Role of Aromatics Fraction of Crude Oils on In-Situ Combustion Performance
Performance predictions of In-Situ Combustion (ISC) process is a challenge as it involves complicated chemical reactions, fluids movement, phase changes, and heat and mass transfer. This study investigates how the aquathermolysis reactions and their chemical products can affect the ISC performance through combination of combustion tube and Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA/DSC) experiments.
Combustion tube experiments were conducted with two different crude oil without water (Swi=0%) and with the presence of water (Swi=34%). Experimental conditions were kept constant (3 L/min air injection rate and 100 psig pack pressure) for all four experiments conducted with two different oil samples. To determine the chemical reactions occurred during combustion tube experiments, the initial crude oil samples and their Saturates, Aromatics, Resins, and Asphaltenes (SARA) fractions were subjected to TGA/DSC experiments under air injection at two constant heating rates with and without water addition. Because during combustion tube experiments, two heating rates were observed, 5°C/min was used to represent the slow heating region (Steam Plateau and Evaporation & Visbreaking) and 20°C/min was used to mimic the rapid heating region (Cracking Region and Combustion Zone). To better understand the complicated mutual interactions of functional groups in crude oil, TGA/DSC experiments were repeated on normal-decane (an alkane), decanal (an aldehyde), decanone (a ketone), and decanol (an alcohol) which may represent the low temperature oxidation (LTO) products. Note that these chemicals have constant carbon number (C10).
The combustion tube experiments showed that Oil1 was able to burn for both conditions (with and without water), while Oil2 could only sustain combustion with water. To study the reason for this difference in burning behavior, the burning behavior of the crude oils and their individual SARA fractions with and without water addition was studied through TGA/DSC experiments. At high heating rate (20°C/min), heat generation does not vary for both crude oil. However, in low heating rates (5°C/min), Oil1 generates higher amount of energy at high temperature oxidation (HTO) zone. We have observed similarities between the decanone (a ketone) burning behaviors with aromatics fractions for Oil1 which may indicate that aromatics fraction may contain ketone functional groups as LTO products Because upon burning, ketones generate higher energy than any LTO products, Oil1 may have functional groups in its structure more like ketones which promotes its combustion more than Oil2. While presence of water does not change the burning behavior of Oil1, we observed that aromatics fraction of Oil2 in the presence of water generates components similar to decanol (an alcohols) burning behavior. Note that alcohols generate more heat than aldehydes upon burning which explains the enhancement of Oil2 burning behavior in the presence of water, however, produced less energy than ketones, hence, combustion performance of Oil2 was poorer than Oil1. Our results suggest that the chemical structure of aromatics fraction is critical for the success of ISC. Water and aromatics fraction interaction at elevated temperature favors ISC reactions.
In-situ combustion (ISC) is a thermal enhanced oil recovery (EOR) method in which air is injected into an oil reservoir and a combustion front sweeps the reservoir in the direction of the gas flow toward the production well (Ramey 1971). ISC is promising not only because it may result in a great oil displacement efficiency but also it increases the produced oil quality by lowering its viscosity significantly (Martin et al. 1958; Sarathi 1998). However, the complex chemical reactions make its performance prediction difficult, as a result, the field application of ISC is limited (Yang and Pope 1998; Abuhesa and Hughes 2008). The heterogeneous nature of the reservoirs makes the chemical reactions more complicated to estimate (Hascakir and Kovscek 2014; Aleksandrov and Hascakir 2015; Ismail et al. 2016). For performance prediction of ISC, some simplified reaction models are implemented which use several different crude oil fractionations (Moore et al. 1992; Kok 1993; Bagci 1998; Ambalae et al. 2006; Murugan et al. 2009; Klock and Hascakir 2015; Ismail et al. 2016; Ismail and Hascakir 2017). Saturates, Aromatic, Resins, and Asphaltenes (SARA) fractionation is one of the most widely used method to determine the reaction kinetics (Kok 1993; Bagci 1998; Speight 1999; Klock and Hascakir 2015). Accordingly, while in in-situ combustion, the role of aromatics and resins fractions are still not very clear, saturates are well-known as the “ignitor” for combustion (Verkoczy and Freitag 1997; Sarathi 1998) and asphaltenes are known as fuel source (coke) of combustion (McCain 1990; Mullins et al. 2012). The coke formation and burning reactions are the key reactions determining the success of ISC (Coats and Redfern 1964; Hascakir and Kovscek 2014).
