Results of a Study of the Physical Properties of Reservoir Oil from the Shinginskoye Field in Tomsk Region

L. A. Pukhlyakov, G. N. Chertenkova, Yu. G. Sazonov

(Presented by Professor A. V. Aksarin)

The oil-bearing capacity of the Shinginskoye field in Tomsk Region is associated with bed Yu-1 of the Vasyugan Formation, which within this field occurs at a depth exceeding 2500 m. The discovery well of the field is well 295. From this well, two samples were taken for investigation of the physical properties of the oil under reservoir conditions. The study was carried out in the laboratory of oil-reservoir physics of the Tomsk Polytechnic Institute on a UIPN-2M apparatus. The work was performed according to the method set forth by V. N. Mamuna and others [1].

The principal physical properties of the oil of the Shinginskoye field for both samples are given in Table 1. More detailed data are presented only for the first sample (Tables 2, 3, 4, and 5). Analogous data for the second sample are not given in the article, since they differ little from the data for the first sample.

The oil of the Shinginskoye field is characterized by a high gas content, which exceeds 100 m³/m³, and by a considerable saturation pressure, the mean value of which reaches 117 atm (Table 1). In this respect, the oil of the Shinginskoye field is similar to the oils of the Strezhevoye and some other fields of Tomsk Region—[3, 4]. Gas liberation from the oil of the Shinginskoye field proceeds more or less uniformly. In particular, at the first stage of degassing, when the pressure was reduced by 25 atm below the saturation pressure, 15.88 cm³ of gas was released from each cm³ of oil. The situation was analogous at the other stages as well (Table 2).

Among the liberated gases, methane occupies first place; in single-stage degassing its share amounted to 63.17 percent by volume. At the same time it was liberated more intensively at the first stages of degassing, where its share reaches 80 percent by volume. At the last stage of degassing its share decreases to 18.95% (Table 3).

Second place among the liberated gases is occupied by propane; in single-stage degassing its share amounted to 14.02 percent by volume, and only third place is occupied by ethane. Its share in single-stage degassing amounted to 11.44%. Next come normal butane (4.57%), isobutane (1.30%), normal pentane (1.0%), isopentane (0.79%), and, finally, hexane (0.33%). The situation with associated gases is analogous in the oils of most other oil fields of Tomsk Region. Second place among them, as a rule, belongs to propane, and only third to ethane.

Table 1

Basic physical properties of reservoir oil from the Shinginskoye field
(well 295, formation Yu-1, reservoir pressure 264 atm, reservoir temperature 103°C)

Unlike methane, the proportion of all associated gases increases as the degassing pressure decreases. This is especially characteristic of propane, whose share at the final stage of degassing exceeds its share at the first stage by a factor of 7.7. The share of ethane increases only up to the penultimate stage (pressure interval 16—8 atm), where it reaches 17.37%, and then even decreases somewhat, reaching 13.59% (Table 3). Thus, the well-known conclusion is confirmed that, as the degassing pressure is reduced, increasingly heavier gases are released from the oil.

Special attention was given to studying the viscosity of the oil from the Shinginskoye field. First, the viscosity of the oil was investigated in the undersaturated state, and then in partially degassed states (Tables 4 and 5). The latter is connected with the fact that, in order to increase the throughput of oil pipelines, the authors had once proposed pumping oil in a partially degassed state at pressures above the degassing pressure [3]. The investigation was carried out at several temperatures and pressures. Its results may be summarized as follows.

With increasing temperature, the viscosity of the oil in all cases decreased smoothly. In this respect, the oil of the Shinginskoye field differs from the oil of the Klyuchevskoye field, whose viscosity had its lowest value at 60°C and, with further heating, increased somewhat [2].

Table 2

Results of stepwise degassing of oil from the Shinginskoye field
(well 295, first sample, temperature 103°C, formation pressure 264 ata)

Table 3

Percentage (volume percent) content of individual fractions of associated gas released from oil of the Shinginskoye field (well 295, first sample) at various stages of degassing

Table 4

Results of the study of the viscosity of oil from the Shinginskoye field
(well 295, first sample) in the undegassed state

Table 5

Results of the study of the viscosity of oil from the Shinginskoye field
(well 295) in the partially degassed state

With a decrease in pressure, as long as it did not fall below the saturation pressure for the undegassed oil or the degassing pressure for the partially degassed oil, the viscosity also decreased. The situation was different with degassing. Each new stage of oil degassing led to a new increase in viscosity. However, this increase remained small, and after degassing at a pressure of 10 atm the viscosity of the oil did not even reach 1 cP. The viscosity of the completely degassed oil from the Shinginskoye field is equal to 4.71 cP (Table 1).

How can the phenomenon described be explained?

Apparently, it is not the loss of all gases that leads to an increase in oil viscosity, but only the loss of those whose main mass escapes at the final stage of degassing, that is, at a pressure below 10 atm.

In this connection, on the basis of Tables 2 and 3, the total amount of gases released from the oil by fractions, in cm³ per cm³ of oil, was determined (Table 6). From it, in particular, it is evident that at a pressure of 8 atm, in each cm³ of oil only 2.9 cm³ of methane, 2.08 cm³ of ethane, 5.99 cm³ of propane, and 2.44 cm³ of normal butane remain in dissolved form. The amount of the remaining gases remains insignificant. Thus, the principal role in reducing the viscosity of oil under reservoir conditions belongs, apparently, not to methane and ethane, but to propane and butane.

Table 6

Total amount of individual gas fractions released from the oil of the Shinginskoye field in the process of stepwise degassing

Amount of gas, cm³, released from each cm³ of oil during the following degassing stages

References

1. V. N. Mamuna, G. F. Trebin, B. V. Ulyaninsky, Experimental Study of Reservoir Oils. GOSINTI, Moscow, 1960.

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

3. L. A. Pukhlyakov, G. N. Chertenkova. A new method for increasing the productivity of oil pipelines. Izvestiya TPI, vol. 217, 1971.

4. L. A. Pukhlyakov, G. N. Chertenkova. Physical properties of oil from the Strezhevoye field in Tomsk Oblast. Izvestiya TPI, vol. 236, 1976.