Subscriber access provided by Kaohsiung Medical University
Fossil Fuels
Experimental investigation of EOR mechanisms of air injection under LTO process: thermal effect and residual oil recovery efficiency Siyuan Huang, Yao Zhang, and James J. Sheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01314 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Experimental investigation of EOR mechanisms of air injection under LTO process: thermal effect and residual oil recovery efficiency Siyuan Huang1, Yao Zhang1, James J. Sheng1,2,* 1 Texas Tech University 2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, China *Corresponding author. Tel.: +1806 834 8477; E-mail address:
[email protected]. Key words: Air injection; Low temperature oxidation; Thermal effect; Enhanced oil recovery. Abstract: AIP (Air injection process) has been applied as an EOR (Enhanced Oil Recovery) method in the light oil reservoir for decades. When high temperature combustion reactions cannot be achieved, the LTO (low temperature oxidation) reactions will dominate the AIP. The exothermic intensity of LTO reactions are much milder than the ones of high temperature combustion reactions, which caused the thermal effect of the LTO reactions been underestimated by researchers for a long time. Beside the thermal effect, questions whether LTO reactions improve recovery efficiency and whether the LTO reactions could produce residual oil from the reservoir need to be answered. In this study, a series of isothermal core flooding tests under different environmental temperature conditions were applied to study the thermal effect on oil recovery. In addition, alternate injection of nitrogen and air was performed to study the LTO effect on oil recovery besides the thermal effect. The experimental results shown that the thermal effect can play a significant role on recovery performance as a higher temperature results in a higher oil recovery factor, where a temperature increment of 40oC by the LTO reactions can result in a 10% recovery factor increase. On the other side, the LTO effect on producing residual oil was not observed in this study. Moreover, despite of the thermal effect, the LTO generated oxygenated compounds will increase the viscosity of the crude oil, which will decrease the recovery efficiency. Therefore, since the thermal effect of LTO works against the viscosity increment effect of LTO, the AIP is recommended only if the thermal effect is more significant comparing to the increased viscosity effect in terms of recovery efficiency.
1. Introduction AIP is an enhanced oil recovery technique in which compressed air is injected into an oil reservoir. The main differences of air injection comparing to other gas injection techniques are the complicated reactions among air, rock and crude oil. During the AIP, three groups of reactions normally take place including: LTO reactions, ITO (intermediate temperature oxidation) reactions, and HTO (high temperature oxidation) reactions. The LTO is an oxygen addition reaction, where the products are water and partially oxygenated hydrocarbons such as carboxylic 1 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
acids, aldehydes, ketones, alcohols, and hydroperoxides [1]. The temperature range for LTO has been extensively studied [2-7], where the typical temperature range for LTO locates between 25oC to 364oC [8]. The HTO reactions are also known as the high temperature combustion reactions, where the main products from the HTO reactions are carbon dioxide, carbon monoxide and water. The typical temperature range for HTO reactions locate from 415oC to 542oC [8]. When developing a heavy oil reservoir, the ISC (in-situ combustion) technique is usually applied which relies on the HTO mode of air injection. In this case, the ignition and combustion sustention are the key factors to determine the success of the project since it was proved that the combustion front can displace extra oil [7]. On the other side, when the AIP is applied in a light oil reservoir, the HPAI (high pressure air injection) technique is considered. In this case, the combustion is not required and the LTO mode air injection mainly dictate the success of the project. For heavy oil development, researchers worked against the occurrence of LTO reactions since oxygenated heavy components will be generated by the LTO reactions, which will increase both oil viscosity and density [9-12]. However, such effect on light oil was considered insignificant as the initial light oil viscosity is low, hence the slightly viscosity increment was not considered to be a problem to the recovery process [13]. Although the exothermic intensity of LTO reaction is much milder comparing to the HTO reaction, the temperature will increase as long as the heat generation is greater than the heat dissipation. The temperature increment by LTO was observed experimentally by Abu-Khamsin et al. [14], where a series of core flooding (air injection) experiments were performed. The highest temperature increment by the LTO was around 10℃, while the author beleived that in the real reservoir condition, the temperature increment would be even greater since the reservoir can provide a better adiabatic environment. Similar tests were performed by Jia et al. [15] where the LTO reactions increased the temperature around 10oC based on the isothermal oxidation tube experiments. Other than the core flooding experiments, kinetic cell experiments including ARC (accelerated rate calorimetry), DSC (differential scanning calorimetry), and SBR (small batch reactor) have also been extensively applied to study the LTO exothermic activity, which all show that the LTO of a light oil has the ability to elevate the temperature [16-18]. Different understanding on the LTO exothermicity causes researchers to consider the HPAI differently. Some researchers believe that the spontaneous ignition is the key to the HPAI, and stated that the combustion propagation can produce extra oil from the oil reservoir after gas flooding [7, 9, 19]. However, other researchers ignore the thermal effect and state that the HPAI in a light oil reservoir can be viewed as a conventional flue gas flooding process, so long as the oxygen in the injected air is removed efficiently in the oil formation by the LTO [12, 20-22]. Furthermore, numerical simulation regarding the HPAI performance and how to overcome the oxygen breakthrough if oxygen was not consumed effectively were extensively studied by different researchers [23-28]. The different understandings on the HPAI technique mainly attributed to the lack of understanding on the LTO reactions. As the controversy remains, more efforts need to be done to better understand the mechanisms of AIP under the LTO mode. In this study, we 2 ACS Paragon Plus Environment
