World Geothermal Congress 2020+1
March - October, 2021

Laboratory Leaching Tests to Investigate Mobilisation of Metals Within Engineered Geothermal Reservoirs


[British Geological Survey, United Kingdom]

Within engineered geothermal systems (EGS), the recirculation of hot fluids would facilitate the extraction of dissolved metals, as well as energy, in surface plant. Potentially, this would generate a second source of revenue, and thus increase the economic attractiveness of EGS development. This concept is the focus of the Combined Heat, Power and Metal extraction from ultra-deep ore bodies project (CHPM2030), where the potential for exploiting hot metal-bearing geological formations at depths greater than 3 km has been investigated. The strategic objective of the CHPM2030 project is to develop a novel technological solution to both make geothermal energy more attractive and to reduce Europe’s dependence on the import of metals and fossil fuels. A key aspect of such a scheme is that sufficient quantities of metals can be mobilised and transported to make the process economic, and critical factors underpinning this are the rates and magnitudes of metal release in the reservoir zone. Laboratory experiments simulating in-situ conditions, using batch and flow-through reactors, have been used at up to 250°C and 300 bar to provide well-constrained data to help to understand and quantify these processes. Such experiments also allow testing of different fluid compositions in order to ascertain if there are specific additives that may improve the metal recovery process. Furthermore, evidence for the degree of leaching may allow assessment of whether it might increase system performance over time - through, for example, silicate and ore mineral dissolution and consequent permeability enhancement. We report a series of leaching tests on rocks from Cornwall, UK, that used a variety of leaching fluids over a range of temperature, pressure and timescale. Increases were found in the concentration of many metals, and were broadly correlated with increasing temperature and acidity. It was also noted that the presence of suitable ligands also increased metal concentration in solution, this included Cl- and especially organics (e.g. acetic acid). Detected elements were grouped as common elements (less economic importance and higher occurrence), and at risk (those having higher economic value defined by the European Union). One of the highest concentrations of common elements was lead (up to 870 ppm using solutions containing acetic acid in batch experiments) and up to 540 ppm in flow-through experiments. Notable concentrations of Fe and Zn are also present in leachates, due to enhanced dissolution of sulphide mineralisation in the samples. Significant concentrations of Al and Si were also found in some leaching solutions, indicating considerable dissolution of matrix silicates, such as quartz and mica, present in the samples. This could be desirable in terms of increasing reservoir permeability and opening flow paths, but if concentrations become too high there is an increased risk of precipitation due to saturation with secondary phases, which could clog fractures and inhibit fluid flow in a geothermal reservoir, and risk fouling boreholes or surface infrastructure. Notable was that dilute acetic acid solutions achieved relatively high concentrations of some metals, but comparatively low concentrations of Al and Si, so limiting the potential for the formation of potentially problematical precipitates. We recognise however, that full extrapolation to a natural setting will require consideration of groundwater chemistry, fluid migration pathways and residence times, variations in surface areas, and decreases in available metals over time, and ongoing work seeks to investigate this. The data gathered here suggests that, given sufficient characterisation of the mineralogy and fluid chemistry of deep geothermal systems, it may be possible to tailor the chemistry of leaching solutions to maximise metal recovery.

        Topic: Geochemistry Paper Number: 14055

         Session 15D: Geochemistry 6 [Tuesday 11th May 2021, 10:00 am] (UTC-8)
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