GENERAL RESEARCH Estimating the Hydrate Safety Margin Using Surface Tension Data of Salt Aqueous Solution Amir H. Mohammadi and Dominique Richon* Centre Energe ´ tique et Proce ´ de ´ s, Ecole Nationale Supe ´ rieure des Mines de Paris, CEP/TEP, 35 Rue Saint Honore ´ , 77305 Fontainebleau, France Petroleum reservoir fluids are normally produced with saline water. One serious problem in production and transportation of these fluids is pipeline blockage due to hydrate formation giving rise to operational problems and other safety concerns. No means of controlling and monitoring are generally available along the pipelines and/or downstream to assess hydrate formation. High safety margins are used in many cases to account for the uncertainties in the operating conditions and to reduce the gas hydrate formation risks. In this work, the possibility of predicting the hydrate safety margin from surface tension data of salt aqueous solutions is investigated using a new equation. The developed method considers only the changes in surface tension of saline water with respect to surface tension of distilled water, and therefore there is no need to have the analytical analysis of the aqueous solution. Independent data are used to examine the reliability of this tool. The predictions of this method are in acceptable agreement with the independent data, demonstrating its reliability for estimating the hydrate safety margin in the presence of salt aqueous solutions. 1. Introduction Gas hydrates are icelike structures in which water molecules, under pressure, form structures composed of polyhedral cages surrounding gas molecule “guests” such as methane and ethane. They can occur in staggering abundance in cold subsea, sea floor, and permafrost environments where temperature and pressure conditions ensure their stability. The natural gas trapped in these deposits represents a potential source of energy many times that of all known natural gas reserves. Gas hydrates can form as well in undersea piping and above-ground pipelines, where they pose a major and expensive problem for the petroleum industry. 1 For pipelines carrying a cocktail of multi- phase fluids including hydrocarbons and formation water with various concentrations of salts, saline water may provide the required protection. However, no means of controlling and mon- itoring are generally available along the pipelines and/or down- stream to assess hydrate formation risk, and therefore high safety margins are used to account for the uncertainties in the operating conditions and for reducing the risk of gas hydrate formation. This work is a continuation of a previously reported study for estimating the hydrate safety margin from aqueous phase properties. 2,3 The possibility of predicting hydrate inhibition effects of saline solutions from surface tension data of the aqueous phase can have a practical use, as measuring the surface tension of the aqueous phase is easier than hydrate suppression temperature measurement. In the present work, a brief review is first made on the surface tension definition, thermodynamics, and measurement methods to provide a better understanding of surface tension phenomena. The predictions of a general predictive method 4 for hydrate suppression temperatures and surface tension data 5 of aqueous phase due to the presence of salt are used for developing a simple equation. Independent data are then used to examine the reliability of this method. The predictions of this equation are in acceptable agreement with the independent data, demonstrating the reliability of the predictive technique developed in this study. 2. Surface Tension Surface tension is an effect within the surface layer of a liquid that causes the layer to behave as an elastic sheet. It is caused by the attraction between the molecules of the liquid, due to various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inward by other molecules deeper inside the liquid, but there are no liquid molecules on the outside to balance these forces. All of the molecules at the surface are therefore subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. Thus the liquid squeezes itself together until it has the lowest surface area possible. 6 Surface tension can therefore be considered as the force along a line of unit length perpendicular to the surface, or work done per unit area. This means that surface tension can also be considered as surface energy. 6 From a thermodynamic point of view surface tension (σ) is defined as 6 where G is the Gibbs free energy and A is the area. Subscripts T, P, and x stand for temperature, pressure, and composition, respectively. There are only empirical equations for calculating the influence of temperature (T) on surface tension. For example: 6 * To whom correspondence should be addressed. Tel.: +(33) 1 64 69 49 65. Fax: +(33) 1 64 69 49 68. E-mail: richon@ensmp.fr. σ ) ( ∂G ∂A ) T,P,x (1) Eo ¨tvo ¨ s equation: σV 2/3 ) k(T C - T) (2) 8154 Ind. Eng. Chem. Res. 2006, 45, 8154-8157 10.1021/ie0608004 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006