Page 2 of 19
Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
performed a series of experimental work to investigate the mechanisms of AIP under LTO mode, where the thermal effect and the effect of LTO reactions besides thermal effect were investigated separately. In addition, the relation between LTO reaction mode of a light oil and the thermal effect was further investigated by performing the numerical simulations. This study can help researchers better understand the mechanisms during AIP under the LTO mode and can also provide insights of air injection. 2. Methodology 2.1. Thermal effect As stated in the previous section, the close to reservoir adiabatic environment is hard to be achieved in the experimental condition. On the other side, it is also not appropriate to neglect the potential thermal effect that could be obtained from the LTO exothermic activity. Hence, in order to investigate the thermal effect during AIP, a series of isothermal core flooding tests under different environmental temperature conditions (80℃, 100℃ and 120℃) are proposed. The lowest temperature test (80oC) is served as the reference test, while a higher temperature test (100oC or 120oC) is used to simulate the situation of more thermal effect. In this way, the thermal effect can be evaluated by comparing the recovery factor among isothermal tests at different temperature conditions. 2.2. LTO effect Fig. 1 shows the schematic of air injection under a LTO process, where three regions including reaction region, flue gas drive region and displaced oil bank region can be identified between the injection well and production well. During the air injection LTO process, the oil produced before gas breakthrough is the un-reacted original oil. Theoretically, the air will react with the residual oil which was left behind the flue gas drive, hence extra oil might be produced by the LTO reactions. In this study, the LTO effect indicates the ability of LTO reactions to produce oil besides the thermal effect of the LTO reactions. The LTO effect was investigated based on both comparative gas flooding experiments and alternative gas injection tests. For the comparative gas flooding experiments, the nitrogen injection is served as the reference test to the air injection test since no oxidation reactions are involved during the nitrogen injection process. The recovery factors are also used to detect the presence of LTO effect by comparing air injection to nitrogen injection at the same operation conditions. For the alternative gas injection tests, both nitrogen injection after air injection and air injection after nitrogen injection tests are proposed. Take the air injection after nitrogen injection test as an example. During this test, the nitrogen is firstly injected to the core to displace the crude oil until no more oil can be produced. Then the injected gas is switched to the air and continued to be operated at the same condition (temperature and pressure) to test if any extra oil can be produced. However, during the gas alternation process, some oil may be produced from different channels of the core, which may mislead to a conclusion that this extra produced oil was due to the LTO effect. In order to avoid this possible misunderstanding, the
3 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
nitrogen injection after air injection is performed. For the comparative tests, the same amount of gas (8 PV) was injected before switching the gas source.
Fig. 1. Schematic of air injection under LTO process 3. Experimental work 3.1. Materials Fig. 2 shows the core plug and crude oil sample used in this study. The Berea sandstone was collected and used in this study with a dimension of 5.308 cm length and 3.874 cm diameter. The measured average helium porosity is 19.01% and the nitrogen permeability is about 200 mD. The crude oil used in this study was collected from the Wolfcamp oil reservoir. The density of this crude oil is 0.83g/cc (38.98 API) and the viscosity of this crude oil is 3.66cp at 25℃ and atmospheric pressure.
Fig. 2. Crude oil sample (left) and core plug sample (right) 3.2. Experimental setup The experimental work consists two parts: core plug saturation and gas injection process. The schematic of the gas flooding apparatus is shown in Fig. 3. A big oven is used to provide 4 ACS Paragon Plus Environment
Page 4 of 19
Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
simulated reservoir temperature for both saturation process and gas injection process. The equipment used for core plug saturation process contains a vacuum pump, a vessel, an accumulator, and a Quizix pump (QX-6000). For the gas injection tests, a composite stainless steel core holder with maximum operating pressure of 10,000 psi is used. The hydraulic oil is pressurized and used to supply the confining pressure to the core holder, where the confining pressure is maintained at 500 psi greater than the injection pressure. The injected gas is obtained from high pressure (6000 psi) compressed gas cylinder. Since two gas sources are needed in this study, both compressed air cylinder and compressed nitrogen cylinder are used. In order to prepare the gas at high temperature condition, two accumulators are used which are placed inside the oven. A BRP (back pressure regulator) is placed at the outlet end of the gas injection test to apply back pressure, the pressure source for the BRP is supplied by a syringe pump (Model 100DX pump). A separator is placed behind the back pressure regulator to separate the produced liquid and gas, where the gas flow rate and breakthrough time are recorded by a gas flow meter (SmartTrak 100) with a readability of 0.02 scc/m (standard cubic centimeters per minute). Also, the produced the liquid is recorded by a weight balance to monitor the recovery efficiency.