There are two dominant reaction types observed during in-situ combustion which aid fuel (coke) formation; low temperature oxidation (LTO) and high temperature oxidation (HTO). LTO reactions take place below 300-400 °C and products of these reactions are oxygenated hydrocarbons such as aldehydes, ketones, and alcohols (Burger and Sahuquet 1972; Sarathi 1998). These LTO products are further oxidized through HTO reactions which take place above 400 °C. The main product of HTO reaction is coke (carbon-rich residue) and combustion is primarily burning of coke (Sarathi 1998). In combustion reaction, formed fuel (coke) is consumed with oxygen and carbon dioxide, water, and carbon monoxide are produced with high amount of energy release. Sustainability of combustion vastly depends on the energy generation through consumption of coke and the coke formation reactions are controlled by the LTO and HTO reactions.
Reservoir rock also contributes to the combustion reactions (Burger and Sahuquet 1972; Burger et al. 1985; Cinar et al. 2011; Kozlowski et al. 2015; Ismail et al. 2016; Aleksandrov et al. 2017; Ismail and Hascakir 2017). The reservoir clays act as catalyst and reduce the activation energy barrier to initiate oxidation and combustion reactions (Kozlowski et al. 2015). The thermal decomposition of carbonate rocks starts at around 600°C (Burger et al. 1985). As the carbonate decomposition is an endothermic reaction, the energy generated during combustion might be used to decompose carbonates rather than to burn the crude oil which reduces the combustion performance significantly (Cinar et al. 2011; Ismail et al. 2016).
Moreover, reservoir initial water saturation may have important contribution to combustion. However, there are limited studies on the effect of water on in-situ combustion (Belgrave et al. 1994; Belgrave et al. 1997; Hascakir et al. 2011; Kudryavtsev and Hascakir 2014).
This paper discusses the factors affecting ISC performance of two crude oil samples with two distinct properties.
The combustion behavior of two crude oils were examined in the scope of this study. The crude oils were characterized first with density at standard conditions and viscosity at 23°C (Prakoso et al. 2015). Then, the crude oil samples were separated into their saturates, aromatics, resins, and asphaltenes (SARA) fractions by following the ASTM D2007-11 (ASTM 2011). Characterization of the two crude oils is summarized in Table 1.
Table 1 Characterization of Crude Oil Samples with density, viscosity, and the weight percent of SARA fractions (Prakoso et al. 2015)
|Sample||Gravity, °API||Viscosity, cP||SARA fractions, wt%|
Note that saturates fraction of crude oils are ignitors and asphaltenes are known as the source for fuel. Because, Oil 2 has greater amount of saturates and asphaltenes than Oil 1, the ignition of Oil 2 may be easier than Oil 1 and Oil2 may form more fuel than Oil 1.
The crude oil samples and their n-pentane insoluble asphaltenes fractions were also analyzed for their elemental composition. Carbon and hydrogen weight percent was determined by using LECO Carlo Erba CHN analyzer and the metal content was determined by Thermo Intrepid Inductively Coupled Plasma (ICP) analyzer. Results in weight percent is given in Table 2. It should be noted that the other elements in Table 2 represent mainly oxygen. Thus, it can be concluded that initial asphaltenes fraction of Oil 2 is highly oxidized already. However, it should also be noted that Oil1 has more oxygen atoms (likely found in the form of oxygenated hydrocarbons) in its deasphalted oil than Oil2.
