Le développement et l'exploitation de réservoirs géothermiques profonds s'accompagnent généralement d'une sismicité induite - un effet secondaire indésirable. Cette recherche est axées sur l'utilisation de simulations numériques pour étudier la propagation des fractures hydrauliques et la réactivation de failles préexistantes lors de la stimulation hydraulique des réservoirs afin de mieux comprendre le comportement du réservoir fracturé et de réduire le risque potentiel de sismicité induite.La sismicité induite est d'abord étudiée du point de vue de l'utilisation de la loi de conservation de l'énergie afin d'expliquer le mécanisme de génération d'ondes élastiques à partir d'une rupture de roche. Ensuite, une approche approximative est proposée pour calculer les accélérations de pointe (PGA) induites par le glissement de faille. Les PGA calculés à la surface du sol servent à évaluer la perception humaine des ondes sismiques et le potentiel de dégradation des structures. / The development and the exploitation of deep geothermal reservoirs are usually accompanied with induced seismicity - an unwanted side effect. This research is focused on using numerical simulations to investigate the propagation of hydraulic fractures and the reactivation of pre-existing faults during the hydraulic stimulation of the reservoirs in an effort to better understand the fractured reservoir behavior and to reduce the potential risk of induced seismicity.The induced seismicity is studied first from the standpoint of using the law of energy conservation in order to explain the mechanism of generating elastic waves from rock failure. Then an approximate approach is proposed to calculate the peak ground accelerations (PGAs) that are induced by the fault slip. The computed PGAs on ground surface are used to assess the human perception of the seismic waves and the damage potential to structures.

Antimony (Sb) is an element of increasing concern as an environmental contaminant. Geothermal systems are a potential source of Sb in the fresh waters of New Zealand’s North Island, but little is known about the element’s behaviour within geothermal fluids, and even less about the eventual fate of geothermally produced Sb. The purpose of this thesis was to determine the factors controlling geothermal Sb behaviour in a range of environments, in order to begin to develop an understanding of the eventual fate of Sb produced from geothermal systems. Factors controlling Sb precipitation at two New Zealand geothermal power stations, were determined using field measurements and geochemical model predictions. Approximately 75 % of the incoming Sb, which ranged from 960 – 1650 μg/kg in the incoming fluids was removed from solution within the Ngawha and Rotokawa power stations. It was found that changes in pH and temperature were the most important factors controlling stibnite (Sb2S3) precipitation. Thermodynamic databases could be used to model this precipitation process, once updated with recent published Sb2S3 solubility data. The mobility of Sb from Sb2S3 precipitates in geothermal features at Wai-O-Tapu and Waimangu, two New Zealand geothermal fields, were investigated. At Wai-O-Tapu, daytime variations in aqueous Sb concentrations from the discharge of Champagne Pool may be due to to changing sulfide-sulfate equilibria coupled with photosynthetic bacterial processes. While daytime concentrations of Sb approached 200 μg/kg, most of the Sb remobilised by such mechanisms appears to be removed by adsorption onto suspended particulate material (SPM) or reprecipitation (as Sb2S3) in an anoxic, low pH lake feature downstream. Concentrations of Sb in the discharge from Alum Lake were below the analytical detection limit (<0.2 μg/kg). At Waimangu, these daytime fluctuations were not observed in the discharge of Frying Pan Lake, and concentrations of Sb were ~13 μg/kg. In the absence of any downstream acidic waters, no precipitation was observed and only minor adsorption onto SPM was observed. Most of the Sb produced from Frying Pan Lake is therefore transported into Lake Rotomahana, the system’s receiving environment. Natural Sb removal processes in receiving (non-geothermal) environments were also assessed. In the Waikato River, Sb concentrations were low (~1 μg/kg), compared to those observed in geothermal environments studied. The most important process was adsorption to SPM, which is enhanced at low (< 5) pH conditions, or in the anoxic base of stratified lakes. In Lake Ohakuri, which was stratified during the summer of 2007, there was also the potential for the removal of Sb as Sb2S3 in the presence of sulfide species that form in the anoxic layer. There was evidence that the adsorption of Sb changes with changing Fe concentrations in suspended particulate material, and therefore Sb adsorption was higher in winter than in summer. The behaviour of Sb was conservative in the Port Waikato estuary at the mouth of the river. Throughout the research, Sb was compared to arsenic (As), a metalloid previously thought to exhibit behaviour similar to Sb in aquatic environments. It was found that while any removal processes shown to affect Sb will also affect As, the inverse did not necessarily apply. Arsenic will adsorb more readily to SPM than Sb and, while there was evidence for bioaccumulation of As by geothermal algae and freshwater macrophytes, there was no such evidence for Sb. Therefore, if geothermally-derived Sb and As did ever significantly contaminate a downstream environment, it should not be assumed that the processes mitigating As contamination will necessarily also apply to Sb.

