Methodology for Designing a Sustainability Assessment Framework for Geothermal Energy Developments

ABSTRACT Geothermal projects have significant socio-economic and environmental impacts, both positive and negative. In order for energy developments to contribute to sustainable developments they must result in positive impacts in all dimensions. Sustainability assessments are valuable tools for policy-or decision-makers for making informed conclusions regarding policy effectiveness and progress toward sustainable development. Currently available assessment frameworks are not suited to assessing geothermal projects, thus a new, specialized framework is required. The methodology for developing a geothermal sustainability assessment framework is described in this paper. INTRODUCTION 1.1 Geothermal Energy and Sustainable Development Energy usage worldwide is increasing. Global energy demand is predicted to grow by more than one-third by 2035, with China, India and the Middle East accounting for 60% of the increase (International Energy Agency, 2012). The use of alternatives such as geothermal energy is set to increase, since the world has only a finite supply of fossil fuels. As well as this, in order to combat climate change and fulfill international agreements, low carbon energy sources such as geothermal energy are now being tapped on a larger scale. In 2008, geothermal energy represented around 0.1% of the global primary energy supply, but estimates predict that it could fulfill around 3% of global electricity demand, as well as 5% of global heating demand by 2050 (Intergovernmental Panel on Climate Change, 2012)

Reservoir engineering studies of small low-temperature hydrothermal systems in Iceland

Geothermal energy provides more than one third of the energy consumed in Iceland. Its primary use is for space heating and most of the 28 public hitaveitur (district heating services) in Iceland utilize small low-temperature geothermal fields that have a natural heat output of only a few 100 kW{sub t} to a few MW{sub t}. All of these small reservoirs respond to production by declining pressure and some by declining temperature. During the 1980's the emphasis in geothermal research in Iceland shifted from exploration to reservoir engineering. The reservoir engineering work carried out concurrent with the exploitation of these small fields includes: testing of individual wells, field wide tests, monitoring the response of reservoirs to long-term production and simple modeling

Hydrology and thermomechanics of liquid-dominated hydrothermal systems in Iceland

Low-temperature hydrothermal activity in Iceland is apparently mostly controlled by dikes and fractures. Conventional methods of production data analysis are not readily applicable in cases of heterogeneous/anisotropic fracture dominated hydrothermal systems. Moreover, the dikes and fractures may control the heat uptake mechanism of low-temperature activity. The free-surface response functions of analytical reservoir models are presented and methods for analyzing production data on the basis of such models are developed. Based on a homogeneous and isotropic half-space model apparent permeability estimates of 0.7 millidarcy are obtained for two low-temperature systems in Tertiary strata in N-Iceland whereas estimates of 5-20 millidarcy are obtained for two systems in Quaternary strata in SW-Iceland. A vertical two-dimensional flow model is, however, more consistent with the apparent linear dike/fracture control of many hydrothermal systems and results in higher permeability estimates. Methods of simulating long term production data by simple lumped capacitor/conductor ladders based on only production/drawdown data are developed and the responses of analytical as well as real systems are shown to be easily simulated by such simple systems. The parameters of simulation ladders also provide information on global hydrological characteristics of hydrothermal systems. A possible dike/fault controlled source mechanism of low-temperature activity in Iceland is considered. This process involves the downward migration of open sections of unwelded quasi-vertical fractures resulting from cooling and contraction of the adjacent rock, in conjunction with vertical heat transfer in the fracture. The rate of downward migration is estimated and found to depend very strongly on the magnitude of the horizontal regional stress. Stress conditions may therefore determine whether a low-temperature system can evolve at a given location as well as determine the intensity of hydrothermal activity