Table 2 Elemental Composition of Deasphalted Oil (DAO) and Asphaltenes (ASP) for both crude oil
|Total||100 wt%||100 wt%|
*Other elements are mainly oxygen and also contains non-metal and non-sulfur elements
†Metals are alkaline metals, alkaline earth metals, semimetals, and transition metals
After characterization studies have been completed, four combustion tube experiments were conducted on these two different oil samples to observe the impact of crude oil composition on ISC behavior. In all four experiments, reservoir rock was simulated by using 20/40 mesh size Ottawa sand which has 32% porosity (Svrcek and Mehrotra 1989; Hamm and Ong 1995) and 34% of the pore space was filled with crude oil samples. The impact of water presence on ISC performance was investigated with two experiments on each oil sample; in the absence (0 volume percent) and presence (34 volume percent) of water. The combustion tube experiments were conducted vertically at 3 L/min air injection rate and 100 psig pack pressure.
Chemical reactions observed during ISC were further investigated through Thermogravimetric and Differential Scanning Calorimetry (TGA/DSC) tests on the crude oil samples, individual SARA fractions, and the blends of individual SARA fractions with water. TGA/DSC tests were started at room temperature(~25 °C) and continued till 900°C under 50 ml/min air injection (nitrogen was also used as the purge gas) (ASTM 2014). Two constant heating rates were applied; 5 and 20°C/min, to mimic the real combustion behavior observed during combustion tube experiments. During combustion tube experiments, low heating rates (~5 °C/min) were observed in the steam plateau and evaporation & visbreaking regions and high heating rates (~20 °C/min) were observed in the HTO and combustion zones (Sarathi 1998; Hascakir 2015).
Results and Discussions
Four combustion tube experiments were conducted on the oil samples. The physical characterization (Table 1) and the elemental composition (Table 2) of those samples were provided in the procedure section. The combustion tube experimental results are summarized by comparing the total experiment time, cumulative oil recovery, heating rate at stable reaction region, and combustion front velocity at stable zone in Table 3. Stable zone is the zone where temperature and front velocity are almost constant over time (Sarathi 1998; Hascakir et al. 2013).
Table 3 Experimental results summary of the previously conducted combustion tube runs (Kudryavtsev and Hascakir 2014; Kozlowski et al. 2015)
|Experiment||Oil||Swi, %||Total Experiment Time, min||Cumulative Oil Recovery, wt%||Heating Rate at reaction zone for stable region °C/min||Combustion Front velocity at stable zone, cm/hour|
|E3||Oil2||0||789||0.00||No reaction zone & propagation|
From Table 3, both combustion tube experiments conducted on Oil 1 (E1 and E2) were sustained and the presence of water did not significantly alter the combustion behavior. However, for Oil2, combustion was not able to sustain without water presence (E3). It is obvious that the contribution of water in combustion reactions differentiate with the crude oil type.
To better understand the role of chemical reactions on combustion sustainability, TGA/DSC experiments were conducted at two different fixed heating rates; low heating rates (5°C/min) and high heating rates (20 °C/min) to mimic the behavior observed during ISC.Figure 1 summarizes those results for two crude oils; top two graphs give the TGA results and bottom two DSC results. TGA results provide the weight loss curves of the samples with increasing temperature. DSC results give idea about the energy generation and reaction types during combustion; in the graphs, peaks represent endothermic reactions and valley represent exothermic reactions (Verkoczy and Jha 1986). Since, in Figure 1-IIA, DSC curves for Oil1 has deeper valley in HTO region (400-600 °C) in which low heating rate was used, Oil1 both with and without water generates higher energy than Oil2. However, at high heating rate (Figure 1-IIB), Oil2 with water has the greatest energy generation.