<p>The resolution power of single Very Low Frequency (VLF) data and multi-frequency Radiomagnetotelluric (RMT) data in delineating conductive structures typical for the sedimentary cover and crystalline basement in Scandinavia is studied with a view to future developments of the technique to increasing the frequency range into the LW radio band. Airborne and ground VLF data are interpreted and correlated with RMT measurements made on the ground to better understand the resolution power of VLF data. To aid in this understanding single and multifrequency VLF and RMT responses for some typical resistivity structures are analyzed. An analytic model is presented for obtaining unique transfer functions from measurements of the electromagnetic components on board an air-plane or on the ground. Examples of 2D inversion of ground and airborne VLF profiles in Sweden are shown to demonstrate the quantitative interpretation of VLF data in terms of both lateral and depth changes of the resistivity in the uppermost crust.</p><p>Geothermal resources are ideal targets for Electromagnetic (EM) methods since they produce strong variations in underground electrical resistivity. Modelling of Magnetotelluric (MT) data in SW Iceland indicates an alteration zone beneath the surface, where there are no obvious geothermal manifestations, in between Hengill and Brennisteinsfjoll geothermal systems. It suggests that a hydrothermal fluid circulation exists at depth. It also proves that the MT method, with its ability to map deep conductive features can play a valuable role in the reconnaissance of deep geothermal systems in active rift regimes such as in Iceland.</p><p>A damped nonlinear least-squares inversion approach is employed to invert Controlled Source Tensor MT (CSTMT) data for azimuthal anisotropy in a 1D layered earth. Impedance and tipper data are inverted jointly. The effects of near-surface inhomogeneities are parameterized in addition to each layer parameter(s). Application of the inversion algorithm to both synthetic and field data shows that the CSTMT method can be used to detect azimuthal anisotropy under realistic conditions with near surface lateral heterogeneities.</p>

Lihir Gold Mine in Papua New Guinea is one of the largest gold mines in the world situated in a seismically sensitive zone. The gold deposit is located in an extinct volcano in close proximity to the sea shore and presents a series of geohazards. Some geohazards are uncommon and include: geothermal outbursts, cavities, water inrush and earthquake/ tsunami. After a major multi-batter (5 benches high) slope failure that occurred on the 1st of October 2009, a team of engineers, lead by the author investigated the incident and made series of recommendations. Arising out of these recommendations, a comprehensive Geohazard Management Plan was formulated by revisiting, revising and putting together all the individual geohazard management plans as a single document. This thesis describes the outcomes of the investigation and presents an overview and systematic approach in formulation of the Geohazard Management Plan, apart from a summary of the gaps that were identified in the existing system, major contributions that were made as well as the expected improvements and constraints in managing these geohazards. / Mining Engineering

Geothermal energy is an important source of clean and renewable energy. This project deals with the study of deep geothermal power plants for the generation of electricity. The design involves the extraction of heat from the Earth and its conversion into electricity. This is performed by allowing fluid deep into the Earth where it gets heated due to the surrounding rock. The fluid gets vaporized and returns to the surface in a heat pipe. Finally, the energy of the fluid is converted into electricity using turbine or organic rankine cycle (ORC). The main feature of the system is the employment of side channels to increase the amount of thermal energy extracted. A finite difference computer model is developed to solve the heat transport equation. The numerical model was employed to evaluate the performance of the design. The major goal was to optimize the output power as a function of parameters such as thermal diffusivity of the rock, depth of the main well, number and length of lateral channels. The sustainable lifetime of the system for a target output power of 2 MW has been calculated for deep geothermal systems with drilling depths of 8000 and 10000 meters, and a financial analysis has been performed to evaluate the economic feasibility of the system for a practical range of geothermal parameters. Results show promising an outlook for deep geothermal systems for practical applications.

