed by migrating fluids as a result of hydraulic fracturing activities. In this work, several flow paths for fracturing fluid, brine and methane are identified and scenarios are set up to qualitatively estimate under what circumstances these fluids would leak into shallower layers. The parametrisation for potential hydraulic fracturing sites in North Rhine-Westphalia and Lower Saxony (both in Germany) is derived from literature using upper and lower bounds of hydraulic parameters. The results show that a significant fluid migration is only possible if a combination of several conservative assumptions is met by a scenario.     

Analytical investigation of hydraulic fracture-induced seismicity and fault activation

More recent public discourse has taken place regarding the potential correlation between seismic activity and hydraulic fracturing in shale gas reservoirs. Public fears about the risk of seismicity stem mainly from past earthquakes induced by conventional deep injections because the two types of projects share similar mechanisms of rock failure and fault activation. Although previous earthquake risks associated with fluid injection were not serious, the situation would be far more problematic if hydraulic fracturing in a shale gas reservoir triggered a similar-sized earthquake due to potential environmental issues. In fact, almost all documented injection-induced earthquakes have been associated with long-duration and high-volume injection rather than short-term (hours) pressurization (e.g., hydraulic fracturing). In general, hydraulic fracturing operations mostly induce microseismic events through rock failure and activation of small fractures. Although shale reservoirs in tectonically active zones pose a high risk of inducing large-magnitude seismic activities, the internal geological conditions and external stimulation conditions are impossible to be satisfied simultaneously to trigger activation of an entire fault and to result in a destructive earthquake during hydraulic fracturing operations.     

超临界CO2分段多簇压裂井间干扰规律研究

水平井分段多簇压裂是非常规油气藏开发的关键技术,在合理利用压裂诱导应力增大储层改造体积的同时,避免井间干扰所导致的裂缝砂堵和压裂窜扰,是压裂工艺优化中的关键科学问题。文章针对超临界CO₂分段多簇压裂的缝间干扰和井间干扰问题,采用流固耦合的扩展有限元方法研究单井及多井裂缝诱导应力演化特征,充分考虑裂缝内超临界CO₂流动和滤失,从非常规油气储层岩性特征、地应力场分布及施工工艺等多方面对压裂扰动应力进行系统研究,揭示单井分段多簇压裂缝扩展机制及应力扰动特征,在此基础上研究多井井间压裂缝干扰规律。结果表明：高水平应力差、高弹性模量的储层中压裂干扰界限较大,低水平应力差、低弹性模量地层需适度增大簇间距,减小簇间干扰;老井压裂后,其邻井压裂缝非对称系数随井间距呈先增大、后减小的趋势;当井间距等于压裂干扰界限时,非对称系数λ达到最大,且井周改造范围最大,但裂缝两翼的非对称性可能导致储层动用不充分。本研究为水平井细分切割压裂和立体式井网设计优化提供理论基础,在“双碳”战略背景下对非常规油气资源高效开发具有重要意义。     

考虑污染物扩散风险的场地地下水污染多层次风险评估方法

地下水中污染物具有迁移性和扩散性,会对下游敏感受体造成威胁,目前场地地下水风险评估主要关注人体健康风险,还未能综合考虑地下水污染的整体风险,尤其是忽视了污染物迁移引起的对下游敏感受体的风险。本研究基于“源-径-汇”模型构建了考虑污染物扩散风险的场地地下水污染风险评估的指标体系与风险评估模式。在指标体系构建方面,重点考虑场地地下水的污染源、迁移路径和敏感受体3个方面。在风险评估模式方面,根据场地不同地下水污染状态开展3个层次的风险评估。基于假想的铬污染场地开展了案例分析,设置了地下水污染状态的4种情景,利用Wexler溶质运移模型计算了地下水污染羽的时空变化,并针对地下水污染的不同层次开展了风险评估。结果表明,在场地地下水污染羽未到达场地边界的2种情形中,场地地下水的风险评分分别为4.0,6.2,分别属于低风险与中风险。在场地地下水污染羽到达或超出场地边界的2种情形中,场地地下水的风险评分分别为7.0,8.8,分别属于中风险与高风险。综合而言,本研究构建的方法能够用来对场地地下水进行系统全面的评估和对比,能够根据风险结果对污染场地进行有效的分级管控,为污染场地的风险管控提供技术支撑。     

