Study of the Component Composition of Associated Gas from the Samotlor Oil Field
Abstract Generated abstract
This study evaluates the composition and quantity of associated gas in downhole oil samples from formations A-1 and B-VIII of the Samotlor field in Western Siberia. Samples were degassed stepwise at reservoir temperature, and the released gases were analyzed chromatographically for hydrocarbons and by chemical gas analysis for carbon dioxide, with gas densities and weight percentages calculated from the measured compositions. The results show that gas from formation A-1 contains more methane and that its oil has a higher gas factor, while decreasing pressure during degassing reduces methane content and increases the relative share of heavier hydrocarbons, especially C3 to C5. Considerable C2 to C5 hydrocarbons remained dissolved after atmospheric degassing, and total associated gas resources were estimated at 13.193 wt.% for A-1 and 15.307 wt.% for B-VIII relative to reservoir oil.
Full Text
STUDY OF THE COMPONENT COMPOSITION OF ASSOCIATED GAS FROM THE SAMOTLOR OIL FIELD
A. V. Kravtsov, N. M. Smolyaninova, V. A. Kuznetsova
(Presented by the scientific-methodological seminar of KhTF)
To evaluate the reserves and component composition of associated gases of the oil of the Samotlor field in Western Siberia, deep oil samples from this field, from formations A-I (well 14) and B-VIII (well 12), were taken and degassed at reservoir temperature on a UIPN-2m unit according to the appropriate methods [1, 2]. Analysis of the selected gas samples was carried out by the chromatographic method using chromatographs GAKh-21 and KhL-4.
Composition of gas and light hydrocarbons dissolved in surface oil*
| Well number | Analysis depth | C₂H₆ on surface oil | C₂H₆ on reservoir oil | C₃H₆ on surface oil | C₃H₆ on reservoir oil | i-C₄H₁₀ on surface oil | i-C₄H₁₀ on reservoir oil |
|---|---|---|---|---|---|---|---|
| 14 (formation A-I) | to C₄ | 0.001 | 0.001 | 0.127 | 0.114 | 0.121 | 0.108 |
| 14 (formation A-I) | to C₅ | 0.001 | 0.001 | 0.127 | 0.114 | 0.121 | 0.108 |
| 12 (formation B-VIII) | to C₄ | 0.006 | 0.005 | 0.203 | 0.181 | 0.181 | 0.161 |
| 12 (formation B-VIII) | to C₅ | 0.006 | 0.005 | 0.203 | 0.181 | 0.181 | 0.161 |
On the GAKh-21, separation of nonhydrocarbon gases and methane was carried out under the following conditions:
stationary phase — zeolite CaX,
column length — 1 m,
thermostat temperature — room temperature,
carrier gas — helium, 100 ml/min.
Hydrocarbon gases up to C₅ were separated on an KhL-4 chromatograph. Analysis conditions:
stationary phase — Vaseline oil (20%) on INZ-600,
thermostat temperature — 50°C,
carrier gas — helium, 100 ml/min,
column length — 4 m.
Determination of the carbon dioxide content in the gas was carried out on a VTI-2 chemical gas analyzer. From the established component composition of the gas, its density under normal conditions was calculated and the volume percentages were converted to weight percentages [3].
The amount of gas released from 1 cm³ of oil during stepwise pressure reduction by one atmosphere, for all samples, increases uniformly with decreasing total pressure in the system. The density of gas and oil by
* After degassing at reservoir temperature.