In geothermal reservoirs and unconventional gas reservoirs with very low matrix permeability, fractures are the main routes of fluid flow and heat transport, so the fracture permeability change is important. In fact, reservoir development under this circumstance relies on generation and stimulation of a fracture network. This thesis presents numerical simulation of the response of a fractured rock to injection and extraction considering the role of poro-thermoelasticity and joint deformation. Fluid flow and heat transport in the fracture are treated using a finite difference method while the fracture and rock matrix deformation are determined using the displacement discontinuity method (DDM). The fractures response to fluid injection and extraction is affected both by the induced stresses as well as by the initial far-field stress. The latter is accounted for using the non-equilibrium condition, i.e., relaxing the assumption that the rock joints are in equilibrium with the in-situ stress state. The fully coupled DDM simulation has been used to carry out several case studies to model the fracture response under different injection/extractions, in-situ stresses, joint geometries and properties, for both equilibrium and non-equilibrium conditions. The following observations are made: i) Fluid injection increases the pressure causing the joint to open. For non-isothermal injection, cooling increases the fracture aperture drastically by inducing tensile stresses. Higher fracture aperture means higher conductivity. ii) In a single fracture under constant anisotropic in-situ stress (non-equilibrium condition), permanent shear slip is encountered on all fracture segments when the shear strength is overcome by shear stress in response to fluid injection. With cooling operation, the fracture segments in the vicinity of the injection point are opened due to cooling-induced tensile stress and injection pressure, and all the fracture segments experience slip. iii) Fluid pressure in fractures increases in response to compression. The fluid compressibility and joint stiffness play a role. iv) When there are injection and extraction in fractured reservoirs, the cooler fluid flows through the fracture channels from the injection point to extraction well extracting heat from the warmer reservoir matrix. As the matrix cools, the resulting thermal stress increases the fracture apertures and thus increases the fracture conductivity. v) Injection decreases the amount of effective stress due to pressure increase in fracture and matrix near a well. In contrast, extraction increases the amount of effective stress due to pressure drop in fracture and matrix.

Multistage hydraulic fracturing of horizontal wells continues to be a major technological tool in the oil and gas industry. Creation of multiple transverse fractures in shale gas has enabled production from very low permeability. The strategy entails the development of a Stimulated Reservoir Volume (SRV), defined as the volume of reservoir, which is effectively stimulated to increase the well performance. An ideal model for a shale gas SRV is a rectangle of length equal to horizontal well length and width equal to twice the half length of the created hydraulic fractures. This project focused on using the Multistage Transverse Fractured Horizontal Wells (MTFHW) for two novel applications. The first application considers using the SRV of a shale gas well, after the gas production rate drops below the economic limit, for low grade geothermal heat extraction. Cold water is pumped into the fracture network through one horizontal well drilled at the fracture tips. Heat is transferred to the water through the fracture surface. The hot water is then recovered through a second horizontal well drilled at the other end of the fracture network. The basis of this concept is to use the already created stimulated reservoir volume for heat transfer purposes. This technique was applied to the SRV of Haynesville Shale and the results were discussed in light of the economics of the project. For the second application, we considered the use of a similarly created SRV for producing hydrocarbon products from oil shale. Thermal decomposition of kerogen to oil and gas requires heating the oil shale to 700 degrees F. High quality saturated steam generated using a small scale nuclear plant was used for heating the formation to the necessary temperature. Analytical and numerical models are developed for modeling heat transfer in a single fracture unit of MTFHW. These models suggest that successful reuse of Haynesville Shale gas production wells for low grade geothermal heat extraction and the project appears feasible both technically and economically. The economics of the project is greatly aided by eliminating well drilling and completion costs. The models also demonstrate the success of using MTFHW array for heating oil shale using SMR technology.

