With the depletion of fossil fuels, geothermal has been gradually entering the field of view on renewable energy in the past decades. A crucial aspect of successful geothermal development mostly depends on site characteristics (Lu, 2018). Tracer tests provide important insights into system characterization, assisting sustainable geothermal reservoir research and management.
One of the most common and useful applications of tracer testing in geothermal is to evaluate and monitor the cooling of the reservoir due to reinjection during the operation. During the past several decades, extensive research has been conducted to study the application of existing tracers and to discover new tracers for a wide spectrum of geothermal fields (i.e., Maier et al., 2015; Nottebohm et al., 2012; Rose et al., 2001).
One approach is to use classical tracers like naphthalene sulfonates, Amino G, and rhodamine WT as thermo-sensitive tracers (Rose and Clausen, 2017, 2014; Rose and Adams, 1994). This approach, however, is limited to high-temperature systems (from 200 to 350 °C).
Another approach, applying to low enthalpy systems (below 200 °C), is based on hydrolysable compounds such as esters, amides, and carbamates (target design possible), leading to defined reaction products (Cao et al., 2018; Nottebohm et al., 2012; Robinson and Tester, 1990; Schaffer et al., 2016). These thermo-sensitive tracer compounds are water-soluble and have different fluorescent properties than their hydrolysis products, which allows for the detection of tracers on site.
Thermo-sensitive tracers could cover a wide range of temperatures depending on their underlying decay mechanism (e.g., Adams and Davis, 1991; Cao et al., 2018; Nottebohm et al., 2012; Rose et al., 1999; Schaffer et al., 2016). In particular, amides and carbamates can be potentially applied for exploitation within the temperature range of 100–150 °C or even higher. Furthermore, the hydrolysis reaction of both amides and carbamates is rather independent of environmental pH/pOH conditions, making these compounds favorable for field applications (Cao et al., 2018; Schaffer et al., 2016). For temperatures lower than 100 °C, esters and carbamates are proposed as good tracers to track temperature distribution (Cao et al., 2018; Nottebohm et al., 2012).
The synthesize of thermo-sensitive tracers also highlights the potential of designing of tracer compounds. Based on the knowledge of tracer properties, tailor-made tracer compounds can be developed with the required properties or effects in hydrogeology. This is knowledge-driven approach and can be used to build a big database for currently available reactive tracers. Moreover, this concept can expand the potential application of tracers in other fields (e.g., to predict environmental risks of hydraulic fracturing, to quantify processes in the hyporheic zone).
These findings are described in the article entitled The feasibility of using carbamates to track the thermal state in geothermal reservoirs, recently published in the journal Geothermics. This work was conducted by Viet Cao, Mario Schaffer, and Tobias Licha from the University of Goettingen.
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