May 10, 2017 - Evolution of Acidic Compounds in Crude Oil during In Situ. Combustion. Renbao Zhao,. â . Jindi Sun,. â . Qiang Fang,. â¡. Yiguang Wei,. â .
Article pubs.acs.org/EF
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 and ‡State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China § Institute of Marine Science and Technology, Shandong University, Jinan, Shandong 250100, People’s Republic of China ∥ Petro Bio Oil Consulting, Tallahassee, Florida 32312, United States ⊥ Department of Chemical and Biomedical Engineering, Florida A&M University/Florida State University, Tallahassee, Florida 32310, United States S Supporting Information *
ABSTRACT: In situ combustion (ISC) process has drawn more and more attention in the development of heavy oil reservoirs as a result of its high recovery efficiency. Although numerous studies have been reported that oil properties exhibit significant changes during the combustion process, the reaction mechanisms and 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 those of acidic components, and no obvious changes in aromatic compound distributions were observed by the positive ion atmospheric pressure photoionization (APPI) FT-ICR MS analysis. Small aliphatic acids were detected in the ISC oils, which were responsible for the high total acid numbers (TANs). 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 oils significantly shifted to a lower carbon number region, with the dominant distribution from 15−40 at the initial state to 10−30 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 ionization (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 of 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 in situ 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 a higher flow rate and condensed by releasing heat in front of low-temperature regions. The oil in the unburned region becomes more mobile as a result of 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 effects and chemical reactions, such as oxygen addition, cracking, and hightemperature oxidation.5 The components of produced oils at © 2017 American Chemical Society
different duration times change drastically in carbon number distributions and functional groups. These chemical changes contribute to the macroscopic property variations, such as a 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. As a result of 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 zone and oil. The uncertainty of the duration time is unfavorable for determining the oxidation reaction time and, hence, makes kinetic studies difficult. Previously, the saturate, aromatic, resin, and asphaltene (SARA) components were widely used to characterize the Received: February 14, 2017 Revised: May 9, 2017 Published: May 10, 2017 5926
DOI: 10.1021/acs.energyfuels.7b00453 Energy Fuels 2017, 31, 5926−5932
Article
Energy & Fuels
Figure 1. Sketch of the three-dimensional model of the THAI process.
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 gas chromatography (GC), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) 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 nonpolar compounds.5 Oils differ in properties as a result of 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 a 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 nonpolar compounds affected by the ISC process and to unravel the possible reaction paths of the crude oil during ISC.
property variation of the crude oil during the combustion process.7 Those interesting results show that the aromatic content decreased with an increasing saturate content, followed by saturate content decreases. Audibert8 reported that the amount of resins plus asphaltenes generally remains unchanged during the combustion in the field. The variation of SARA contents, however, was not adequate to explain the oilupgrading mechanism at the molecular level. The difficulty remained of determining whether the tested oil samples were 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 The 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 a huge amount of heat and produce carbon oxides at an even higher temperature region of around 400−600 °C. In the field, the 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 100 acid homologues and nearly 3000 chemical formulas containing O2, O3, and O4 classes with carbon numbers ranging from 15 to 55 have been identified in the past few 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 a flame ionization detector (GC− FID) proved to be efficient in providing preliminary qualitative and quantitative information on the samples.17 To obtain the
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The 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 L. The oil sample was obtained from the Karamay oil field located in northwest of the Xinjiang province in China. The oil viscosity was around 9000 mPa s, which is measured by a RheoStress 6000 rheometer (Thermo Fisher Scientific Co., Ltd.) at the temperature of 30 °C. A total of 100 kg of oil and sand with a 1:8 weight ratio (oil/ sand) was completely mixed and packed into this model (container). On both sides of the container, there were in total 19 thermowells fixed upon it and each inserted with one thermocouple to make online temperature measurement. The air injection well was fixed in the bottom of the container, while the production well as well as the funnel well were also installed and mounted onto the upper flange. 5927