Table 1
Composition of the gas during stepwise degassing of reservoir oil with collection into a common volume*
| Component | 60 atm, % vol., reservoir A-1 | 60 atm, % vol., reservoir B-VIII | 60 atm, % wt., reservoir A-1 | 60 atm, % wt., reservoir B-VIII | 40 atm, % vol., reservoir A-1 | 40 atm, % vol., reservoir B-VIII | 40 atm, % wt., reservoir A-1 | 40 atm, % wt., reservoir B-VIII | 20 atm, % vol., reservoir A-1 | 20 atm, % vol., reservoir B-VIII | 20 atm, % wt., reservoir A-1 | 20 atm, % wt., reservoir B-VIII | 16 atm, % vol., reservoir A-1 | 16 atm, % vol., reservoir B-VIII | 16 atm, % wt., reservoir A-1 | 16 atm, % wt., reservoir B-VIII | 6 atm, % vol., reservoir A-1 | 6 atm, % vol., reservoir B-VIII | 6 atm, % wt., reservoir A-1 | 6 atm, % wt., reservoir B-VIII | 0 atm, % vol., reservoir A-1 | 0 atm, % vol., reservoir B-VIII | 0 atm, % wt., reservoir A-1 | 0 atm, % wt., reservoir B-VIII |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C₁ | 92.23 | 90.55 | 82.08 | 79.80 | 91.90 | 89.44 | 81.76 | 77.00 | 90.88 | 87.90 | 79.30 | 74.20 | 90.49 | 86.52 | 78.30 | 71.20 | 88.75 | 84.52 | 74.50 | 66.20 | 82.41 | 70.43 | 61.50 | 44.10 |
| C₂ | 1.64 | 3.10 | 2.92 | 5.48 | 1.70 | 3.28 | 3.03 | 5.65 | 1.94 | 3.64 | 3.39 | 6.15 | 2.05 | 3.96 | 3.55 | 6.51 | 2.39 | 4.75 | 4.00 | 7.45 | 3.35 | 6.00 | 5.00 | 7.50 |
| C₃ | 1.86 | 2.83 | 4.55 | 6.90 | 2.06 | 3.38 | 5.04 | 8.20 | 2.58 | 4.11 | 6.17 | 9.60 | 2.71 | 4.70 | 6.45 | 10.70 | 3.50 | 6.40 | 8.10 | 13.70 | 5.86 | 12.78 | 12.10 | 22.00 |
| iso-C₄ | 0.46 | 0.38 | 1.48 | 1.22 | 0.48 | 0.44 | 1.53 | 1.35 | 0.76 | 0.55 | 2.36 | 1.64 | 0.90 | 0.64 | 2.81 | 1.90 | 1.02 | 0.97 | 3.11 | 2.74 | 2.66 | 2.28 | 5.60 | 5.15 |
| n-C₄ | 0.89 | 0.44 | 2.88 | 1.40 | 0.91 | 0.54 | 2.92 | 1.68 | 0.96 | 0.86 | 3.02 | 2.63 | 1.00 | 1.07 | 3.13 | 3.20 | 1.30 | 1.70 | 3.96 | 4.80 | 2.10 | 4.78 | 5.70 | 10.80 |
| iso-C₅ | 0.20 | 0.25 | 0.84 | 0.99 | 0.19 | 0.24 | 0.73 | 0.92 | 0.23 | 0.26 | 0.90 | 0.99 | 0.24 | 0.29 | 0.95 | 1.05 | 0.30 | 0.41 | 1.14 | 1.44 | 0.99 | 1.20 | 3.34 | 3.37 |
| n-C₅ | 0.22 | 0.35 | 0.90 | 1.39 | 0.21 | 0.31 | 0.81 | 1.20 | 0.23 | 0.32 | 0.90 | 1.22 | 0.24 | 0.35 | 0.92 | 1.26 | 0.31 | 0.42 | 1.16 | 1.48 | 0.96 | 1.15 | 3.23 | 3.27 |
| C₆ | — | — | — | — | — | — | — | — | — | — | — | — | — | 0.05 | — | 0.22 | — | 0.20 | — | 0.84 | 0.13 | 0.89 | 0.45 | 2.97 |
| [[unclear: component label]] | 2.31 | 1.82 | 3.60 | 2.80 | 2.22 | 1.87 | 3.46 | 2.82 | 2.08 | 2.02 | 3.17 | 2.99 | 2.04 | 1.83 | 3.10 | 2.63 | 1.90 | — | 2.80 | — | 1.69 | — | 2.18 | — |
| CO₂ | 0.20 | 0.28 | 0.75 | 0.02 | 0.30 | 0.50 | 0.74 | 1.18 | 0.34 | 0.25 | 0.79 | 0.58 | 0.33 | 0.59 | 0.79 | 1.33 | 0.54 | 0.63 | 1.23 | 1.35 | 1.45 | 0.49 | 0.90 | 0.84 |
* Degassing was carried out at reservoir temperature: 65°C for reservoir A-1 and 75°C for reservoir B-VIII.
