Assessment of Kinetic Conditions of Quartz Geothermometer Application: Experiment and Modeling

Abstract

A quartz geothermometer (QG) makes it possible to determine the temperature of a deep geothermal reservoir (GR) using SiO₂ concentration (m) in a solution outflowing from this reservoir on the surface. An error made in the initial QG modeling led to the underestimation of the quartz precipitation rate and thus expanded the region of QG application. Another disadvantage of this modeling was that it ignored the possibility of precipitation of metastable silica modifications. To eliminate these shortcomings, a new numerical QG modeling was performed by the finite-difference method using new kinetic data. The reliability of data was assessed by their involvement in modeling the slow cooling of the quartz–water system and comparison of the obtained results with the experimental results of this process. The best agreement between the experiments and calculations was obtained using two-stage SiO₂ precipitation when different kinetic constants were applied above and below the amorphous silica (AS) solubility for the description of deposition of AS and other metastable silica modifications, respectively. The results of the new QG simulation using new kinetic data were similar at the same ratio of the two initial parameters that characterize the deposition surface area normalized to the water mass in the system (S/M) and the rate of solution rise (v). The real boundary values of this ratio, S/M and v, were determined, at which the model predicts the correct QG values for different temperatures of the solution in GR and at the surface. Kinetic equations used in the simulation ignore many peculiarities of the silica precipitation mechanism. An experimental study of these peculiarities will provide a more realistic QG model close to the real natural processes.