Organic Molecules as Sorbing Tracers for the Characterization of Fracture Surface Areas in Enhanced Geothermal Systems (EGS)

Abstract

The specific fracture surface area to volume ratio A/V is a crucial parameter for assessing the heat exchange efficiency and, thus, allowing for the success evaluation of hydraulic stimulations in geothermal reservoirs. Unfortunately, this ratio cannot directly be determined with existing techniques. Based on the assumption that A/V is proportional to the surface area available for sorption, several inorganic cations were recently proposed as sorbing (cation exchanging) tracers to derive A/V values. However, the main disadvantages of inorganic ions are the limited number of available ion types, their potential geogenic background, and their challenging detection at trace concentrations in the field. To overcome these limitations and to improve the applicability of sorbing tracers, the spectrum of potentially suitable chemical substances as sorbing tracers is broadened by considering organic molecules. The organic tracers discussed in this work are cationic and fluorescent, therefore, highly water soluble and easy to measure at very low concentrations. Furthermore, different molecular structures can be selected or even individually adapted according to the prevailing reservoir conditions. Additional sorption processes with the reservoir rocks beside cation exchange may also play a role allowing the application of these organic molecules even in highly saline reservoirs. First results from lab column experiments with different organic amines under varying conditions (different salt concentrations) are presented emphasizing the potential of using organic molecules as sorbing tracers in the future.