A NEW METHOD FOR INCREASING THE THROUGHPUT OF OIL PIPELINES

L. A. Pukhlyakov, G. N. Chertenkova

(Presented by Professor L. V. Aksarin)

The most widespread method of increasing the throughput of oil pipelines is heating the oil at the head stations. In particular, it has found application in the pumping of highly viscous Mangyshlak oils, which become unsuitable for pumping at a temperature of 32°C. During pumping, the temperature here is maintained within the range from 65 to 35°C \[4\].

Other methods of increasing the throughput of oil pipelines are also known, for example, heat treatment, which consists in first heating the oil to 60–80°C and then rapidly cooling it to −18°C (\[5\], pp. 1–18), vibration heating \[2\], injection into the oil of liquid solvents, for example gasoline, and also pumping oil together with water through pipes whose inner surface has hydrophilic properties \[1\].

Each of these methods has certain advantages and disadvantages. In particular, heating the oil and injecting gasoline into it have the advantage that they are applicable to any oils. Their common disadvantage is high cost. Heat treatment and vibration heating are cheaper methods; however, not every oil reduces its viscosity as a result of their application. Finally, pumping oil together with water through pipes whose inner surface has hydrophilic properties also has defects. One of them is the overloading of the oil pipeline with water.

In studying the viscosity of oils from fields of the Tomsk Region, it was established that with the loss of associated gases during the transition of oil from reservoir conditions to surface conditions, their viscosities increased sharply. In particular, oil from the Klyuchevskoye field in the degassed state under reservoir conditions (at a temperature of 78°C and a pressure of 252 atm) had a viscosity of 0.89 centipoise. But under surface-

conditions after complete degassing—4.84 centipoise [3]. The viscosities of oils from other fields of Tomsk Oblast changed in an analogous manner (Table 1).

Table 1.

Viscosities of oils from fields of Tomsk Oblast under reservoir and surface conditions

It is interesting to note that partial degassing of the oil did not lead to a sharp increase in its viscosity. Partially degassed oil differed little in viscosity from completely degassed oil, as can be seen from the example of the oil from the Strezhevoye field (Table 2).

Table 2

Results of a study of the viscosity of oil from the Strezhevoye field in a partially degassed state

Similarly, during degassing the viscosity of oil from the Olenevskoye field changed (Table 3).

Table 3

Results of a study of the viscosity of oil from the Olenevskoye field in a partially degassed state

From the materials presented it is evident that partially degassed oils at a temperature of 20°C have viscosities on the order of 1.2–1.4 centipoise, which is four to five times less than the viscosity of fully degassed oil at the same temperature.

Thus, it appears probable that the oils of the fields of Tomsk Oblast should be pumped not in the fully degassed state, but after degassing at a pressure on the order of 12–20 atm. In doing so, however, one important condition must be observed. Nowhere in the pipeline should the pressure fall to the degassing pressure. Otherwise gas will begin to evolve from the oil, and this will lead to an irrational increase in the volume of liquid in the pipeline and to a decrease in its throughput.

Is such a method of increasing the throughput of oil pipelines applicable when pumping oils from other regions, for example, Mangyshlak oils? Apparently, yes. In any case, if Mangyshlak oil under reservoir conditions did not possess greater mobility than at the surface, it would be impossible to extract it from the reservoir.

Proceeding from what has been said, the following technology is proposed for preparing oil for pumping and for the pumping itself. The oil extracted from wells is degassed in a high-pressure separator (the specific pressure, in each individual case, is determined experimentally) and is freed of water. Then, by means of a high-pressure pump (up to 50 or 100 atm), it is injected into the oil pipeline and moves along it to the first pumping station. At the intake of this station, the pressure must exceed the degassing pressure by five to ten atm. In the remaining sections of the oil pipeline, the pumping conditions must be the same as in the first.

If oil production at a given field is carried out by the deep-well pumping method, then, in order to prevent preliminary degassing of the oil, it is necessary to ensure that the pump is submerged below the liquid level by at least 150 m.

The described method of increasing the throughput of oil pipelines is less expensive than any other methods, for it is based not on any special measures, but on the preservation of associated gases released in the range of relatively low pressures (up to 12–20 atm). On the other hand, it is also more reliable than some others. For example, heated oil may cool in some section of an oil pipeline, and if it is an excessively viscous oil, similar to Mangyshlak oil, this will lead to the failure of an entire section of the pipeline. It is easier to prevent the loss of gas by oil during pumping through the pipeline than the loss of heat.

There is one more advantage of the method described, namely that it can be used in combination with other methods, in particular simultaneously with heating, which is especially important when pumping highly viscous oils of the Mangyshlak type.

REFERENCES

1. V. E. Gubin. Transportation of viscous oils and petroleum products through main pipelines. Transport and Storage of Oil and Petroleum Products, 1967, No. 11.

2. N. V. Kalashnikov, V. I. Chernikin. Vibroheating of viscous petroleum products. Gostoptekhizdat, 1961.

3. L. A. Pukhljakov, G. N. Chertenkova. On the viscosity of oil from the Klyuchevskoye field of Tomsk Oblast under reservoir conditions. Izvestiya TPI, vol. 218, 1970.

4. E. Sokhrin. From Mangyshlak to the Volga. Newspaper Sovetskaya Rossiya, No. 213 (4332), September 12, 1970.

5. V. I. Chernikin. Pumping of viscous and solidifying oils. Gostoptekhizdat, 1958.

Errata Noticed