Applications of Molecular Simulation in Oil and Gas Production and Processing
Résumé
Molecular simulation is an emerging technique which consists in performing a detailed simulation of microscopic systems involving typically a few hundreds of molecules. On the basis of these simulations, appropriate statistical averages are performed to derive fluid properties that can be compared with experimental measurements. The purpose of the article is first to provide the reader with basic notions of molecular simulation. Then, application examples are given in several fields of the oil and gas industry. In reservoir engineering, the examples relate to the properties of poorly known hydrocarbons, the thermal properties of natural gases, and the phase equilibria and volumetric properties of acid gas-hydrocarbon mixtures. The application to gas production and processing is illustrated by the phase equilibria involving methanol (a common solvent or hydrate inhibitor) and the solubility at high pressure of gases in polymer materials. In hydrocarbon processing, the solubility of hydrogen sulfide in hydrocarbons at high temperature is discussed. Petroleum Industry Applications of Thermodynamics Applications de la thermodynamique dans l'industrie pétrolière INTRODUCTION The essence of molecular simulation is to consider explicitly all the molecules of a small size system and to determine the behaviour of the latter from a careful computation of the interactions between its components. Moreover, the position of most atoms must be explicitly computed, in order to account for the difference in geometry between individual molecules (Fig. 1). These computations take much more computer time than classical thermodynamic models: from a few hours to several weeks of computing time, depending on system complexity. Yet, these methods are increasingly used in the oil and gas industry to compute thermodynamic properties. Why is this so? There is no short answer, and several kinds of reason must be quoted. Figure 1 Example of a simulation box of a mixture of cyclohexane and H 2 S in the liquid phase. Cyclohexane molecules are represented by six anisotropic united atoms and H 2 S molecules by a single united atom. Firstly, molecular simulation is the unique way to gather the prediction of all thermophysical properties in a single theoretical framework, which is statistical thermodynamics [1-3]. It is sufficient to provide a receipe to compute the potential energy of the system from the positions of its atoms or molecules, and all the system's properties can be derived by statistical thermodynamics. This holds for equilibrium properties like vapour pressure, as well as for transport properties like viscosity [4-6]. Of course, finding the adequate receipe for computing the potential energy without exceeding reasonable computing times is not an easy task, and the existing receipes will certainly be improved in the years to come. Nevertheless, this field is sufficiently mature that it can provide reliable predictions in many instances, as will be illustrated in the present article. In these cases, molecular simulation has shown an unprecedented ability to extrapolate prediction from small molecules to large ones, from low pressures to high pressures, and from low temperatures to high temperatures. The second major reason to consider molecular simulation is the difficulty of modelling many systems with classical thermodynamic models (equations of state, group contribution methods, activity coefficient models, etc.) in absence of experimental data. Despite the large amount of experimental data in the literature, the oil and gas industry faces several problems where this kind of difficulty is encountered: – systems containing toxic or corrosive components like hydrogen sulfide or organo-mercuric compounds; – heavy hydrocarbons, which are not commercially available as pure chemicals to perform measurements; – unstable systems at process temperature; – properties of microporous adsorbents prior to their synthesis; – systems at extreme pressures. In such cases, the extrapolation capacity of molecular simulation makes it the safer way to predict thermophysical properties. This is especially the case when specific interactions occur at the molecular scale, as in zeolitic micro-porous adsorbents [7], or when polar systems are considered. In the present article, we will first give a brief introduction to molecular simulation methods. Then we will provide an overview of application examples in reservoir engineering, gas production and hydrocarbon processing. However, we will restrict these examples to fluid phase properties. It must be stressed that molecular simulation of fluid phases is a vast field, that is impossible to treat in detail within such an article. We will therefore frequently refer the readers to articles or manuals where they will find complementary information.
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