Clogging of shallow geothermal doublets: hydrologic, geochemical and microbiological observations in a pilot site
Résumé
Well and surface equipments (exchangers, filters) clogging is a major issue that affects many geothermal doublets in shallow aquifers (open-loop groundwater heat pump). In the case of wells, injection wells in particular, clogging of screens, porosity of gravel pack filters, and more widely the pores of the aquifer lead to severe losses in hydraulic performance in a long run. These clogging processes involve complex interactions between abiotic chemical processes (precipitation of carbonate minerals; iron and manganese oxides), physical processes (mobilisation and precipitation of particles) and biological processes (mineral precipitation linked to bacterial metabolism or biofouling). Each of these processes is well known and documented individually. However, prediction of clogging phenomena using a global approach that could integrate parameters related to operating conditions (temperature, pressure, flow rate...) as well as water chemical, physical, and biological characteristics is a scientific challenge. Except for carbonates species, the small temperature variation (<15°C) induced by geothermal doublet operation seems to have a limited effect on the concentration of the major chemical components of groundwater but does impact redox equilibria. The main impact on hydrochemical characteristics and equilibrium states of the groundwater induced by geothermal doublets operating at injection temperatures below 30°C is related to mixing of waters with different chemical facies (Bonte et al, 2013; Possemiers et al, 2016). From a biological point of view, the microbial communities responsible for clogging are highly sensitive to temperature variations, redox variations, and operating conditions (Vetter et al, 2012). Redox equilibria and their influence on biochemical reactivity are very sensitive to very small changes in physico-chemical parameters. Such variations might occur on surface or in-depth installations due to the impact of pumping and/or injection. The impact of mixing of waters with different chemical compositions on biochemical reactions in a context of heterogeneous geological environments is a research question debated over recent years. We present the initial results of a research project carried out jointly by Antea Group and the Geosciences Rennes laboratory, launched in April 2015 aiming at studying the origin of clogging problems and analyzing the effects of preventive and curative treatments. In this framework, field investigations of geothermal doublets are performed to evaluate and identify the possible origins of clogging phenomena. For selected doublets, a real-time monitoring of operating conditions and water physico-chemical characteristics is performed. In addition, chemical and bacteriological analyzes are carried out on deposits and water samples. The first challenge is linked to the selection of the most relevant geochemical and hydrogeological tools to characterise clogging of geothermal doublets under operation. The second issue is related to the characterisation and simulation of biogeochemical reactions induced by the operation of a geothermal doublet. In this context, the evaluation of reaction mechanisms and controlling parameters is critical to assess the clogging risk of an installation and to select the appropriate preventive or curative solutions.