Key Words:- Crude oil, Egyptian central Gulf of Suez, Chemical fingerprint, Capillary gas chromatography, Carbon number distribution.

Transcription

1 Chemical Fingerprinting of some Egyptian crude oils by gas chromatography Ashraf Yehia El-Naggar Chemistry department, Faculty of Science, Taif University, Kingdom of Saudi Arabia Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt. Abstract: Four crude oil samples were collected from El Hamd well oil Field located in Egyptian central Gulf of Suez and investigated by exaing the suitability of their chemical fingerprint via capillary gas chromatography (CGC). (CGC) assists to investigate carbon number distribution of n-paraffin s and to detere relative percentages of paraffin s. The distribution of n-paraffin s could be taken as indication of the genetic origin of the crude oils. The maximum abundance at C 17, C 18, and C 19 reflects the marine origin of these crude oils. Key Words:- Crude oil, Egyptian central Gulf of Suez, Chemical fingerprint, Capillary gas chromatography, Carbon number distribution. I. INTRODUCTION The hydrocarbons including saturates and aromatics represent the main constituents of petroleum crude oil. On the other hand, the petroleum refiners are mostly interested in the amount of the successive distillation fractions (e.g. gasoline, naphtha, kerosene, gas oil) and the chemical composition or physical properties of these fractions. The classification of crude oils is based on the content of the various structural types of hydrocarbons, namely paraffin s, naphthenes and aromatics. In addition, bulk composition of the residual fraction and Maltene should be considered. Marquart et al. [1] analyzed the n-paraffins separated by urea adduction by GLC. The GC separation was carried out on a 9-ft column packed with 2 % Carbowax 1 on Chromosorb W. The column temperature was programmed from 75 oc to 25 oc at 6 oc -1. They recommended squalane as the internal standard. Jokuty et al. [2] used GC-FID to analyze saturates, aromatics, resins, asphaltenes and waxes in 3 crude oils. They also developed a method to detere the oil adhesion to a test surface. Lai and Song [3] detered temperature-programmed retention indexes for over 15 pure compounds (alkanes, alkenes, naphthenes, polycyclic aromatic hydrocarbons) using two capillary columns with different stationaryphase polarities at three heating rates. Despite the high resolving power and extended working range of modern capillary GC, a substantial proportion (3-9%) of fresh and biodegraded crude oils is still having unresolved percentage and chromatograms of crude oils often contain humps or so-called unresolved complex mixtures (UCMs) under the peaks represent of separated paraffinic hydrocarbons. A few attempts to characterize hydrocarbon UCMs have been made recently [4-8]. A. Samples II. EXPERIMENTAL Four crude oil samples were collected from wells of El Hamd oil field located in the central Gulf of Suez, namely: H-1, H-2, H-3 and H-4. These oil samples are representative for the the crude oils in Egyptian central Gulf of Suez. B. Analytical Techniques 1. Each Crude oil sample was distilled up to 2 C at atmospheric pressure. The residual fraction (> 2 C) was deasphalted according to IP-143 standard procedure. The deasphalted fraction (maltene) was separated into saturates, naphthens, aromatics and resins by liquid column chromatography. The column used was packed with 1:1 alua overlying silica gel. Saturates, aromatics and resins fractions were obtained by successive elution with n-pentane, toluene and methanol, respectively. 44

