Institutt for kontinentalsokkelunderskelserCONTINENTAL SHELF INSTITUTE, NORWAYHkon Magnussons gt. 1 B Postboks 1883 7001 Trondheim. Norway Tlf. (07) 920611
Telex 55434 IKU N Telegram NORSHELF Telefax (07) 920924 (Aut)
REPORT TITLE/ TITTEL
HYDROCARBON CHARACTERISATION OF WELL 3 1 / 5 - 2 .
CLIENT/ OPPDRAGSGIVER _ ,_
Saga Pejpofeum f T . s . " ~ " ~"~~ "- n - * RESPONSIBLE S?NTtSTAPBQi5JEKTANSVARUG _ _ .. _ ^
Susanmh Betts' v & _. -_r4? ; ^ ,, ,"a
NO OF PAGES/ANT SIDER
NO OF ENCLOSURES/ANT BILAG
One oil and one gas sample from 31/5-2 were analysed.
The oil (B-8116) is a mature biodegraded paraffinic oil.
The gas (B-8971) associated with the oil is 80% methane and shows dep-letion of n-alkanes of carbon number above C (relative to branchedand cyclic compounds of similar molecular weight).
KEY WORDS/STIKKORD31/5-2 TROLL
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Gas chromatography results
GC-MS analysis of saturated steranes and terpanes
GC-MS analysis of aromatic hydrocarbons
Fraction boiling below 210 C.
Weight of oil fractions.
% composition of the oil.
C,-Cg hydrocarbons,yg and %.
Results of gas analysis.
Data from gas analysis.
Results of carbon isotopic analyses.
Sterane/triterpane molecular ratios (maturity),
Sterane/triterpane molecular ratios
(source and maturity).
Figure 1: Gas chromatogram Cp-Cg hydrocarbons.
Figure 2: Gas chromatogram of gas sample.
Figure 3: Saturated fraction gas chromatograms.
Figure 4: Branched/cyclic fraction gas chromatogram.
Figure 5: Aromatic fraction gas chromatogram.
Figure 6: Mass chromatograms - terpanes.
Figure 7: Mass chromatograms - steranes.
Figure 8: Mass chromatograms - aromatic hydrocarbons,
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IKU was supplied with one oil and one gas sample from this well,the two samples together make up a representative reservoir fluid.
The following information on collection of the oil and gas samples wassupplied by GECO.
Flash of reservoir fluid to stock tank conditions
Flash conditionsGas oil ratioBo at 250 bargBo at bubble pointDensity of oil at 15 CMolecular weight of oilStandard conditions
-o250 barg, 67.6 C to atmosphere and 15 C.60.5 sm3/m3.1.158 m3/m3.1.170 m3/m3.
3894.4 kg/m251.for gas volumes = 15C and 1 atm.for oil volumes = 15C and atmospheric
Saga supplied information on reservoir temperature (67.6C) and pres-sure 156 barg.
The following analyses were carried out on the oil sample:
) - Measurement of API gravity of the oil.C2-Cg hydrocarbon characterisation.Measurement of the fraction boiling under 210C.Fractionation of the fraction boiling over 210C by MPLC (mediumpressure liquid chromatography).Molecular sieving of the saturated fraction into normal andbranched/cyclic al kanes.Gas chromatography of the saturated/branched and cyclic/aromaticfractions.GC-MS of saturated fraction (m/z 191, 217 and 218).GC-MS of aromatic fraction (m/z 231 and 253).6 C isotope analysis of saturated and aromatic hydrocarbon frac-tions.
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C4"C10 a n a l y s i s was carried out on a HP 5880 A gas chromatograph equip-ped with a 50m x 0.2mm (I.D.) fused silica column coated with OV-101.Helium was used as carrier gas at lml/min. The inlet split ratio was1:60. The temperature program was 35C (5 min) - 8C/min. - 200C (2min.) which gave good resolution from C- upwards. Quantitation was car-ried out using the same standard gas as for C,-C5+ analysis.
Total orgSftjc carbon
Bulk samples were cra-sjied in a mortar. Aliquots of the samples were thenweighed into Leco crucibt&Sx.and treated three times with hot 10% HC1 toremove carbonate, and washed I^Mmes with distilled water to remove tra-ces of HC1. The crucibles were thenx|rt-aced on a hot plate and dried for24 hours. The total organic carbon (TOC)Content of the dried sampleswas determined using a Leco CR12 carbon analysers
Evaporation of the light components in fluid samples
Prior to chromatographic separation of oil/condensate samples, the frac-tions boiling below 210C were removed by heating the samples to constantweight at 210C is obtained. The heating is performed at atmosphericpressure.