stages during differential degassing also increases as the pressure is reduced, which indicates an increase in the gas content—especially in the last stage—of heavy hydrocarbons \(C_4—C_5\). The total amount of gas liberated from \(1\ \text{m}^3\) of oil during its contact degassing for samples from wells 14 (formation A-1) and 12 (formation B-VIII) is, respectively, 95.01 and 78.38 \(\text{m}^3\).
The composition of the associated gas liberated during degassing into the total volume is presented in Table 1. The associated gas from formation A-1 (well 14) is characterized by a higher methane content than the gas from formation B-VIII (well 12), but the gas content of oil from formation A-1 is higher than that of formation B-VIII.
As should be expected, with decreasing pressure in the system during the process of stepwise degassing, the methane content in the associated gas decreases, while the content of hydrocarbons \(C_3—C_5\) increases.
After degassing of a reservoir oil sample to atmospheric pressure, a considerable amount of hydrocarbons \(C_2—C_5\) remains dissolved in it. The yield and composition of the gases remaining dissolved in the degassed oil were determined by us according to the procedure of Laboratory No. 2 of VNIINP and are presented in Table 2.
It is evident from Table 2 that oil degassed at reservoir temperature contains considerable amounts of heavy components—mainly hydrocarbons \(C_4\) and \(C_5\). The sum of dissolved hydrocarbons up to \(C_4\) and up to \(C_5\) differs for the investigated oil samples (from wells 14 and
Table 2
in surface and reservoir samples of Samotlor oil (% by weight, relative to oil)
| \(n\text{-}C_4H_{10}\), on surface oil | \(n\text{-}C_4H_{10}\), on reservoir oil | \(n\text{-}C_5H_{12}\), on surface oil | \(n\text{-}C_5H_{12}\), on reservoir oil | \(n\text{-}C_5H_{12}\), on surface oil | \(n\text{-}C_5H_{12}\), on reservoir oil | Total, on surface oil | Total, on reservoir oil |
|---|---|---|---|---|---|---|---|
| 0.596 | 0.537 | — | — | — | — | 0.845 | — |
| 0.596 | 0.537 | 0.652 | 0.588 | 0.993 | 0.895 | 2.490 | 2.243 |
| 0.940 | 0.837 | — | — | — | — | 1.330 | — |
| 0.940 | 0.837 | 0.779 | 0.693 | 1.291 | 1.150 | 3.400 | 3.027 |
well 12) and ranges, respectively, from 0.845 to 1.330 wt.% and from 2.490 to 3.400 wt.% relative to degassed oil. Methane and non-hydrocarbon gases are completely volatilized during degassing and are absent from the surface oil sample.
Conclusions
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Stepwise degassing was carried out on downhole oil samples from formations A-1 (well 14) and B-VIII (well 12) of the Samotlor field in Western Siberia at reservoir temperature.
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It was established that the total resources of associated gases amount to 13.193 wt.% for formation A-1 (well 14) and 15.307 wt.% for formation B-VIII (well 12), calculated relative to reservoir oil.
Literature
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“Method for sampling and studying downhole samples of reservoir oils from the Tomsk and Novosibirsk regions,” report of NTSU, Novosibirsk, 1966.
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V. N. Mamuna, G. F. Trebin, B. V. Ulyaninskii. Experimental Study of Reservoir Oils. GOSINTI, 1960.
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A. S. Smirnov, A. I. Shirkovskii. Gas Production and Transportation. Gostoptekhizdat, 1967.