PHYSICAL PROPERTIES OF OIL FROM THE STREZHEVOY FIELD IN TOMSK OBLAST

L. A. Pukhlyakov, G. N. Chertenkova

(Presented by Professor A. V. Aksarin)

The physical properties of oil change greatly when it passes from reservoir conditions to surface conditions. In particular, its density and viscosity increase, the volume it occupies decreases, etc. This is due to the fact that under reservoir conditions oil is under high pressure and contains a large amount of dissolved associated hydrocarbon gases. Knowledge of these properties is necessary for calculating oil reserves and for designing the development of fields.

To investigate the physical properties of oil from the Strezhevoy field, two oil samples were taken under reservoir conditions from well No. 15, from bed Yu-1, at a depth of 2572–2565 m, that is, at a pressure of 256.8 atm and a temperature of 86°C. The samples were delivered in sampling vessels to the reservoir-oil laboratory of the Tomsk Polytechnic Institute, where they were studied on a UIPN-2M apparatus according to the method described by V. N. Mamuna et al. [1]. The sampling date was December 19, 1969; the date of completion of the analysis was January 14, 1970. The latter circumstance is very important, since during prolonged storage of oil in sampling vessels it may lose a significant amount of the gases contained in it.

In the course of the investigation, the saturation pressure was determined first of all. For this purpose, the oil samples in the press of the apparatus were compressed to specified pressures, and the volumes corresponding to these pressures were measured. Initially, at high pressures, considerable changes in pressure were accompanied by very limited changes in volume, from which it follows that the oil behaved here as a typical liquid, that is, gas had not yet begun to separate from it. Then the smallest changes in pressure began to be accompanied by large changes in volume, from which it follows that gas had begun to separate from the oil. Then the initial volume of oil was subtracted from the volumes obtained, that is, the volume it occupied at reservoir pressure. Their differences ($\Delta N$) were plotted along the abscissa axis, and the corresponding pressures along the ordinate axis (Fig. 1). The obtained points (circles filled with ink) formed two broken lines, one of which corresponded to the first sample (I), and the second to the second (II). The break points of these lines gave us the saturation pressures. In the first case it proved equal to 148 atm, and in the second to 154 atm (Fig. 1). The average between them gave a value of 151 atm. This value was also taken as the saturation pressure for the oil of the Strezhevoy field.

Next, the coefficient of compressibility of the oil was determined, expressing the change in its volume as a function of changes in pressure at pressu-

...at pressures above the saturation pressure. This coefficient is calculated from the formula

\[ \alpha=\frac{\Delta V}{V\Delta P}, \tag{1} \]

where \(V\) is the initial volume of the oil sample, and \(\Delta V\) is the change in this volume when the pressure changes by an amount \(\Delta P\). This coefficient does not remain constant. As the pressure increases it decreases, and at a pressure of 256 atm (reservoir pressure) it averaged \(19.7 \cdot 10^{-5}\ \text{l/atm}\), while at a pressure close to the saturation pressure (in the interval 154–160 atm) it averaged \(29 \cdot 10^{-5}\ \text{l/atm}\).

Next, by the method of contact degassing, the gas content per unit volume of oil \(f_v\) was determined.

After contact degassing had been carried out, multistage degassing was performed. Its results are not given.

After multistage degassing of the oil, the density and viscosity under surface conditions were determined; then, on the basis of the data from contact and multistage degassing, the oil formation-volume factor, its shrinkage, and other characteristics were calculated. The results of these studies are given in Table 1.

In the course of contact and multistage degassing, the oil samples transferred from the sampler to the UIPN press were initially completely used up. Therefore, for studying the viscosity of the oil in the undegassed and partially degassed states, new portions of oil were transferred to the UIPN press. In this case the oil of the first sample proved sufficient only for determining its viscosity in the undegassed state. The study of its viscosity in the partially degassed state was carried out using the second sample. The results of these studies are given in Tables 2 and 3.

Fig. 1. Graphs of the relationship between volume differences and pressures for oil from the Strezhevoy field. I—the curve of the first sample, with a break at 148 atmospheres, and II—the curve of the second sample, with a break at 154 atmospheres. The circles filled with ink correspond to points obtained from experimental data. The circles not filled with ink correspond to the break points.

It can be seen from these tables that, in the partially degassed state, the oil of the Strezhevoy field has a fairly high mobility. In particular, after degassing it at 20 atm and bringing it to a pressure of 30 atm, its viscosity did not exceed 1.25 centipoise. Meanwhile, after complete degassing, the oil acquired a viscosity of 5.6 centipoise, i.e., exceeded this value by a factor of 4.5.

It follows from what has been said that if the oil of the Strezhevoy field is pumped in the undegassed state at a pressure above the...

Table 1

Physical properties of oil from the Strezhevoy field

Table 2

Results of the study of the viscosity of oil from the Strezhevoy field
in the ungassed state at various pressures

Table 3

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

saturation pressure, or in the partially degassed state at a pressure above the degassing pressure, then the capacity of the oil pipeline will become higher.

REFERENCES

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