Thomas Wallroth

Approaches to the modelling of hdr reservoirs: A review

Abstract A classification of HDR models by their representation of the reservoir geometry is presented together with the ‘building-block’ sub-processes which embody much of the coupled physics necessary for HDR modelling. A number of current modelling approaches are reviewed. The need for integrating stimulation and circulation models is identified. The need for a good, though not necessarily fully explicit, representation of the reservoir fracture geometry is necessary as a vehicle to express the coupled physics. Limitations on the computational tractability of explicit network models, when strongly coupled physics are added, suggests the types of model that may hold the most promise for future work.

Stress and rock mechanics issues of relevance to HDR/HWR engineered geothermal systems: review of developments during the past 15 years

This paper reports the findings of the Stress and rock mechanics working group of the Academic Review of Hot Dry Rock/Hot Wet Rock (HDR/HWR) Engineered Geothermal Systems convened in Sendai, Japan in 1997. Key developments in the fields of stress and rock mechanics that are relevant to the development of HDR/HWR systems and that have occurred since the last Academic Review in 1982 are described. Rock mechanics is here taken to include basic studies of fluid flow through fractures. Key unresolved issues that are important for HDR/HWR systems are also discussed.

Induced microseismicity and reservoir growth at the Fjällbacka hot dry rocks project, Sweden

Although still relatively uncommon, microseismic detection is accepted as a valuable technique in monitoring large-scale hydraulic injection or extraction from rock at depth. Even through microseismicity may potentially provide information on rock mass behaviour that is unavailable from other remote techniques, the interpretation of microsiesmic activity is often restricted to the analysis of their locations. This restricted usage is primarily due to our current limited understanding of fundamental mechanisms of microseismic energy release and their relations to the mechanics of rock deformation. In this paper, microseismic data will be used to investigate the mechanism of reservoir growth during viscous fluid injection into a jointed granitic rock mass at the Fjallbacka Hot Dry Rock geothermal energy site in Sweden. It will be demonstrated that using simple seismological and rock mechanics interpretation techniques it is possible to obtain a valuable insight into the behaviour of a jointed rock mass during fluid injection. It will be shown through the combined interpretation of microseismic fault plane solutions and in situ stress data, using the simple rock mechanics concept of peak shear strength, that reservoir growth took place through the shear failure of shallowly dipping natural joints due to sufficiently elevated pore fluid pressures.

Current status of seismic and borehole measurements for HDR/HWR development

Seismic and borehole measurements provide significant information about HDR/HWR reservoirs that is useful for reservoir development, reservoir characterization, and performance evaluation. Both techniques have been widely used during all HDR/HWR development projects. Seismic measurements have advanced from making passive surface measurements during hydraulic fracturing to making passive observations from multiple boreholes during all phases of HDR/HWR development, as well as active seismic measurements to probe regions of the reservoir deemed to be of interest. Seismic data provide information about reservoir extent, locations and orientations of significant fractures, and areas of thermal drawdown. Recent advances include the ability to examine structures within the seismically active zone using statistics-based techniques and methods such as seismic tomography. Seismic method is the only means to obtain direct information about reservoir characteristics away from boreholes. Borehole measurements provide high-resolution information about reservoir characteristics in the vicinity of the borehole. The ability to make borehole measurements has grown during the course of HDR/HWR development as high temperature tools have been developed. Temperature logging, televiewer logs, and electrical property measurements have been made and shown to provide useful information about locations of fractures intersecting wellbores, and regions where water leaves and enters injection and production wellbores, respectively.

Hot dry rock research experiments at Fjällbacka, Sweden

A number of in-situ experiments, aimed at investigating geological, hydrogeological and hydromechanical aspects of HDR reservoir development were carried out at the Fjallbacka test site in western Sweden between 1984 and 1995. By means of hydraulic stimulations, a roughly horizontal reservoir that connects two 500 m deep wells was created. An open-loop circulation test performed between the wells gave a flow recovery of about 50%. All major fluid injections have resulted in microseismic activity due to shear failures. A cross-hole seismic survey of the inflated reservoir provided some additional evidence of the distribution of hydraulically active fractures.

