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1.
Hydraulic stimulation and geothermal reservoir operation may compromise the rock mechanical stability and trigger microseismic events. The mechanisms leading to this induced seismicity are still not completely understood. It is clear that injection causes an overpressure that reduces the effective stress, bringing the system closer to failure conditions. However, rock instability may not result only from hydraulic effects, but also from thermal effects. In fact, hydro-mechanical (i.e., isothermal) models often fail to reproduce field observations because the injection of cold water into a hot reservoir induces thermal stresses due to rock contraction. Thus, rock instability is likely to result from the superposition of hydraulic and thermal effects. Here, we perform coupled thermo-hydro-mechanical and hydro-mechanical simulations to investigate the effects of cold water injection in a fracture zone-intact rock system. Results show that thermal effects induce a significant perturbation on the stress in the intact rock affected by the temperature drop. This perturbation is likely to trigger induced seismicity in the surroundings of critically oriented fractures near the injection well. Hydro-mechanical simulations show that the behavior depends on the orientation of the faults and on the initial stress tensor. In the direction of the fractures, where the strains are more constrained, total stress increases with increasing pressure; thus, deviatoric stress increases or decreases depending on the initial stress state. The comparison between hydraulic and thermal effects shows that, when the largest confining stress acts perpendicular to the fractures, thermoelastic effects dominate and could trigger induced seismicity. 相似文献
2.
本文从应力场角度分析了1976年唐山地震发生的可能力源。震前唐山地区除受有太平洋板块和印度板块的作用外,主要受有附近的热力源的作用。以渤中为中心的下辽河—渤中—黄骅上地幔隆起的高温区和沧东断裂水平温度梯度带的存在,使唐山地区位于热应力集中区。本文根据已有的观测结果,提出了将温度变化区简化为圆形、狭长矩形和椭圆形等三个力学模式,分别求得它们的解析解,并计算了它们的热应力场。结果表明:在增温区以内,正应力都是压应力,剪应力很小。在增温区以外,法向应力为压应力,周向应力为张应力,水平剪应力较大,但各个应力分量随着距离增加而衰减,只在增温区的边缘地带达到最大值,其数值有几百巴的数量级。唐山恰好位于该增温区的边缘地带,表明热力源是促使唐山地震发生的重要原因。 除唐山地震外,华北地区近年发生的几个大地震如邢台、河间、渤海、海城地震,以及1983年发生的菏泽地震,均位于热应力的高值地带,这些地震的孕育和发生显然也与热力源的作用有关。 相似文献
3.
—Forced torsional oscillation techniques have been used to explore the seismic-frequency shear mode viscoelasticity of specimens of two crustal rocks (Cape Sorell quartzite and Delegate aplite), cycled between room temperature and 700°C under conditions of moderate confining pressure. The anisotropy and intergranular inhomogeneity of thermal expansivity in these materials give rise to large deviatoric stresses, resulting in thermal cracking at temperatures above a pressure-dependent threshold temperature, associated with the onset of very pronounced temperature sensitivity of the shear modulus, in general accord with the predictions of fracture mechanics models. For Delegate aplite in particular, the shear modulus behaves reproducibly during multiple thermal cycles at different confining pressures, consistent with the notion that the thermal cracks are of low aspect ratio (minimum/maximum dimension), and are therefore readily closed by the prevailing confining pressure once the thermal stresses are removed. Marked frequency-dependent dissipation of shear strain energy is observed on heating each rock to temperatures ≥ 500°C, although the attenuation varies significantly with prior thermal history, probably as a result of progressive dehydration and relaxation of deviatoric stresses. Temperature and pressure dependent crack densities for Delegate aplite have been estimated by comparison of the observed shear moduli with those expected for a crack-free aggregate. In parallel with the forced oscillation tests, measurements have been made of the rate at which (argon) pore pressure equilibrium is re-established following a perturbation. Combination of these results, which provide a proxy for permeability, with the inferred crack densities indicates that the variation of permeability with crack density is well described by a percolation model with a threshold crack density of ～0.2. 相似文献
4.
