PHYSICAL PROPERTIES OF OIL AND PERMEABILITY OF THE B-Kh RESERVOIR OF THE SEVERNOYE FIELD IN TOMSK OBLAST

L. A. Pukhlyakov, G. N. Chertenkova

(Presented by Professor A. V. Aksarin)

Knowledge of the physical properties of oil and the permeability of the reservoir is necessary for calculating its reserves and designing development. In this connection, in the laboratory of oil-reservoir physics of the Tomsk Polytechnic Institute, at the request of the Tomsk Territorial Geological Administration, a study was begun of the physical properties of oils from fields of Tomsk Oblast, among which is the Severnoye field.

For the study, an oil sample was taken from well 204, which had been perforated in the interval 1868–1879 m and tested under several regimes. The initial reservoir pressure was 183 atm, and the reservoir temperature was 73° C. The study was carried out according to the procedure described by V. N. Mamuna et al. [1] on the UIPN-2M apparatus.

The principal physical properties of the oil of the Severnoye field are given in Table 1. From the gas liberated in the process of contact degassing, a sample was taken for analysis. The analysis showed that this gas has the following composition (in volume percent): methane 69.01, ethane 4.49, propane 9.56, isobutane 2.63, normal butane 7.24, isopentane 1.57, normal pentane 1.88, butane 0.81, carbon dioxide 1.09, nitrogen 1.72.

Stepwise degassing was carried out at four pressures (stages): 60 atm, 40 atm, 20 atm, and at atmospheric pressure (754 mm Hg). The results of this study are given in Table 2.

Special attention was devoted to the viscosity of the oil. The results of this study are given partly in Table 1, partly in Table 3 and in Fig. 1.

Determination of the reservoir permeability was carried out from inflows into the well under stabilized regimes according to the method of L. A. Pukhlyakov [2, 3]. The work was performed in the following order.

Table 1

Basic physical properties of oil from the Severnoye field, Tomsk Oblast
(B-Kh reservoir, well 204, filter interval 1868–1877 m)

Table 2

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

Table 3

Results of the study of the viscosity of oil from the Severnoye field, Tomsk Oblast,
at various temperatures and pressures

1. The length of the filter per one opening was determined. In this case, the number of openings proved to be 132, the total length of the perforated intervals 1100 cm, and the sought value 6.818 cm.

1. It was determined whether the perforation density of well 204 is high or low. For this purpose, the reservoir thickness per one opening (6.818 cm) was compared with the value \(2.55 r\), where \(r\) is the radius of the well before lowering the casing string, which in our case is equal to 9.5 cm. Thus, this value proved to be 24.22 cm, and the perforation density is high.

Fig. 1. Viscosity curves of oil from the Severnoye field, Tomsk Oblast. The solid curves correspond to the viscosities of live oil at pressures of 183, 150, and 120 atmospheres. The accompanying numerical characteristics express the pressures at which the viscosity measurements were made. The dotted curves correspond to the viscosity of degassed oil. Their numerical characteristics express the degassing pressure.

1. The radius of influence of the openings was determined. At high perforation density the calculation is made by the formula

\[ s = 0.5 \sqrt{\frac{2 \pi r h}{n}}, \tag{1} \]

where \(h\) is the length of the perforated part of the string in cm, equal in our case to 900 cm; \(n\) is the number of openings, equal to 132. As a result of the calculation we obtain 10.1 cm.

1. The geometric characteristic of the zone of influence of the openings was determined. In the case of channels at the openings and high perforation density, this characteristic is calculated by the formula

\[ G_s = \frac{1}{y} \ln \frac{(y+\lambda)s}{(y+s)\lambda} - \frac{0.25}{r+y} \ln \frac{s+y}{\lambda+y} + 0.0625 \frac{s-\lambda}{(r+y)^2} - \ldots, \tag{2} \]

where \(y\) is the length (depth) of the channel beyond the cement ring in cm, \(\lambda\) is the mean radius of the channel at the openings in cm, \(r\) is the radius of the well before lowering the casing string in cm, and \(s\) is the radius of influence of the openings in cm.

In the case where there are no channels at the holes (a loose formation) and the perforation density is high, the geometric characteristic of the zone of influence of the holes is calculated by the formula

\[ G_s^*=\frac{1}{\lambda}-\frac{1}{s}-\frac{1}{4r}\ln\frac{s}{\lambda} +0.0625\frac{s-\lambda}{r^2} -0.0117\frac{s^2-\lambda^2}{r^3}+\ldots, \tag{3} \]

where \(\lambda\) is the radius of the hole in cm; the remaining notation is the same as in formula (2). In the case under consideration the formation is competent; the calculation is carried out by formula (2), into which, substituting \(y=2.0\ \text{cm}\), \(\lambda=0.2\ \text{cm}\), \(r=9.5\ \text{cm}\), and \(s=10.1\ \text{cm}\), we obtain \(G_s=1.079\ 1/\text{cm}\).

