Hongying Li

A generalized model for predicting non-Newtonian viscosity of waxy crudes as a function of temperature and precipitated wax☆

Precipitated wax, shear and thermal history have pronounced effects on viscosity and rheological behavior of waxy crudes. On the basis of mechanism of waxy crude rheology, a shear-rate-dependent viscosity model has been developed by applying theory of suspension rheology. This model is characterized by its capability to predict viscosities of crude oils with various thermal and shear history and beneficiated with pour-point-depressants (PPD). Once viscosities at only two temperatures above the wax appearance temperature and apparent viscosities at one temperature in the non-Newtonian regime are known, viscosities or apparent viscosities at any temperatures above the gel point can be predicted by using the model together with the concentration of precipitated wax at that specified temperature. Verification by using 3458 viscosity data points ranging from 5 to 2900 mPa s from 33 virgin crudes and 14 PPD-beneficiated crudes with various thermal and shear history shows that the model predicts viscosities with an absolute average deviation of 7.43%. Furthermore parameters of rheological models such as the consistency coefficient K and the flow behavior index n of the power law model may be obtained by regressing predicted viscosity data and corresponding shear-rates.

Correlations Between the Pour Point/Gel Point and the Amount of Precipitated Wax for Waxy Crudes

Abstract: The amount of precipitated wax is one of the key factors that governs the flow properties of waxy crudes. Experimental results of 24 crudes have shown that approximately 2 wt% precipitated wax is sufficient to cause a virgin waxy crude gelling. Accordingly, a correlation between the pour point and the temperature at which 2 wt% of wax has precipitated out from a crude oil; i.e., T c (2 wt%), and a correlation between the gel point and T c (2 wt%) have been developed. The proposed correlation of the pour point is in accordance with the correlation developed by Letoffe et al. (1995) on the basis of 14 crudes from eight countries. The development of the gel point correlation and further verification of the pour point correlation indicate that there is a relationship between the gelling of virgin waxy crudes and the amount of precipitated wax. According to these correlations and the amount of precipitated wax, which can be determined only with a little sample by thermodynamic models or Differential Scanning Calorimetry (DSC) experiment, the gel point and pour point of virgin waxy crude can be predicted even if the oil sample is very limited. Heat treatment and chemical treatment can greatly improve flow behavior of waxy crudes, and more precipitated wax is present when the beneficiated (thermally beneficiated or PPD-beneficiated) crude oils gel. Experimental results showed that approximately two or three times the amount of precipitated wax presents at the gelling temperature when the oils were in their beneficiated (thermally beneficiated or PPD-beneficiated) conditions.

Advances in rheology and flow assurance studies of waxy crude

Flow assurance is one of the core issues in safe and economical operation of waxy crude pipelines. Its essence lies in flow and heat transfer of the crude. In the past 10 years, the authors’ team has achieved a lot of innovative results in aspects of waxy crude rheology, flow assurance assessment, and pipelining technologies on the basis of decades of studies. The rheological characteristics of waxy crude are much better understood, and a method for quantitatively simulating the effect of flow shear was developed based on some theoretical breakthroughs. Studies of the mechanism of waxy crude rheology have been deepened to the quantitative level. After successful development of efficient numerical-algorithms, accurate simulations have been achieved for various complex flow and heat transfer situations in waxy crude pipelining, and a reliability-based approach to flow assurance assessment has been set up. New pipelining technologies have been developed such as batching pour-point depressant-(PPD-) treated multiple-waxy-crudes, intermittent transport of waxy crudes through long-distance pipelines, and batching hot and cold crudes. By their application, a series of problems hindering safe, efficient and flexible operation of waxy crude pipelines were tackled, demonstrating that transportation technologies for waxy crude have advanced to a new and high level.

