Wax deposition and gel formation have been crucial problems to the oil industries. The deposition of waxes would lead to gelation which blocks flow in pipelines and interrupts steady production of oil at offshore fields. The plugged gel needs to be restarted by applying a pressure that is much higher than the steady pressure in normal production flow, requesting for higher capacity pumps and piping dimensions. However, the available conventional restart pressure over-predicts pumping pressure and pipeline dimensions as it neglects the occurrence of thermal shrinkage and the resulting gas voids formed within the gelled crude oil and assumes waxy crude as single phase and incompressible fluid. The paucity of researches on quantification and locating of voids within the gel across and along the pipelines maintains the assumption made earlier to this date. There have been different researches conducted to prove that the over predicted restart pressure and piping dimensions result in excessive unnecessary cost to oil industries. This paper reviews the transportation of waxy crude oil at offshore fields and the previous researches on the behavior of waxy crude oil in Newtonian and non-Newtonian regions. In addition, the restart-ups process and the associated problems encountered, flow assurance and management of waxy crude oil, flow improvers of waxy crude oil gel following sufficient cooling are thoroughly discussed. It further reviews previous researches on thermal shrinkage and intra-gel voids formation in waxy crude oil which would help in the process of developing vigorous wax deposition model and restart pressure equations. Furthermore, different mechanisms used to ease restart pumping of gelled crude oil in pipelines are highlighted.

Flow start-up and transportation of waxy crude oil in pipelines-A review

Chemical inhibition is one of the methods to mitigate deposition of wax in pipelines; however, the efficiency of the inhibitors needs to be validated in the laboratory prior to field applications. Cold finger, as the most commonly used experimental setup in the field, has quite a different shear regime and temperature field compared to the flow loop and field production pipeline. This paper investigates the flow field (cold finger and flow loop) effect on the wax deposition and the performance of wax inhibitors under the same initial temperature difference and comparable shear stress conditions. The normalized deposit mass, wax content, carbon number distribution, and wax mass flux were investigated in these two different flow fields, and the performance of wax inhibitors was also investigated. Moreover, the results from this study are expected to serve as a basis for selecting proper inhibitors for field applications based on laboratory testing.

Effect of the Flow Field on the Wax Deposition and Performance of Wax Inhibitors: Cold Finger and Flow Loop Testing

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 ...

The Effect of Operating Temperatures on Wax Deposition

Investigation of inhibitors efficacy in wax deposition mitigation using a laboratory scale flow loop

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 mass-transfer phenomena at the oil–deposit interface. In addition, this study also revealed that the dynamics of the competition between all of thes...

Counterintuitive effects of the oil flow rate on wax deposition

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 acc...

Single-Phase Wax Deposition Experiments

Influence of wax inhibitor on fluid and deposit properties

While diffusion as the major mechanism for wax deposition has been investigated in past decades, wax gelation has mostly been studied in quiescent conditions and is considered to be less significant than diffusion in flow conditions. In this study, gelation has been observed as a major mechanism for the formation of wax deposits in oil/water stratified flow. The experiments are carried out in a state-of-the-art flow loop using a North Sea gas condensate and formation water. The flow map study using reflex camera and X-ray tomography reveals that most of the completely stratified flows occur at low total flow rates of oil and water, which correspond to low shear stresses at the wall. It was found that the carbon number distributions of the wax deposits formed in this region have very low fractions of heavy components and are very close to the distribution of the deposit that is only formed by gelation. It was further revealed that the deposit thickness increases with increasing degree of gelation, which co...

Lene Amundsen

Papers

Single-Phase Wax Deposition Experiments

The Effect of Operating Temperatures on Wax Deposition

Counterintuitive effects of the oil flow rate on wax deposition

Influence of wax inhibitor on fluid and deposit properties

Wax Deposition in Stratified Oil/Water Flow