Microseismic impact assessment for shale-gas stimulation (MIA)

水力增产是目前油气行业发展最快的一个领域。水力增产是指在高压下将流体注入岩石中，形成从“致密”（低渗透率）岩石中开采碳氢化合物所需的裂缝。页岩气开采已经改变了美国的经济格局。据估计，到2050年，美国可能实现能源自给自足，而且由于页岩气正在取代煤炭作为燃料，美国的二氧化碳排放量大幅减少。煤炭再次成为英国主要的电力燃料，而欧洲现在正从美国进口煤炭。因此，人们对开采更清洁的页岩气有着相当大的兴趣。然而，这些活动并非没有争议。人们早就知道，地下流体注入有可能引发地震事件。除了地震活动带来的直接危害外，重新激活的断层可能会提供从水库到地面的水力连接，从而导致对含水层污染和地表泄漏的担忧。在这里，我们提出了一个长期的地震监测策略，以监测诱发地震活动，并指导减轻其可能造成的风险。被动地震监测是指探测非常小的地震或微地震活动。许多人为活动可以产生微地震事件（震级在-2.0到-4.0之间的地震，其释放的能量相当于从厨房柜台上掉下一加仑牛奶）。这些事件可以显示裂缝网络的发育和流体的运移位置，为盖层完整性的破坏提供潜在的早期预警信号。此外，这些活动有时会引发更大的事件（例如最近在兰开夏郡发生的M2.3事件）。虽然这种规模的事件不太可能造成重大破坏，但它们引起了当地居民的关注。布里斯托尔大学在过去的10年里一直在这一领域进行研究，并且在技术方面处于领先地位，因为行业规范是简单地定位事件。我们已经开发了一套软件，可以表征震源，监测裂缝网络的发展，并具有半实时跟踪流体在地下运移的潜力。布里斯托尔大学微地震活动项目（BUMPS）项目是由8家石油和天然气公司赞助的联合工业项目（JIP）。进入第三阶段，重点已经从技术开发转移到解决非常规资源区运营商面临的关键问题。拟议的催化剂资金将扩大项目的范围，包括环境影响监测。稀疏地震台网可用于监测页岩气区的诱发地震活动。在目前的“红绿灯”系统下，运营决策将基于震级为0.0 - 0.5的事件。因此，需要对诱发事件的震级进行准确和独立的估计。对于震级如此之低的地震来说，要做到这一点是一项具有挑战性的任务，而目前英国的地震目录只完成了2.0级。因此，如果我们要明确地确定在增产活动期间和之后检测到的任何事件的原因，就需要对这些低震级的地震活动进行基线调查。我们将在英国目前天然气勘探的地区部署一个稀疏的地震仪网络（见支持信）。这种网络提供了对地震震级的独立估计，这是对诱发地震活动的“红绿灯”监测所需要的信息。对于监管目的和公众信心来说，这样一个地震网络是由独立于油气运营商的机构建立和运营的，这一点非常重要。为此，我们将为页岩气增产的环境影响监测制定一套最佳实践指南。

Hydraulic stimulation - where fluids are injected at high pressure into rocks to create the fractures needed to produce hydrocarbons from 'tight' (low permeability) rocks - is currently the fastest growing sector of the oil and gas industry. Shale gas exploitation has been an economic game changer in the USA. It has been estimated that the USA could be energy self-sufficient by 2050 and the country has reduced its CO2 emissions substantially, as shale gas is displacing coal as a fuel. Coal has once again become Britain's dominant fuel for electricity, and Europe is now importing coal from the USA. Hence there is considerable interest in exploiting cleaner shale gas. Nevertheless, these activities are not without controversy. It has long been known that subsurface fluid injection has the potential to trigger seismic events. As well as the direct hazard posed by seismic activity, reactivated faults may provide a hydraulic connection from the reservoir to the surface, leading to concerns about aquifer contamination and surface leakage. Here we propose a long-term seismic strategy to monitor induced seismicity and guide the mitigation of the risks it may pose.Passive seismic monitoring refers to detecting very small earthquakes or microseismicity. Many anthropogenic activities can create microseismic events (earthquakes with magnitudes between -2.0 to -4.0, which release the energy equivalent of dropping a gallon of milk from a kitchen counter). Such events hold the telltale signs of fracture network development and where fluids are migrating, providing potential early warning signs of any breach in caprock integrity. Furthermore, these activities can sometimes trigger much larger events (such as the recent M2.3 event in Lancashire). Although events of such magnitude are unlikely to cause significant damage, they represent a source of concern for local populations. Bristol University has been doing research in this area for the past 10 years and is ahead of the curve in technology, as the industry norm is to simply locate events. We have developed a suite of software that characterises the seismic source, monitors fracture network development, and holds the potential to track fluid migration through the subsurface in semi-real-time. The Bristol University Microseismicity Projects (BUMPS) project is a joint-industry-project (JIP) sponsored by 8 oil and gas companies. Entering into its third phase, the focus has shifted from technique development to answering key questions facing operators in unconventional resource plays. The proposed catalyst funding will broaden the scope of the project to include environmental impact monitoring. Sparse seismic networks can be used to monitor induced seismicity in shale gas regions. Under the current 'traffic-light' system, operational decisions will be taken based on events with magnitudes of 0.0 - 0.5. Accurate and independent estimates of induced event magnitudes are therefore required. To do so robustly is a challenging task for events of such low magnitudes and current UK seismic catalogues are only complete to magnitude 2.0. Therefore, baseline surveys of seismicity that are complete down to these low magnitudes will be needed if we are to unambiguously determine the cause of any events detected during and following stimulation activities. We will deploy a sparse network of seismometers in a region of current gas exploration in the UK (see letter of support). Such networks provide an independent estimate of event magnitude, information that is needed for 'traffic-light' monitoring of induced seismicity. It is important for regulatory purposes and public confidence that such a seismic network is established and operated by institutions independent from the oil and gas operators. To this end, we will develop a set of best practice guidelines for environmental impact monitoring of shale gas stimulation.