Tidal tilt observations in the Krafla geothermal area in North Iceland

A brief tilt and strain survey was conducted in the Krafla-Namafjall area in the North of Iceland during August of 1979 in order to study the feasibility of applying solid earth tidal observations in the exploration of volcanic geothermal systems. The rationale of the survey was based on the expectation that various types of geological structures such as rift zones and magma chambers can lead to observable distortions of the local solid earth tidal amplitude field. The field procedure consisted in measuring the local tidal amplitudes and comparing them with normal theoretical amplitudes at the same location. The Krafla volcanic complex is a central volcano traversed by a N-S trending fissure swarm, that has been tectonically and volcanically active since December 1975. Accompanying this activity have been periodic inflations of the Krafla caldera, presumably caused by a flow of magma into a local magma chamber and resulting in long term tilts of the order 0.5 μrad/day that have been observed at two sites south and southeast of the caldera. In computing theoretical amplitudes the effects of the ocean tides need to be estimated. In the case of north Iceland they are found to be of the same magnitude as the solid earth tides. The amplitudes of the M₂ ocean loading tilt at Krafla are estimated to be 0.066 μrad and 0.032 μrad for the NS and EW components respectively. The most noteworthy result was obtained at a site in the Namafjall geothermal area inside the active Krafla fissure swarm. The ratios of the observed to the theoretical M₂ tidal tilt amplitudes at this site as estimated by a least squares spectral analysis method are found to be 0.9 ± 0.3 and 3.2 ± 1.5 for the NS and EW components respectively. On the basis of some simple order-of-magnitude estimates we can exclude one of the numerous nearby fractures as a possible cause for the EW tilt anomaly and conclude that it is most likely to be generated by a large body of magma below the Krafla fissure swarm. Due to thermoelastic noise other tilt data obtained during this survey turned out to be less reliable. However, our work at a site east of the fissure swarm and southeast of the caldera indicated a possible anomaly. The strain data are highly contaminated by thermal noise and could not be successfully analyzed. These results tend to confirm that tidal tilt observations can be of use in explorations of volcanic and geothermal systems. Our work indicates that a few improvements of the simple field techniques adapted may enhance the quality of data. These include (1) increasing the instrument resolution, (2) selecting sites with surface layers that are incapable of transmitting thermal stresses and (3) obtaining more extensive higher quality temperature recordings, that should enable the thermal noise to be largely removed

Detailed three-dimensional modeling of the Botn hydrothermal system in N-Iceland

A detailed three-dimensional numerical model has been developed for the low-temperature hydrothermal system at Botn in Central North Iceland. It is based on a conceptual reservoir model which has evolved during two decades of geothermal research in the area and on the 10 year production history of the system. The model consists of (1) A powerful recharge system at depth, (2) a shallow production reservoir and (3) a cold ground-water system at the surface. About 10 million tons of hot water have been extracted from the production reservoir since late 1981. The presence of the powerful recharge system results in a very slow long-term pressure decline. Flow of water in the production reservoir appears to be controlled by a highly permeable, vertical fracture-zone confined by low-permeability rocks. Cold ground-water flows down into the fracture-zone during production causing some cooling of the extracted water

Sustainable and Environmentally-Sound Development Strategies Addressed Through International Collaboration

ABSTRACT Collaborative research into sustainable and environmentally-sound development strategies is carried out under the auspices of the IEA Geothermal Implementing Agreement (IEA-Geothermal) (www.iea-gia.org), through the Tasks of its Annex 1, Environmental Impacts of Geothermal Development. Cooperation amongst member countries facilitates knowledge sharing and exchanges of geothermal operational and modeling experience. This is vital in order to learn from past successes and mistakes. By growing investor confidence in the long-term sustainable development of geothermal resources, and avoiding or mitigating adverse local environmental effects, we can materially advance global efforts to mitigate the much more serious adverse effects of climate change resulting from fossil-fuel carbon emissions. Analysis of long-term historical performance of developed geothermal reservoirs, together with simulations of their likely future performance using reservoir models, leads to important conclusions regarding optimizing sustainable strategies for future development. A key factor is the choice of initial and subsequent staged capacity installments; these are justified by increasingly more-sophisticated reservoir simulation models. The objective is to avoid excessive pressure or temperature draw-down, but to allow for sufficient reservoir response to provide good history matching. A second key factor is the ability to adapt reinjection strategies (location, depth, fluid chemistry and temperature) as new information from monitoring of production/injection effects becomes available. The third key factor is the early recognition of the dynamic response of a resource to its utilization, with good information collected on the source location, chemistry and temperature of induced recharge fluids. Improved tracer technology helps characterize parameters such as permeability, diffusion and fluid storage between injection and production sectors. Better calibration of reservoir models improves characterization of the permeability structure and boundary recharge parameters that dictate long-term reservoir behavior. Over very long timescales (>100 years) reservoirs are likely to trend toward a pseudo steadystate wherein induced mass and heat recharge almost balance the net mass and heat that can be extracted. Other options for sustainable development, however, might involve cyclic or intermittent energy extraction ('heat grazing') wherein parts of a large heat resource may be developed and recovered in rotation. Strategies must also take into account potentially adverse local environmental effects. An alternative long-term strategy is to use the acquired knowledge and simulated behavior from early production stages to plan deeper drilling, by targeting the primary up-flows. Over time, the shallow parts of a resource are 'retired' and bore-holes tap directly into higher enthalpy and more productive sectors of the resource. Challenges associated with this strategy include the need to reduce the cost of deep drilling, and to develop technologies to deal with super-critical and potentially corrosive reservoir fluids. However, the rewards could be significant