页岩气开采的相关实验、模型和环境效应

页岩气是一种重要的非常规天然气资源,正在改变世界能源、经济和政治格局。渗透率是评价页岩气藏商业开采可行性的重要参数之一,由于页岩的致密性,页岩气的流动机理不同于常规气藏,因此,页岩储层渗透率测试和页岩气流动模型已成为当前国际研究的热点课题之一。在对页岩气开采技术简单介绍的基础上,综述了页岩储层渗透率测试的试验和理论研究现状,分析了气体吸附对页岩渗透率的影响。阐述了页岩气流动模型的最新进展,分析了双重孔隙模型描述气体迁移的准确性,提出了描述均匀储层中页岩气解吸-扩散-渗流多级运移模型。评述了页岩气开采的温室效应和对地下水的影响,并简单介绍了适合页岩气开采的新技术即无水压裂开采技术,即采用CO2对页岩气藏分段压裂,同时将CO2埋存于废弃井中。最后,对页岩储层渗透率测试和页岩气流动模型研究的新发展以及无水压裂技术进行了展望。     

重庆涪陵页岩气勘查开发区环境地质调查进展

紧密结合国家非常规能源勘探开发的重大战略,聚焦重庆涪陵页岩气勘查开发区环境地质调查工作,总结了研究区岩溶发育特征与分布规律,评价了涪陵页岩气开发区区域水资源量和地下水质量。在此基础上,收集分析国内外资料,梳理了页岩气勘查开发所引发的6类环境(地质)问题及勘查、钻井、压裂、开采、闭井等5个阶段需关注的地质环境问题或风险等,其中涪陵页岩气勘查开发区主要诱发水土污染、地质灾害和大气污染等环境(地质)问题; 基本识别了2种地下水污染模式、3种污染类型及可能的地下水污染风险途径; 初步确定了涪陵页岩气勘查开发区污染指示性特征因子,并形成了页岩气勘查开发区地质环境影响评价指标体系,可指导页岩气勘查开发的环境地质调查。该研究为页岩气国家新型清洁能源的绿色开发和开发区生态文明建设提供支撑。     

重庆涪陵页岩气勘查开发区环境地质调查进展

紧密结合国家非常规能源勘探开发的重大战略,聚焦重庆涪陵页岩气勘查开发区环境地质调查工作,总结了研究区岩溶发育特征与分布规律,评价了涪陵页岩气开发区区域水资源量和地下水质量。在此基础上,收集分析国内外资料,梳理了页岩气勘查开发所引发的6类环境(地质)问题及勘查、钻井、压裂、开采、闭井等5个阶段需关注的地质环境问题或风险等,其中涪陵页岩气勘查开发区主要诱发水土污染、地质灾害和大气污染等环境(地质)问题; 基本识别了2种地下水污染模式、3种污染类型及可能的地下水污染风险途径; 初步确定了涪陵页岩气勘查开发区污染指示性特征因子,并形成了页岩气勘查开发区地质环境影响评价指标体系,可指导页岩气勘查开发的环境地质调查。该研究为页岩气国家新型清洁能源的绿色开发和开发区生态文明建设提供支撑。     

页岩气开发的地下水环境背景值、监测指标及污染示踪方法研究——以焦石坝区块为例

页岩气开采过程中水力压裂及废水回注可能带来的环境问题受到越来越多关注。由于水力压裂开采技术涉及的污染物种类众多,因此建立敏感性监测指标对于识别潜在的地下水污染具有重要意义。本文以中国涪陵页岩气田焦石坝区块为例,首先确定了地下水环境背景值（包括水化学、同位素及溶解气）;通过页岩水与浅层地下水的端元对比,确定了地下水环境敏感性监测指标与污染示踪方法。结果显示浅层地下水TDS在146~402 mg·L-1之间,属于HCO₃-Ca·Mg型,地下水属于年轻地下水（含3 H、14C接近100 pmC）,地下水甲烷含量均小于0.01 mg·L-1,水中溶解气的甲烷（CH₄）体积比低于0.006 4%,δ13C-CH₄总体小于-50‰,为生物成因甲烷（显著区别于热成因页岩气）。涪陵页岩气田焦石坝区块页岩水TDS约为海水两倍,为大气降水成因,显著区别于北美的（蒸发）海水起源。根据页岩水和浅层地下水水文地球化学特征的差异,建立了确定地下水敏感性监测指标的框架,识别出焦石坝区块地下水敏感性指标（7项）。页岩气开发对地下水潜在的污染主要包括气体污染和溶解固体组分污染。对于气体,建立了甲烷含量及同位素组成的端元,并且应用惰性气体进行了两个端元识别;对于溶解固体组分污染,基于浅层地下水背景值数据,可利用本文给出的敏感性指标进行判别。本项研究对于中国页岩气开发的地下水环境保护具有重要的意义,有助于完善页岩气开发过程中地下水环境监测和潜在污染示踪。     

