Silica solubility and dissolution kinetics at high saline geothermal conditions

This dataset contains solubility data for silica as a function of time, temperature and salinity. The dataset supports Chapter 2 in the deliverable “Report on mineral solubility and precipitation at high salinities, DOI: https://doi.org/10.48440/gfz.4.8.2023.001 from the H2020 project REFLECT.

The silica material used as solid substrate for the dissolution studies was pro analysis sea sand (purified by acid washing and calcinated for analysis) from Merck. The sand grain size (125-250 µm) included in the experiments was obtained by sieving the material. The sieved powder was washed with tap water to remove fine grains from the samples, and dried prior to experiments. The BET surface area of the sand was measured to 0.69 m2/g and the weighted mean particle size distribution (PSD) was 118 µm. The crystallographic structure was determined by X-ray diffraction analysis (XRD) and this analysis showed that the sample contained mainly low-quartz (minimum 95% w/w) with a few unidentified impurities. SEM/EDS maps of the silica powder showed essentially pure silica with minor Al impurity. Some grains or regions are enriched in Al and K, suggesting some aluminium silicate. Some minor spots rich in Ti, Fe and Cr were also detected.

The experiments conducted to study silica solubility at equilibrium conditions were performed at five different temperatures (100, 125, 150, 175 and 200°C) and four salinities (NaCl concentrations 50.9, 103.6, 215.7 and 338.1 g/kg H2O). The columns containing SiO2 and NaCl solutions where isolated for a reaction time of six days before fluid sampling (Table1 “Silica solubility at high saline geothermal conditions”).

The experiments conducted to study silica solubility kinetics were performed for different time periods (from 1 hour up to 144 hours) to study solubility as a function of time. These tests were conducted at 200°C with NaCl concentration 50.92 g/kg and 338.09 g/kg H2O (Table2 “Silica solubility kinetics at high saline geothermal conditions”).

The experimental setup consists of packed static columns. Maximum four columns (length 40 cm, i.d. 10.22 mm, stainless steel SS316) packed with the material to study can be placed in parallel within the setup. Porous metal frits (HC276) are placed at the outlet and inlet of the columns to prevent entrainment of the material. Approximately 50 g of dried SiO2 powder is required to fill a column completely and the pore volume was measured gravimetrically to be approximately 15 ml. Two Gilson 307 high performance liquid chromatography (HPLC) pumps are included in the setup. One for filling and displacing column pore fluid and one for diluting the fluid prior to sampling, preventing precipitation of dissolved silica due to depressurization and cooling. Pressure was maintained by a dome loaded backpressure regulator (BPR) from CoreLab at the column outlet and liquid samples were collected using a fraction collector (Gilson FC203B). The setup of columns and inlet/outlet valves was placed in a heating cabinet (Memmert). The columns were thermally insulated to prevent instabilities in temperature and hence pressure when opening the heating cabinet during sampling.

The columns are flooded with degassed NaCl fluid at a low flow rate and pressurized initially to 25 bars while temperature is increased slowly to the desired level. The time of start is noted, the brine pump is shut off, and the individual columns isolated by closing inlet and outlet valves. After a period (hours, days, or weeks) samples are withdrawn from the columns and diluted at the mixing point by re-opening the valves and operating both HPLC pumps. A dilution factor of 8.5 is selected to prevent precipitation. For each sampling five samples of 2 ml is collected (totally 10 ml of fluid). The two first samples are considered to contain mainly dead volumes from tubing, fittings and valves and are therefore discharged. The three last samples represent the pore fluid from the column. These samples are analysed for Si and NaCl concentration. The NaCl concentration was analysed to keep control of the dilution step of the sampling process.

SiO2 and NaCl concentrations were analysed using inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma optical emission spectrometry (ICP-OES). The elements Si and Cl (ICP-MS) or Si and Na (ICP-EOS) were detected.

The Si concentration from the analysis was reported as mg/L solution. From this concentration the concentration of SiO2 in the samples were calculated and reported as mol/kg H2O. The conversion from liter solution to kg H2O was done using the OLI software for density calculations.