Feasibility study for the Thelamork low-temperature system in N-Iceland

The Thelamork low-temperature geothermal system in N-Iceland has for the last decade been considered as a possible source of hot water for the Akureyri District Heating Service. A productive well was drilled in the summer of 1992 afer 10 years of geothermal research in the area Following that a feasibility study was performed in order to determine whether harnessing the geothermal gwtem for space heating would be economical. This study consisted of a nine month full scale production test along with partial reinjection and tracer tests. It also involved careful monitoring of production rates, water level changes and chemistry. Finally, the data collected were analyzed on the basis of simple reservoir models. The results of the analysis indicate that the system will sustain a production of 19-20 I/s, initially at 91 degrees C, for the next 10 years, given that 3 l/s will be reinjected. However, a cooling of 9-12 "C is predicted due to infiltration of colder groundwater and the reinjection The results also suggest that hamessing the geothermal system will be economical, despite the high cost of exploration and an 11 km insulated pipeline to Akureyri

Crustal Conditions Favoring Convective Downward Migration of Fractures in Deep Hydrothermal Systems

Cooling magma plutons and intrusions are the heat sources for hydrothermal systems in volcanic settings. To explain system longevity and observed heat transfer at rates higher than those explained by pure conduction, the concept of fluid convection in fractures that deepen because of thermal rock contraction has been proposed as a heat-source mechanism. While recent numerical studies have supported this half a century old hypothesis, understanding of the various regimes where convective downward migration of fractures can be an effective mechanism for heat transfer is lacking. Using a numerical model for fluid flow and fracture propagation in thermo-poroelastic media, we investigate scenarios for which convective downward migration of fractures may occur. Our results support convective downward migration of fractures as a possible mechanism for development of hydrothermal systems, both for settings within active zones of volcanism and spreading and, under favorable conditions, in older crust away from such zones.publishedVersio

Proceedings of the Workshop on Geothermal Reservoir Engineering

Well SN-12 in the Seltjarnarnes low-temperature field in SW-Iceland was drilled to a depth of 2714 m in the fall of 1994. The well appeared to be almost non-productive at the end of drilling. A comprehensive ten day stimulation program was, therefore, initiated. The program involved, firstly, high-pressure wellhead injection and, secondly, high-pressure injection below a packer placed at 1412 m depth. After about twelve hours of wellhead stimulation the pressure dropped suddenly, indicating that the well had been stimulated. At the same time the water level response increased suddenly in two near-by monitoring wells. During the second stimulation phase (packer at 1412 m) the well appeared to be stimulated even further. The well eventually produced about 35 l/s with a drawdown of roughly 60 m, and the stimulation had increased the yield of the well by a factor of nearly 60. Thus well SN-12, which appeared to be almost non-productive at the completion of drilling, had turned into a good production well. It is believed that during the stimulation some previously closed fractures, or interbed contacts, reopened connecting well SN-12 to the main fracture system of the geothermal reservoir