Predicting the Lifetime of Geothermal Doublets Analytically

Abstract

A direct-use geothermal doublet can deliver low-emission heat for decades. How long exactly it can deliver heat for, is hard to predict. It depends on geological and technical characteristics and on the minimum required temperature of the water that is produced from the subsurface. Production temperature is typically constant for the first years or decades, and begins to decrease once the colder, re-injected water reaches the producer well. This evolution can be calculated numerically when subsurface conditions are well-known, but that is not usually the case. Especially in the planning phase of doublets, when practical experience is not yet available for new locations, this complicates economic projections. As direct-use geothermal doublets often have high investments costs and narrow profit margins, this uncertainty can hold back permits and investments in a reliable, sustainable source of heat. It would be useful to be able to quickly estimate production temperature as a function of time for any geological and technical scenario. Therefore, in this study, we develop a method to provide such an estimate. We combine an analytical calculation of thermal breakthrough time (following Gringarten and Sauty, 1975) with a parameterized equation for production temperature after thermal breakthrough. This allows us to calculate expected system lifetime. The average root mean squared error (RMSE) of our predicted temperature curves is 0.7K. The estimates are returned nearly instantly and allow to analyze and compare temperature evolution and lifetime across large numbers of scenarios.