Thermodynamic modeling can be used for predicting of asphaltene deposition and precipitation by using polydisperse data of heavy organic fractions. Accurate measurement of asphaltene molecular weight distribution at reservoir conditions could enhance the performance of model predictions. However, in the best of our knowledge there is no reported experience in the literature that extracts the molecular weight distribution of asphaltene in both solid and liquid phases at different reservoir conditions. In this work the high pressure, high temperature asphaltene measurement setup has been used, and the asphaltene precipitation amount due to pressure reduction and CO2 injection processes were measured for one of the Iranian heavy crudes. The molecular weight distributions of asphaltene in liquid phase at different gas compositions as well as total asphaltene content were measured by GPC apparatus. Colloidal thermodynamic model applied and the asphaltene molecular weight distributions in both solid and liquid phases were predicted at different conditions. New experimental method has been proposed for investigation of asphaltene MW distribution behavior with thermodynamic changes (pressure depletion and CO2 injection) at reservoir condition. In addition, asphaltene average MW (number or weight average or both) for each sample has been measured. It has been observed that asphaltene MW distribution shifted to the right (toward higher MW values) with addition of CO2 and pressure depletion above bubble point. Reversely, its distribution shifted to the left by reducing pressure below bubble point. Measured average distributions confirmed that asphaltene aggregates grow with pressure depletion and CO2 injection and their sizes decrease with pressure depletion below bubble point. Our obtained results clearly illustrate that the molecular weight distribution of asphaltene in solid and liquid phases is sensitive to composition changes at reservoir condition. Predictive capabilities of colloidal model are critically evaluated on the measured data of asphaltene molecular weight distribution in liquid phase.
Asphaltenes are the heaviest component in crude oil. They are defined as the portion of oil that is soluble in aromatics like benzene and toluene and insoluble in straight chain n-alkanes (light aliphatic) such as n-pentane or n-heptane [1, 2]. Asphaltene precipitation and deposition have long been recognized as problems in the production facilities. It can threaten the economic recovery of oil, or increase considerably the cost of production. The changes in reservoir pressure, temperature, and composition can cause asphaltene to precipitate from the crude oil [1].
Several models have been developed to predict asphaltene precipitation behaviour of crude oil. Solid and colloidal thermodynamic models are two powerful models which are highly accepted by researchers in prediction of asphaltene behaviour. Solid model was developed by Nghiem at 1993 [3]. In this model, the precipitated asphaltene is represented as pure solid with the oil and gas modelled with an equation of state. CMG software currently uses this model to predict asphaltene precipitation behaviour. Hirschberg applied Flory-Huggins model for asphaltene precipitation [4]. Asphaltene was assumed to be homogeneous in this model. Later on Mansoori and Jiang [5] applied Scott and Magat [6] heterogeneous model on asphaltene precipitation.
