RESULTS OF A STUDY OF THE PHYSICAL PROPERTIES OF OIL FROM THE OLENYE FIELD IN TOMSK OBLAST

G. N. CHERTENKOVA

(Submitted by Professor A. V. Aksarin)

For the study of the physical properties of oil from the Olenye field in Tomsk Oblast, oil was taken from well 123 by means of a bottom-hole sampler. In this case, inflow into the well occurred in the intervals 2542–2549 and 2555–2560 m from bed Yu-1 of the Vasyugan suite of Upper Jurassic age. The reservoir pressure at the moment the sample was taken was 256.15 atmospheres, and the reservoir temperature was 91°C.

The study was carried out according to the method set forth by V. N. Mamuna et al. [2]. The principal characteristics obtained in the study proved to be as follows. The saturation pressure was 120 atmospheres; the density of the separated oil, 0.862 g/cm³; gas content referred to unit volume, 71.50 cm³/cm³; gas content referred to unit weight of separated oil, 82.99 cm³/g; the average density of the gas released during single-stage or contact degassing, 1.240 g/liter; the volumetric coefficient of the oil, 1.300; oil shrinkage, 23.08%; the density of the oil under reservoir conditions, 0.731 g/cm³; the average coefficient of gas solubility, 0.590 cm³/cm³·atm. The coefficient of compressibility of the oil proved to differ at different pressures. Specifically, at a pressure of 256–148 atmospheres it was equal to \(14.96 \cdot 10^{-5}\) 1/atm, at a pressure of 148–128 atmospheres \(18.55 \cdot 10^{-5}\) 1/atm, and at a pressure of 128–120 atmospheres \(23.15 \cdot 10^{-5}\) 1/atm.

The composition of the associated gases released in the process of contact or single-stage degassing, according to the data of V. A. Kuznetsova, proved to be as follows (in volume percent): methane 74.36, ethane 7.71, propane 5.26, isobutane 2.04, normal butane 2.22, isopentane 2.58, normal pentane 2.95, hexane 0.47, and nitrogen 2.41.

The quantities of gases released from the oil in the process of multistage degassing at each stage, and their densities, are given in Table 1.

Special attention in the study of the oil from the Olenye field was devoted to its viscosity. The viscosity was studied above all at pressures higher than the saturation pressure, that is, in the presence of dissolved gases in it. In this case the study was carried out at various temperatures and pressures. As a result of this study it was established, first, that the viscosity decreased with a decrease in pressure. The viscosity curves corresponding to higher pressures lay above the curves corresponding to lower pressures (Fig. 1). However, this decrease is not in direct dependence on pressure. This is evidenced by

Table 1

Results of multistage degassing of an oil sample from the Olenye field, Tomsk Oblast

Fig. 1. Viscosity curves of oil from the Olenye field, Tomsk Oblast. The solid curves correspond to the viscosities of non-degassed oil at pressures of 256, 200, and 150 atmospheres. The accompanying numerical labels indicate the pressures at which the viscosity measurements were made. The dashed curves correspond to the viscosities of degassed oil. Their numerical labels indicate the degassing pressure.

indicates that the distances between these curves are not the same. This is especially evident in the temperature range of 50–80°C (Fig. 1).

More interesting were the changes in viscosity with changes in temperature. There is an opinion, expressed in particular by J. Amyx et al. [1, p. 291], according to which the viscosity of oil decreases with increasing temperature. Proceeding from this opinion, many authors, …

For example, V. S. Ugolev and V. I. Musinov [3], in order to increase inflow into wells, propose heating their bottomholes. Several types of installations have been developed for this purpose. A study of the viscosity of oil from the Olenye field in Tomsk Oblast showed that, as temperature increases, this parameter does not always decrease. Moreover, sometimes viscosity increases with increasing temperature. In fact, the viscosity curves for oil from this field before degassing (solid curves in Fig. 1) are at first directed downward (viscosity decreases), and then begin to rise upward (viscosity increases). Thus, it turns out that this oil in the non-degassed state has its lowest viscosity at a temperature of 60°C.

In addition, the viscosity of oil from this field was determined at various degrees of degassing: 90 atmospheres, 60 atmospheres, 25 atmospheres, and 12 atmospheres (dashed curves in Fig. 1). During degassing, the viscosity of the oil increased sharply (the indicated curves in Fig. 1 are located, as a separate group, somewhat above the viscosity curves for non-degassed oil). As for the quantitative relationship between the viscosities of non-degassed and partially degassed oils, it is relatively small. Specifically, the viscosity of non-degassed oil lies within the range 0.45–1.0 centipoise, while the viscosity of partially degassed oil lies within the range 0.8–1.5 centipoise.

Complete degassing of the oil produces an enormous effect in increasing viscosity. The viscosity of completely degassed oil from the Olenye field is 6.7 centipoise. This latter circumstance is apparently connected with the fact that the oil loses the bulk of its associated gases in the final stages of degassing, when the pressure is reduced from 12 atmospheres to atmospheric pressure (Table 1).

From the study carried out, one very important practical conclusion follows: in the process of developing an oil field, degassing of the reservoir must not be allowed, since otherwise the viscosity of the oil will increase sharply and its extraction from the formation will become impossible. As for already degassed fields, such as the Patos field in Albania, the best method of increasing the recovery of such fields may be considered the injection of hydrocarbon gases into them.

The second practical conclusion following from this study is that the best method of increasing formation recovery will be the injection of gas into it. The injection of water into oil reservoirs, which has become widespread in recent years, is less effective in this respect, since water does not contribute to reducing the viscosity of oil.

LITERATURE

1. J. Amix, D. Bass, R. Whiting. Physics of the Petroleum Reservoir (trans. from English). Gostoptekhizdat, 1962.

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

3. V. S. Ugolev, V. I. Musinov. Thermal Methods in Oil Production. Gostoptekhizdat, 1959.