Long-Time Kinetic Impact on Key Factors Affecting Asphaltene Precipitation

Abstract

The employment of predictive techniques combining kinetic and thermodynamic analyses is the succinct solution to effectively control asphaltene precipitation during crude oil production. Although thermodynamic processes and conditions have been well studied in the literature, the effect of long-time kinetics on the key factors affecting the precipitation of asphaltenes was not critically studied. This work employed a model oil for asphaltene precipitation long-time kinetic observations. Filtration and confocal microscopy experimentations for time periods of 0–7200 min were utilized to study asphaltene yields and sizes at room and high temperatures (25, 50, and 70 °C), rotation speeds (60 and 150 rpm), and precipitant concentrations (50 and 60 wt %). The results from both experiments were fitted with DLVO models. The experimental results confirmed that the effects of temperature, rotation speeds, and precipitant compositions on asphaltene precipitation were significantly affected by time. The asphaltene yields increased from 27 to 83% within 7200 min when the heptane concentration increased from 50 to 60 wt %. Conversely, increasing temperatures from 25 to 70 °C reduced the cumulative asphaltene yields by 20–40% when observed for a long time (0–7200 min). Significant reductions in the mean equivalent diameter (MED) of precipitating asphaltenes upon increasing temperature within the studied timeframes were observed via the confocal experimentations. The rotation speeds also showed an inverse relationship with asphaltene yields and particle size. A reduction from 30 to 50% of the yield was observed as the rotation speeds of the system were increased from 60 and 150 rpm. The results of this work confirmed the significant impact of contact time on the precipitation of asphaltenes. In addition, DLVO theory employed to predict the experimental data obtained from the confocal microscopy fitted the data with mean absolute errors ranging from 4.44 to 5.15, which showed significant limitations upon increasing temperature. This development is progress toward enhancing the predictability of a production route devoid of asphaltene-related problems.

Advantages of a Millifluidic Approach to the Characterization of Long-Time Solvency Effects on Bulk Asphaltene Precipitation

“…17 Other common methods to quantify the extent of precipitation of asphaltene solids as a function of solvent composition include optical microscopy or centrifugation and filtration to measure the total concentration of precipitated solids. 1,17,28,29 Among these studies, equilibration time of the sample was not considered in detail, and it was assumed that steady state was reached over the duration of the experiment. However, inherent complexities of asphaltene dispersions, including optical density, adsorption to substrates and containers, and limited molecular properties to track, lead to varied results that are sensitive to measurement technique and time allotted for sample aging.…”

“…Because asphaltene precipitation is driven by solvency effects, , and solubility varies greatly based on the polarity of the solvent environment, , the microstructures that form in bulk dispersions, and subsequent aggregation and precipitation behaviors, are highly dependent on the solvent conditions. , Various techniques have been applied to quantify aggregation and precipitation of asphaltene solids, although the determination of the precipitation onset and mechanistic models are currently a matter of discussion. ,, Techniques include near-infrared and UV–vis spectroscopy methods to identify the onset and conditions required for asphaltene precipitation by measuring the alkane fraction in which the absorption deviates from Beer’s law . Other common methods to quantify the extent of precipitation of asphaltene solids as a function of solvent composition include optical microscopy or centrifugation and filtration to measure the total concentration of precipitated solids. ,,, Among these studies, equilibration time of the sample was not considered in detail, and it was assumed that steady state was reached over the duration of the experiment. However, inherent complexities of asphaltene dispersions, including optical density, adsorption to substrates and containers, and limited molecular properties to track, lead to varied results that are sensitive to measurement technique and time allotted for sample aging.…”

“…However, to take these measurements, the precipitated solids were removed and weighed, thus stopping the experiment at a set time point and longer-time effects were not investigated. It has been determined that the rate of precipitation can vary with solvent composition, resulting in precipitation times varying and the measured values for the fraction of precipitated asphaltenes would vary depending on the experimental time scales. , Without in situ techniques, the long-time effects of solvency regarding bulk stability or steady-state behaviors of these dispersions will remain poorly understood. In macroscale experiments, the opacity of asphaltene systems makes it difficult to visualize precipitation or deposition of solids without halting the experiment.…”