Thermo-Hydro-Mechanical (T-H-M) Impacts of Glaciation and Implications for Deep Geologic Disposal of Nuclear Waste

Abstract The thermo-hydro-mechanical impacts of extreme climate change on the lithosphere down to depths at which deep repositories might be sited have been simulated. The effects of glaciation, including ice sheet and permafrost development, have been studied using site-specific data by combining four models. A climate model provides the forcing function, and ice sheet, permafrost, and coupled hydromechanical models are used to assess impacts. It is concluded that glaciation occurs on a timescale and has impacts on a depth scale that require it to be analysed in a safety analysis for deep lithosphere disposal of long-lived radionuclides in areas that have been prone to glaciation in the past. The simulations have provided valuable insight about processes and mechanisms likely to influence the long-term performance of a repository, the geosphere, or both. The key impacts are discussed, and appropriate methods identified.

Characterisation of a fractured reservoir using microearthquakes induced by hydraulic injections

Abstract Microseismic activity is induced by a number of engineering activities which involve fluid injection or extraction in naturally fractured rock masses. Evidence collected during the last 15 years of Hot Dry Rock (HDR) geothermal energy research has clearly indicated the very strong positive correlation between induced microearthquake locations and fluid flow paths within the fractured rock mass. Microseismic monitoring is now used regularly in HDR research as the only effective means of mapping hydraulic stimulations and monitoring hydromechanical processes during operation of the fractured reservoir. Many of the techniques that have been developed through HDR research are directly applicable to the hydrocarbon industry for the monitoring of hydraulic fracturing treatments, waterflood operations and reservoir depletion. This paper provides a brief introduction to the hydromechanical interactions that result in induced microseismicity and presents case studies from three European HDR sites, where the monitoring of microseismic activity has proved vital in targeting production wells, mapping the progress of stimulation and waterfloods, and understanding the hydromechanical behaviour of the fractured system.

Modelling of a european prototype hdr reservoir

Abstract The European Hot Dry Rock Geothermal Energy Project, located in the Rhine graben at Soultz-sous-Forets, Alsace, France, is entering a new phase in its development. Over the next few years the existing HDR system will be developed to form an operational Scientific Prototype HDR System. This paper provides an introduction to the collaborative reservoir modelling studies undertaken as part of the European Programme. In particular the paper addresses the general methodology adopted in the reservoir design process and focuses on one of the preliminary objectives of the study, assessment of the minimum HDR doublet separation required to meet the thermal performance objectives during circulation. Two “preliminary reservoir designs” are adopted as starting points for the study, the first based on exploitation of large scale planar fractures, the second on the development of a modular (multi-cell) system based on 3 cells supporting 51/s production each. Estimates were obtained using models based on both idealised geometry and empirical observations of reservoir circulation at the Camborne School of Mines (CSM) HDR project. The results indicate that a wellbore separation of around 400m would be required for the multi-cell system to achieve the required thermal performance of 10% thermal drawdown, or less, during 10 years circulation at 151/s production. Whereas, the wellbore separation required for the single fracture design would be in excess of 650m.

A Finite-Element Study of Potential Coupled Hydromechanical Effects of Glaciation on a Crystalline Rock Mass

Abstract A number of studies related to past and on-going deep repository performance assessments have identified glaciation/deglaciation as major future events in the next few hundred thousand years capable of causing significant impact on the long term performance of the repository system. Bench Mark Test 3 (BMT3) of the international DECOVALEX III project has been designed to study the coupled hydro-mechanical (H-M) impacts of glaciation and deglaciation on the long-term (up to 100 000 years), post-closure performance of the geosphere in which a hypothetical repository is located. The BMT3 is a generic exercise based on simplified geological, hydrogeological and rock mechanical characteristics of a crystalline rock research area in the Canadian Shield. This paper presents the site-scale coupled hydro-mechanical finite-element modelling studies conducted by the AECL and CTH teams. Interim results suggest that coupled hydro-mechanical effects, transient effects and fracture zone structural geometry are important.

Estimating the spatial variability of specific capacity from a Swedish regional database

A regional database incorporating well information has been used to determine likely values of parameters controlling groundwater flow in south-east Sweden based on an assessment of the specific capacity using geostatistics. Variography is the basis for geostatistical simulation conditioned by measurements of the specific capacity in local wells. In the absence of sufficient data to develop a stochastic model for the hydraulic conductivity at depth, it is suggested that a general model describing its vertical distribution be adopted using profiles established in deep wells, guided by conditionally simulated estimates.