The fraction of light components is determined as the weight differencebetween the original sample and the amount that is left after the heat-ing.
The fraction of the oil boiling above 210 C was separated into saturatedfraction, aromatic fraction and non hydrocarbon fraction using a MPLCsystem with hexane as eluant (Radke et al., Anal. Chem., 1980). The vari-ous fractions were evaporated on a Buchi Rotavapor and transferred toglass vials and dried in stream of nitrogen.
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Molecular sieve adsorption
The sample containing 2mg of n-alkanes was dissolved in 35ml of cyclo-hexane and lgs of Molecular Sieve pellets (5) which had been activatedat 300C in 24 hours, were added. This mixture was then refluzed forabout 24 hours. While the solution was still hot, the sieve pellets wereremoved from the solution by filtering. The solvent was then removed ona Buchi Rotavapor. GC analysis were performed on the samples, using thesame conditions as for the other GC analysis.
The normal al kanes were recovered from the Molecular Sieve pellets bydestruction of the pellets with hydrofluoric acid. The solution was ex-tracted with boric acid and hexan, and the solvent was then removed ona Buchi Rotavapor. GC analysis were performed on the samples, using thesame conditions as for the other GC analysis.
Gas chromatographic analysis
The C2-Cg hydrocarbons of the oil were determined on a Carlo ErbaFractovap GC. The column used was a 30m fused silica capillary columncoated with SE-54. The temperature program applied was 50C (2min.) to180C at 4C/min.
The saturated, the branched/cyclic and the aromatic hydrocarbon frac-tions were each diluted with n-hexane and analysed on a HP 5730A. TheGC is equipped with a 15m OB-1 fused silica column and hydrogen (ca.2.5 ml/min.) is used as carrier gas. Injections are performed in splitmode (split ratio 1:10). The temperature program applied is 80C (2 min.)to 280C at 4C/min.
The data processing for all the GC analyses was performed on a VG Mul-tichrom lab data system.
Gas chromatography - mass spectrometry (GC-MS)
GC-MS analyses were performed on a VG Micromass 7O-7OH GC-MS-DS system.The Varian Series 3700 GC was fitted with a fused silica 0V-1 capillarycolumn (30m x 0.3mm i.d.). Helium (0.7kg/cm ) was used as carrier gasand the injections were performed in split mode (1.5^1, split ratio 1:15).The GC oven was programmed from 70C to 280C at 4C/min. after an ini-
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tial isothermal period of 2 minutes.
The saturated hydrocarbons were analysed in multiple ion mode (MID) at
a scan cycle time of approximately 2 sees. Full data collection was app-
lied for the aromatic hydrocarbons at a scan time of 1 sec/decade. The
mass spectrometer operated at 70eV electron energy and an ion source
temperature of 200C. Data acquisition was done by VG data systems.
Peak identification was performed applying knowledge of elution patterns
in certain mass chromatograms. Calculation of peak ratios was done from
peak height in the appropriate mass chromatograms.
6 C isotope analysis
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The specific gravity of B-8116 at 60F = 0.8934
API0 gravity = 26.8
This is a medium gravity oil. The gravity is probably higher now thanit was originally due to biodegradation and consequent loss of lighterhydrocarbons.
Fraction boiling below 210C (see table 1)
Oil composition (see tables 2 and 3)
A biodegraded paraffinic oil.
The distribution of C2-Cg hydrocarbons shows a similar trend to thatseen in the nC,g+ fraction. The n-alkanes have been depleted by biodegra-dation. Table 4 and figure 1 show the relative weights and percentageweights of C2-Cg compounds present in the sample. The light hydrocarbonratios used by Thompson, 1979 to estimate type of source and maturityare probably not valid for this sample because it is biodegraded. Inaddition to this it has not been possible to determine some of the neces-sary compounds, e.g. heptane.
The results have been tabulated (table 5). A similar trend of depletionof n-alkanes is also seen in the gas sample (above C.). The iC,/nC. ratiois approximately 6.5 and iC5/nC5 is 4.8. A similar predominance of bran-ched