2 2. Sulfur (S%) and density (API gravity) of the crude oil were measured according to ASTM D-4294 and ASTM D-452 procedures, respectively. 3. Saturated hydrocarbon fractions were subjected to capillary gas chromatography in order to investigate carbon number distribution of n-paraffins and detere relative percentages of paraffins. The instrument used was Agilent 689 Series equipped with flame ionization detector (FID). Oven temperature was programmed from 1 C to 3 C at fixed rate of 3 C -1. HP-1 fused silica capillary column (6 m X.53 mm X.5 μm) was used for the analysis. The chromatograms of the four samples (H-1, H-2, H-3 and H-4) are shown in Figs.1-4. III. RESULTS AND DISCUSSION Gas chromatograms of total hydrocarbons in the crude oil samples are shown in Figs. 1 to 4. The GC/FID chromatograms of the crude oil samples are characterized by a monotonically decreasing homologous series of normal alkanes and isoalkanes with slightly even and odd carbon preference extending from n-c 13 to C 35 in El Hamd wells. The distributions are typical of marine organic matter from alga and/or bacteria [9]. The distribution of n-alkanes and isoprenoids in this region as shown in the chromatograms is diagnostic of marine organic faces source with contribution to the biomass from algae and plankton [1]. The mode of distribution of n-paraffins could be taken as indication of the genetic origin of the crude oils. Results of n-paraffins distribution are given in Table 1 and Figs.1-4. The maximum abundance at C 17, C 18, and C 19 reflects the marine origin of these crude oils. Carbon preference index (CPI) can be calculated for C + 15 of n-paraffins fraction which represents odd / even carbon numbers. The CPI values are close to unity ranging from.992 to 1.54 (Table 2) indicating that the studied samples are mature crude oils [11]. From the results of the studied crude oil samples in Table 3, it has been found (Table 4) that the aromatic hydrocarbons are the major type (> 5 %) in the four crude oils range from to % and low saturates/aromatics ratios ranging from.52 to.87. Also, the crude oils contain ~ 25 % of naphthenes and paraffins range from to 24.6 and 16.73to 22.6 respectively. Therefore, these crude oils are classified as aromatic intermediate oils. This class contains heavy crude oils of low waxy content (API ranging between 2.1 and 23.45, Table 2). The crude oils contain ~ 3% of Asphaltene and resins (ranging between 8.11 to 8.84 % and 2.71 to 31.1 % respectively (Tables 2, 3) and high sulfur contents ( wt %) is indicative of marine origin [11]. Carbon number distribution of the studied crude oil samples were shown in Figure 5. It has been found that each oil sample has a significant finger print differ compared with the other finger prints of other crude oils. In this paper, a simple method for identification of outlying samples from the set of samples based on finger printing and carbon number distribution is presented. Results obtained for analysis of f our crude oil samples through separation of various hydrocarbons by using capillary gas chromatograph with flame ionization detector are discussed. IV. CONCLUSIONS 1. The detered API gravity, sulfur percent and gross compositions (saturates, aromatics, asphaltenes, and resins compounds) classified the crude oils as aromatic intermediate oils. This class contains heavy oils of low waxy content and characterized by high maturity level. 2. Normal alkane distribution, isoprenoids (pristane and phytane, isoprenoid/n-alkanes) indicate that the crude oils derived from mixed organic sources (mainly marine with few input from terrestrial origin) belong to carbonate oil type, deposited in transitional environments under reducing-oxidizing conditions. 3. The GC/FID of the crude oil samples are characterized by a monotonically decreasing homologous series of normal alkanes and isoalkanes with slightly even and odd carbon preference extending from n-c 13 to C 35 in El Hamd wells. The distributions are typical of marine organic matter from alga and/or bacteria. The distribution of n-alkanes and isoprenoids in this region is diagnostic of marine organic faces source with contribution to the biomass from algae and plankton. 45

3 FID1 A, (G474.D) C3 2 1 C12 C FID1 A, (G471.D) Fig. 1: Gas Chromatogram of Saturated Fraction of H-1 Well C Fig. 2: Gas Chromatogram of Saturated Fraction of H-2 Well. FID1 A, (G475.D) C Fig. 3: Gas Chromatogram of Saturated Fraction of H-3 Well. FID1 A, (G473.D) Fig. 4: Gas Chromatogram of Saturated Fraction of H-4 Well. 46

4 C12 C28 C29 C3 C31 C32 C33 ISSN: Carbon number Well Name H-1 H-2 H-3 H-4 C C C C C C C C C C C C C C C C C C C C C C Table 1: Relative Distribution of n-paraffins (wt %) of the Crude Oils H-1 47

5 C28 C29 C3 C12 ISSN: H H H-4 Fig. 5: Distribution Curve of n-paraffins for the studied crude oil samples Table 2: Geochemical Parameters Derived from GC Analysis Well Name API Pr/n-C 17 Ph/n-C 18 Pr/Ph CPI H H H H Table 3: Distribution of Hydrocarbon Types Well Name Relative Percentages of Hydrocarbon Types, wt% Paraffins Naphthenes Aromatics H H H H

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