CHOOSING HOW TO BOOST WAXY CRUDE LINE FLOW DEPENDS ON OIL'S QUALITIES

Nour A. El-Emam, Abdel Wahab A. Bayoumi, Ismail M. El-Gamal, A. Abu-Zied
Al-Azhar University
Cairo

In studies of methods for improving pipeline flow of waxy crudes, dilution by either gasoline or kerosine proved more effective than heating or the addition of flow improving chemicals.

Gasoline proved the most effective diluent in the tests of three Egyptian crude oils.

Another method-heating-also led to a significant improvement in the rheological properties of the tested crudes. Wax content of such crudes affects heating efficiency, however, and the greater the wax content in the crude, the better the improvement in the Theological properties.

Flow improvers also improved flow properties-to different extents-of two of the studied crudes. Another was unaffected by any chemical additive.

Additionally, a computer program was developed that calculates the pressure-loss reduction resulting from the improved flow properties of the tested crudes regardless of the method used.

This program led to the conclusion that dilution by gasoline is the best technique for one type of waxy crude represented by GPY-3, while heating is the best method for the other two crude types represented by M-96 and Khalda. Table 1 presents the characteristics of the three studied crude oils.

Final selection of the suitable technique for a specific crude, however, must be decided not only on the basis of such technical considerations as are presented here, but also on the economics for each case.

TECHNIQUES

Some crude oils are so waxy that their transportation by ordinary pipelines is difficult and sometimes impossible.

Several techniques have been proposed for improving the flow properties of such waxy crude oils to make their transportation by pipeline easier. These techniques include the following:

• Heating the crude in the pipeline with or without insulation (OGJ, Apr. 8, 1968, p. 57; Jan. 25, 1982, p. 179)1 *Diluting with light hydrocarbon distillates

• Blending with low-wax content crudes (OGJ, Sept. 18, 1978, p. 159)

• Using water either as an emulsifier or as a layer between the crude and the inside wall of pipe (OGJ, Mar. 6, 1989, p. 39)

• Using chemical additives (pour-point depressants, flow improvers, paraffin inhibitors, wax crystal modifiers)2-5

• Using certain heating/cooling cycles to modify wax crystal growth and thus lower the pour point (crude-oil conditioning).67

The main objective of the present work was to select the appropriate technique of flow improvement for each of certain Egyptian waxy crude oils-GPY-3, M-69, and Khalda. This selection has been achieved by a series of experiments carried out on the crudes using three different techniques: dilution, heating, and chemical treatment.

Evaluating the flow properties of these waxy crude oils involved both rheological measurements and pour-point testing. The rheological properties were measured by a rotational viscometer (Rheotest II); the pour point was determined according to the ASTM-D97 procedure.

• Dilution. Two light petroleum distillates were used for dilution: straight-run gasoline and kerosine. Their characteristics are listed in Table 2.

  The two petroleum distillates were added individually in 5, 10, 15, and 20 vol % to four identical portions of each crude-oil sample. The diluted portions were subjected to pour-point determination and Theological measurements at 15 C.

• Heating. Tests subjected the four identical samples to Theological measurements at 15, 20, 30, and 40 C.

• Chemical treatment. Three flow improvers (two synthesized and one commercial) were used for improving the flow properties of the three tested crudes. These are:
  1. A synthesized flow improver of the ester type (Lauryl palimitate) melting point 39 C.

  2. A synthesized flow improver of the polymer type (Cetylester of maleic anhydride-vinyl acetate copolymer) average mole wt 39,000 and polydispersity 1.369.

  3. A commercial flow improver/paraffin inhibitor denoted (Petrolite CF-2448).

The three flow improvers were added individually to each tested crude-oil sample in concentrations ranging from 100 to 1,000 ppm.

In some cases, this range of concentration was too low to cause any flow improvement. Therefore, the concentration range was extended to 10,000 ppm in cases of Khalda crude (with all studied flow improvers) and M-96 crude (with polymer type only).

CASSON EQUATION

For the Theological measurements, the obtained shear stress-shear rate data have been fitted to Bingham, Herschell-Bulkley, and Casson Theological equations.

It is observed that the measured data are well fitted to the Casson equation which can be expressed as follows: 8-10

T0.5 = K1 + K2 (du/dr)0.5

(A nomenclature box is provided.)

A statistical analysis determined whether the results (rheology or pour point), obtained under different conditions, are statistically different.11

The t-test method employed indicated that a statistical difference exists between any two sets of data.

DILUTION

Fig. 1a illustrates the relationship between the volume percentage of diluent and Casson yield stress (K1) for the tested crude oils at 15 C. This figure makes clear that as the volume percentage of diluent increases, the Casson yield stress decreases for all tested crude oils.

This figure also shows that Khalda crude is much more affected by both gasoline and kerosine than the other two crudes. Also evident is that the improvement resulted from gasoline is better than that obtained by kerosine in all cases.

Fig. 1a shows, as well, that K1 approaches 0 at gasoline in the range of 15-20 vol % for the three tested crude oils.

