Thermodynamic Solubility Models to Predict Asphaltene Instability in Live Crude Oils

Abstract

This paper describes a thermodynamic solubility model to predict the asphaltene stability/instability of live crude oil systems under reservoir conditions. The model uses the Hildebrand solubility parameter of stock tank oil to predict the solubility parameter of the live oil under reservoir conditions by incorporating the dissolved gas composition along with the pressure−volume−temperature (PVT) properties of the live oil. Precipitation and deposition of asphaltenes from crude oils resulting from changes in pressure, temperature, and composition have a huge economic impact on the oil industry. Due to the uncertainty in mitigating and preventing asphaltene problems, an accurate prediction of asphaltene instability could help to minimize asphaltene remediation costs. While the Hildebrand solubility parameter concept has been previously used to determine the onset point of asphaltene precipitation in the stock tank oils, it does not capture the effect of dissolved gases on the onset solubility parameter. An empirical correlation between the onset solubility parameter and the molar volume of precipitants has been used to estimate the effect of dissolved gas on the onset solubility parameter of live oil. However, extrapolation of this correlation to higher temperatures and pressures is only valid over a very limited range. Consequently, a model was developed to predict the onset solubility parameter under live oil conditions based on the thermodynamic partial Gibbs free energy rule. Solubility parameters of the stock tank oil were determined and coupled with the equation of state models based on the PVT data of the live oils to predict the asphaltene instability of a live system. To validate the model, high temperature and high pressure experiments were performed using blends of live oil and miscible injection gases and the onset points of asphaltenes were determined. This model is capable of predicting asphaltene instability under live conditions over a wide range of temperatures and pressures.

A review on methods of determining onset of asphaltene precipitation

“…They identified the intersection point of these two trends as the asphaltene precipitation onset. Kraiwattanawong et al (2007) stated that selecting the minimum point of light absorbance versus volume fraction of precipitant plot results in an inaccurate determination of the asphaltene precipitation onset. They showed that plotting the derivative of light absorbance with respect to volume fraction of precipitant versus volume fraction of precipitant provides a more precise value of asphaltene precipitation onset.…”

On the effect of temperature on precipitation and aggregation of asphaltenes in light live oils

“…nC 5 or nC 7 ) at standard lab conditions. The majority of the previous works on thermodynamics of asphaltene stability are based on the assumption that asphaltene destabilization is an instantaneous phenomenon and the kinetics effects associated with asphaltene precipitation and aggregation have not been considered . The many experiments reported to date indicate that asphaltene aggregation is a gradual and time‐dependent process that occurs very slowly …”

A new approach for quality control of reservoir fluid composition data using live oil solubility parameter

“…reduction of live oil solubility parameter from initial pressure to bubble point pressure) is also because fluid expanding results in decreasing interactions among molecules/particles [44]. After the bubble point is reached, dissolved gas starts escaping from the liquid phase causing the live oil solubility parameter to increase with a pressure reduction because gas has low solubility parameter [44,45]. As a result of above discussion, the solubility parameter of live oils is found to be a linear function of their densities measured at high pressures and temperatures.…”