This thesis aims to predict the probable undiscovered geothermal systems through investigation of spatial relation between geothermal occurrences and its surrounding geological phenomenon in Western Anatolia. In this context, four different public data, which are epicenter map, lineament map, Bouger gravity anomaly and magnetic anomaly maps, are utilized. In order to extract the necessary information for each map layer the raw public data is converted to a synthetic data which are directly used in the analysis. Synthetic data employed during the investigation process include Gutenberg-Richter b-value map, distance to lineaments map and distance to major grabens present in the area. Thus, these three layers including directly used magnetic anomaly maps are combined by means of Boolean logic model and Weights of Evidence method (WofE), which are multicriteria decision methods, in a Geographical Information System (GIS) environment. Boolean logic model is based on the simple logic of Boolean operators, while the WofE model depends on the Bayesian probability. Both of the methods use binary maps for their analysis. Thus, the binary map classification is the key point of the analysis. In this study three different binary map classification techniques are applied and thus three output maps were obtained for each of the method. The all resultant maps are evaluated within and among the methods by means of success indices. The findings reveal that the WofE method is better predictor than the Boolean logic model and that the third binarization approach, which is named as optimization procedure in this study, is the best estimator of binary classes due to obtained success indices. Finally, three output maps of each method are combined and the favorable areas in terms of geothermal potential are produced. According to the final maps the potential sites appear to be Aydin, Denizli and Manisa, of which first two have been greatly explored and exploited since today and thus not surprisingly found as potential in the output maps, while Manisa when compared to first two is nearly virgin.

This study aims the real-time monitoring of gases (CO2, N2, O2, H2, H2S, CH4, He, Ar) discharging from natural pools in the Tekke Hamam geothermal field (Denizli) in addition to the geochemical characterization of the field along with the Kizildere geothermal field, both located at the eastern segment of the B&uuml / y&uuml / k Menderes Graben. The continuous gas monitoring experiment (November 2007-October 2008) conducted in the Tekke Hamam geothermal field has revealed temporal variations in the gas compositions, gas flow rate and pool temperature. Different variation components, such as daily variation profiles and peak/Multi-day signals, are detected in the monitored data, which are mainly correlated with shallow and deep processes involving mainly meteorological factors and seismicity induced variations, respectively. Particularly, the coupled variations in the gas compositions and flow rate seem to correlate with seismicity induced permeability modifications within the subsurface during the absence of significant meteorological factors, such as high rainfall and varying atmospheric pressure. The noble gas characterization of the fields have revealed both high 3He/4He and 4He/20Ne isotopic ratios, suggesting a mantle contribution of about 18% for Kizildere and 34% for Tekke Hamam, whereas the other noble gases (Ar, Ne, Kr, Xe) are of atmospheric nature. The different mantle contributions observed in both fields can suggest a different mantle-He flux variably contaminated by crustal helium. The chemical (cation-trace element-anion) and stable isotopic (&amp / #948 / 18O-&amp / #948 / D) contents of the thermal waters reveal high temperature water-rock interaction accompanied by the effects of deep origined gases (mainly CO2 and H2S) discharging from the fields.

Die Broschüre beschreibt Möglichkeiten der Wärmeversorgung von sächsischen Gartenbaubetrieben mit Geothermie in Abhängigkeit von geologischen Standortfaktoren und Betriebsstrukturen/ Nutzungskonzepten. Ziel des Projektes war, für die Beheizung von Gewächshäusern den Einsatz von Geothermie zur Grundlastversorgung zu untersuchen. Hervorzuheben ist hierbei die Verknüpfung der geologischen/geothermisch und gärtnereitechnischen Komponente. Die Ergebnisse zeigen, welches geothermische System die wirtschaftlichste Variante für Gewächshäuser und deren Nutzung ist. Das Projekt stellt eine Entscheidungsgrundlage für die Nutzung von Geothermie in sächsischen Gartenbaubetrieben dar.