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Evolution of Acidic Compounds in Crude Oil during In-situ Combustion Renbao Zhao, J.D Sun, Qiang Fang, Yiguang Wei, Guixue Song, Chunming Xu, Chang Samuel Hsu, and Quan Shi Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017
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Evolution of Acidic Compounds in Crude Oil during In-situ Combustion Renbao Zhao†, Jindi Sun†, Qiang Fang ‡,Yiguang Wei†, Guixue Song§, Chunming Xu‡, Chang Samuel Hsu∥, ‡, and Quan Shi‡*
†
State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum,
Beijing 102249, China ‡
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,
China §
Institute of marine science and technology, Shandong University, Jinan 250100, China
∥
(a) Petro Bio Oil Consulting, Tallahassee, FL 32312 USA; (b) Department of Chemical and
Biomedical Engineering, Florida A&M University/Florida State University, Tallahassee, FL 32310 USA
ABSTRACT: In-situ combustion (ISC) process has drawn more and more attention in the development of heavy oil reservoir due to its high efficiency and oil recovery. Although numerous studies have been reported that oil properties exhibit significant changes during the combustion process, the reaction mechanisms and the evolution of oil components are still not well understood. In this work, the compounds of produced oils collected from a three dimensional simulated production model (container) at different duration times after combustion being initiated and the original oil were characterized at the molecular level using gas chromatography (GC), gas chromatography – mass spectrometry (GC-MS) and high-field Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Both aromatic and acidic components were analyzed. The aromatic components showed relatively more stable characteristics than that of acidic components, no obvious changes in aromatic compounds distributions were observed by the positive ion atmospheric pressure photoionization (APPI) FTICR MS analysis. Small aliphatic acids were detected in the ISC oils which were responsible for the high total acid numbers (TAN). The acidic Ox (x=1-3) compounds, which have major contributions to the increase in TAN, were generated in greater abundances compared to that of the original crude oil. The carbon number distributions of the O1 and O2 classes in the produced
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oils significantly shifted to a lower carbon number region, with the dominant distribution from 15 - 45 at the initial state to 10 - 40 at the longest duration time. The double bond equivalent (DBE) values decreased during the combustion process. The generated acidic O1 components with DBE values less than 4 were also found in negative ion electrospray (ESI) analysis, indicating the oxidation of hydrocarbons to alcohols.
1. INTRODUCTION Heavy oil is considered as an unconventional oil. The thermal methods have been proven to be efficient in its recovery, instead of traditional mining methods.[1] Among all the thermal methods, in-situ combustion (ISC) is well recognized for its efficiency to recover heavy oils. The ISC method has been conducted in a block with more than 50 wells in Xinjiang, China since 2009. Generally, in the ISC process a small portion of the oil is consumed as fuel to be burned insitu underground with injected air. A huge amount of heat is generated to increase the reservoir temperature.[2] The lighter components that are easily evaporated have higher flow rate, and condensed by releasing heat in front of low temperature regions. The oil in the unburned region becomes more mobile due to significant reduction in viscosity. The oil bank is then formed with increasing oil saturation to be flooded into the production well for collection. Oil oxidation during the ISC process involves extremely complex reactions over various temperature regions.[3, 4]
The properties of the oils are varied when they experience different evaporation effect and
chemical reactions such as oxygen addition, cracking and high temperature oxidation.[5] The components of produced oils at different duration times change drastically in carbon number distributions and functional groups. These chemical changes contribute to the macroscopic property variations such as significant drop in viscosity.[6] The characterization of complex heavy oil and its reaction kinetics with oxygen, which are believed to be great challenges, are essential for a better understanding of the ISC process, especially at the molecular level. Due to the multiphase fluid flows coupled with complicated chemical reactions in porous media, samples obtained sequentially from the physical simulation model or production well do not necessarily correspond to the duration time in the model or the reservoir. The duration time in our model refers to the contact time of oxygen in the combustion
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zone and oil. The uncertainty of the duration time is unfavorable for determining oxidation reaction time, and hence makes kinetics studies difficult. Previously, the SARA (saturates, aromatics, resins and asphalts) components were widely used to characterize the property variation of the crude oil during the combustion process.[7] Those interesting results show that the aromatics content decreased with increasing saturate content, followed by the saturate content decreases. Audibert[8] reported that the amount of resins plus asphaltenes generally remain unchanged during the combustion in the field. The variation of SARA contents, however, was not adequate to explain the oil upgrading mechanism at the molecular level. The difficulty remained of determining whether the tested oil samples being from cracking or just diluted by condensed light components. When considering the reaction kinetics of the hydrocarbons, especially for producing the heavy heteroatom-containing O, N, and S-components, the oxidation kinetics and the reaction products are severely influenced by their functional groups and molecular structures.