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Low-Temperature Oxidation Characteristics and Its Effect on the Critical Coking Temperature of Heavy Oils Liang Zhang, Junyu Deng, Lei Wang, Zhenya Chen, Shaoran Ren, Changhao Hu, and Shoujun Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502070k • Publication Date (Web): 31 Dec 2014 Downloaded from http://pubs.acs.org on January 2, 2015
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Low-Temperature Oxidation Characteristics and Its Effect on the Critical Coking Temperature of Heavy Oils 1*
ZHANG Liang , DENG Junyu1, WANG Lei2, CHEN Zhenya3, REN Shaoran1, HU Changhao4, ZHANG Shoujun5 1 School of Petroleum Engineering, China University of Petroleum (Huadong), Qingdao 266580, China; 2 Hancheng Branch, Coalbed Methane Company Limited, CNPC, Hancheng 715409, China; 3 School of Petroleum Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China 4 Research Institute of Petroleum Exploration and Development, Liaohe Oilfield, CNPC, Panjing 124000, China; 5 Shuguang Branch Company, Liaohe Oilfield, CNPC, Panjing 124000, China *Corresponding author, Tel.: +86 15053259740 E-mail:
[email protected],
[email protected] Address: No 66, Changjiangxi Road, Huangdao District, Qingdao, Shandong, China, 266580
ABSTRACT: Air assisted steam stimulation (huff and puff) is an innovative process for enhanced heavy oil recovery. In this paper, the low temperature oxidation (LTO) and coking experiments were conducted to reveal the LTO characteristics of heavy oils and its effect on the oil critical coking temperature. The O2 consuming ability and oxidation activity of different oils, in a temperature range of 80-170 oC, were analyzed, including typical light and heavy oil samples. The experimental results show that heavy oils have a higher oxidation activity at low temperature than that of light oils. Heavy oils can effectively consume O2 in the injected air and produce a certain amount of CO2 under reservoir conditions. In terms of the difference in LTO characteristics of different oils, a more general LTO reaction kinetics model of crude oils in reservoir was established. A parameter, Rdec (ratio of hydrocarbon oxides participating in decarbonylation to total hydrocarbon oxides produced from oxidation), is introduced to indicate the O2-to-CO2 conversion features of different oils during LTO process. The Rdec, Ea and ko of heavy oils are usually lower than those of light oils, which can well reflect the LTO characteristics of heavy oils and demonstrate that heavy oils are more easily oxidized at low temperature. However, due to more heavy oil components generated during LTO process, the critical coking temperature of heavy oils exposed to air environment will decrease (by 120 oC from 400 oC to 280 oC for super heavy oil). It should be noted that the effect of LTO process on critical coking temperature is different for different heavy oils, and the induced coking risks are also different in different heavy oil reservoirs, which need further studies in lab and field. KEY WORDS: heavy oil, steam stimulation, air injection, low temperature oxidation, oil coking temperature, enhanced oil recovery
1. INTRODUCTION Steam stimulation (huff and puff process) is an effective technique for heavy oil recovery. However, for the conventional steam stimulation technique, its performance in the late stage of heavy oil development usually becomes worse. Some major production problems, such as low reservoir pressure and low oil-steam rate ratio are often encountered, which can lead to a low oil recovery after a large number of cycling. Therefore, changing the development mode of heavy oil fields is considered, by converting steam stimulation to steam flooding, SAGD (Steam Assisted Gravity Drainage) [1-2], or other techniques. For the heavy oil reservoirs which are not suitable for such changes in development mode, the thermal chemical agents (such as surfactants, viscosity reducers and urea) and gas injection (such as CO2 and N2 injection) are usually employed to assist and improve the steam huff-and-puff effect. Gas assisted steam stimulation can increase formation pressure, keep steam dryness, reduce heat loss, expand steam swept volume, and form a solution gas drive in heavy oil reservoir. However, this technique is often subject to the difficulty in generation or purchase of N2 and CO2
