Nanoscale Investigation of Asphaltene Deposition under Capillary Flow Conditions

Abstract

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.

Characterization of the Interfacial Material in Asphaltenes Responsible for Oil/Water Emulsion Stability

“…The smooth nanosheets could easily fold and wrinkle and their thickness was about 15–35 nm, which is in excellent agreement with previous studies. ,− They provided a large surface area on which asphaltene aggregates (Figure c) and fine clay particles (Figure e) adsorbed from the bulk solution, increasing the surface roughness. A recent study showed that small- and medium- sized asphaltene clusters are more prone to adsorb on surfaces than larger ones, due to their high mass diffusivity . The diameter of these clusters is less than 20 nm, and they can be seen as scattered patches on some nanosheets, as shown in Figure d.…”

“…A recent study showed that small-and medium-sized asphaltene clusters are more prone to adsorb on surfaces than larger ones, due to their high mass diffusivity. 30 The diameter of these clusters is less than 20 nm, and they can be seen as scattered patches on some nanosheets, as shown in Figure 3d. On the contrary, the clay platelets are about 100−200 nm large and fall within the size of Gibbsite clays investigated by Coelho and coworkers.…”

Study of Asphaltene Deposition onto Stainless-Steel Surfaces Using Quartz Crystal Microbalance with Dissipation

“…This bell-shaped curve was observed for both low and high dilutions and was also reported in an earlier study of crude oil adsorption onto gold surfaces. 42,63,78 The asphaltene fraction that has precipitated out at the solubility with 70 vol% heptane content seems to be the fraction that produces the largest deposition. From an earlier study of asphaltene solubility fractions using fluorescent depolarization technique, 79 it was found that different solubility fractions have the same molecular species but differ in the amount of each species.…”

Yen–Mullins Model Applies to Oilfield Reservoirs

“…The aggregation number of ∼7 for nanoaggregates is shown most clearly by surface-assisted laser desorption ionization mass spectrometry. ,, Combined small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) show similar aggregation numbers and also show that the nanoaggregate has a structure with a PAH stack in the interior and peripheral alkanes in the exterior. − Similar values for the CNAC and aggregation number have been obtained by direct current (DC) conductivity, − NMR, ,, and centrifugation. , The second hierarchical aggregation threshold corresponds to cluster formation at the critical cluster concentration (CCC) of 10 –3 mass fraction, which was first shown by resolving the kinetics of floc formation , and was confirmed by DC conductivity , and centrifugation. , Both of these methods obtain cluster aggregation numbers of <10 nanoaggregates. SANS and SAXS studies obtain comparable aggregation numbers for asphaltene clusters. − ,, AFM imaging also confirms the size of clusters …”

“…[101][102][103]112,113 AFM imaging also confirms the size of clusters. 114 In this paper, asphaltene gradients in oil reservoirs are examined with a focus on testing the validity of the Yen− Mullins model for reservoir crude oils. EoS modeling for the different reservoirs requires independent assessment of the state of equilibrium, as discussed.…”