Viscosity Prediction of Non-Newtonian Waxy Crude Heated at Various Temperatures

Viscosities are important parameters for design and operation of crude pipelines. The heating temperature is the major factor affecting viscosities of waxy crude below the wax appearance temperature. Below the abnormal point, waxy crude exhibits non-Newtonian flow behavior with the viscosity dependent on the shear rate. Both of these make determination of the non-Newtonian viscosities of waxy crude a very time-consuming job. On the basis of the model for predicting non-Newtonian viscosity of waxy crudes as a function of temperature and precipitated wax, an approach to predict non-Newtonian viscosity of waxy crude heated to various temperatures has been developed only based on a few measurements. The accuracy of prediction by this approach has been verified by 468 viscosity data from the Daqing crude heated at various temperatures. The totally average relative deviation between the measured and predicted viscosity is 9.42%.

Transportation of Waxy Crudes in Batch Through China West Crude Oil Pipeline With Pour-Point-Depressant Beneficiation

The West Crude Oil Pipeline, 1500km long, a newly constructed pipeline, was put into operation in August 2007. Several crudes with different wax content and thus different pour point/gel point are pumped through the pipeline in batch, with the gel point ranging from −10°C to 10°C. In winter, the lowest ground temperature will be as low as 2°C at the depth where the pipeline is buried. The pour-point-depressant (PPD) was used to reduce gel point of the waxy crudes to ensure safety of pipeline operation. To make sure that the selected PPD reduce gel point effectively, field tests were conducted from September to November in 2007 to prepare for winter operation. The PPD was injected to the waxy crudes at the initial station, and gel points and densities were measured at the initial station and several intermediate stations along the pipeline. The test results show that the PPD may effectively reduce the gel point of the crude, but the effectiveness depends on the composition of crude oil; the reduced gel point of the crude may recover during pipelining; reheating temperature at the intermediate stations may seriously affect the flow properties of the PPD beneficiated crude. Reheating the PPD beneficiated Tuha crude oil to the temperature range of 20 ∼ 35°C results in poor flow properties. This indicates that the reheating temperature at the intermediate stations must be properly controlled to avoid falling into this temperature range. Based on this field test, the pipeline was safely operated for last winter.Copyright

Hydraulic Characteristics of Pipelines Transporting Hot Waxy Crudes

Waxy crudes are generally pipelined by means of heating. In general, the friction loss of a pipeline decreases with decreasing flow rate. This is the case of isothermal pipeline. However, a hot oil pipeline operated at low flow rate might show a contrary case, i.e. friction-loss increases with decreasing flow rate. This is an unstable operation state and may result in disastrous consequence of flow ceasing if tackled improperly. For a waxy crude pipeline, this may also be exaggerated by the non-Newtonian flow characteristics at temperatures near the pour point. That is to say, there may exist a critical flow rate for pipelines transporting heated waxy crude, and in order to ensure safe operation, the flow rate of a pipeline transporting hot oil should be no less than this critical flow rate. Based on theoretical analysis and can study, the hydraulic characteristics of pipelines transporting hot waxy crudes was investigated, and an empirical model was developed correlating the critical flow rate QC and the pipelining parameters, such as the average overall heat transfer coefficient, the ground temperature, the heating temperature, etc. Another relationship was found between TZC , the outlet temperature of the pipeline corresponding to the critical flow rate, and the critical flow rate. This TZC is also the lowest pipeline outlet temperature that ensures the normal pipelining operation state. Case study on a 720mm O.D. pipeline transporting heated Daqing waxy crude with a pour point of 36 °C showed that the TZC was in a range of 31∼34.2°C.Copyright

The effect of dynamic cooling on the flow properties of PPD-treated crude oil

Abstract By means of DSC, test of flow parameters, and microscopic observation, this paper studied the relation between the influence on the low temperature flow properties and that on the waxy crystal of the PPD-treated crude oil due to different final temperature of dynamic cooling. It has been experimental confirmed that the influence caused by the final temperature of dynamic cooling on low temperature flow properties and waxy crystal vary from the different final temperature of dynamic cooling. With this temperature increases, the depressive effects first get worse, then improved and finally kept nearly unchanged at higher temperature.