| The CO2-H2O-NaCl system, a typical representative of many fluid systems in the nature, has long been concerned and studied by people. The P-V-T-x (pressure-volumn-temperature-composition) properties of this system is essential for discussing those important issues including geological fluids and petrogenetic and metallogenie mechanisms. However, the currently existed P-V-T-x data are really constrained and those data at temperature range from 273 to 473 K and pressure over 40 MPa are still lacking, which seriously limits people’s full recognition on properties of this system. The aim of this dissertation is to experimentally investigate the P-V-T-x properties (density/volume property and phase equilibrium property) of the CO2-H2O-NaCl system within temperature, pressure, and composition condition at 273.15-573.15 K,10-120 MPa, xco2=0-saturated and mNaCl=0～5 mol/kg, respectively, using the newly designed high pressure optical cell (HPOC) technique. The obtained data fill the gap of previous P-V-T-x research and are contributed to 1) detailing the density/volume behavior; 2) developing two modified empirical models (one for the binary CO2-H2O and the other for the ternary CO2-H2O-NaCl system) that are more reliable for predicting experimental P-V-T-x data; 3) making some tentative interpretation on the macroscopic behaviors via Raman spectroscopic analyses; and 4) calculating isochores of fluid inclusions and proposing some new insights into the issue of the CO2 geological sequestration and the important role of CO2 in the hydrothermal ore forming fluids.The discussions and conclusions of this paper are as follows:(1) The method using HPOC to make in situ observation on the P-V-T-x properties of the CO2-H2O-NaCl system is proved to be of high efficiency and accuracy, and reliable data could be measured through systematical calibration of the raw data. Within the P/T/x condition ranges by this paper, the measurement error of CO2 solubility is less than 5%, and the accuracy for determining volumn data is better than 0.1% using HPOC.(2) It is indicated by the volume data derived for the binary CO2-H2O and the ternary CO2-H2O-NaCl system from the primarily measured length change ratios of the column-shape samples that density of the CO2-H2O-NaCl) solutions decreases with increasing the pressure, while increases with rising the pressure and salinity when other conditions are remained. However, adding of CO2 below and above the transition temperature (Tt) has contrary effects on the density of (NaCl-)H2O solutions, with pCO2-(NaCl)H2O>p(NaCl)H2O when T<Tt and pco2-(NaCl)H2O<P(NaCl)H2O (p is the density of the corresponding system)when T<Tt, respectively. At pressure of 30 MPa,Tt for the CO2-H2O system is around 484 K, which is consistent with that reported in the literature. Notably, Tt shows a positive dependence on the pressure-raised by about 100 K when increasing the pressure from 30 MPa to 120 MPa. On the other hand, Tt is decreased by around 50 K by the increased NaCl concentrations from 0 to 5 mol/kg. Also, the data by this study illustrate that the apparent molar volume (Vφ) of CO2 in CO2-(NaCl-)H2O solutions sees an obvious reliance on the pressure but an negligible dependence on concentration of CO2. At temperatures lower than 298.15 K, the effect of temperature on Oη is little. By contrast, when T>298.15 K, Vφ increases dramatically with increasing the temperature. Two modified volume equations of state (EOS) for CO2-H2O and CO2-H2O-NaCl system are developed based on two new sets of parameters regressed with data of this study. These equations show better performances on predicting experimental density/volume data with accuracy reaching the level of 0.1% than previous models and thus are thought to have stronger reliance and wider applicability.(3) Raman spectroscopic investigations on H2O、CO2、CO2-H2O and CO2-H2O-NaCl systems show that, firstly, the structure of water is strongly destroyed by the increasing temperature, due to the changed bond angle of ∠O-O-O and bond length of R (O…O) within the hydrogen bonding structure, as a consequence of which the fully hydrogen bonded water (FHW) is transformed into partially hydrogen bonded water (PHW) and then the property of the bulk water changes significantly. Secondly, increasing the temperature, the structure breaking ability of NaCl on water structure is gradually reduced and turns ignorable at 513 K because of the strengthened Na+-Cl-interaction (conbined ion pairs) and the weakened Cl-H2O interaction. Thirdly, saturated CO2 shows hardly observable effect on Raman spectra of H2O(-NaCl), which indicates that the predominant interaction between CO2 and H2O molecules is Van der Waals’force, and there is no notable interaction between CO2 and Na+/Cl-/NaCl. Finally, the remarkably strengthened relative intensity of the hot bands in the Femi diad of CO2 suggests that the linear structure of CO2 molecule may bend and the resulted increased polarity of CO2 molecule is explained to enhance the solubility of CO2 in polar H2O above 353 K. Accordingly, it is concluded that it is the H2O-H2O interaction, H2O-CO2 interaction, Cl-H2O interaction and the Na+-Cl- that control the P-V-T-x behaviors of the CO2-H2O-NaCl system. Construction of models on the background of molecule theory should consider change of mater’s iner structure induced by raising the temperature.(4) The modified EOS of this study could be reliably used to calculating isochores for the CO2-H2O-NaCl-type fluid inclusions. It is shown by a case study of fluid inclusion that the EOS developed by this paper can provide good basis for understanding ore forming process. At the temperature regions below and above the Tt, CO2 has distinct effect on density of hydrothermal fluids:when T>Tt, CO2 decreases density of fluid while raises the density of fluid when T<Tt. Then, in the epithermal environment, adding of CO2 in the aqueous fluid leads aggregation of metal-bearing fluids, which is called "Favourable Metallogenic Locus", and meanwhile, CO2 produces a large immiscible region and "boil" the fluid. This process is good to precipitation and enrichment of the metals. On the other hand, in the porphyry environment whose surrounding temperature is higher, CO2 could promote migration of ore forming fluids upwards into lager space and induces strong local convection in the fluid, which accelerates hydrothermal alteration and formation of veinlet-disseminated ores in the vicinity of the porphyry regimes. CO2 sequestration and storage (CSS) into geologic saline aquifers needs taking into accounts the shift of storage mechanisms caused by existence of 7t for the density property of CO2 saline aqueous solutions, which would affect the storing efficiency of CO2. So, it is recommended that the temperature, pressure and salinity condition should be evaluated in details before carrying out CS S so as to ensure the capacity, efficiency and long-term safety of CSS. It is further revealed by the modeling that the improved models by this study show stronger reliance in evaluating CSS in geologic saline formations.In the future work, we recommend further qualitative investigation on the CO2-H2O-NaCl system and quantitative measurements in lager P-T ranges, considering some shortcomings such as the lack of systematic investigation on P-V-T-x properties of the CO2-rich phase in this paper. |
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