DOI: 10.1021/acs.energyfuels.7b00453 Energy Fuels 2017, 31, 5926−5932
Article
Energy & Fuels
mm inner diameter × 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, and then held at 300 °C for 40 min. In mass spectrometry (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/s. 2.4. High-Resolution MS. 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 MS experiment. It was equipped with 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 the Supporting Information and have been described elsewhere.24,25
The horizontal production well union was composed of one slotted casing and one tube. The casing with an inner diameter of 10 mm is cut evenly into 80 slots with one end and screwed onto the upper flange, while the tube with a smaller inner diameter of 4 mm inserted 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 packing to realize a moveable sealing manner. Downstream of the TPS, the filter, liquid condenser (container with a volume of 100 mL), and backpressure regulator were equipped sequentially and finally connected with a gas analyzer. The temperature and gas concentration (CH4, O2, CO, and 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 TPS. The flue gas was cooled and filtered through a filter (a sand pack container with a 60−100 mesh) 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. 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
3. RESULTS AND DISCUSSION The crude oil and three effluent (produced) oils collected at different duration (combustion) times (after the ignition) were selected to investigate the compositional changes in the ISC process. The combustion times and TANs of the oils were listed in Table 1. 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. Because 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 Therefore, the nonmonotonic 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 Karamay oil sample 0. The oil is recovered from the reservoir by steam
Table 1. Heat Treatment and TAN of Oil Samples sample reaction (duration) time (h) temperature (°C) TAN (mg of KOH/g) C (wt %) H (wt %) H/C N (wt %) O (wt %)
0
1
2
3
0
5
9
13
30 5.89
160−220 10.45
220−280 6.23
240−320 22.75
86.05 11.59 1.62 0.27 1.15
83.99 11.55 1.65 0.04 1.48
85.69 12.25 1.72 0.05 1.00
83.88 10.39 1.49 0.20 2.73
times are affected more by the heat and oxidation than earlier duration times, causing significant viscosity reduction of oil during the ISC process. Prior to the GC−FID and FT-ICR MS analyses, the dewatering process should be performed on the produced oils. The samples were centrifuged to remove water and sand completely. About 7−12 mg of each sample was dissolved in 1 mL of 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 APPI analysis, the samples were diluted in toluene to a concentration of 0.2 mg/mL. 2.2. GC. The samples were analyzed by an Agilent HP-5 column (60 m × 0.25 mm inner diameter × 0.25 μm) in 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, and then held at 300 °C for 40 min. Both the injector and detector were operated at 300 °C. 2.3. Gas Chromatography−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
Figure 2. GC−FID chromatograms of the original oil (0) and produced oils (1, 2, and 3) at different duration times.
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, 5928
DOI: 10.1021/acs.energyfuels.7b00453 Energy Fuels 2017, 31, 5926−5932
Article
Energy & Fuels
distribution in the m/z 60 extracted ion chromatogram. The C7 and C8 acids can also be observed clearly from the total ion chromatogram, with asymmetric peak shapes. Considering that the nonpolar GC column used in this study is not suitable for acid analysis and, hence, the peak height should be higher with a polar GC column, the content of aliphatic acids was considerably high for a crude oil. This is consistent with an extraordinarily high TAN value (22.75). The Karamay crude oil was severely biodegraded in the reservoir, and no normal alkanes were detected by GC−FID, even by GC−MS. Hence, the presence of fatty acids is significant for the understanding of the heavy oil molecular structures. We speculate that these aliphatic acids were the oxidation products of normal alkanes, which were generated from the thermal cracking of large molecules, especially the polar compounds in resin and asphaltene fractions of the crude oil. High TAN value crude oils have a corrosion problem for refinery processing facilities, and small molecular acids are considered to have more acidity and corrosivity; therefore, the high acid content of the ISC produced oil would bring refining problems for the oil utilization. Figure 4 shows the mass spectra obtained by positive ion APPI FT-ICR MS (on the left) and the DBE values versus carbon number plots (on the right). As a characteristic of
except in the retention time region of