— We addressed effects of in situ stress on the formation of flow pathways in fractured rocks in geothermal reservoirs, especially for HDR projects. Here we focused on fractures which are critically-stressed, causing shear slip in a current stress field. The sliding is likely to break sealing in the fractures and, as a result, to increase their permeability. Such a mechanism is possibly significant under high-temperature conditions at geothermal fields because of temperature enhancement on chemical reactions for the sealing. We present a procedure to estimate the orientation of the critically-stressed fractures relative to axes of in situ principal stress with the aid of the Mohr diagram. The procedure allows us to evaluate intuitively how the orientation changes with factors such as magnitude of in situ principal stresses and pore pressure. We applied the procedure to estimate possible orientations of the critically-stressed fractures in major HDR test sites. Results show that overall alignments of microseismicity during hydraulic stimulation are within predicted ranges for possible orientations of the critically-stressed fractures. Furthermore, it was found that if the state of in situ stress is not favorable to cause sliding of natural fractures, it tends to lead a high wellhead pressure at hydraulic stimulation and a high recovery rate at circulation. On the other hand, if the state of in situ stress is favorable for sliding, it tends vice versa to lead a low wellhead pressure at hydraulic stimulation and a low recovery rate at circulation. 相似文献
5.
Pore pressure changes in a geothermal reservoir, as a result of injection and/or production of water, result in changes of stress acting on the reservoir rock and, consequently, changes in the mechanical and transport properties of the rock. Bulk modulus and permeability were measured at different pressures and temperatures. An outcropping equivalent of Rotliegend reservoir rock in the North German Basin (Flechtinger sandstone) was used to perform hydrostatic tests and steady state fluid flow tests. Permeability measurements were conducted while cycling confining pressure; the dependence of permeability on stress was determined at a constant downstream pressure of 1 MPa. Also, temperature was increased stepwise from 30 to 140 °C and crack porosity was calculated at different temperatures. Although changes in the volumes of cracks are not significant, the cracks control fluid flow pathways and, consequently, the permeability of the rock. A new model was derived which relates microstructure of porosity, the stress–strain curve, and permeability. Porosity change was described by the first derivative of the stress–strain curve. Permeability evolution was ascribed to crack closure and was related to the second derivative of the stress–strain curve. The porosity and permeability of Flechtinger sandstone were reduced by increasing the effective pressure and decreased after each pressure cycle. 相似文献
6.
7.
Paul Segall 《Pure and Applied Geophysics》1992,139(3-4):535-560
Earthquakes can be induced by fluid extraction, as well as by fluid injection.Segall (1989) proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction. Here, I present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries. The results ofGeertsma (1973) for a thin disk reservoir with uniform pressure drop are recovered as a special case. Predicted surface subsidence agrees very well with measured leveling changes over the deep Lacq gas field in southwestern France. The induced stresses are finite if the reservoir pressure changes are continuous. Computed stress changes are on the order of several bars, suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production. 相似文献
8.
Fluid Pressure Variation in a Sedimentary Geothermal Reservoir in the North German Basin: Case Study Groß Schönebeck 总被引：1，自引：0，他引：1
Ernst Huenges Ute Trautwein Björn Legarth Günter Zimmermann 《Pure and Applied Geophysics》2006,163(10):2141-2152
The Rotliegend of the North German basin is the target reservoir of an interdisciplinary investigation program to develop a technology for the generation of geothermal electricity from low-enthalpy reservoirs. An in situ downhole laboratory was established in the 4.3 km deep well Groβ Schönebeck with the purpose of developing appropriate stimulation methods to increase permeability of deep aquifers by enhancing or creating secondary porosity and flow paths. The goal is to learn how to enhance the inflow performance of a well from a variety of rock types in low permeable geothermal reservoirs. A change in effective stress due to fluid pressure was observed to be one of the key parameters influencing flow properties both downhole and in laboratory experiments on reservoir rocks. Fluid pressure variation was induced using proppant-gel-frac techniques as well as waterfrac techniques in several different new experiments in the borehole. A pressure step test indicates generation and extension of multiple fractures with closure pressures between 6 and 8.4 MPa above formation pressure. In a 24-hour production test 859 m3 water was produced from depth indicating an increase of productivity in comparison with former tests. Different depth sections and transmissibility values were observed in the borehole depending on fluid pressure. In addition, laboratory experiments were performed on core samples from the sandstone reservoir under uniaxial strain conditions, i.e., no lateral strain, constant axial load. The experiments on the borehole and the laboratory scale were realized on the same rock types under comparable stress conditions with similar pore pressure variations. Nevertheless, stress dependences of permeability are not easy to compare from scale to scale. Laboratory investigations reflect permeability variations due to microstructural heterogeneities and the behavior in the borehole is dominated by the generation of connections to large-scale structural patterns. 相似文献
9.