Fig. 3. Schematic of the gas flooding apparatus
3.3. Experimental procedures 3.3.1. Core plug saturation In this study, the water saturation effect was not considered. Also, saturating the core with a single fluid is more practical to meet the repeatability requirement. Therefore, for the core plug saturation process, the core plug was saturated under reservoir temperature inside the oven by injecting reservoir temperature oil. The core was firstly placed in the core holder, where the confining pressure was supplied at 50 psi. Then, the inlet valve of the core holder was closed and the outlet valve was opened to vacuum the free gas which was left inside the core sample. The vacuum process was performed for one day. During this period, air, nitrogen and oil were loaded into corresponding accumulators where the oven was set to the test reservoir temperature. Normally, it takes around 3 to 4 hours for the whole experimental environment to reach the 5 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 19
thermal equilibrium stage. In this study, we wait even longer until the vacuum process has been completed. After the vacuum process, the outlet valve of the core holder was closed and the confining pressure was increased to 1500 psi to start the saturation process by injecting oil at a constant pressure of 800 psi for six hours. The saturated core plug was then took out from the core holder and the weight of the saturated core was measured. The saturation results were shown in Table 1. It is observed that around 99.8% oil saturation was reached ultimately, which proves that this method can be effectively applied to saturate the core plug. In addition, the volume expansion factor due to temperature change was also considered during the saturation process. The unit volume change with temperature difference can be expressed by Eq.1: ܸ݀ = ܸ ∗ ߚ ∗ (ݐ − ݐ )
Eq. 1
where the dV is the change in volume (cc); ߚ is the volumetric temperature expansion coefficient (1/℃); ݐ is the final temperature (℃); ݐ is the initial temperature (℃). The experimentally measured saturation weight of the core under 100℃ for six hours was 140.672 g, and the estimated saturation weight according to the volume expansion equation was 140.703g, which validated the saturation method proposed in this section. Moreover, the gas dissolution effect was considered by comparing the live oil tests with the dead oil tests. The live oil was prepared in accumulator 3 (shown in Fig. 3) before the core plug saturation process. The nitrogen with 300 psi was injected to accumulator 3 and then the accumulator was heated by the oven to the test reservoir temperature. Then, the accumulator 3 was pressurized to the saturation pressure of 800 psi in this study, and the accumulator was maintained at that pressure for one day to reach equilibrium and fully saturation. After the live oil was prepared, the undissolved nitrogen was pumped out through the bypass line, and the saturated oil was injected to the core plug with back pressure at 800 psi in order to maintain the gas saturation. The saturation process was performed for one day to make sure reproducible initial oil saturation can be achieved for each experiment. Core bulk volume, cc Dry weight, g Porosity,% Oil density, g/cc Saturated weight @ 21 ℃, g Saturated oil weight @ 21℃, g Oil volume, cc Oil saturation, %
Table 1. Core saturation results Initial temp, ℃ 62.6 Final temp, ℃ 131.6 β, 1/℃ 19.01 0.83 dV, cc 141.4 dm, g
21 100 0.001 0.9 0.7
9.9
Calculated saturation weight @100 ℃, g 140.703
11.9 99.8
Measured saturation weight @100 ℃, g
6 ACS Paragon Plus Environment
140.672
Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
3.3.2. Gas injection As mentioned in the methodology section, both nitrogen injection and air injection tests are performed in this study. During each test, the pressure difference was controlled at 40 psi, where the back pressure was 800 psi and the injection pressure was 840 psi. The injection pump was applied at a constant pressure mode. The produced oil was separated from the produced gas at the separator and the weight of the produced oil was constantly measured to obtain the recovery factor. Each test will be stopped when no more oil was produced, and the ultimate recovery factor was calculated based on the produced oil comparing to the initial oil saturation. Also, the breakthrough time was monitored by the gas flow meter at the outlet of the gas injection apparatus. For both nitrogen and air injection tests, isothermal tests were conducted under 80℃, 100℃ and 120℃, respectively. The reason for choosing these temperatures is because the typical reservoir temperature ranges from 70oC to 140oC. For field which was considered to implement the AIP, a high reservoir temperature is preferred. While due to our experimental limitation, the highest temperature we can applied for the oven is 120oC, as a result, temperatures as 80oC, 100oC and 120oC were selected for performing the AIP experiments. When performing the gas alter injection tests, similar approach for single gas injection tests was adopted except that after each injection test, the injection gas will be altered to the other gas source to test if any extra oil can be produced. In this study, both air injection after nitrogen injection and nitrogen injection after air injection were performed. The injected gas volume was measured by recording the volume of gas remained inside the accumulator which was used to contain the gas. In order to minimize the experimental error, the same core plug was used repeatedly in this study. After each test, the core was placed in the Soxhlet Extractor for cleaning and dried by the oven. In order to guarantee the experimental accuracy, before each test, the dry weight of the core plug was measured to make sure the weight difference is within 0.1 gram to the initial dry weight. Then, the saturation process and the injection process will be repeated as described before. The gas injection pressure, confining pressure, back pressure and reservoir temperature used in the tests are listed in Table 2. Table 2. Experimental conditions for the gas injection process Temperature, oC 80 100 120