At 20 vol % of kerosine, the value of K1 approaches 0 in case of GPY-3, while Khalda and M-96 still show a positive value. This confirms, again, that the effect of gasoline on the tested crude oils is better than that of kerosine.

Fig. 1b illustrates the relationship between the volume percentage of diluent and Casson plastic viscosity (K2) and shows that gasoline improves Casson plastic viscosity better than kerosine. Also, the figure makes clear that K2 decreases as the diluent percentage increases.

The rate of K2 reduction, however, is higher for the crude having higher initial value; i.e., the reduction in Khalda plastic viscosity (K2) is the highest of all crudes.

How gasoline and kerosine dilutions affect the pour points of the investigated crude oils appears in Fig. 1c. This figure illustrates that the diluents clearly affect the pour point of all studied crude oils.

This figure also shows a direct proportion between the volume percentage of the diluent and the reduction in the pour point; i.e., as the volume percentage increases the pour point reduction increases as well.

Fig. 1c shows, also, that the initial pour point of GPY-3 crude (20 C.) has been reduced to - 10 C. and -4 C. because of dilution by 20 vol % gasoline and kerosine, respectively. The pour point of M-96 crude (26 C.), on the other hand, has been reduced to 20 C. and 5 C. because of dilution by 20 vol % gasoline and kerosine.

Fig. 1c also indicates that the 29 C. initial pour point of Khalda crude has been reduced to - 1 C. and 5 C. as a result of dilution by 20 vol % of gasoline and kerosine, respectively. This means that the effect of gasoline on the pour point of the tested crude oils is more desirable than that of kerosine.

HEATING

Fig. 2a clearly shows that, as the temperature increases, the Casson yield stress (K1) decreases for the three tested crude oils. The reduction in Casson yield stress differs from one crude to another and was found to be higher for the crudes having higher initial values of K1.

At 40 C., all tested crude oils behave as Newtonian fluids; i.e., the values of Casson yield stress equal 0.

At temperatures up to 30 C., the tested crude oils follow the non-Newtonian behavior with a yield stress. Table 1 and Fig. 2a also indicate a direct proportion between the wax content in the crude oil and the reduction rate in the Casson yield stress (K1).

The intersect of the two curves of M-96 and GPY-3 crudes can be interpreted in the light of this proportion.

Fig. 2b makes clear that the values of Casson plastic viscosity decrease slightly as the temperature increases, the extent of decrease in K2 depending upon the temperature. The greatest reduction appears for Khalda crude oil.

CHEMICAL TREATMENT

Figs. 3a and 3b illustrate how chemical treatment affects Casson yield stress (K1) for GPY-3 and M-96 crudes, respectively.

Fig. 3a indicates that K1 remains constant up to 100 ppm and after that decreases with increasing concentration. At any concentration, the reduction in K1 obtained by the polymer additive is the highest, while that obtained by the ester type is the lowest. In other words, the polymer is the best additive for improving the flow properties of GPY-3 crude oil.

Fig. 3a also shows that the GPY-3 crude approaches Newtonian behavior when treated by 500 ppm of the polymer additive and still shows a non-Newtonian behavior after treatment with 1,000 ppm of the other two additives.

Fig. 3b illustrates how the three types of additives affect K1 for M-96 crude oil.

As the concentration increases, K1 decreases and reaches 0 at 1,000 and 10,000 ppm of the paraffin inhibitor and polymer-type additives, respectively. In other words, M-96 crude oil approaches the Newtonian behavior at 1,000 ppm of paraffin inhibitor and 10,000 ppm of polymer. It is worth mentioning that the ester has no effect on M-96 crude oil up to 10,000 ppm.

How chemical treatment affects Casson plastic viscosity (K2) is shown in Figs. 3c and 3d.

Fig. 3c shows the effect of the three additives on GPY-3 crude oil. This figure shows that chemical treatment has no effect on K2 until the additive concentration reaches 100 ppm. After that, K2 decreases with the increase in the additive concentration.

Fig. 3c also shows that K2 of GPY-3 crude is more affected by the polymer than by the other two additives (ester and paraffin inhibitor).

Fig. 3d indicates that the values of K2 decrease with increasing concentration of both polymer and paraffin inhibitor. The used polymer causes the greatest reduction in K2 but at higher concentrations (1,000-10,000 ppm). Experiments indicated that the ester type has no effect at all on K2 in the case of M 96 crude up to 10,000 ppm.

Figs. 3e and 3f illustrate how chemical treatment with the three types of additives affects pour points of the tested crude oils.

Fig. 3e, showing the effect of the three additives on the pour point of GPY-3 crude oil, shows that the polymer results in the lowest pour point, while the ester type shows the highest value.

There is no change in the pour point of GPY-3 crude at concentrations greater than 500 ppm in cases of polymer and ester. The paraffin inhibitor still shows a reduction in the pour point, however, up to 1,000 ppm.

To compare the effect of the three additives on the pour points of the tested crudes, a pour point of 10 C. is taken as a datum. This temperature represents, more or less, the lowest temperature in Egypt during winter.