If these results are combined with the results given in Table 3, it is observed that the higher the energy generation does not specifically mean higher the oil recovery but higher the combustion front velocity (this discussion is obtained by comparing E2 and E4 results given in Table 3. Also, note that the weight loss till reaching 100-150 °C in TGA graphs given in Figure 1, corresponds to water; water evaporation also varies at high and low heating rates according to oil type. Oil 2 losses the water at higher temperature than Oil 1 at low heating rates (Comparison of black (Oil1) and red (Oil2) concrete curves in Figure 1-IA and the evaporated water consumes less energy for Oil2 than Oil 1 (Comparison of black (Oil1) and red (Oil2) concrete curves in Figure 1-IIA).
Figure 1 Combustion behavior of crude oil samples at low (5°C/min) and high (20°C/min) heating rates
To further investigate the reasons in the difference of ISC behavior for two oil samples, TGA/DSC experiments were also performed on SARA fractions of both oil at low (5°C/min) and high (20°C/min) heating rates. TGA results (weight loss graphs) are given in Figure 2and DSC results are giveninFigure 3.
TGA results (Figure 2) provide an overview of which crude oil fraction interacts with water more for the water presence cases. While saturates fraction of Oil 2 interacts with water when lower heating rates were applied, saturates fraction of Oil 1 loses water at water evaporation temperature immediately (Figure 2-A-I). For aromatics fraction vice-versa behavior was observed at high heating rate (Figure 2-B-II). The resins fractions of both crude oil interact more with water at high heating rate (Figure 2-B-III) than at low heating rate (Figure 2-A-III). The asphaltenes fractions of crude oils act differently than any other fractions; while asphaltenes of Oil 1 interacts with water more at low heating rate (Figure 2-A-IV), asphaltenes of Oil 2 interacts with water more at high heating rate (Figure 2-B-IV). Without water cases do not show a significant difference other than for asphaltenes at low heating rate (Figure 2-A-IV); while asphaltenes of Oil 1 (black dashed curve in Figure 2-D-I) has been preserved without consumption till high temperatures, the consumption of Oil2 asphaltenes (red dashed curve in Figure 2-A-IV) starts earlier.
DSC results are given in Figure 3. These curves provide information on heat flow for different crude oil fractions alone (dashed lines) and after blending the fractions with water (concrete lines) for both Oil 1 (black curves) and Oil 2 (red curves). The peaks observed in DSC curves indicate the occurrence of endothermic reactions and the peaks observed in DSC curves shows the temperature region for exothermic reactions. It is observed that at low heating rate, presence of water in the system does not generate the exothermic reactions, thus, the energy should be stored in the chemical bonds, when the high temperature region is reached (in where high heating rates are observed for ISC), the exothermic reactions contribute the formation of deep valley (high energy generation).
Figure 2 Weight loss behavior of crude oil fractions with and without water addition at low (5°C/min) and high (20°C/min) heating rates
Figure 3 Heat flow behavior of crude oil fractions with and without water addition at low (5°C/min) and high (20°C/min) heating rates
The reaction kinetics study summary given in Figure 2 and Figure 3 shows that SARA fractions from both oil samples exhibit different combustion behaviors in the presence and absence of water phase in the system. Note that the reaction kinetics of individual fractions are studied in this work to develop understanding on the role of each SARA fraction during ISC through a simplified approach. However, it should be noted that the reaction kinetics study of individual SARA fractions is still too complicated to fully grasp ISC mechanisms. Consequently, we conducted similar reaction kinetics analyses on hydrocarbons with known molecular formula. We have selected four different hydrocarbon groups; alkanes, aldehydes, ketones, alcohols which represents the simplified LTO products, to investigate how the reaction kinetics tests of these four groups resemble to the reaction kinetics of SARA fractions from Oil 1 and Oil 2. This way, we aim to identify the functional groups present in SARA fraction causes differences in burning behavior of bulk crude oil.