Confining Pressure, psi 1500 1500 1500
Injection Pressure, psi 840 840 840
Back Pressure, psi 800 800 800
4. Experimental results and discussion 4.1. Thermal effect of AIP The thermal effect of AIP was studied by comparing recovery efficiency under different experimental temperature conditions, where a higher temperature condition is used to simulate the temperature increment that was caused by the LTO reactions. Fig. 4 shows the recovery 7 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 19
profile of air injection tests under 80oC, 100oC and 120oC, respectively. The thermal effect was observed since the recovery factor increased with the increment of the test temperature, where the lowest recovery factor of 63.4% occurred at test temperature of 80oC, and the highest recovery factor of 74% occurred at the test temperature of 120oC. The measured viscosity and temperature relation of the crude oil is shown in Fig. 5, which shows that the viscosity decreased from 2.7 cP to 1.1 cP when temperature increased from 80oC to 120oC. The viscosity effect on the production rate can be revealed based on the Darcy rate equation as shown in Equation 2, where k is the permeability, Darcy; A is the cross-sectional area of the core, cm2; ߂ܲ is the pressure drop, atm; μ is the fluid viscosity, cP; L is the length of the core, cm; and Q is the flow rate, cm3/s. If the viscosity decreased from 2.7 cP to 1.1 cP and other factors remained unchanged, the production rate will increase around 2.5 times under a single-phase flow condition. ܳ=
௱
Eq. 2
ஜ
Under an immiscible two-phase flow condition, for this case is air displacing oil, the BuckleyLeverett equation is applied to demonstrate the viscosity effect on the recovery efficiency as shown in Fig. 6. It can be seen that the decrement of viscosity caused the recovery factor increased around 12% at the breakthrough point, which is close to the isothermal air injection experimental results as shown in Fig. 4, where the recovery factor increased around 10.6% based on the temperature increment. From this analysis. It reveals that the recovery efficiency increment in the isothermal core flooding tests at different temperature conditions is mainly attributed to the viscosity reduction.
The gas breakthrough was detected around 0.8 PV (pore volume) of gas injection for all three isothermal air injection tests. It can be seen that most of the crude oil was produced at the breakthrough point, which is consistent with the core flooding tests and slim tube air injection tests reported by Montes et al. [7]. After breakthrough, only a little amount of oil was produced before 4 PV of gas injection, and no more oil was recovered till 8 PV of gas injection. Although, the recovery performance is not as good as the one where combustion front was developed and well sustained as reported in the combustion tube tests in ref [7], this study shows that the thermal effect under the LTO mode can still play a significant role on recovery performance as a higher temperature will result in a higher oil recovery factor. As mentioned previously, extensive researches have been performed on the LTO exothermic intensity of crude oil, which show that the temperature increment is feasible to be achieved by the LTO reactions of a light oil as long as the heat generation is higher than the heat dissipation. However, as the reactivity of a light oil is highly dependent on the crude oil intrinsic properties, and the heat dissipation is determined by the reservoir environment, the reactivity and the oxidation reaction mechanisms of a light oil and the reservoir heat loss conditions should be well understood in order to better evaluate the thermal effect. 8 ACS Paragon Plus Environment
Page 9 of 19
Moreover, the nitrogen injection at 80oC was performed and the recovery performance was shown in Fig. 7. It can be seen that at 80oC, the recovery performance of nitrogen injection is better than the air injection. Also, this phenomenon was observed with both live oil injection tests and dead oil injection tests, which means that the difference of recovery factor between nitrogen injection and air injection at the same temperature condition was not caused by the gas dissolution differences of different gases. Based on our understanding, the reason causes nitrogen showing a higher recovery factor than the air injection is due to part of the oxygen been consumed without generating flue gas during the air injection. The consumption of the oxygen causes shrinkage of the injected gas volume. As a consequence, the injected pressure was decreased unless more air was injected to the core plug. Although, a small portion of the oxygen was consumed which decreased the recovery efficiency, it is noticeable that the impact of crude oil consumption by LTO reactions is less significant comparing to the increased oil recovery by the thermal effect of LTO reactions as the recovery efficiency of air injection at 100oC and 120oC are higher than the nitrogen injection at 80oC. To conclude, this study shows that the recovery efficiency of air injection can be much more promising than the nitrogen injection if thermal effect can be achieved with sufficient crude oil exothermic activity and a good adiabatic reservoir environment. The thermal effect of light oil LTO reactions was further investigated by numerical simulation which will be discussed in a later section. 0.8 0.7 RF, fraction
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
0.6 0.5 80C Air injection 100C Air injection 120C Air injection
0.4 0.3 0.2 0.1 BT @ 0.8 PV 0 0
1
2
3 4 5 PV of gas injection
6
7
8
Fig. 4. Recovery performance of isothermal air injection tests at 80oC, 100oC and 120oC, respectively.
