# Oil Sludge Deposition in Storage Tanks: A Case Study for Russian Crude Oil in Mo-he Station
###### Abstract
He divided the deposition into asphaltene deposition (ADE) and wax deposition (WDE), based on which he determined the deposition mechanism and estimated the deposition amount. After analyzing the composition of five crude oil deposits, [PERSON] et al. [?? ] suggest that asphaltene and wax would not interact in the precipitation process, but simply adsorb and settle each other. In addition, similar to [PERSON], they classified the deposits. For the wax dominated sediments, the results showed that the paraffin content and average carbon content in the sediments were higher than those in the crude oil products, because the sediments contained oil residue, asphaltene and a small amount of inorganic salt particles. For the deposits dominated by asphaltenes, the solubility of asphaltenes in sediments was lower than that in crude oil, because they contained some aromatic hydrocarbons and polar substances. This also shows that wax and asphaltene settle together. However, how the two interact and what forces are produced between them need to be further studied at the molecular level. [PERSON] et al. [?? ] described the self-combination and mutual combination of wax and asphaltene through DLA theory (diffusion limited aggregation), and replaced the consideration of particle interaction with adhesion coefficient by assuming that the adhesion coefficient between asphaltenes was 0.8, the adhesion coefficient between wax and asphaltene was 0.4, and the adhesion coefficient between waxes was 1.0. The change curve of the fractal dimension lays a foundation for exploring the coprecipitation of wax and asphaltene from the molecular level. Many scholars analyze the settlement process from the experimental point of view. [PERSON] et al. [17] used the repeated depth pipette method to trace the viscosity, density, and solid content of four sampling points with three different sources of crude oil as the oil source. They finally obtained four stages of sedimentation: chaotic period, temporary stability period, accumulation, and sedimentation period. [PERSON] [17] studied stratification and deposition during crude oil storage through field tests and simulation experiments. In order to obtain the area fraction of the wax crystal, [PERSON] [17] used polarized light microscopy (PLM) to observe the wax crystal distribution in crude oil. The results show that with the increase of storage time, the wax crystal concentration in the oil phase decreases gradually, and finally tends to be stable. [PERSON] [17] et al. injected the oil into the sedimentation tank for standing sedimentation, and obtained the curve of the deposition rate versus deposition time and sampling port height. The results show that the deposition rate increases with the storage time, and the increasing rate changes from fast to slow; the higher the height from the bottom of the tank, the smaller the deposition rate. The most closely related problem to production practice is how to predict sludge deposition. Due to the different settling mechanisms of various substances in crude oil, the interaction between them and the influence on sedimentation are not clear. Therefore, it is difficult to predict the amount of sludge deposition according to the type of components. At present, there are three popular methods to predict oil sludge deposition: the thermodynamic model method, colloidal model method and mathematically model method, all of the three are mathematically complicated. The current popular method is to use the solubility model to predict the deposition of wax and asphaltene. This model considers that wax or asphaltene are dissolved or partially dissolved in oil. They interact with other components in crude oil, and their settlement can be predicted by gas-liquid or solid-liquid equilibrium. The modeling of wax deposition in crude oil is based on the thermodynamic description of solid wax and oil-liquid equilibrium.