562
Ind. Eng. Chem. Res. 1999, 38, 562-564
Compositional Analysis of Oil Samples at High Pressure Ivar M. Dahl* and Elisabeth M. Myhrvold† SINTEF Applied Chemistry, P.O. Box 124, Blindern, 0314 Oslo, Norway
Hanne M. Øren Saga Petroleum A/S, P.O. Box 490, 1301 Sandvika, Norway
A new, simple method of analyzing light crude oil up to C24, sampled at high temperatures and pressures, has been developed. The method avoids time-consuming depressurizing procedures. The method utilizes a liquid sampling valve, which endures moderate heating at relatively high pressures. The global composition and number-average molecular weight of the sample is determined with sufficient accuracy. Combined with a PVT software package, the gas/oil ratio and the composition of the gas phase by degassing at 15 °C and 1 bar can be predicted. Introduction Oil found in deep reservoirs in the North Sea is sampled at high temperatures and pressures. To determine the global composition of the oil, the samples are degassed at specified conditions and the gas and oil fractions are separately analyzed. The analyses of the separated gas and oil fractions are later recombined to determine the global composition. Degassing of the oil is a time-consuming procedure and at least two analyses are necessary to determine the global composition. The scope of this work has been to develop a method for analyzing the complete pressurized sample in one operation. This procedure could improve accuracy as well as shorten analysis time. The benefit of the depressurizing method, namely of determining the gas/oil ratio directly, might be rather superfluous, as modern computational tools1 for phase equilibrium calculations2,3 are able to calculate this from the global composition. In fact, for condensate fields, which may give oil with a gas/oil volume ratio in excess of 1000, the computational procedure may have advantages. Experimental Section The experimental setup is schematically illustrated in Figure 1. A liquid sampling valve (LSV) from Valco (C 412242 Y with a 2.5 µL external loop) was used to inject the sample on the gas chromatograph (GC). The valve is specified to tolerate high pressure (206 bar) and temperature (150 °C). The floating piston in the oil sampler (Proserv, S1-690-64 titanium cylinder) can be moved by pumping in ethylene glycol with a Minipump VS (Milton Roy) at 5 mL/min, or by N2 pressure. The following procedure was used for liquid sampling. The sample vessel was heated in an oven to achieve the original sampling temperature (tor). The transfer lines were pressurized with N2 to achieve the original sampling pressure (por ) 29 bar). Valve V1 was pressurized with ethylene glycol by means of a HPLC pump, and when the backside pressure reached por, valves V1 * To whom correpondence should be addressed. E-mail:
[email protected]. Tel.: (+47)22 06 73 00. Fax: (+47)22 06 73 50. † E-mail address:
[email protected].
Figure 1. Schematic drawing of the high-pressure sampling unit.
and then V2 were opened. Approximately 5 mL of the oil sample was fed through the LSV and into the Hoke vessel before the valves were closed and the sample was injected. After injection, the oil in the Hoke vessel was depressurized, and the degassed oil collected for subsequent offline injections (with and without addition of the internal standard). Then 2 wt % of ethyl-tert-butyl ether was added to the degassed oil as the internal standard. The oil used for the investigation was a partly degassed condensate sample from the North Sea. The injection system is schematically shown in Figure 2. The large loop size in the valve (2.5 µL), and the correspondingly large split ratio, were chosen to minimize the relative amount of sample remaining in the tubing after injection. After the LSV valve was turned, 7.8 V was applied to the tubing (1/16 in. stainless steel, measured resistance ∼0.6 Ω/m) over a total length of 35 cm in order to heat and vaporize the sample as completely as possible. This corresponds to 1 kW/m. The heating was applied for a period of 30 s. The He flow was 170 N‚mL/min through the LSV, and 240 N‚mL/min through the MFC (mass flow controller from Hi-Tech, 0-300 mL of He/min) into the injector. This corresponds to a split ratio of 410 (with a column flow of 1 N‚mL/min). The GC was a Hewlett-Packard 5890 with two columns/two detectors mounted parallel to one another in one injector. The column for the analysis of permanent gases was a Poraplot Q (25 m × 0.32 mm i.d. with a 20
10.1021/ie980244g CCC: $18.00 © 1999 American Chemical Society Published on Web 12/09/1998
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 563 Table 1. GC Run of a C6-C44 Standard compound
calibrated (wt %)
analyzed (wt %)
nC6 nC7 nC8 nC9 nC10 nC11 nC12 nC14 nC16 nC18 nC20 nC24
6.32 6.32 8.42 8.42 12.63 12.63 12.63 12.63 10.53 5.26 2.11 2.11
6.20 6.26 8.55 8.22 12.97 12.83 13.19 12.44 10.07 5.38 2.07 1.81
Table 2. Comparison of C13+ Determination (of Pressurized Sample), by Internal Standard and by Integration. A Degassed North Sea Condensate Sample
Figure 2. Schematic drawing of the injection system.
