Efficient Modelling of Asphaltene Precipitation

Abstract

Abstract An efficient modelling technique based on the representation of the precipitated asphaltene as a pure dense phase is presented. The success of the approach is based on the division of the heaviest component in the oil into a nonprecipitating and a precipitating component. The characterization of these components is discussed. This model was able to make quantitative predictions of experimental data from the literature as well as additional data from industry. This was achieved with only a small number of adjustable parameters (two or three). The mechanistic aspect of the model with regards to colloidal nature of asphaltene/resin micelles is also discussed. An algorithm for three-phase flash calculations with asphaltene precipitation is described. Introduction Asphaltene precipitation from reservoir fluids during oil production is a serious problem because it can result in plugging of the formation, wellbores and production facilities. A description of asphaltene problems and remedial actions in many areas throughout the world are given in [1,2,3,4]. Currently, mechanical and chemical cleaning methods of wellbores are being improvised to maintain production, but these methods are time-consuming and expensive. Asphaltene precipitation also occurs frequently during enhanced-oilrecovery by gas injection which impedes seriously the recovery. A model for correlating and predicting asphaltene precipitation is highly desirable because it would allow the design of injection/production schemes such that asphaltene precipitation can be avoided or minimized. Asphaltenes are heavy hydrocarbon molecules that are in colloidal suspension in the oil, stabilized by resins adsorbed on their surface [5,6,7]. Changes in pressure, temperature and composition may cause asphaltene precipitation. The nature of the precipitated asphaltene is still under investigation. Two scenarios are possible [2]:the asphaltene/resin micelles precipitate essentially unaltered andthere is a dissociation of the asphaltene/resin micelles which cause the precipitation of the asphaltene. In field situations, asphaltenes will first flocculate and depending on the flow dynamics may or may not deposit. Only laboratory experiments are considered in this paper and all the flocculated asphaltenes will be referrred to as precipitated asphaltenes. Modelling approaches Several approaches for modelling asphaltene precipitation have been reported in the petroleum literature: Solubility model The first important approach in modelling asphaltene precipitation in petroleum engineering is due to Hirschberg et al [5]. Vapor-liquid equilibrium calculations with a cubic equation of state were first applied to split the mixture into a vapor and a liquid phase. The liquid was then divided into three components: asphaltene, resin and a component for the remaining oil and solvent. P. 375^