The intrinsic characteristics of crude oils, combined with the pressure and low temperature of the seabed, can lead to flow assurance problems, compromising production. The low temperature causes a decrease in the wax solubility, eventually reaching the wax appearance temperature (WAT). Precipitated crystals can cause deposition on the tube walls and can affect production as they increase the pressure drop, decreasing flow during production. Depending on molecular weight, molecular structure and carbon number distribution, waxes can be called macrocrystalline and microcrystalline. Macrocrystalline wax mainly has a carbon number distribution of approximately C20 to C40 and has a linear molecular structure, so hydrocarbons tend to form large plates. Microcrystalline wax generally has chains with carbon numbers between C30 to C70 and its crystals tend to form smaller crystals. In the present work, the impact on gel strength was evaluated as a function of wax type (macrostructure and microstructure), wax concentration and applied shear rate. Understanding these impacts makes it possible to improve the models of wax deposition and, consequently,
the methods of preventing and mitigating the phenomenon.
Tag: Wax deposition
Evaluation of the effect of wax concentration in crude oils on the dimensionless temperature plateau
One of the main challenges regarding crude oil production under low temperatures in deep seas concerns the wax deposition in pipelines. This issue presents a high level of complexity and the governing mechanism is still under investigation. Considering the deposition phenomenon and seeking to improve the knowledge about the governing mechanism and the main variables influencing the final behavior, the present work analyzes how different oil compositions and mixtures, under different flow conditions, impact the deposit wax thickness. It also compares the dimensionless temperature for different wax concentrations, which allows to evaluate the behavior of the temperature gradient, and verifies the behavior of the plateau under different Reynolds numbers. Finally, the mechanism proposed in this work is compared with the behavior proposed by the main governance mechanisms discussed in the literature in the last decades.
Evidence that wax deposition is a phase transition rather than a molecular diffusion phenomenon
Over the past five decades, wax deposition has been widely considered a mass transfer-controlled phenomenon. Despite the highly inaccurate predictions, engineers cannot accurately predict the final thickness of the deposit, the hypothesis that wax deposition is a mass transfer phenomenon was not commonly questioned, but this has recently changed. This paper shows evidence that wax deposition is limited by phase transition (heat transfer), by analyzing a vast experimental matrix previously presented in the literature and clearly showing that the thickness decreases as the Reynolds number increases, which cannot be explained by molecular diffusion alone, also by showing that the Reynolds number does not influence the ratio between the deposit’s thermal resistance and the total thermal resistance (dimensionless temperature) for all cold flow experiments, which is further evidence of phase transition. When comparing the limits of the molecular diffusion approach with the experimental data, without any fitting parameter, one observes that not only the experimental data cannot be predicted, but the trend is also incorrect. When using the phase transition model (heat transfer), the accuracy in the thickness prediction is high, which is evidence that what limits the wax deposition is the phase transition. This shows that heat transfer equations can accurately predict wax deposition thickness. Since all wax deposition simulators have the heat transfer calculations, to improve their predictions, one must only implement a single boundary condition.
Critical Review on Wax Deposition in Single-Phase flow
Wax deposition is a costly problem for the O&G industry, especially for pipelines in cold environments. For at least three decades, the scientific community has overwhelmingly agreed that molecular diffusion is the main mechanism for wax deposition. There are, however, severe problems with models based on molecular diffusion. They rely on untested hypotheses and several empirical correlations; hence, they can hardly predict the experimental data from laboratory. For real fields, the prediction is no better than an educated guess – heuristic solutions. Several research areas in wax deposition need to be better understood, and these are discussed in detail here, with a highlight to the most important concern: the controlling mechanism. Is wax deposition indeed a mass transfer controlled phenomenon? What is the evidence supporting this “general knowledge”? Is it possible that, for some conditions, mass transfer is dominant, and for others, the phase transition mechanism is dominant? Apart from this, we also discuss other issues: the accuracy of empirical correlations for diffusivity, the behavior of crystals in the deposit and how that influences the general deposit behavior, non-Newtonian influence on heat transfer and mass transfer, among others. Wax deposition is a complex topic that has been reviewed over and over. In this review, however, we focus on both presenting what has been discussed in the literature and make a critical analysis. The goal is to increase the general knowledge by highlighting a number of gaps and challenges related to this complex and financially exorbitant issue.