| 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 ( more than 100 years) reservoirs are likely to trend toward a pseudo steady-state 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. |