淮南采煤沉陷区积水来源的氢氧稳定同位素证据

淮南是我国东部重要的能源基地,由于长期地下采煤,地表形成大面积的采煤沉陷区并积水,造成严重地质灾害。针对于此,部分学者提出利用采煤沉陷区建立"平原水库"解决周边地区干旱年份农田缺水问题的设想。然而,一方面,由于煤层上覆几百米厚的新生代沉积,采煤塌陷形成的沉陷裂隙是否沟通了不同含水层之间的水力联系,并因此改变了这个地区的地下水系统,成为区域水资源评价需要了解的一个重要科学问题;另一方面,建立"平原水库"需要有稳定的补给水源,采煤形成的沉陷裂隙如果沟通了地下不同深度含水层的水力联系,是否使地下水成为塌陷区除降雨外的重要补给来源,这就成为评价"平原水库"水资源潜力的重要参考依据。氢氧稳定同位素是示踪天然水体水来源的重要手段,笔者在淮南矿区采集了旱季和雨季的浅层地下水、河水、雨水、沉陷区的积水等不同水体的水样23件,分析了其氢氧稳定同位素组成并与深层地下水进行对比。结果表明:雨季和旱季,该地区采煤沉陷区积水的氢氧稳定同位素组成都非常接近大气降水的氢氧稳定同位素组成,而与深层地下水的氢氧稳定同位素组成相差较大,说明采煤沉陷区的积水来源主要是大气降水补给。采煤沉陷区的沉陷裂隙贯穿了整个新生代地层,使地表水发生下渗与在深部与深层地下水发生不同程度的混合,而深层地下水尚不是"平原水库"的稳定补给源。     

水力裂缝穿越非连续面扩展时的断裂过程研究

非连续面发育是非常规油气储层的显著地质特征之一,水力裂缝能否穿越非连续面扩展会关系到压裂的改造效果。为研究水力裂缝穿越非连续面扩展时断裂过程区（fracture process zone,简称FPZ）发育特征,采用自主设计的可视化压裂试验装置对含预制摩擦界面的砂岩平板试件开展水力压裂试验。基于数字图像相关法实时监测了水力裂缝正交穿越界面扩展过程中的位移及应变场特征。试验结果表明,水力裂缝穿越界面扩展之前,断裂过程区已经开始跨越界面发育;裂缝能否穿越界面扩展在FPZ的初始发育阶段已经注定,不受FPZ内应力软化过程影响。基于Renshaw-Pollard准则建立了考虑FPZ边界范围的裂缝穿越非连续面扩展准则,并通过前人及文中试验数据进行了可靠性验证。相比而言,改进准则更准确地考虑了裂缝前端线弹性断裂力学的适用范围。研究发现FPZ长宽比对裂缝穿越界面扩展准则有显著影响,相同条件下,FPZ长宽比越大,裂缝正交穿越界面扩展所需要的摩擦系数下限值越小。     

Pressure transient analysis of multi-fractured horizontal wells in tight oil reservoirs with consideration of stress sensitivity

Multi-fractured horizontal well (MFHW) is an effective technique to develop unconventional reservoirs. Complex fracture network around the well and hydraulic fractures is formed during the fracturing process. Fracture network and hydraulic fractures are the main seepage channel which is sensitivity to the effective stress. However, most of the existing models do not take the effect of stress sensitivity into account. In this study, a new analytical model was established for MFHW in tight oil reservoirs based on the trilinear flow model. Fractal porosity and permeability were employed to describe the heterogeneous distribution of the complex fracture network, and the stress sensitivity of fracture was considered in the model. The Pedrosa substitution and perturbation method were applied to eliminate the nonlinearity of the model. By using the Laplace transformation method, the analytical solutions in Laplace domain were obtained. Then, validations were performed to show that the model is valid. Finally, sensitivity analysis was discussed. The presented model provides a new approach to the estimation of fracturing effect and can be also utilized for recognizing formation properties of tight oil reservoirs.     