µm coating) (thermal conductivity detector), and the column for hydrocarbon analysis was a HP PONA (50 m × 0.2 mm i.d. 0.5 µm coating) (flame ionization detector). The temperature program was 40 °C isothermal (15 min), to 70 °C (20 min), to 130 °C (20 min), isothermal at 130 °C (20 min), to 250 °C (12 min), and isothermal at 250 °C (60 min). Response factors for the oil components are taken from ref 4. The gas/liquid separation and compositional analysis of the fractions were performed in a standard manner (single-stage separation, gas/liquid equilibration at 15 °C and 1 bar). Calculations of phase compositions were made with the PVTsim software package.1 Results and Discussion Sampling Precision and Accuracy. A reproducible sampling size is all-important to the quality of the analysis. Repeated analyses during the test period have been performed on mixtures of pure compounds in the C5-C10 boiling area, to determine the precision and accuracy of the system. Real oil samples is expected to give somewhat higher uncertainties than this mixture. The mixture was injected with the sampling valve at room temperature. The relative precision of the peak area determination was 0.3%. The accuracy, which could be determined by comparing the normalized results from the integration, using the tabulated response factors,4 and the known composition of the standard, was also 0.3% relative. Thus, there is no systematic error in the analysis set up to carbon numbers C12 produced lower areas than should be expected. By injecting a reference sample (nC6-nC44) from ChemServices with intermittent heating of the transfer lines, we could determine components eC24. Components gC28 were not eluted from the column, neither with a syringe injection nor with LSV injection. The relative wt % of the compounds in the C6-C24 region is shown in Table 1.
run
C13+ wt % in original sample from integration
C13+ wt % in original sample using internal standard
1 2 3 4
33.9 33.9 32.4 32.1
31.7 32.7
Table 3. Number-Average Molecular Weight of the Oil by Different Methods area integration
use of internal standard
freezing point depression
108.6 ( 3
107.7 ( 3
112.8 ( 7
The standard deviation between calibrated and analyzed values is 0.25 wt % absolute, with no systematic variation with the boiling point. This means that the components are quantitatively transferred to the injector, at least up to C24. At higher carbon numbers the chromatographic system seems to be the limiting factor, since there is no elution of C28 or heavier compounds from the column at the given chromatographic conditions. Heavy-Component Peak, Baseline Correction, Internal Standard. With the chosen chromatographic conditions, individual peaks from oil samples will be identified only for compounds C12 will elute as a broad curve with some resolved peaks on top. The area of this broad and largely unresolved peak may contain a considerable amount of the total oil sample, even for lighter oils. In the same region a blank run will give a drift in the baseline because of column bleeding. This drift must be corrected to give a more precise area; how to do this is not evident, but of some importance. An examination of gas chromatograms of different runs show some baseline differences between the runs. The area determination of the C13+ fraction in the present method is done by “tangential skimming” of the baseline from elution times of 74140 min. An alternative to the determination of this area by integration is to estimate the amount of C13+ from the known concentration of an added internal standard. In Table 2, the >C12 weight fraction is estimated by integration and baseline subtraction, and by addition of the internal standard. Ethyl-tert-butyl ether (ETBE) is used as internal standard, as it elutes at a retention time where there is no overlap with the oil components. This determination of the C13+ fraction is made on the degassed, excess oil, sampled in the Hoke vessel (Figure 1), as described previously. Assuming that the C7-C10 fraction is negligibly vaporized in the degassed sample, the original C13+ content can easily be calculated from
564 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999
Mn )
Figure 3. Molar distribution of carbon number fractions in the condensate. Table 4. Gas Content and Gas Composition for a Standard Flash (15 °C, 1 bar) [Measured and Calculated Values]
N2 CO2 C1 C2 C3 C4 C5 C6 C7+ gas/oil ratio (mL/mL)
measured from separation and gas analysis (mol %)
calculated1 from high-pressure method and PVTsim (mol %)
calculated1 from traditional recombination and PVTsim (mol %)
0.3 6.35 41.26 20.68 16.03 9.44 3.60 1.13 1.21 41