Colin M. Sayers 《Geophysical Prospecting》2023,71(1):114-124
Geothermal resources have potential for providing cost-effective and sustainable energy. Monitoring of production-induced changes in geothermal reservoirs using seismic waves requires understanding of the elastic properties of the rock and how they change due to injection of fluids and opening and closing of natural and hydraulic fractures. P- and S-wave velocities measured in a granitic geothermal reservoir using sonic logging are systematically lower than those predicted using the composition of the rock. Cracks may occur in granitic rocks from tectonic stresses and from the thermal expansion mismatch between differently oriented anisotropic crystals. An isotropic orientation distribution of microcracks causes a significant reduction in both the P- and S-velocities, consistent with the observed sonic P- and S-velocities. Vertical fractures cause a difference in the velocity of vertically propagating shear waves polarized parallel and perpendicular to the fractures. An assumption that the lower measured velocities are caused by the presence of vertical fractures is inconsistent with the sonic data. This is because vertical fractures cause a decrease in slow S-wave velocity that greatly exceeds the decrease in P-wave velocity, in contrast to the observed data. The growth of vertical fractures in the geothermal reservoir may be monitored using the difference in velocity of the fast and slow shear waves, while the change in P-velocity in a crossplot of measured P- and slow S-velocities is useful for estimating the ratio of the normal-to-shear compliance of the fractures. 相似文献
10.
采集夏县中心地震台温泉水及周边水点样品进行水化学组分特征分析。利用矩形图、Na-K-Mg三角图等方法,初步分析温泉水的水质类型、水-岩平衡状态、热储温度以及循环深度等,并结合氢氧同位素组成特征,初步分析温泉水补给来源。研究表明,夏县中心地震台温泉水化类型属于Na-Cl·SO4型,水-岩反应属于部分成熟水,热储温度为148.8℃,循环深度为3.91 km,补给源主要为大气降水,温泉为断裂型温泉。以上结果可为该台流体异常分析提供基础研究资料,为夏县地区水化研究奠定基础。 相似文献
11.
Porosity, permeability, and fluid flow in the Yellowstone geothermal system, Wyoming 总被引：3，自引：0，他引：3
Patrick F. Dobson Timothy J. Kneafsey Jeffrey Hulen Ardyth Simmons 《Journal of Volcanology and Geothermal Research》2003,123(3-4):313-324
Cores from two of 13 U.S. Geological Survey research holes at Yellowstone National Park (Y-5 and Y-8) were evaluated to characterize lithology, texture, alteration, and the degree and nature of fracturing and veining. Porosity and matrix permeability measurements and petrographic examination of the cores were used to evaluate the effects of lithology and hydrothermal alteration on porosity and permeability. The intervals studied in these two core holes span the conductive zone and the upper portion of the convective geothermal reservoir. Variations in porosity and matrix permeability observed in the Y-5 and Y-8 cores are primarily controlled by lithology. Y-8 intersects three distinct lithologies: volcaniclastic sandstone, perlitic rhyolitic lava, and non-welded pumiceous ash-flow tuff. The sandstone typically has high permeability and porosity, and the tuff has very high porosity and moderate permeability, while the perlitic lava has very low porosity and is essentially impermeable. Hydrothermal self-sealing appears to have generated localized permeability barriers within the reservoir. Changes in pressure and temperature in Y-8 correspond to a zone of silicification in the volcaniclastic sandstone just above the contact with the perlitic rhyolite; this silicification has significantly reduced porosity and permeability. In rocks with inherently low matrix permeability (such as densely welded ash-flow tuff), fluid flow is controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the 0.6-Ma Lava Creek ash-flow tuff. In this core, the degree of welding appears to be responsible for most of the variations in porosity, matrix permeability, and the frequency of fractures and veins. Fractures are most abundant within the more densely welded sections of the tuff. However, the most prominent zones of fracturing and mineralization are associated with hydrothermal breccias within densely welded portions of the tuff. These breccia zones represent transient conduits of high fluid flow that formed by the explosive release of overpressure in the underlying geothermal reservoir and that were subsequently sealed by supersaturated geothermal fluids. In addition to this fracture sealing, hydrothermal alteration at Yellowstone appears generally to reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred. 相似文献
12.
Induced Microearthquake Patterns in Hydrocarbon and Geothermal Reservoirs: Six Case Studies 总被引：2，自引：0，他引：2
W.S Phillips J.T. Rutledge L.S. House M.C. Fehler 《Pure and Applied Geophysics》2002,159(1-3):345-369
—?The injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs. By deploying sensors downhole, data sets have been collected that consist of a few hundred to well over 10,000 induced events. We find that most induced events cluster into well-defined geometrical patterns. In many cases, we must apply high-precision, relative location techniques to observe these patterns. At three sedimentary sites, thin horizontal strands of activity are commonly found within the location patterns. We believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress. At a massive carbonate and two crystalline sites, combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns. In addition, the fine-scale seismicity patterns often evolve systematically with time. At sedimentary sites, migration of seismicity toward the injection point has been observed and may result from slip-induced stress along fractures that initially have little resolved shear. In such cases, triggering events may be critical to generate high levels of seismic activity. At one crystalline site, the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures. We hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs. 相似文献
13.