Proposal for applying a component-based mixture approach for ecotoxicological assessment of fracturing fluids

Hydraulic fracturing is increasingly being used to produce gas from unconventional resource sites for energy supply. Therefore, concerns about risks of this technology related to human health and the environment have to be addressed. Among the major issues is the potential contamination of surrounding water systems by chemical additives used in fracturing fluids. In this study, the ecotoxicological hazards of fracturing fluids, both, their individual components (chemicals) as well as their mixtures (product) were assessed using a component-based mixture approach. For five exemplary fracturing fluids, 40–90 wt% of the contained substances could unambiguously be defined in their chemical identity. The concentrations used in the applied fluid mixture were considered as (maximum) exposure concentrations. For components with mass fractions between 10 and 74 wt%, the effect concentrations for acute and chronic toxicity of fish, daphnia and algae were retrieved from experimental databases and through predictive modeling. The hazard indices calculated from the ratio of exposure to effect concentration were >1 for all fracturing fluids, using different scenarios. This indicated a hazard from the undiluted fracturing fluids. The assessment framework presented in this study allows for dealing with data gaps and uncertainties in a tiered fashion and in particular accommodates for combined effects resulting from chemical mixtures. It might be employed for ecotoxicological risk assessment of products containing chemical mixtures and optimization of their environmental performance.     

页岩气开采压裂液技术进展

页岩气是一种分布广泛且储量丰富的非常规能源,但由于其储层具有孔隙度小及渗透率低等特性,一直以来页岩气未得到大规模开采。直到近20年,页岩气开采技术才得到飞速发展,这主要得益于水力压裂技术的进步。压裂液是水力压裂的重要组成部分,其性能的优劣直接影响水力压裂施工的成败。通过调研国内外文献,结合国内外压裂液技术的发展历程,分析了页岩气开采中几种常用压裂液的优点、适应性及存在的问题,综述了两类新型的无水压裂技术。结合我国页岩气储层的特殊性及压裂技术发展的现状,提出了适于我国页岩气开采的压裂液技术发展的建议,并特别强调了在页岩气开发初期重视环保的重要性。     

天然裂缝复杂程度对致密储层破裂特征的影响

地层内部总存在孔隙、裂隙、层理、裂缝、断层等结构特征从而体现为地层地质非均质性,并在人工注采过程中发生不同尺度的破裂而体现为跨尺度的裂缝时空延展特征和致裂机理。以致密油为代表的非常规油气藏开采开发在我国油气产能中占比越来越大,并往往利用水力压裂储层改造技术产生人工裂缝以增强油气渗流而实现其规模经济开发;同时,这类低渗透油气藏亦是CO₂压裂、注入和封存的重要对象。文章基于真三轴大型物理模型水力压裂实验和工程尺度下水力压裂微地震监测结果,通过不同尺度裂缝/裂隙活动所致声发射和微地震事件的时空展布研究岩体多尺度裂缝的破裂特征。研究表明,无论是相对稳定的真三轴水力压裂,还是实际地质工程尺度下的水力压裂微,岩石原生裂缝、裂隙分布趋势和方位对裂缝扩展具有主控作用。     

单轴加载条件下页岩层理角度对水力压裂缝扩展规律影响研究

水力压裂作为一种改造储层渗透性、压裂增产的技术,对页岩气开采具有重要意义。为研究射孔附近水力压裂过程中页岩各向异性特征对破裂压力及裂缝扩展的影响规律,开展了单轴试验条件下不同层理角度的页岩水力压裂试验。研究表明:页岩的破裂压力存在明显的各向异性,破裂压力随层理角度的分布曲线呈U型分布,其中0°和90°破裂压力最大,30°最小;页岩的破裂形态主要有两种,一种为沿着最大主应力方向即竖直方向起裂并延伸,另一种模式为裂缝先沿着最大主应力方向起裂并延伸,延伸过程中直接穿过层理面,随后渐渐转向为沿层理面方向扩展;破裂机制则包括拉张破坏和拉张剪切混合破坏。研究结果对于深入了解页岩裂缝起裂和延伸机理、水力压裂施工设计等具有重要的意义。     

A Review on Shale Reservoirs as an Unconventional Play – The History, Technology Revolution, Importance to Oil and Gas Industry, and the Development Future

As a milestone of the entire energy industry, unconventional resources have inevitably swept the world in the last decade, and will certainly dominate the global oil and gas industry in the near future. Eventually, the “unconventional” will become “conventional”. Along with the rapid development, however, some issues have emerged, which are closely related to the viability of unconventional resources development. Under the current circumstances of low crude oil and gas price, coupled with the prominent environmental concerns, the arguments about the development and production of unconventional resources have been recently heated up. This work introduced the full-blown aspects of unconventional resources especially shale reservoirs, by discussing their concepts and definitions, reviewing the shale gas and shale oil development history and necessity, analyzing the shale plays’ geology and petroleum systems with respects to key hydrocarbon accumulation elements and mechanisms, and summarizing the technology resolution. This study also discussed the relevant key issues, including significant estimation uncertainty of technically recoverable resources, the equivocal understanding of complex geology preventing the production and technologies implementation optimization, the difficulties of experiences and technologies global expanding, and the corresponding risks and uncertainties. In addition, based on the latest production and exploration data, the future perspective of the unconventional resources was depicted from global unconventional resources assessments, technology development, and limitations constraining the development.     

