ON SOME PHYSICAL PROPERTIES OF OIL FROM THE LUGINETSKOYE FIELD

L. A. Pukhlyakov, G. N. Chertenkova

(Presented by Professor A. V. Aksarin)

To study the physical properties of the oil from the Luginetskoye field, samples were taken from two wells. The first was from well 159, horizon Yu-1, perforation interval 2,325–2,333 m, and the second from well 162, horizon Yu-1, perforation interval 2,342–2,348 m. In the card accompanying the first sample, the value 228.6 ati was indicated as the reservoir pressure; in the card accompanying the second sample, 243.2 ati. The reservoir temperature was the same in both cases—81°C. The samples were transferred into containers at a pressure of 220–232 ati and delivered to the laboratory of oil-reservoir physics of the Tomsk Polytechnic Institute, where they were studied.

The study was carried out on the UIPN-2M apparatus according to the procedure described by V. N. Mamuna et al. [1]. First of all, the saturation pressure was determined. For this purpose, the oil sample transferred into the apparatus press was heated to 81°C (reservoir temperature) and compressed to the value of the reservoir pressure; the resulting sample volume was recorded on the press scale. Then, using the manual drive of the press, the sample volume was increased in such a way that the pressure inside the press decreased by 10–20 atmospheres. At the same time, both the sample volume and the pressure inside the press were recorded. The operation was repeated several times.

The obtained changes in volume \(\Delta N\) were plotted along the abscissa axis, and the corresponding pressures \(\Delta P\) along the ordinate axis. As a result, two graphs were obtained which, strange as it may seem, differ fundamentally from one another.

The graph corresponding to the first sample (Fig. 1) consists of two slightly concave curves: one with a slope of 26.4 at/cm\(^3\) and the other with a slope of 2.75 at/cm\(^3\), or, taking into account that the sample volume was 145 cm\(^3\), and recalculating for a sample volume of 100 cm\(^3\)—38 at/cm\(^3\) and 4.0 at/cm\(^3\), respectively. The point of their intersection corresponds to a pressure of 104 ati. This value was adopted as the saturation pressure for the first sample. Thus, the curve with a slope of 38 at/cm\(^3\) was taken as the curve of the liquid state of the sample, and the curve with a slope of 4.0 at/cm\(^3\) as the curve of the two-phase state of the first sample.

On the graph of the second sample (Fig. 2), one curve was obtained with a continuously decreasing slope. In the first section its slope is 3.54 at/cm\(^3\), in the second 2.75 at/cm\(^3\), in the third 1.66 at/cm\(^3\), and in the last 1.14 at/cm\(^3\), or, taking into account that the sample volume was 106 cm\(^3\) and recalculating for a sample volume of 100 cm\(^3\), we obtain 3.75; 2.9; 1.76 and 1.2 at/cm\(^3\).

Fig. 1. Graph for determining the saturation pressure of oil taken from well 159 of the Luginetskoye field. The graph shows a break in the curve corresponding to the moment at which gas bubbles appear in the oil.

In interpreting the data obtained, the authors’ opinions differed. In the opinion of G. I. Chertenkova, the point of intersection of the next-to-last and last segments of the given curve, which corresponds to a pressure of 209 atm, is the point at which degassing of the second sample begins.

In the opinion of L. A. Pukhlyakov, gas bubbles were already present in some quantity in this oil sample at the very beginning of the experiment, that is, at a pressure of 243 atm. Evidence of this is the fact that even on the first segment of the curve under consideration its slope, 3.75 atm/cm³, does not reach the value of the average slope of the second curve of the first sample (4.0 atm/cm³), which was taken above as the curve of the two-phase state of the sample. In other words, in L. A. Pukhlyakov’s opinion, the saturation pressure of this sample exceeds the reservoir pressure.

These samples differ sharply not only in the values of saturation pressure, but also in other parameters. Thus, the gas content, referred to unit volume, is 80.86 cm³/cm³ for the first sample and 116.99 cm³/cm³ for the second sample (Table 1); the formation volume factor is 1.297 for the first sample and 1.393 for the second. All this suggests that in the present case

Fig. 2. Graph for determining the saturation pressure of oil sampled from well 162 of the Luginetskoye field.

Table 1

Results of the study of the principal physical properties of oil from the Luginetskoye field

different wells penetrated different pools. This is also indicated by the considerable difference in reservoir pressures (228.6 atm for the first sample, 243.2 atm for the second).

A second conclusion also suggests itself. Well 159 penetrated an ordinary oil pool. Well 162 penetrated a gas-oil pool near the gas-oil contact. In accordance with this is the fact that in the associated gas of the second oil sample (taken from well 162) the proportion of methane is too high. It reaches 88.68 volume percent (Table 2). Meanwhile, in the associated gases of most oil fields of Tomsk Oblast, for example, Strezhevo, Pervomaiskoye, and others, the methane content lies within the range of 50–60 volume percent. The proportions of propane and butane are unusually small here (1.50 and 0.73 volume percent); they are smaller than the proportion of ethane (2.99 volume percent). But in the associated gases of the oils of most fields the proportions of propane and butane exceed the proportions of ethane. One gains the impression that the second sample of oil from the Luginetskoye field (taken from well 162) proved to be saturated with natural gas from the gas cap.

Table 2

Composition of the associated gas evolved from oil of the Luginetskoye field, taken from well 162

The results of stepwise degassing of both oil samples are given below (Tables 3 and 4).

Table 3

Results of stepwise degassing of oil from the Luginetskoye field, taken from well 159

Table 4

Results of stepwise degassing of oil from the Luginetskoye field, sampled from well 162

REFERENCES

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