9 ACS Paragon Plus Environment
Energy & Fuels
16 14
Viscosity, cP
12 10 8 6 4 2 0 0
20
40
60 80 Temperature, oC
100
120
140
Fig. 5. Viscosity-temperature relation of the tested crude oil
1 0.9 0.8 0.7 0.6 fg
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 19
0.5
80C Air 120C Air
0.4 0.3 0.2 0.1
68% RF @ 120oC
56% RF @ 80oC
0 0
0.1
0.2
0.3
0.4
0.5 Sg
0.6
0.7
0.8
0.9
Fig. 6. Viscosity effect to the recovery efficiency during air displacing oil
10 ACS Paragon Plus Environment
1
Page 11 of 19
0.7 0.6 0.5 RF, fraction
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
0.4 0.3 0.2
80C Air injection 80C N2 injection
0.1 BT @ 0.8 PV 0 0
1
2
3 4 PV of gas injection
5
6
7
8
Fig. 7. Recovery performance of isothermal air injection test and nitrogen injection test at 80oC. 4.2. LTO effect of AIP As mentioned in the methodology section previously, the LTO effect of AIP in this study indicates the ability of LTO reactions to produce crude oil besides the thermal effect of LTO reactions. In order to study the LTO effect on producing residual crude oil, both alternate gas injection and comparative gas injection tests were performed. Fig. 8 shows the recovery performance of the comparative tests between air injection and nitrogen injection at 80oC, 100oC and 120oC, respectively. For all the tests, most of the crude oil was recovered before breakthrough, where the breakthrough occurred at around 0.8 PV of gas injection. Also, it was observed that despite of the injected gas type, a higher temperature results in a higher recovery factor, which is attributed to the viscosity decrement with increased temperature. For all three isothermal temperature tests, the nitrogen injection tests show higher recovery efficiency than the air injection tests, and the reasons cause this phenomenon were discussed as follow. The first reason is due to the crude oil consumption by LTO reactions during air injection since no oxidation reactions are involved during the nitrogen injection tests. The second reason is because of the viscosity increment by the LTO reaction. As mentioned in the introduction section, the oxygen addition reaction is the dominant reaction type in LTO reaction, where the oxygenated compounds will be generated, which are more viscous than the original hydrocarbon components. As a result, the overall viscosity of the crude oil was increased during the AIP. In addition, since no flue gas was generated in the oxygen addition reaction, the oxygen consumption resulted with a shrinkage of injected gas volume. As a consequence, the injected pressure was decreased. On the other side, since most of the crude oil was recovered before 4 PV of gas injection, and no 11 ACS Paragon Plus Environment
Energy & Fuels
more oil was produced till 8 PV of gas injection, the LTO effect on producing residual oil was not observed for all 80oC, 100oC and 120oC conditions. 0.9 0.8 0.7 RF, fraction
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 19
0.6 0.5 0.4 80C Air injection 80C N2 injection 100C Air injection 100C N2 injection 120C Air injection 120C N2 injection
0.3 0.2 0.1 BT @ 0.8 PV
0 0
1
2
3 4 5 PV of gas injection
6
7
8
Fig. 8. Comparative isothermal gas injection tests between air injection and nitrogen injection on LTO effect Other than the comparative gas injection tests, the alternate gas injection tests were performed, which are mainly used to study the LTO effect on recovering residual oil. For an air injection after nitrogen injection test, the nitrogen was firstly injected for around 8 PV till only residual oil was left, and then the air was injected. The reverse alternate gas injection tests as nitrogen injection after air injection were also performed in case some residual oil was produced from different channels of the core after switching the injected gas. The results of alternate gas injection tests are shown in Fig. 9. According to Fig.9, it can be seen that similar to previous experiments, most of the crude oil was recovered before breakthrough, and the change of injection gas cannot produce residual oil under all three experimental temperature conditions. Based on both comparative gas injection tests and alternate gas injection tests, it was revealed that the LTO cannot recover residual oil, and most of the crude oil was produced before gas breakthrough. Also, it was observed that despite of the thermal effect, the LTO reactions will increase the crude oil viscosity, hence decreases the recovery efficiency, which is opposite to the thermal effect in terms of recovery efficiency. Therefore, if thermal effect cannot be obtained, because when heat generation is not much higher than the heat dissipation, the LTO dominant AIP technique will behave even worse than the nitrogen injection in terms of recovery efficiency.
