| Title | What is the End Point for Geothermal Developments: Modeling Depletion of Geothermal Fields |
|---|---|
| Authors | Allen W. Clotworthy, Jim V. Lawless, Greg Ussher |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Resource Estimation, Stored Heat, Reservoir Modeling, Reinjection, Corrosion, Scaling |
| Abstract | Any finite quantification of the capacity of a geothermal resource implicitly involves a start point and an end point for energy extraction. The issue addressed here is, why does commercial energy extraction cease from a geothermal resource, and what are the implications for resource capacity estimation? In a simple stored heat estimate with no natural heat or fluid recharge over the project lifetime, the implicit assumption is that the project will cease when all of the available energy has been extracted. So the “failure mode” is a temperature decline. Even that is a significant oversimplification - in a system with reinjection, energy extraction will have to cease when the fluid coming out of the production wells drops below the minimum inlet temperature requirement of the power plant. But at that time there will be a temperature and pressure gradient laterally through the reservoir from the reinjection to the production wells, so the average resource temperature at that time will be less than the power plant inlet temperature. Further considerations to take into account are: heat loss up production wells, which could be considerable; heat loss between the wells and power plant; heat loss between the separators and power plant and reinjection wells; and heat gain down the reinjection wells. There are power systems aspects to consider as well as parasitic pumping etc. loads. Furthermore, if the production temperature declines over the lifetime of the project the power plant efficiency would drop and the production pumping requirements will change. With dynamic reservoir simulation the failure mode is often found to be pressure decline rather than temperature. Based on practical experience of geothermal systems that have been exploited for a long period of time, there are other possible failure modes including: premature reinjection returns leading to a low percentage energy recovery; fluid depletion; groundwater incursion causing undesirable chemical effects such as scaling and corrosion as well as enthalpy decline; or excessive environmental effects on the surface. |