1. In the case of incomplete penetration of the formation by perforation, the geometric characteristic of the flow-constriction zone \(G_\delta\) is calculated. For a one-component perforation incompleteness it is expressed by the formula

\[ G_\delta=\frac{n}{h}\ln\frac{(h+x_0)\delta_1}{(h+\delta_1)x_0}, \tag{4} \]

where \(h\) is the length of the perforated part of the well in cm; \(\delta_1\) is the length of the incompleteness component—in the case under consideration, with a formation interval of \(1868\text{--}1895.1\ \text{m}\) and a perforation interval of \(1868\text{--}1877\ \text{m}\), this parameter is equal to \(1810\ \text{cm}\); and \(x_0\) is the radius of influence of the filter, expressed by the relation

\[ x_0=r+y+s, \tag{5} \]

which in the case under consideration takes the form

\[ x_0=9.5+2.0+10.1=21.6\ \text{cm}. \tag{6} \]

Substituting the known parameters into expression (4), we find that in our case the geometric characteristic of the flow-constriction zone is equal to \(0.491\ \text{cm}\).

1. The geometric characteristic of the plane-radial-flow zone was determined. In the presence of a flow-constriction zone, the following formula is used for this purpose:

\[ G_R=\frac{n}{H}\ln\frac{R}{\delta_{\max}}, \tag{7} \]

where \(H\) is the total formation thickness in cm—in our case \(2710\ \text{cm}\); \(\delta_{\max}\) is the largest incompleteness component in cm—in our case \(1810\ \text{cm}\); and \(R\) is the well radius of influence in cm. This parameter, in turn, is calculated by the formula

\[ R^2=\frac{2V_\Sigma}{\pi HmaP_s}, \tag{8} \]

where \(V_\Sigma\) is the volume of oil recovered during the formation test, in \(\text{cm}^3\); \(m\) is the formation porosity as a fraction of unity, in our case \(0.24\); \(\alpha\) is the oil compressibility in \(1/\text{at}\), in our case \(16.3\cdot10^{-5}\ 1/\text{at}\); \(P_s\) is the pressure at the outer boundary of the zone of influence of the holes, in atm, calculated by the method of successive approximations using the formula

\[ P_s=(P_{\text{pl}}-P_z)\frac{G_\delta+G_R}{G_s+G_\delta+G_R}. \tag{9} \]

1. The inflow to the well under reservoir conditions was determined in \(\text{cm}^3/\text{s}\). The calculation was carried out by the formula

\[ Q_{\text{pl}}=11.574\,Q_{\text{pv}}\cdot b, \tag{10} \]

where \(Q_{\text{pv}}\) is the inflow to the well, measured under surface conditions in \(m^3/day\); \(b\) is the oil formation volume factor in fractions of unity, in the present case 1.304.

In the first flow regime, well 204 operated for 29 hours; the pressure drawdown was 25.7 ati, and the production rate was \(14.4\ m^3/day\). Using formula (10), we find that its production rate under reservoir conditions was \(217.3\ cm^3/sec\), and the total withdrawal was \(22689\ thousand\ cm^3\). Then, using formulas (7, 8, and 9), we find that the depression at the external boundary of the zone of influence of the openings was \(8.257\ at\), the radius of influence of the well was \(4044\ cm\), and the geometric characteristic of the plane-radial flow zone was \(0.039\ 1/cm\).

1. The data obtained were substituted into the formula for the pressure drop during inflow under stabilized regimes:

\[ P_{\text{pl}} - P_{3} = \frac{Q_{\text{pl}}\mu}{2\pi k_{\text{ef}}h} \left(G_s + G_{\delta} + G_R\right), \tag{11} \]

where \(P_{\text{pl}}\) and \(P_3\) are the reservoir and bottomhole pressures in ati, in our case 183 and 157.3 ati, respectively; \(\mu\) is the oil viscosity in centipoises, in our case \(0.74\ cp\); and \(k_{\text{ef}}\) is the effective permeability of the formation, in darcies. The other parameters are explained above.

Substituting the available data into formula (11), we find the permeability of the formation in the first regime. It proved to be \(0.01237\ darcy\), or \(12.37\ millidarcies\). Under the other regimes, the permeability proved to be approximately the same.

References

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

2. L. A. Pukhlyakov. Derivation of the inflow formula for a hydrodynamically imperfect well. Proceedings of TPI, vol. 201, 1972.

3. L. A. Pukhlyakov. Method for determining the permeability of formations in unsealed zones from the results of well tests under stabilized regimes. Information Sheet No. 100-74. Tomsk TsTNI, 1974.