Effect of dispersed water on the paraffin crystallization and deposition of emulsified waxy crude oil via dissipative particle dynamics

Highlights

• Influence of water on nucleation and crystallization of emulsion is discussed.

• The nucleation and structural strength of wax crystal is determined by critical water cut.

• Water deteriorates the low-temperature rheological behavior of emulsions.

Abstract

The gelation of emulsions is a notorious problem during the transportation of crude oil. In this study, the impact of dispersed water on the paraffin crystallization behavior in waxy crude oil was investigated using the mesoscopic dissipative particle dynamic method. The results indicated that water cuts affect the nucleation of wax crystals. The homogeneous nucleation of waxy crystals is important in systems when the water cuts are lower than the critical value (47.75 mol% of the studied systems), whereas heterogeneous nucleation becomes dominant when the water cuts are larger than the critical value. In addition, the water accelerates the nucleation and build-up of the crystal structure. Furthermore, the wax appearance temperature of the system experiences a steady increase when the water cut is lower than the critical value. The wax appearance temperature increases abruptly when the water cuts are larger than the critical value. This indicates that the injection of water further deteriorates the low-temperature rheological behavior of the emulsions. The critical water cut also impacts the competitive capture relationship between the water molecules and paraffin molecules.

Introduction

Currently, most of the extracted crude oil has a high freezing point, high wax content, and high viscosity [1]. Generally, crude oil comprises saturated alkanes, aromatic hydrocarbons, paraffin wax (C18–C50), asphalt, resin, and so on [2], [3]. Wax crystals would gradually precipitate from the oil phase when the temperature of the system is below the wax appearance temperature (WAT) [4], which results in the safety of flow security, equipment corrosion, and economic losses in the oil industry [5].

In addition, because secondary and tertiary (even quaternary) oil recovery are used in the exploitation of current oil fields, the water cut of waxy crude oil is increasing dramatically. Generally, emulsions can be divided into water-in-oil (W/O) and oil-in-water (O/W) emulsions [6]. The emulsification of water in waxy crude oil affects the normal operation of oil fields. Thus, the multi-phase transportation technology of emulsified W/O waxy crude systems has gradually become a prevalent method in both land and offshore oil fields [7].

The abovementioned problems have been extensively investigated via experiments and theoretical models. The cold finger apparatus [8] and flow loop experiments [9] are widely employed to predict the wax molecule diffusion rate and solubility [10]. In addition, laser scanning confocal microscopy [11] and cross-polarized microscopy [12], [13] are also used to investigate the wax crystal structure. However, most previous studies were based on single-phase flow theory. For multiphase emulsified waxy oil, the influence of dispersed water on the wax crystallization process is still controversial. Fan et al. [14] stated that emulsified water causes flow obstructions on the wax molecules, which results in the tortuosity of wax diffusion paths. Sun et al. [15] reported that the strength of the gel structure was influenced by the water cut in crude oil. More specifically, the gel strength of the emulsions is enhanced by increasing the water cut. Merad et al. [16] studied the relationship between the rheological properties and flow behavior of Pickering emulsions. Their results indicated that when the water is cut up to 50 wt%, the yield stress and viscosity exhibit an exponential increase, whereas the phase inversion point is not reached (W/O emulsion to O/W emulsion). However, Kyeongseok et al. [17] concluded that the water droplets destroy the emulsion gel structure, which results in a decrease in the gel point and yield stress. Piroozian et al. [18] concluded that dispersed water droplets can be adsorbed by paraffin crystallization at temperatures below the WAT, and the viscosity of free oil decreases with an increase in water cut and after the inversion point (W/O emulsion to O/W emulsion). Wang et al. [19] investigated the influence of water cut on the restart of model waxy crude oil flows. The results indicated that the pressure required to restart the gelled crude oil decreases with an increase in the water cut, which means that the yield stress and the gel strength of waxy crude oil decreases. In addition, Pasco et al. [13], [20] observed that the critical water cut (approximately 50%) influences the positive or negative effects of water on wax gel strength. In other words, the gel structure and morphology are subject to synergy and competition between the paraffin wax and water droplets. Therefore, the impact of dispersed water on paraffin crystallization and deposition in crude oil systems should be further studied. Vargas et al. [21] studied the influence of dispersed water on the yield stress of gelled waxy crude oil. They observed that the yield stress tends to initially decrease and then increase as the water cut increases, which revealed the competition between the number of crystal nuclei and the size of the final crystals. The effect of water droplets on the gel strength and rheological properties of crude oil is uncertain. For most common W/O emulsions in the oil industry, the most widely accepted perspective is that the presence of dispersed water droplets deteriorates the overall rheological and thermodynamic properties. However, the influence varies with the operating conditions. Therefore, more in-depth research is required to address this problem.

With the development of computer technology over the past decades, microscopic and mesoscopic models have also been employed as effective tools to study paraffin crystallization, phase transition, morphology characteristics, and aggregation behaviors. At the microscopic scale, molecular dynamics (MD) simulations have been widely used to reveal these mechanisms. For example, using MD, San-Miguelet et al. [22], [23] simulated the deposition process of paraffin wax molecules (C28) on different surfaces of Fe₂O₃ and observed that the paraffin molecules are easily deposited on the crystal face (0001). Wu et al. [24] employed MD methods to investigate the effect of ethylene–vinyl acetate pour point depressants (EVA PPDs) on wax deposition. Gan et al. [25], [26] analyzed the effect of the nucleus shape on the homogeneous and heterogeneous nucleation of wax. Chen et al. [27], [28] simulated the influence of electric and magnetic fields on the physical parameters of waxy crude oil. Our previous MD study [29] investigated the deposition of paraffin molecules on the surface. The interaction mechanism between wax molecules and PPD molecules was also discussed. In addition, mesoscale dissipative particle dynamics (DPD) simulations are widely employed in this field. Using DPD, Bustamante-Rendón et al. [30] observed that anionic and cationic surfactants can effectively reduce the surface tension of the oil–water interface of crude oil. Wang et al. [31] used DPD to verify their experiment on the effect of Janus nanoparticles on the oil–water surface tension. Ivan et al. [32] simulated the deposition of asphaltene on hematite (Fe₂O₃) using MD and DPD. David et al. [33] used DPD to simulate the crystallization process of single- and multi-component linear alkanes and isoparaffins. Rao et al. [34] studied the mesoscopic morphology and evolution mechanisms of a paraffin phase change material using DPD. However, to the best of our knowledge, the impact of dispersed water on paraffin crystallization and deposition has rarely been investigated via MD or DPD.

Because the paraffin molecules have a macromolecular structure, our previous MD research was inefficient in simulating the emulsified waxy crude oil system [29]. Consequently, in this paper, the impact of dispersed water on paraffin crystallization and deposition is discussed using the DPD method. This paper is also expected to provide useful insights into the interaction of the fluid–solid interface.