This study discusses experimental and modeling results of asphaltene aggregation and deposition using various nalkanes as precipitants to destabilize asphaltenes from a crude oil. The amount of asphaltenes precipitated as a function of precipitant carbon number and concentration was obtained after monitoring the slow kinetic aggregation process. A geometric population balance was used to estimate the asphaltene−asphaltene collision efficiency during bulk aggregation. The results revealed that, for a fixed volume fraction of precipitant, the collision efficiency decreases with increasing precipitant carbon number, resulting in slower aggregation. The tendency for asphaltenes to deposit was measured using capillary flow experiments under similar conditions. Similar asphaltene deposition behavior was obtained when the results were normalized by the asphaltene solubility and other experimental factors. A modified aggregation model was applied to the results and revealed that the difference between the asphaltene and solution solubility parameters is a dominant predictor of asphaltene aggregation. The time required to form an initial deposit inside the capillary apparatus was also found to correlate with the difference between asphaltene and solution solubility parameters. However, the deposition rate of asphaltenes in the capillary apparatus did not correlate with the collision efficiency or solubility parameter difference, contrary to initial expectations. The results suggest that mass transport barriers in the apparatus provided sufficient resistance to deposition as to limit observable correlation between the deposition rate and collision efficiency.

Time-resolved size and structure measurements of asphaltenes while in the process of precipitating were monitored for the first time using ultra-small-angle X-ray scattering. The results revealed that asphaltenes precipitating from a heptane-toluene mixture demonstrate a hierarchical structure of an asphaltene-rich phase (e.g., droplet) that further agglomerates into fractal flocs. The fractal flocs that form by the agglomeration of the asphaltene-rich phase are what is commonly detected by optical microscopy above the precipitation detection point. The surface of the asphaltene-rich phase is initially rough and transitions to a smooth interface, as would be expected for a highly viscous liquid. Simultaneous small-angle X-ray scattering measurements were also performed to investigate the structure of soluble asphaltenes, providing comprehensive structural characterization from the nanometer- to micrometer-length scales as a function of time. Further, the results demonstrate that the size and concentration of asphaltenes remaining in solution (e.g., soluble asphaltenes) do not change during precipitation, whereas the structure of insoluble asphaltenes varies. The ability to measure the properties of asphaltenes as they undergo precipitation opens new opportunities for understanding the fundamental mechanisms of asphaltene deposition and aggregation and the impact of chemical inhibitors to alter these processes. The universality of these conclusions and how specific properties vary as a function of asphaltene source and solution properties can provide valuable insight into asphaltene behavior.

In this work, kinetic asphaltene precipitation was investigated using temperature fluctuations. Asphaltene precipitation was previously identified as a fully reversible process by altering the solution pressure or composition but only partially reversible using temperature changes. Slow kinetic asphaltene precipitation plays a critical role in the accurate monitoring of asphaltene precipitation, and previous reversibility studies need to be revisited in light of this phenomenon. Previous studies used a combination of precipitant addition and temperature changes to conclude that precipitated asphaltenes do not fully redissolve when the mixture temperature is changed. Modeling results reveal that precipitated asphaltenes should not be expected to redissolve, regardless of the magnitude of temperature changes, after a precipitant (e.g., dodecane) is added to the mixture. Consequently, this study was designed to isolate the influence of slow kinetics, precipitant addition, and temperature changes on the solubility and reversibility of asphaltene precipitation. Temperature cycling experiments were performed to investigate the reversibility of asphaltene precipitation and revealed that the process is fully reversible with temperature changes. This finding reinforces that, for the system in this study, asphaltene phase behavior is controlled by equilibrium thermodynamics and not a colloid stabilization model.

This study proposes a novel approach to investigate the diffusion-limited deposition of asphaltenes in flow lines to better understand their nanoscale behavior at interfaces and aid in the development of more accurate remediation methods and modeling tools. Experiments were first designed by flowing asphaltene-in-toluene solutions through capillary polyetheretherketone tubes and imaging their cross-sectional areas using high-resolution scanning electron microscopy. A two-step digital image analysis using machine-learning concepts was applied and consisted of a (1) denoising process by analyzing the local and global bias and variance and (2) binarization process to improve the quality of image segmentation. As a result of polydispersity, particles on the tube surface were categorized into nanoaggregates (1.5–4 nm), small clusters (SCs, 4–10 nm), medium clusters (MCs, 10–20 nm), large clusters (LCs, 20–100 nm), and extra-large clusters (XLCs, >100 nm). A Langmuir adsorption isotherm was measured in toluene with an adsorption free energy of −29 kJ/mol, in agreement with previous work. Nanoaggregates and SCs were the main constituents of the adsorption layer as a result of their high mass diffusivity. A competing behavior between aggregation and adsorption was observed as the asphaltene concentration increased in toluene. Enhanced self-assembly in the bulk phase led to a continuous decrease in the number of adsorbed particles. Adding n-heptane to toluene at different volume ratios prompted the deposition of MCs with a peak in the particle size, number, and mass density observed in the vicinity of the onset of precipitation. These clusters are potential precursors to fouling because they constitute the building blocks of larger particles that grow over time on the surface. Limiting their deposition could be achieved by either increasing the flow rate or introducing chemical inhibitors that promote the formation of larger aggregates in the bulk phase under given flow conditions. The novel insights gained from this study reveal that MC-rich petroleum fluids are more prone to flow assurance challenges and that effective flow enhancers are those that promote the aggregation of MCs into particles that are too large to deposit.

Inorganic solids are often present in real heavy oil systems, but are typically absent in asphaltene laboratory studies. For the first time, we investigate the influence of inorganic solids on the kinetic precipitation of asphaltenes. In contrast to potentially slow kinetics in homogeneous liquid petroleum mixtures, rapid kinetic precipitation of asphaltenes was observed when inorganic solids were present in the system. A combined homogeneous aggregation and diffusion-limited heterogeneous nucleation model was developed to quantify the rate of asphaltene precipitation under the explored experimental conditions. The rate of heterogeneous nucleation was generally observed to be faster than the rate of homogeneous aggregation; however, this trend was reversed as the solvent strength decreased and greater quantities of asphaltenes precipitated. The inorganic solids were characterized, and kaolinite clay was observed in the studied samples. This investigation leads to a clearer understanding of the complex asphaltene aggregation process that occurs in real and heterogeneous systems. The competing pathways of asphaltene precipitation provide novel insight into asphaltene precipitation and deposition, in addition to new experimental strategies to measure asphaltene solubility.