Determination of water content of natural gases : state of the art
Résumé
Natural gases normally contain significant quantities of water. During production, transportation and processing operations, undesired amounts of dissolved water may condense altering the physical state from vapor to condensate, gas hydrate and / or ice. Condensate formation may lead to corrosion and / or two-phase flow problems. The formation of gas hydrate/ice could result in pipelines blockage and shutdown. Accurate knowledge of phase behavior in water + natural gas system, especially near and inside the gas hydrate or ice formation regions, is therefore of interest to avoid these problems. Experimental data for water content of gases at low temperatures, especially near and inside the gas hydrate or ice formation regions, are rare and scarce. This is partly due to the fact that the water content is very low at these conditions and hence generally difficult to measure it. Furthermore, reaching equilibrium at low temperatures is a time consuming process. The aim of this communication is to present advances in predicting water content of natural gases. After review of the existing methods reported in the literature for estimating water content of gases, we introduce useful remarks to reduce experimental information required to determine water content of gas in equilibrium with liquid water, gas hydrate or ice. A semi-empirical method is then introduced for determining the water content of natural gases in equilibrium with liquid water or ice. This method allows calculating the water content using liquid water / ice vapor pressure and molar volume of liquid water / ice as well as pressure and temperature of the system. A mathematical correction factor based on the McKetta - Wehe chart, which is a function of gas gravity and the system temperature, can be used to take into account the effect of heavy hydrocarbons on the water content of gases in equilibrium with liquid water. In order to extend the capability of this method to sour gases, a mathematical correction factor, which is a function of concentration of acid gases in the system and the system temperature as well as the system pressure, can be used for taking into account the effect of acid gases on the water content of gases in equilibrium with liquid water. Another semi-empirical method for determining water content of methane-rich hydrocarbon gas in equilibrium with gas hydrates is then introduced. The methode estimates the water content of methane using the vapor presure of the empty hydrate lattice and the partial molar volume of water in the empty hydrate as well as pressure and the temperature of the system. In order to extend the capability of this tool for determining water content of methane-rich hydrocarbon gas in equilibrium with gas hydrates, a correction factor, which is a function of gas gravity and the pressure, can be used.