| Abstract |
A high temperature-pressure apparatus, located in Water-Gas Laboratory at GNS Science Wairakei, has been used to simulate greywacke-brine interaction and formation scaling mechanisms at 154oC and 27 bars with a continuously-flowing fluid. This laboratory experiment reproduced an injection scenario and allowed determination of equilibration of the greywacke reservoir rock, forming the Taupo Volcanic Zone basement in New Zealand, with injection brine that is high in dissolved components (950 mg/kg of dissolved silica). For the simulation, 24 g of coarsely-crushed greywacke, 1-2 mm size, has been placed in the core holder. At room temperature and 1 ml/hr, reacted fluid samples contained measureable concentrations of Ca and Mg reaching 77 and 6 mg/kg, respectively, due to either carbonate dissolution or cation exchange with clay minerals forming the fine grained matrix. At 154°C and 27 bars, the reacted water remained strongly oversaturated with respect to silica even if a loss of 240 mg/kg of SiO2 was observed. The overgrowth of secondary amorphous silica is responsible of this process. In the run products, the fine grained matrix is covered with a thick layer of amorphous silica, and quartz and feldspar surfaces are also coated but to a lesser extend. Under geothermal conditions, the precipitation of Ca-Mg-Fe bearing aluminosilicates, most likely smectite and chlorite, probably occurred because of the drop of Mg, Ca, Al and pH observed in effluents. Na and K concentrations remained stable during the simulation. These results are quite specific to this experiment and cannot be considered as representative of greywacke-brine interactions because host rock characteristics (chemistry, surface area, etc.), injected fluid chemistry, gas composition, flow rate, temperature and pressure are important factors that dictate the mechanisms of dissolution-precipitation processes. |