Molecular dynamics simulation of the diffusion crystallization mechanism of the binary alkane mixture nC5H12–nC24H50 under the water-wetting condition of a pipe wall.

[Display omitted] • With the decrease of temperature, the order degree of the system is strengthened and the fluidity is weakened. • The diffusion coefficient of the H 2 O molecule is greater than that of the n C 5 H 12 molecule by a factor of ∼2. • At the same temperature, the diffusion coefficient of the n C 24 H 50 molecule in n C 5 H 12 is greater than that in H 2 O. • At temperatures of 50–20 °C, n C 24 H 50 molecules precipitate from the oil and gather on the pipe wall surface. Wax deposition on the inner wall of oil pipes, sucker rods, oil- and gas-gathering pipelines and equipment caused by wax crystals has been a challenging problem in the petroleum industry. At present, most of the marine terrestrial transitional facies and continental shale oil and gas being explored and developed in China have high-wax-content volatile oil or condensate, and many shale oil and gas wells have been seriously blocked by wax deposition. In this work, the wax deposition behavior of the binary alkane mixture n C 5 H 12 – n C 24 H 50 at different temperatures under the condition of water wetting of a pipe wall was studied through molecular dynamics simulation. When the temperature is 130–70 °C, the wax molecules are loosely distributed without obvious aggregation, and the pipe wall is mainly occupied by water molecules, exhibiting a water-wetting state. With the decrease in temperature, the distance between n C 24 H 50 molecules decreases correspondingly, which enhances the interaction between wax molecules and increases the probability of mutual aggregation. When the temperature is 50 °C, a small amount of n C 24 H 50 molecules pass through the water molecular layer and gather on the pipe wall. When the temperature is <50 °C, the binding energy between the crystal surface of the pipe wall and the molecule n C 24 H 50 exhibits a first-order phase transition, and the molecule n C 24 H 50 can more easily bind to the pipe wall than the molecule H 2 O. When the temperature is 40–20 °C, the aggregation amount of n C 24 H 50 molecules on the pipe wall further increases, exhibiting an obvious crystallization tendency, and a small amount of n C 5 H 12 molecules begin to aggregate with n C 24 H 50 molecules through the water molecular layer on the pipe wall. At the same temperature, the diffusion speed of n C 24 H 50 molecules in n C 5 H 12 is faster than that in H 2 O. [ABSTRACT FROM AUTHOR]