A geochemical study of thermal and cold springs, stream waters and gas emissions has been carried out in the Mt. Amiata geothermal region.The cold springs and stream waters do not seem to have received significant contribution from hot deep fluids. On the contrary, the thermal springs present complex and not clearly quantifiable interactions with the hot fluids of the main geothermal reservoir.The liquid-dominated systems in the Mt. Amiata area, like most of the high-enthalpy geothermal fields in the world, are characterized by saline, NaCl fluids. The nature of the reservoir rock (carbonatic and anhydritic), and its widespread occurrence in central Italy, favor a regional circulation of “Ca-sulfate” thermal waters, which discharge from its outcrop areas. Waters of this kind, which have been considered recharge waters of the known geothermal fields, dilute, disperse and react with the deep geothermal fluids in the Mt. Amiata area, preventing the use of the main chemical geothermometers for prospecting purposes. The temperatures obtained from the chemical geothermometers vary widely and are generally cooler than temperatures measured in producing wells.Other thermal anomalies in central Italy, apart from those already known, might be masked by the above-mentioned circulation. A better knowledge of deep-fluid chemistry could contribute to the calibration of specific geothermometers for waters from reservoirs in carbonatic rocks. 相似文献
14.
Enhanced geothermal systems (EGS) are an attractive source of low-carbon electricity and heating. Consequently, a number of tests of this technology have been made during the past couple of decades, and various projects are being planned or under development. EGS work by the injection of fluid into deep boreholes to increase permeability and hence allow the circulation and heating of fluid through a geothermal reservoir. Permeability is irreversibly increased by the generation of microseismicity through the shearing of pre-existing fractures or fault segments. One aspect of this technology that can cause public concern and consequently could limit the widespread adoption of EGS within populated areas is the risk of generating earthquakes that are sufficiently large to be felt (or even to cause building damage). Therefore, there is a need to balance stimulation and exploitation of the geothermal reservoir through fluid injection against the pressing requirement to keep the earthquake risk below an acceptable level. Current strategies to balance these potentially conflicting requirements rely on a traffic light system based on the observed magnitudes of the triggered earthquakes and the measured peak ground velocities from these events. In this article we propose an alternative system that uses the actual risk of generating felt (or damaging) earthquake ground motions at a site of interest (e.g. a nearby town) to control the injection rate. This risk is computed by combining characteristics of the observed seismicity of the previous 6 h with a (potentially site-specific) ground motion prediction equation to obtain a real-time seismic hazard curve; this is then convolved with the derivative of a (potentially site-specific) fragility curve. Based on the relation between computed risk and pre-defined acceptable risk thresholds, the injection is increased if the risk is below the amber level, decreased if the risk is between the amber and red levels, or stopped completely if the risk is above the red level. Based on simulations using a recently developed model of induced seismicity in geothermal systems, which is checked here using observations from the Basel EGS, in this article it is shown that the proposed procedure could lead to both acceptable levels of risk and increased permeability. 相似文献
15.
—?The reservoir structure of the Soultz HDR field has been investigated by examining induced microearthquake multiplets. Microseismic events with similar waveforms have been selected from microseismic data obtained during a 1993 hydraulic fracturing experiment. Precise relative arrival times and source locations have been determined by cross-spectrum analysis. The cross-spectrum analysis decreased the residual from 0.75?ms to 0.1?ms. The estimated orientations of the multiplet planes are consistent with fracture orientations detected in Soultz boreholes. A comparison between the stress field and the orientation of structural planes suggests that the structural planes were under a critical condition of frictional slip. 相似文献
16.
增强型(或工程)地热系统是指从地壳深部低孔渗的干热岩体中,采用人工工程方法形成储层,经济地采出具有相当数量的热能.这种早期被称之为干热岩的技术首次在美国新墨西哥州芬登山得到示范.本文以芬登山增强型地热系统为例,通过综述其发展历程,论述了增强型地热系统钻井与储层激发、工质流动与储层测试、储层性能评价等关键问题,总结了从芬登山增强型地热系统研究中获得的认识和启示.芬登山地热项目发展的经验表明,增强型地热储层是通过重新打开本来存在、但被密封的、多重连通的节理组,或通过水力压裂创造新的流动通道和换热面,利用复杂裂隙网络中的流循环,从储层系统中开采热能.芬登山实践表明,在稳态条件下增强型地热系统的持续运行是可能的,芬登山地热项目对世界和我国深层地热发展具有重要指导意义. 相似文献
17.