12 ACS Paragon Plus Environment
Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
To conclude, since the thermal effect based on the LTO exothermic activity works against the LTO effect (viscosity increment due to the formation of the oxygenated compounds) in terms of recovery performance, when considering AIP, researchers need to evaluate which effect will be more significant comparing to the other. If the thermal effect is dominant, the AIP could be considered, otherwise the AIP is less preferred. The findings on LTO effect also reveal that the exothermic characteristic of crude oil is crucial for determining the AIP performance as a stronger exothermic activity will not only decrease the viscosity of the crude oil under LTO mode, but also can shift the reaction mode from LTO to HTO, which is often known as the spontaneous ignition.
Fig. 9. Alternate gas injection tests on LTO effect
5. Numerical simulation study of EOR mechanisms of LTO mode AIP In this section, the significance of thermal effect and LTO effect were further investigated by numerical simulation. The simulation was performed by using the thermal simulator CMGSTARS. The Wolfcamp light oil used in the experimental work was also applied in the model, where the corresponding kinetic data and kinetic model in the LTO stage were obtained and developed in previous studies [2, 18]. The kinetic model contains three reactions with eleven components. The detailed fluid properties can be found in ref. [18]. A one dimension Cartesian grid of 5 * 1 * 1 with 5 active blocks in the x direction was built to simulate the core plug, where the total grid block size was set to 5.3 cm * 3.87 cm * 3.87 cm in x, y, z directions, respectively. The lab scale model was validated based on the air injection test as shown in Fig. 10 and the 13 ACS Paragon Plus Environment
Energy & Fuels
main reservoir properties of the model are listed in Table 3. The perforated injection well and production well are located in (5 1 1) and (1 1 1), respectively. In addition, the phase behavior was estimated with Gas-liquid K-values, which are functions of pressure, temperature, and composition that can be obtained through CMG-WinProp PVT module. Table 3. Main parameters used in the lab scale simulation model. Parameter Reference depth (cm) Porosity (dimensionless) Horizontal permeability (mD) kv/kh (dimensionless) Oil saturation (dimensionless) Reference pressure (kPa) Original reservoir temperature (oC) Rock volumetric heat capacity (J/(cm3*oC)) Rock thermal conductivity (J/(cm*min*oC)) Water thermal conductivity (J/(cm*min*oC)) Oil thermal conductivity (J/(cm*min*oC)) Gas thermal conductivity (J/(cm*min*oC)) Temperature of injected gas (oC)
Value 0 0.19 200 1 0.998 5800 80 2.35 1 0.36 0.077 0.083 80
70 60 50 RF
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 19
40 Simulation
Experiment
30 20 10 0 0
5
10 15 PV of gas injection
20
Fig. 10. Model validation of air injection and air injection test at 80oC 14 ACS Paragon Plus Environment
25
Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
It is known that the exothermic behavior of a crude oil can be characterized by kinetic data (frequency factor and activation energy) and enthalpy values of the reactions. Also, as discussed previously, the heat loss condition plays a significant role on the overall thermal effect of AIP. If the reservoir has a better adiabatic condition, it requires less heat generation from the LTO reactions to increase the in-situ temperature and enhance the recovery performance accordingly. Here, in order to study the thermal effect of LTO reactions, two reservoir models with different environmental conditions are considered: one is at typical reservoir condition, where the thermal properties of the reservoir are shown in Table 3; and the other is at adiabatic condition, where no conductive heat loss is considered between over/under burden strata. Besides the heat loss condition effect, the typical kinetic data range of a crude oil LTO reaction was tested, where the frequency factor ranges from 0.1 s-1 to 105 s-1 and activation energy ranges from 20 kJ/mole to 70 kJ/mole [8]. The simulation results are shown in Fig. 11. It was seen that air injection under both typical reservoir environment (base case) and perfect adiabatic environment did not show any temperature increment, where two temperature curves overlapped on each other. This phenomenon reveals that the absence of thermal effect was not caused by the non-perfect adiabatic reservoir environment, but attributed to the insufficient crude oil exothermic activity. This statement was further validated by researching the effect of crude oil kinetic data, which represents the reactivity of the crude oil. In Fig. 11 (a), it was seen that a lower activation energy (20 kJ/mole) and a higher frequency factor (105 s-1) resulted in a stronger exothermic behavior, which increased the temperature for around 25oC. In addition, Fig. 11 (b) also shows that a higher temperature increment will result with a higher recovery performance, where the low activation energy and high frequency factor case presents a slightly higher recovery factor than the base case. The simulation study shows that the intrinsic exothermic characteristics of crude oil are critical to the performance of AIP. The oil which is lack of exothermic intensity should be screened out if the operators expect to gain advantages of thermal effect by performing AIP.