Fig. 3e shows that about 200 ppm of the polymer additive is enough to reduce the pour point of GPY-3 from its initial value (20 C.) to the reference value (10 C.). About 350 ppm of paraffin inhibitor or 500 ppm of ester are needed to reduce the pour point of this crude to the reference value.

Fig. 3f shows the effect of adding both polymer and paraffin inhibitor on the pour point of M-96 crude oil. About 160 ppm of paraffin inhibitor is enough to reduce the pour point of M-96 crude from 26 to 10 C., in comparison with 4,500 ppm of polymer additive.

The ester type has no effect on M-96 crude oil.

PRESSURE-LOSS AFFECTED

The final target of any improvement in the rheology, as well as in the pour point of crude oils, is reduction of pressure loss during pipeline transportation.

A computer program, devised for this research to calculate the pressure loss, is based on the recommended procedure for Casson model.12 The program's flow chart, equations, and listing appear in an accompanying box.

Pressure loss was calculated at all tested conditions for a 12-in. pipeline.

To illustrate the effects of the three investigated techniques relative to each other, the untreated pour point (p) was taken as a reference temperature. Departure from this temperature, higher (by heating) or lower (by dilution and chemical treatment), served as a basis for this comparison.

The pressure-loss reduction (AP,) was calculated at different temperatures relative to the reference temperature with the following equation:

[See original book for equation.]

The calculation procedure is as follows:

• Dilution or chemical treatment
  1. Select some temperatures lower than the pour point of the respective crude.

  2. Obtain the volume percentages of diluent corresponding to the selected temperatures from Fig. 1c.

     The corresponding chemical concentrations are obtained from Figs. 3e and 3f for GPY-3 and M-96 crudes, respectively.

  3. Determine the corresponding values of K1 using Figs. 1a, 3a, and 3b and do the same for K2 using Figs. 1b, 3c, and 3d.

  4. Use the obtained values of K1 and K2 and the proposed computer program to calculate the pressure loss (DELTA P).

  5. Determine the pressure loss reduction (DELTA Pr) with the provided equation.

• Heating
  1. Select some temperatures higher than the pour point of the respective crude.

  2. Determine the corresponding values of K1 and K2 using Figs. 2a and 2b, respectively.

  3. Repeat Steps 4 and 5 above to calculate the pressure loss (DELTA P) and pressure-loss reduction (DELTA Pr).

Fig. 4 shows the results obtained.

Fig. 4a shows that the addition of gasoline to GPY-3 crude oil has led to the best improvement in comparison with the other two techniques. This is indicated by the highest pressure-loss reduction resulting from dilution by gasoline.

Fig. 4b illustrates that the heating technique is the best for M-96 crude oil regarding the pressure-loss reduction in the pipeline. Fig. 4c shows that heating the Khalda crude oil has resulted in a greater reduction in the pressure loss than that obtained by dilution. The chemical treatment has no effect in this regard.

REFERENCES

1. Myers, R. W., "An Electrically Heated Buried Gathering System Transports High-Pour Point Crude Oil," JPT, pp. 890-894, June 1978.

2. Brod, M., Deane, B.C., and Rossi, F., "Field Experience with the Use of Additives in the Pipeline Transportation of Waxy Crudes," J. of the Institute of Petroleum, Vol. 57 (1971), No. 554, pp. 110-114.

3. Ells, J.W., and Brown, V.R., "The Design of Pipelines to Handle Waxy Crude Oils," J. of the Institute of Petroleum, Vol. 37 (1971), No. 555, pp. 175-183.

4. Price, R.C., "Flow Improvers for Waxy Crudes," J. of the Institute of Petroleum, Vol. 57 (1971), No. 554, pp. 106-109.

5. Towell, A.J., "Performance, Testing and Developments of Lub oils, Fuels and Chemicals," First International Seminar, Misr Petroleum Co., Cairo, June 4-7, 1977.

6. Russell, R.J., " The Pumping of 85 F. Pour Point Assam (Nahorkotiya) Crude Oil at 65 F.," J. of the Institute of Petroleum, Vol. 57 (1971), No. 554, pp. 117-128.

7. Szilas, A.P., Production and Transport of Oil and Gas (Budapest: Acadamia Kiado, 1975).

8. Govier, G.W., and Aziz, K., The Flow of Complex Mixtures in Pipes, Van Nostrand Reinhold Co., New York, 1972.

9. Skelland, A.H.P., Chapter 1 of Non-Newtonian Flow and Heat Transfer, John Wiley & Sons Inc., New York, 1967.

10. Mohammed, H.M., The Rheological Properties of Egyptian Crude Oils, masters thesis (1987), faculty of engineering, Cairo University.

11. Stanley, L.T., Chapter 2, Practical Statistics for Petroleum Engineers, Petroleum Publishing Co., Tulsa, 1973, pp. 39-50.

12. El-Emam, N. A., The Rheological Characteristics of non-Newtonian Thixotropic Crude Oils Under Pressure, PhD thesis (1980), Technical University of Heavy Industries, Hungary.

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