[9] Carbon bond cleavage will occur through breaking either the alkyl side chains on the ring or aliphatic bridges between aromatic and naphthenic units.[9] Temperature is another key factor to determine the reaction pathway. The oxygen addition happens in a low to middle temperature region of around 100-300°C, followed by hydrocarbon cracking in the middle region of around 280-400°C, with new compounds formed.[10] The coke oxidation reactions generate huge amount of heat and produce carbon oxides at an even higher temperature region of around 400-600°C. In the field, total acid number (TAN) is commonly used as an important indicator of the degree of alteration of the recovered oil from each well.[11] The variation of TAN values can be used to represent the degree in oxidation of produced oils during the combustion process.[8] In crude oils, over one hundred acid homologues, nearly 3000 chemical formulas containing O2, O3 and O4 classes with carbon numbers ranging from 15 to 55, have been identified in the past decades.[12, 13].The acidic O2 compounds are typically formed by CH2 in the side chain of cyclic species with two oxygen atoms connected as a “COOH” group.[14] The TAN values can also be affected by decomposition of carboxylic acids at high temperatures.[15, 16] Gas chromatography with flame ionization detector (GC-FID) proved to be efficient in providing preliminary qualitative and quantitative information of the samples.[17] In order to obtain the even more precise information on the carbon number distributions as well as functional groups of the components for the whole crude oil including non-volatiles in GC, FT-ICR MS
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with different ionization methods are used. For example, laser desorption ionization (LDI)[18] has been reported for identifications of Ox class and naphthenic acids. Electrospray ionization (ESI) has been found highly efficient in analyzing polar compounds containing heteroatoms.[13,
19]
Atmospheric pressure photoionization (APPI) has been proposed for analyzing non-polar compounds.[5] Oils differ in properties due to different relative abundances of main hydrocarbon components with different carbon numbers and molecular structures as well as functional groups.[20] Previous works indicated that the major fractions of heavy oils are normally composed of fractions with boiling point higher than 200°C, and the components of production oils were greatly affected by the initial composition of the heavy oils.[21] For the characterization of the compounds eluting during the ISC process at the molecular level, GC-FID and FT-ICR MS were used. The primary focus of this work is to elucidate the variation of the polar and non-polar compounds affected by the ISC process, and to unravel the possible reaction paths of the crude oil during ISC. 2. EXPERIMENTAL 2.1 Sample Preparation Toe-to-heel air injection (THAI), an ISC process, simulation model was designed with a vertical injection well and a horizontal production well fixed into a cylindrical combustion chamber made of Hastelloy alloy with a volume of 48 liter. The oil sample was obtained from Karamay oil field located in northwest of Xinjiang province in China. The oil viscosity was around 9000 mPa·s, which is measured by RheoStress 6000 rheometer (Thermo Fisher Scientific Co. Ltd) at the temperature of 30 °C. A total of 100 kg of oil and sand with 1:8 weight ratio (oil to sand) were completely mixed and packed into this model (container). On both side of the container, there were totally 19 thermowells fixed upon it and each inserted with one thermocouple to make online temperature measurement. Air injection well was fixed into the bottom of the container while the production well as well as the funnel well were also installed and mounted onto the upper flange. The horizontal production well union was composed of one slotted casing and one tube. The casing with an inner diameter of 10mm is cut evenly into 80 slots with one end and screwed onto the upper flange, while the tube with a smaller inner diameter of 4mm inserting into the casing and connected to the three-phase-separator (TPS) with a ball valve. The connecting point between the tube and the upper flange was sealed with graphite
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packing to realize moveable sealing manner. At the downstream of the TPS, the filter, liquid condenser (container with a volume of 100ml) and backpressure regulator were equipped sequentially and finally is connected with gas analyzer. The temperature and gas concentration (CH4, O2, CO, CO2) data were monitored by a computer (shown in Figure 1). More detailed procedures of the THAI process can be found elsewhere.[22] The ignition was started when the leak check and preheating process were completed. With the volume of fire chamber increased, the average temperature also increased. The heated oils that flow into the slotted casing well through drainage were displaced by the flue gas at a faster flow rate. Oil and water that flooded by the flue gas were swept into a three-phase-separator. The flue gas was cooled and filtered through filter (a sand pack container with mesh 60-100) and container to remove liquid before going to the gas analyzer. The outflowed oils were collected from the separator at different times after the ignition started. The physical simulation model, which is shown in Figure 1, can tolerate a pressure of 2.5 MPa at the maximum temperature of 650 °C.[23] In the THAI process, a significant decrease in viscosity of the oil generated with plenty of foamy oils was observed. In order to expose the oxidation mechanism, the TAN values of the samples were measured as reference for monitoring the evolution of the acidic components. The values of the original oil and its effluent oils collected at different duration times are listed in Table 1. The oils collected at later duration times are affected more by the heat and oxidation than earlier ones, causing significant viscosity reduction of oil during the ISC process.