页岩储层-超临界CO2-模拟压裂液相互作用实验研究及其环境意义

近年来,页岩气作为一种重要的非常规天然气来源,逐渐成为国内外关注的热点。当前的研究工作总体侧重于页岩气开采技术的改进与环境监测,对页岩气开采过程中水岩相互作用还少有报道。本文采用模拟实验,选取储层页岩样品,在温度90 ℃、压力10 MPa环境下,开展了CO2压裂液页岩相互作用实验研究。主要探讨页岩气开采过程中,由于常规水力压裂液以及后期超临界CO2的增注,造成的储层岩石矿物的变化以及返排液成分的变化。实验结果显示：压裂液能使岩石矿物发生溶蚀,超临界CO2的存在会进一步加剧溶蚀反应的进行,促使孔隙变大并产生更多的微孔隙,为页岩气提供更多的运移通道,有利于油气的运移。但是,存在的环境威胁不容忽视：一方面,岩石中会溶解出大量的Ca、Mg、Si元素和少量Fe、Mn等金属元素,超临界CO2、富有机质页岩以及压裂液中表面活性剂等物质在高温高压酸性环境条件下,容易生成挥发性有机物残留在地层中,极有可能沿着岩石破碎的孔隙、断裂发生迁移或泄露,从而对地下含水层造成污染;另一方面,多种成分进入高矿化度溶液体系经返排回到地表,也会增大处理返排液的难度。本文取得的实验数据和成果有助于理解页岩气储层在开采过程中可能发生的水岩相互作用过程及其潜在的环境风险。     

中国非常规油气沉积学新进展

经过近20年不断探索,中国陆上非常规油气勘探开发取得了重大进展。2019年全国非常规油气产量占油气总产量的23%,2020年非常规油气产量接近7 000万吨油当量,标志着中国进入非常规油气革命发展新阶段。非常规油气沉积学作为非常规油气地质学理论体系的重要组成部分之一,受到越来越多的关注和重视,形成了“陆相深水砂质碎屑流等重力流沉积模式”、“海陆相富有机质页岩沉积模式”、“细粒沉积岩发育微纳米级孔喉系统”、“多地质事件沉积耦合形成非常规油气甜点区（段）”等重要认识。专辑主要是由从事非常规油气相关的沉积学专家对中国近几年非常规油气层系沉积研究新进展的系统性和及时性总结,内容涵盖了鄂尔多斯、四川、松辽、渤海湾、准噶尔等近50个大中型及中小型含油气盆地,地层时代跨度自元古代至新生代,涉及致密油/页岩油、页岩气、致密气、煤层气、油页岩油等非常规油气层系（段）近30个。这些研究成果为我国非常规油气资源勘探开发提供了重要理论基础与技术支撑。提出未来非常规油气沉积学需以非常规油气工业开发的“甜点箱体”和“甜点群”为重点研究方向,指导非常规油气资源高效勘探开发。     

Geomechanical Characteristics of Gas Shales: A Case Study in the North Perth Basin

Gas shales are one type of unconventional reservoirs which have attracted significant attention for gas production in recent years. Gas production from very tight shales requires employment of hydraulic fracturing as a stimulation technique. To design hydraulic fracture operation the mechanical properties of the targeted and surrounding formations should be estimated. Also, the magnitude and orientation of in situ stresses in the field need to be known to estimate the fracture initiation and propagation pressures. This study focuses on gas shale characteristics in the North Perth Basin and uses data corresponding to well Arrowsmith-2 (AS-2) which is the first dedicated shale gas well drilled in Western Australia. A log-based analysis was used to build the rock mechanical model (RMM). The RMM results were used to set up a hydraulic fracturing laboratory experiment. The test was done in the presence of three principal stresses to mimic the real field stress conditions. The test results include the pressure–time curve which was used to estimate the initiation and propagation pressure at that depth. The results were used to draw some practical conclusions related to hydraulic fracturing operation in the field.