利用FLAC,模拟了双轴压缩岩样的破坏过程及剪应力异常.在峰值强度之前及之后,岩石的本构模型分别取为线弹性及莫尔库仑剪破坏与拉破坏复合的应变软化模型.本文仅分析了3个具有较大应力降的周期的剪应力异常及塑性区分布.在第1及第2周期,剪破坏分别发生在试样的两侧及背景空区内部,前兆明显,地震属于断错型.在第3周期,背景空区之外的新剪切破裂带引起断错型地震;空区内部的老破裂带引起走滑型地震,未观测到前兆.在塑性区边缘,剪应力梯度较高.破裂带位置剪应力值反而较低.尽管在应力-应变曲线的软化阶段之前,不同单元的剪应力表现出千姿百态的复杂形态,但尚有规律可循.若几个单元都位于某条破裂带上,则剪应力的变化可能是同步的,或演变规律类似.鉴于原地复发地震类型可能不同,前兆将有差别. 相似文献
18.
During geothermal power production using a borehole doublet consisting of a production and injection well, the reservoir conditions such as permeability k, porosity φ and Skempton coefficient B at the geothermal research site Gross Schoenebeck/Germany will change. Besides a temperature decrease at the injection well and a change of the chemical equilibrium, also the pore pressure p p will vary in a range of approximately 44 MPa ± 10 MPa in our reservoir at ?3850 to ?4258 m depth. This leads to a poroelastic response of the reservoir rocks depending on effective pressure p eff (difference between mean stress and pore pressure), resulting in a change in permeability k, porosity φ and the poroelastic parameter Skempton coefficient B. Hence, we investigated the effective pressure dependency of Flechtinger sandstone, an outcropping equivalent of the reservoir rock via laboratory experiments. The permeability decreased by 21% at an effective pressure range from 3 to 30 MPa, the porosity decreased by 11% (p eff = 6 to 65 MPa) and the Skempton coefficient decreased by 24% (p eff = 4 to 25 MPa). We will show which mechanisms lead to the change of the mentioned hydraulic and poroelastic parameters and the influence of these changes on the productivity of the reservoir. The most significant changes occur at low effective pressures until 15 to 20 MPa. For our in situ reservoir conditions p eff = 43 MPa a change of 10 MPa effective pressure will result in a change in matrix permeability of less than 4% and in matrix porosity of less than 2%. Besides natural fracture systems, fault zones and induced hydraulic fractures, the rock matrix its only one part of geothermal systems. All components can be influenced by pressure, temperature and chemical reactions. Therefore, the determined small poroelastic response of rock matrix does not significantly influence the sustainability of the geothermal reservoir. 相似文献
19.
大姚地热动态特征分析 总被引：3，自引：1，他引：3
通过大姚高精度水温观测井水地质、井孔条件的分析，震例观测资料、实验观测数据的处理，以及井水水质分析结果，表明大姚水温周期性变化的突出位置是井深75m左右，根本原因是该地层石膏脉稳定释热，含水层热水与井内冷水混合过程中，在通道内形成钙盐类沉淀物（如CaSO4CaCO3），堵塞对流通道，含水层内外压差突破被阻塞通道，热水反复侵入井内，便形成水温周期性变化。地震的孕育、发生和调整过程，由于应力场作用，产生附加地热场，水温基值发生变化，周期畸变或消失，这种附加地热动态是可以恢复的。所以认为大姚地热动态是井区特殊地层地热信息的反映。 相似文献
20.
秦岭北缘断裂带温泉水渗流特征与地震活动水平研究 总被引：1，自引：0，他引：1
前人对秦岭北缘断裂带出露温泉水的氢氧同位素、He同位素和SiO2含量进行了计算,得到了温泉水的热储温度和循环深度。在此基础上,运用温度场作用下的热水垂向运移数学模型演算了循环深度和断层带渗透性之间的关系。根据1970～2008年的区域台网资料,对断裂带进行了分段地震活动多参数组合方法分析,并结合历史中强地震(公元前781年～公元2008年)的空间分布特点,分析总结了断层带不同区段渗透特征和地震活动的关系。最后对秦岭北缘断裂带的渗透性成因进行了解释。 相似文献