These control experiments were conducted on decane (C10H22), decanal (C10H20O), decanol (C10H22O), and decanone (C10H20O). All of these chemicals have 10 carbons and they all have saturated hydrocarbon chain. But they are investigated under different hydrocarbon groups; decane is a normal alkane, decanal is an aldehyde, decanol is an alcohol, and decanone is a ketone. Note that apart from decane, other chemicals are selected due to their oxygen content to as representative of the LTO products observed in ISC (Burger et al. 1985). Figure 4 provides the heat flow behavior (DSC) of these chemicals at 5 and 20 °C/min heating rates.
Figure 4 – Heat flow behavior of C10 hydrocarbon functional groups: decane (dark blue), decanal (light green), decanol (dark green), and decanone (dark blue).
In Figure 4, the similar pattern in both low and high heating rates without water, but different temperature ranges for the heat flow peak was observed. Without water, both low and high heating give a similar heat flow behavior which indicates alcohol releases the greatest heat followed by ketone and aldehyde. Addition of water have significantly increases all the heat release especially for aldehyde (Figure 4B-I). We have observed similarities between the decanone (a ketone) burning behavior with aromatics fractions for Oil 1 which may indicate that aromatics fraction may contain ketone functional groups. However, in the aromatics fraction of Oil 2, we observed similar burning behavior represents decanal which is an aldehyde. Because upon burning, ketones generate more heat than aldehydes (Salooja 1965; Escobar et al. 2004), Oil 1 should have functional groups in its structure more like ketones which promote its combustion more than Oil 2. With the addition of water to these chemicals, the energy consumed in endothermic reactions is observed much higher (Figure 4-B-II). A more intriguing difference is how the addition of water changes the placement of the endothermic peaks as functions of temperature. While presence of water does not change the burning behavior of Oil 1 (Figure 3), we observed that aromatics fraction of Oil 2 in the presence of water generates components similar to decanol (an alcohols) burning behavior (Comparison of Figure 2 and Figure 3 with Figure 4). Note that alcohols generate more heat than aldehydes upon burning (Salooja 1965) which explains the enhancement of Oil 2 burning behavior in the presence of water, however, produced less energy than ketones, hence, Oil 1 exhibits better combustion behavior than Oil 2. In other words, if the heat released during ISC is the greater than the heat consumed than combustion process sustained (Belgrave et al. 1993; Sarathi 1998; Hascakir and Kovscek 2014). And ketones produces more energy upon burning than any other oxygenated hydrocarbons (Escobar et al. 2004; Serinyel et al. 2010).
In this study, combustion behavior of two different oil samples were examined with four combustion tube experiments. We mainly investigated the impact of water presence and the oil type on combustion kinetics through TGA/DSC experiments. The chemical reactions study for the crude oil samples and their individual SARA fractions with the presence and absence of water were completed. Control experiments on an alkane, an aldehyde, a ketone, and an alcohol were conducted to further simplified the reactions occur during combustion tube tests.
It can be concluded that the burning behavior of oil is affected by the presence of water. Water interacts more with the aromatics fraction in the crude oil. The importance of aromatics fraction on ISC dynamics was studied for the first time and it was found that the presence of ketones functional groups in aromatics fraction can enhance the combustion process. Thus, it might be beneficial to study the oxidation products of every fraction prior to ISC application. Note that while saturates are reported as ignitor and asphaltenes are reported as fuel source mainly for in-situ combustion, with this study, we can conclude that both aromatics and resins are viscosity reducer.
The presence of water removed an overpowering endothermic reaction from the resins fraction of Oil 2, leading to the conclusion that the resins fraction of Oil 2 was the reason for combustion quenching in the combustion tube experiments without water. The burning behavior of the aromatics are depending on the oxygenated functional group existed in aromatics fractions. As ketones showed the only significant change in the presence of water, ketones in deasphalted oil may be responsible for the natural burning behavior of crude oils.
Hence, our results suggest that the chemical structure of aromatics fraction is critical for the success of ISC and water and aromatics fraction interaction at elevated temperature favors ISC reactions.
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Figure A-1 – TGA graphs of C10 hydrocarbon functional groups: decane (dark blue), decanal (light green), decanol (dark green), and decanone (dark blue).
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