Co-Deposition Mechanism of Waxes with Asphaltenes and Scales in a Vertical Wellbore of Gas Condensate Well

With the increased exploitation of gas condensate resources, the problem of wax deposition in wellbores is increasingly severe. Hence, this paper reveals the co-deposition mechanism of vertical gas condensate wellbores with waxes, asphaltenes and scales from a microscopic viewpoint by molecular dynamics (MD) simulation techniques. Several experiments were also utilized to determine the basic physical properties of the condensate. To avoid inaccuracies caused by simplifying the condensate multiphase system using a single component, eight typical components were selected to enhance modelling precision according to actual well fluid characteristics. The wellbore, condensate, asphaltene and scale systems modelled by the MD simulation techniques were validated with the corresponding data from the National Institute of Standards and Technology (NIST), and this actual error of the results after stimulating was less than 10 %, which met the error criteria and verified the construction of the system model as realistic and feasible. The simulations investigate the co-deposition mechanism of waxes with asphaltenes and scales in vertical wellbores at three levels: diffusion mechanism, aggregation properties and co-deposition behavior. The simulation results show that as the waxy components in the models increases, the aggregated and adhered condensate layer at the wellbore becomes thinner and the "replacement" between the waxy components and condensate molecules begins happening, and this deposition phenomenon gradually strengthens. When in the asphaltene-condensate system, the promotion effect on the deposition of wax molecules becomes more obvious as these asphaltene components increases, but up to 2% asphaltene components, the wax deposition behavior is inhibited conversely. Simultaneously, asphaltenes with a larger variety and number of heteroatoms contribute more significantly to this co-deposition behavior of waxy components and asphaltene components. For these scale-condensate systems, the most prominent influence for the aggregation and deposition behavior of waxy components is exerted by sulphate scale, with the most noticeable co-deposition effect. Its interaction energy (absolute value) is the highest, reaching 3840 kJ/mol. The results in the article can offer a basis and instruction to this targeted management for wax deposits in vertical wellbores of gas condensate wells.