15 ACS Paragon Plus Environment
Energy & Fuels
110
Temperature, oC
105 100 95 Temp_Air inj_base
90
Temp_Air inj_low Ea and high Ar 85 Temp_Air inj_adiabatic 80 75 0
5
10 15 PV of gas injection (a)
20
25
70 60 50 RF
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 19
40 RF_Air inj_base 30
RF_Air inj_low Ea and high Ar
20
RF_Air inj_adiabatic
10 0 0
5
10 15 PV of gas injection (b)
20
25
Fig. 11. Effect of kinetic data and adiabatic condition on (a) temperature increment and (b) recovery performance
6. Conclusions The following conclusions are drawn from this study.
16 ACS Paragon Plus Environment
Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
•
•
•
•
The isothermal core flooding tests under different environmental temperature conditions can be effectively applied to evaluate the thermal effect of AIP by comparing the recovery factor among isothermal tests at different temperature conditions. For AIP at the LTO mode, the thermal effect can play a significant role on recovery performance as a higher temperature results in a higher oil recovery factor, which was mainly due to the viscosity reduction by the temperature increment. The LTO effect on producing residual oil was not observed in this study. Also, despite of the thermal effect, the LTO generated oxygenated compounds will increase the viscosity of the crude oil, which results in decreasing recovery efficiency. Since the thermal effect based on the LTO exothermic activity works against the LTO effect (viscosity increment due to the formation of the oxygenated compounds) in terms of recovery performance, when considering AIP, researchers need to evaluate which effect will be more significant comparing to the other effect. If the thermal effect is dominant, the AIP could be considered, otherwise the AIP is less preferred.
References 1. Burger, J.G., Sahuquet, B.C., 1972. Chemical aspects of in-situ combustion – heat of combustion and kinetics. Society of Petroleum Engineers Journal 3599-PA. Vol 253 PP 410422. 2. Huang, S., Jia, H., & Sheng, J. J. (2016a). Research on oxidation kinetics of tight oil from Wolfcamp field. Petroleum Science and Technology, 34(10), 903-910. 3. Huang, S., Jia, H., & Sheng, J. J. (2016b). Effect of shale core on combustion reactions of tight oil from Wolfcamp reservoir. Petroleum Science and Technology, 34(13), 1172-1179. 4. Sarathi, P.S., 1999. In-situ combustion handbook- principles and practices. BDM Petroleum Technologies. Bartlesvillem Oklahoma. DOE/PC/91008-0374. 5. Greaves, M., Ren, S.R., 2000. Improved residual light oil recovery by air injection (LTO process). Journal of Canadian Petroleum Technology, 39(1), 57-61. 6. Gutierrez, D., Taylor, A., Kumar, V., Ursenbach, M., Moore, R., Mehta, S., 2008. Recovery factors in high-pressure air injection projects revisited. SPE Reservoir Evaluation & Engineering. 11 (6), 1097–1106. 7. Montes, A.R., Gutierrez, D., Moore, R.G., Mehta, M.G., 2010. Is high-pressure air injection (HPAI) simply a flue gas flood. Journal of Canadian Petroleum Technology, SPE-133206PA. 8. Huang, S., & Sheng, J. J. (2017). Discussion of thermal experiments’ capability to screen the feasibility of air injection. Fuel, 195, 151-164. 9. Gutierrez, D., Skoreyko, F., Moore, R. G., Mehta, S. A., & Ursenbach, M. G. (2009). The challenge of predicting field performance of air injection projects based on laboratory and numerical modelling. Journal of Canadian Petroleum Technology, 48(04), 23-33.
