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Description
Geothermal water from Gassum sandstone aquifer has provided renewable energy in Denmark since the late 1970s . Normally the heat depleted brines are reinjected back into the same aquifer as a disposal solution. In recent years, it has been shown that hypersaline fluids can be used to produce energy by pressure retarded osmosis (PRO). The energy production from hypersaline fluids is called SalPower electricity generation, which is based on the osmotic pressure difference between the draw (brine) and the feed (freshwater). By pressurizing the draw solution, a hydro turbine can convert the water flux across a semipermeable membrane into electricity \cite{Madsen2017}. Power densities above 5 W/{$m^2$} can be obtained if the PRO process is operated at pressure up to 70 bar. The idea of harvest more energy out of the heat depleted geothermal brines is attractive. The main issue with the overall process is the larger amounts of diluted brine that shall be handled. Reinjection can be considered as a feasible solution; nonetheless, the complex fluid-fluid and rock-fluid interactions that are taking place upon reinjection should be addressed properly in order to avoid technical and environmental issues. The present study aims to determine the implication of those dynamic interactions in a possible injectivity reduction (formation damage) due to diluted geothermal brine reinjection.
Isothermal titration calorimetry (ITC) can provide insights into complex fluid-fluid and rock-fluid interactions that can occur upon diluted geothermal brine reinjection. According to Cobos et al.\cite{Cobos2018}, ITC is a highly sensitive microcalorimetry technique that provides accurate adsorption enthalpy values which are used to elucidate the interfacial-phenomena at the rock-aqueous interface. In the ITC experiments, 100 mg of Berea sandstone powder ($<$ 100 $\mu$g) was placed in a reaction vessel and 200$\mu$L of Thisted geothermal brine (TB) was added to the particles. The titration ampule was lowered step by step into the calorimeter until it reached its final position. 10 injections of 9.948 $\mu$L of two times diluted brine (2D*TB) were titrated into the calorimetric cell after 1 hour of equilibrium time.
The result of the salinity difference between the injected fluid (2DTB) and the formation brine is dynamic fluid$-$fluid and rock$-$fluid interactions. It was found that a water-water network perturbation occurred when 2DTB was added to the sandstone$-$formation water system. On the other hand, an ion exchange process between the rock and the injected fluid also took place inside the porous media. A set of composition variation changes could stabilize the clay particles that are loosely attached to the pore surface. This is because 2D*TB has a lower ionic strength than TB. Consequently, polyvalent cations can be adsorbed onto mineral lattice while monovalent cations are being desorbed to keep the equilibrium in the solution. The direct implication of those dynamic interactions is a reduction in the possible formation damage as shown in \cite{Kia1987}. The authors acknowledge that special care should be taken when designing a reinjection scheme to avoid Fe(III) oxides precipitation inside the porous media.
References
[1] A. Mahler, B. Røgen, C. Ditlefsen, L. Nielsen, T. Vangkilde-Pedersen, Geothermal energy use, country update fordenmark, European Geothermal Congress 2013, (2013)
[2] H. Madsen, S. Søndergaard, J. Muff, E. Søgaard, Pressure retarded osmosis from hypersaline solutions: Investigating commercial FO membranes at high pressures, Desalination, 420, (2017)
[3] J. Cobos, P. Westh, E. Søgaard, Isothermal Titration Calorimetry Study of Brine−Oil−Rock Interactions, Energy &Fuels, 32, 7338-7346,(2018)
[4] S.F. Kia, H.S. Fogler, M.G. Reed, Effect of Salt Composition on Clay Release in Berea Sandstones, SPE 16254, (1987)
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