The effect of ethylene-vinyl acetate copolymer on the formation process of wax crystals and hydrates

Abstract

Ethylene-vinyl acetate copolymer (EVA) as a kind of effective polymeric pour point depressant has been extensively used in the pipeline transportation of crude oil to inhibit wax deposition and improve the low temperature fluidity of crude oil. In this work, molecular dynamics simulations were performed to investigate the effect of EVA on wax-hydrate coexistence system to evaluate the application potentiality of EVA to the flow assurance of deep-sea oilegasewater multiphase flow system. Our simulation results reveal that wax molecules gradually stretched and stacked from random coiling to a directional and ordered crystalline state during the process of wax solidification. The strong affinity of polar vinyl acetate side chains of EVA to neighboring water molecules made the EVA molecule prefer being in a curly state, which disrupted the ordered crystallization of surrounding wax molecules and delayed the solidification rate of wax cluster. In addition, it is found that EVA cocrystallized with wax molecules to form eutectic when the wax was fully solidified. The simulation results of hydrate nucleation and growth show that the EVA molecule displayed a two-sided effect on gas adsorption of wax crystals, which was the key factor that affected the nucleation and growth of hydrates in the methane-water system. The nonpolar hydrocarbon backbone of EVA increased the diffusion rate of methane and water, allowing more methane to diffuse to the surface of wax crystals, reducing the methane concentration in aqueous solutions and inhibiting the hydrate formation. On the other hand, the nonpolar vinyl acetate chains had a repulsive effect on methane, which reduced the adsorption area of methane on the eutectic surface and decreased the adsorption threshold value of the wax crystal. The excluded methane molecules would continue dissociating in the aqueous phase and participating in the nucleation and growth process of hydrates. Therefore, the probability of hydrate formation would be increased. It was worth noting that the inhibition performance of EVA on hydrate formation mainly played a significant role in the system with small wax crystal, while its hydrate promotion effect played a dominant role in the system with lager wax crystal. In summary, EVA could significantly inhibit both of the wax and hydrate deposition for the wax-gas-water multiphase system with low wax content. When the wax content in the system was high, the role of EVA was mainly played in the alleviation of wax crystallization rather than the gas hydrates. The results of the present work can contribute to a better understanding of EVA on wax deposition and hydrate formation, and provide theoretical support of the potential industrial applications of EVA.

1. Introduction

With the exploration and exploitation of the onshore and deep-sea oil and gas resources, the products gradually change from single-phase to multiphase (oil, gas, and water) flows. During the multiphase transportation process, wax deposition and hydrate formation can easily occur under appropriate thermodynamic conditions, leading to pipeline blockage, potentially significant financial losses, and safety issues [1e4].

Currently, for the crude oil single-phase flow systems, the most widely employed approach to alleviate the wax deposition problem involves incorporating pour point depressants (PPDs), which could improve the low-temperature flowability of waxy crude oil with a small dosage (tens to hundreds parts per million) [5e7]. Representative PPDs comprise polymethacrylate, ethylene-vinyl acetate copolymer (EVA), maleic anhydride copolymer, poly-α-olefin copolymers, and their derivatives [8e14]. Among them, EVA as a kind of effective polymeric PPDs has been extensively studied [15]. The EVA molecule is composed of polar vinyl acetate (VA) and nonpolar alkyl long chains of different lengths, in which the VA content directly affects the crystallization ability of EVA copolymers [16,17]. The main chain of the EVA molecule preferred to be adsorbed in parallel with the wax surface involving the polar VA group staying upside of the surface to interfere with their crystallization.