The behavior of waxy crude oils in subsea production lines has been successfully investigated in a 2 in. deposition flow loop. A North Sea waxy gas condensate was used to investigate wax deposition in turbulent single-phase flow under different temperature and flow conditions. A reliable and accurate procedure for determination of wax thickness and wax roughness from pressure drop, weight, and laser measurements has been developed. The laser technique is a new and promising method to measure the spatial distribution of wax thickness, which was not captured by the traditional pressure drop and weighing methods. These experiments have led to an increased understanding of the mechanisms of wax deposition, which is needed to develop more-accurate models based on physical effects. These models are then the basis for a more accurate prediction of the rate of wax deposition in production lines. The main finding is that molecular diffusion is indeed the central mechanism that steers wax deposition but that an accurate quantitative description also needs to take the wax composition of the deposit and the effects of shear stress into account. However, for higher oil temperatures it was found that the wax deposit's structure changes from the well-known smooth homogeneous type to a new irregular, patchy type. This deposit cannot be described by the traditional diffusion models. In addition, the experiments were used to confirm that the laboratory-scale measurement techniques that are typically used to determine wax appearance temperature do result in a temperature that coincides with the temperature where wax starts to deposit under realistic flow conditions.

In this study, a deposition model called the Michigan Wax Predictor (MWP) was used to establish criteria as to whether the rate of wax deposition increases or decreases as the oil/coolant temperature is changed in a series of flow-loop experiments. The model was able to predict the effects of the temperature conditions on wax deposition without applying any adjustable parameters. Despite the fact that many previous studies have used the "thermal driving force" to characterize the effect of temperature on wax deposition, it was not the most comprehensive predictive parameter, as it neglects the importance of the solubility curve on wax deposition. This study has revealed that the most important factors affecting deposition are the mass driving force and the shape of the solubility curve. The major impact of the shape of the solubility curve is to affect the change in characteristic mass flux for wax deposition the when the oil and the coolant temperatures are changed. In particular, the differences in the shapes of the solubility curve can be used to explain the discrepancies on the effect of the oil temperature on wax deposition observed in different deposition studies.

The experimental trend of a reduced deposit with an increasing flow rate has been observed in a series of wax studies. Despite the fact that many previous studies intuitively attribute the reason to the "shear removal", the role of heat and mass transfer was frequently overlooked as the true explanation. In the current study, the Michigan Wax Predictor (MWP) was applied to elucidate this trend by analyzing the growth rate of the wax deposit in a series of flowloop experiments from first principles. The model was able to predict the experimentally observed decrease in deposit thickness with an increasing oil flow rate without any adjustable parameters. It was found that three effects exist to affect wax deposition when the oil flow rate is changed, and each one can either increase or decrease the growth rate of the deposit. These effects focus on the heat-and masstransfer phenomena at the oil−deposit interface. In addition, this study also revealed that the dynamics of the competition between all of these three effects can vary as time progresses and that the overall behavior of the wax deposit growth is eventually determined by the most dominant effect of the three. These results have provided important insight for the effects of the oil flow rate on wax deposition.

No abstract

Wax precipitation and deposition are major flow assurance problem in crude oil production and transportation. Knowledge of the amount of wax that precipitates from the crude oil at different temperatures, delineated by the solubility curve, is necessary for accurate predictions of wax deposition in subsea pipelines. The solubility curve is obtained from the wax precipitation curve, which is determined using centrifugation and high temperature gas chromatography (HTGC). However, previous studies have overestimated the amount of precipitated wax by assuming that all heavy alkanes or heavy n-alkanes exist solely in the solid phase. This work addresses this issue by conducting mass balances on the noncrystallized carbon numbers in the centrifuged cake and in the crude oil. A new equation was developed to obtain the solid fraction of the centrifuged cake and crude oil. The wax precipitation curve developed by this new method was compared with the curves determined using the previous methods and differential scanning calorimetry (DSC). The curve determined by this new method was consistent with the results obtained by DSC, while previous methods overpredicted the amount of precipitated wax.