In situ Raman spectroscopic study of diffusion coefficients of methane in liquid water under high pressure and wide temperatures
Introduction
Methane is commonly found in sedimentary basin at temperatures of at least 473 K [1]. Diffusion coefficients of methane in water at high pressure and high temperature near the reservoir conditions are fundamental quantities in the calculations of the rates of transport caused by concentration gradients, especially in unfractured clay, shale, and other geological materials having low permeability [2], [3]. However, most experimental studies on the diffusion coefficients of methane in water were performed at temperature below 353 K and at one atmosphere or rather low pressures, due to the difficulties of accurate measurement on the concentration of gas in the liquid at higher temperatures and high pressures. In the last decades, some indirect methods were developed by measuring the changing properties of diffusing gas such as volume, pressure, without measuring the liquid compositions [4], e.g., the gas pressure decay method [3], the dissolved gas volume method [5]. These indirect methods have some advantages, for example, they are simple and easy. But these indirect methods need to correlate the measured changes of gas pressure or volume to the composition dissolved in the liquid to obtain the diffusion coefficient, which may introduce some uncertainties. The gas pressure decay method can perform at relatively high pressure, e.g., 35 MPa [3], but it is not performed at constant pressure, thus it is not able to reveal clearly the pressure effect to the diffusion coefficient of methane in water.
Raman spectroscopy has been used for quantitative analysis for several decades [6]. At constant temperature the concentration of dissolved methane is proportional to the Raman band intensity ratio of methane and water [7], [8], [9], [10]. Thus, we can use the quantitative Raman spectroscopic method to monitor in situ the concentration changes during methane diffusion in water under high pressure and elevated temperature. We have previously applied such quantitative Raman spectroscopic methods to determining the diffusion coefficients of methane in water at room temperature and under two pressures [9]. In this study, the work was extended to the wider temperature and pressure ranges, from 273 to 473 K and from 5 to 40 MPa, to study the temperature and pressure effects on the diffusion coefficients of methane in water.
Section snippets
Experimental
The experimental setting (Fig. 1) and procedures for study of methane diffusion at high pressures and wider temperatures are extended from our Raman spectroscopic observation at room temperature [9]. The main improvement is that the capillary high pressure optical cell was mounted inside a Linkam CAP500 heating–cooling stage (for details, see [11]), so that the diffusion process can be observed at a constant temperature from 273 to 473 K with precisions of ±0.1 K from 273 to 373 K, and ±0.3 K from
Results and discussion
Diffusion of methane in water was observed at the wider temperature and pressure ranges, from 273 to 473 K and from 5 to 40 MPa. The concentration of methane in water in the diffusion cell changes as a function of time and distance (Fig. 3). At the spot near the interface (usually observed within 10 μm to the gas–liquid interface), the observed concentrations are always close to the equilibrium values.
Conclusions
Diffusion of methane in liquid water was studied with quantitative Raman spectroscopy under high pressure and wide temperatures, 19 data of the diffusion coefficients of methane in water were obtained from 273 to 473 K, and from 5 to 40 MPa. It is concluded that, diffusion coefficients of methane in water show Arrhenius behavior only at higher temperatures, the temperature dependence of the diffusion coefficients of methane in water can be fitted with Speedy–Angell power-law. The pressure effect
Acknowledgments
We thank Dr. I-Ming Chou and Dr. R.C. Burruss for their kind help and guidance for many years on the Raman spectroscopic research on Geo-fluids. We are grateful to Dr. Rui Sun, and two anonymous reviewers for their critical reviews and helpful comments. This work was partly supported by the National Basic Research Program of China (the 973 Program, Grant No. 2009CB219503), National Sciences Foundation of China (No. 41102154), and the Fundamental Research Founds for National University, China
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