17 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
10. Belgrave, J.D.M., Moore, R.G., Bennion, D.W., 1993. A comprehensive approach to in-situ combustion modeling. 20250-PA SPE Journal paper. SPE Advanced Technology Series. Vol. No.1 PP98-107 11. Gui, B., Yang, Q.Y., Wu, H.J., Zhang, X., 2010. Study of the effects of low temperature oxidation on the chemical composition of a light crude oil. Energy & Fuels 2010, 24, 11391145. 12. Niu, B., Ren, S., Liu, Y., Wang, D., Tang, L., & Chen, B. (2011). Low-temperature oxidation of oil components in an air injection process for improved oil recovery. Energy & Fuels,25(10), 4299-4304. 13. Fassihi, M. R., Meyers, K. O., & Baslie, P. F. (1990). Low-temperature oxidation of viscous crude oils. SPE reservoir engineering, 5(04), 609-616. 14. Abu-Khamsin, S. A., Iddris, A., & Aggour, M. A. (2001). The spontaneous ignition potential of a super-light crude oil. Fuel, 80(10), 1415-1420. 15. Jia, H., Zhao, J. Z., Pu, W. F., Li, Y. M., Yuan, Z. T., & Yuan, C. D. (2012). Laboratory investigation on the feasibility of light-oil autoignition for application of the high-pressure air injection (HPAI) process. Energy & Fuels, 26(9), 5638-5645. 16. Sarma, H. K., Yazawa, N., Moore, R. G., Metha, S. A., Okazawa, N. E., Ferguson, H., & Ursenbach, M. G. (2002). Screening of three light-oil reservoirs for application of air injection process by accelerating rate calorimetric and TG/PDSC tests. Journal of Canadian Petroleum Technology,41(03). 17. Chen, Z., Wang, L., Duan, Q., Zhang, L., & Ren, S. (2013). High-pressure air injection for improved oil recovery: low-temperature oxidation models and thermal effect. Energy & Fuels, 27(2), 780-786. 18. Huang, S., & Sheng, J. J. (2017b). An innovative method to build a comprehensive kinetic model for air injection using TGA/DSC experiments. Fuel, 210, 98-106. 19. Moore, R. G., Mehta, S. A., & Ursenbach, M. G. (2002, January). A guide to high pressure air injection (HPAI) based oil recovery. In SPE/DOE Improved Oil Recovery Symposium. Society of Petroleum Engineers. 20. Clara, C., Durandeau, M., Quenault, G., & Nguyen, T. H. (2000). Laboratory studies for light-oil air injection projects: potential application in handil field. SPE Reservoir Evaluation & Engineering, 3(03), 239-248. 21. Greaves, M., Ren, S. R., Rathbone, R. R., Fishluck, T., & Ireland, R. (1998, January). Improved residual light oil recovery by air injection (LTO process). In Annual Technical Meeting. Petroleum Society of Canada. 22. Ren, S. R., Greaves, M., & Rathbone, R. R. (1999). Oxidation kinetics of North Sea light crude oils at reservoir temperature.Chemical Engineering Research and Design, 77(5), 385394. 23. Wang, Y., Ren, S., Zhang, L., Deng, J., Peng, X., & Cheng, H. (2018). New insights into the oxidation behaviors of crude oils and their exothermic characteristics: Experimental study via simultaneous TGA/DSC. FUEL-GUILDFORD-, 219(1), 141-150. 18 ACS Paragon Plus Environment
Page 18 of 19
Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
24. Jia, H., & Chen, H. (2018). The Potential of Using Cr 3+/Salt-Tolerant Polymer Gel for Well Workover in Low-Temperature Reservoir: Laboratory Investigation and Pilot Test. SPE Production & Operations. 25. Bhattacharya, S., Belgrave, J. D., Mallory, D. G., Moore, R. G., Ursenbach, M. G., & Mehta, S. A. (2017). Investigation of Thermal Fingerprint in Accelerating-Rate Calorimetry for AirInjection Enhanced-Oil-Recovery Processes. SPE Journal, 22(02), 548-561. 26. Pu, W., Liu, Z., Li, Y., Wei, B., Jia, H., & Kong, L. (2017). A novel insight of laboratory investigation and simulation for high pressure air injection in light oil reservoir. Journal of Natural Gas Science and Engineering, 38, 333-344. 27. Jia, H., & Chen, H. (2018). Using DSC technique to investigate the non-isothermal gelation kinetics of the multi-crosslinked Chromium acetate (Cr3+)-Polyethyleneimine (PEI)-Polymer gel sealant. Journal of Petroleum Science and Engineering, 165, 105-113. 28. Huang, S., & Sheng, J. J. (2018). Feasibility of spontaneous ignition during air injection in light oil reservoirs. Fuel, 226, 698-708.
19 ACS Paragon Plus Environment