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Figure 1. A sketch of the three dimensional model of THAI process.
Prior to the GC-FID and FT-ICR MS analyses, dewatering process should be performed on the produced oils. The samples were centrifuged to get rid of water and sand completely. About 7-12 mg of each sample was dissolved in 1 mL toluene in a concentration of 7-12 mg/mL. In the negative ion electrospray ionization (ESI) FT-ICR MS experiments, 20 µL of each solution was taken and diluted with 1 mL of toluene/methanol (1:3, v:v) mixture. In addition, a 15 µL 28% NH4OH solution was spiked into the 1 mL analyte to promote the ionization. In the atmospheric pressure photoionization (APPI) analysis, the samples were diluted in toluene to a concentration of 0.2 mg/mL. 2.2 Gas chromatograph The samples were analyzed by an Agilent HP-5 column (60 m × 0.25 mm i.d. × 0.25 µm) in an Agilent 7890A GC equipped with a flame ionization detector (FID). The oven temperature was held at 40 °C for 10 min, then programmed from 40 to 70 °C at 4 °C /min, followed by ramping from 70 to 300 °C at a rate of 8 °C /min, then held at 300 °C for 40 min. Both the injector and the detector were operated at 300 °C. 2.3 Gas chromatograph-mass spectrometry (GC-MS) The GC-MS analyses were carried out on an Agilent 7890A (GC)-5975C (MS) GC-MS system. An Agilent HP-5 column (60 m × 0.25 mm i.d. × 0.25 µm) was used with an oven temperature program as follow: 35 °C for 10 min, then programmed from 35 to 300 °C at a rate of 5 °C /min, then held at 300 °C for 40 min. In the MS, electron impact (EI) ionization at 70 eV was used. The mass range was set to m/z 35-500 with a scan rate of 1 scan per second.
2.4 High Resolution Mass Spectrometry A Bruker Apex-Ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet (operating at 9.0 T) was used in the ultrahigh resolution mass spectrometry experiment. It was equipped with atmospheric pressure photoionization (APPI) and negative electrospray ionization (-ESI) sources to analyze compounds with different functionality. A list of the peak masses is generated by screening out noises for signal-to-noise (S/N) ratios less than 4. The experimental conditions and the data process were listed in Supporting Information and have been described elsewhere.[24, 25]
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3. RESULTS AND DISCUSSION The crude oil and three effluent (produced) oils collected at different duration (combustion) time (after the ignition) were selected to investigate the compositional changes in the ISC process. The combustion times and the total acid numbers (TAN) of the oils were listed in Table 1.
Table 1 Heat treatment and TAN of Oil samples. Samples
#0
#1
#2
#3
Reaction (duration) Time, h
0
5
9
13
Temperature, °C
30
160-220
220-280
240-320
TAN, mg KOH/g
5.89
10.45
6.23
22.75
C, wt%
86.05
83.99
85.69
83.88
H, wt%
11.59
11.55
12.25
10.39
H/C
1.62
1.65
1.72
1.49
N, wt%
0.27
0.04
0.05
0.20
O, wt%
1.15
1.48
1.00
2.73
Table 1 shows significant increases in TAN with duration time except for sample #2. The changes in TAN are related to the changes in the amounts of acidic components. Since the process was carried out at high temperatures with oxygen, it was believed that oxidation occurred with hydrocarbons. However, it is also known that carboxylic acids are unstable at high temperatures, which could decompose into CO2 and lead to decreases in TAN values.[16] So, the non-monotonic changes in TAN values shown in Table 1 can be contributed by the generation and decomposition of carboxylic acids. Figure 2 displays the GC-FID chromatograms of the Karamay oil sample #0. The oil is recovered from the reservoir by steam injection and can be regarded as the original state without any oxidation reaction. The oils produced from the block of the ISC process are very close to the location of sample #0 and have never been analyzed at the molecular level previously. These figures present the preliminary distribution results of the oil components.. It has been reported that the chromatogram is correlated to the property of the oil influenced by the characteristics of the thermal evolution and biodegradation during the oil formation in the reservoir.[26] In comparison of the four samples shown in Figure 2, there are no obvious changes except in the
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retention time region of
