Quantifying the contribution of heat recharge from confining layers to geothermal resources

Abstract

Geothermal operations are expanding and increasingly contributing to the current energy supply. Assessing the long-term operable lifetime of these projects is complicated as the reservoirs they produce from are often deep and subsurface properties are uncertain and spatially variable. The minimum lifetime of a geothermal project usually considers the heat in place in a geothermal reservoir, but not the heat flow from the confining layers into the reservoir during operation. For the economic feasibility and optimal design of a geothermal project it is the key to capture this process, as this allows more accurate prediction of the long-term extraction temperature. Previous studies are not conclusive on the contribution of vertical recharge from the confining layers. This research evaluates the contribution of recharge to the heat production for geothermal projects. In a simulation study with an idealised, homogeneous geothermal system, the reservoir thickness, well placement and production rate are varied to investigate their respective influence on thermal recharge. The results show that the recharge from the vertical confining layers may contribute considerably to the total energy output of a geothermal well. Due to recharge, geothermal systems produce more heat than the heat in place and under specific conditions the produced heat can be more than five times as large. The largest contribution of the recharge of vertically confining layers can be expected under conditions of thin reservoirs and a long interaction time between the injection water and confining layers (i.e. large well spacing, low production rate). The insights of this study may help to optimise the design and operation of individual geothermal projects for optimal utilisation of a geothermal reservoir for energy supply.