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gas. Rich and cheap air is a good injection fluid for oilfield development. Air injection technology has been applied in many oil reservoirs worldwide. The ISC (in situ combustion) technique is an effective method to enhance heavy oil recovery mainly through steam flooding and viscosity reduction as a result of heat generated by high temperature oxidation (HTO) reaction at high pressure [3-4]. For light oil reservoirs, the low temperature oxidation (LTO) reaction mainly occurs during air injection process which is more like N2 injection but with an additional thermal effect [5-7]. Field tests have being conducted in a few light oil reservoirs in china recent years, including Zhongyuan oilfield, Baise oilfield, Shanshan oilfield, and Changqing oilfield [8-10]. Results from the Zhongyuan oilfeld have shown significant benefits of air and air foam injection via the LTO process. Up to 4% original oil in place (OOIP) of incremental oil production had been achieved in the pilot region during 3 years of air injection [11]. No matter for heavy oil or for light oil, in order to avoid the risk of potential explosion induced by O2 breakthrough in producers, the selected oil reservoirs for air injection should have good oxidation characteristics to consume most of the injected O2 in the air (reducing the O2 content in produced gas to a safe level) [6,7,12,13]. Air assisted steam stimulation is a new process to improve the thermal recovery effect in heavy oil reservoir through LTO mechanism. In a typical and safe process, air and steam (over 200 oC) are injected underground simultaneously for several days, and then air injection is stopped, leaving the steam injected continuously to displace the air deep into the oil reservoir. Subsequently, the injection well is shut in and the LTO reaction between oil and air goes on. When most of O2 is consumed and the well head pressure becomes steady, the injection well opens for oil production. Air assisted steam stimulation has the advantages as other gas assisted steam, such as maintaining reservoir pressure, lessening steam condensation, and enlarging steam swept volume. It can also bring the benefits of LTO thermal effect and flue gas flooding, which can further increase the oil mobility, raise the steam swept volume and improve the effectiveness of steam soaking process. At present, a few laboratory experiments and field applications have been conducted and proved that this technique is beneficial for improving steam stimulation (e.g. the pilot test in the Du80 block in Liaohe oilfield, China [14]). However, its EOR mechanisms are still not clear, especially such as why heavy oils usually present different LTO characteristics from light oils, and the effect of LTO process on enhanced heavy oil recovery. In addition, change of oil composition induced by LTO will have an effect on the critical coking temperature of heavy oils. Coking reaction may be triggered at a high reservoir temperature improved by LTO process. It is harmful to reservoir properties, which needs to be verified. In this paper, the LTO characteristics of heavy oils and its effect on the oil coking temperature are studied through lab experiments. The O2 consuming ability and oxidation activity of heavy oils under different temperatures are evaluated in comparison with typical light oil. The coking temperatures of heavy oil before and after LTO reaction were investigated. Based on the experimental results, a more general LTO reaction kinetics model of heavy oils under reservoir conditions was established. The study is expected to provide a deep insight into the LTO mechanisms of air assisted steam stimulation in heavy oil reservoirs.
2. EXPERIMENTAL SECTION 2.1. Equipment. The LTO experimental equipment was used to assess the LTO characteristics of heavy oils and its influence on the oil coking temperature. Its schematic diagram is shown in Figure 1. It mainly consists of a stainless steel reactor with a volume of 100 mL (anti-pressure up to 30 MPa and anti-temperature up to 500 oC), a digital-control thermo-tank (heating temperature up to 500 oC with a control precision of ±0.1 oC), a temperature sensor (precision of ±0.1 oC), a pressure sensor (precision of ±0.01 MPa), a high pressure gas tank (air or N2), and a data acquisition system. In addition, the JH106A portable gas analyzer (precision of ±0.01%mol), and the H3860A infrared gas analyzer (precision of ±0.01%mol) were used to measure the O2 and CO2 contents in the tail gas in the reactor after LTO experiment, respectively. 2.2. Materials. (1) three typical dehydrated oil samples were used, including two heavy oils and one light oil:
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the sample 1 is super heavy oil from the Du80 block, Liaohe oilfield, China; the sample 2 is ordinary heavy oil from the Biqian 10 block, Henan oilfeld, China; and the sample 3 is light oil from the Ming 15 block, Zhongyuan oilfield, China; (2) self-prepared formation water with 5%NaCl; (3) quartz sand with mesh size of 80-120. The basic properties of these three oil samples are listed in Table 1.
Figure 1 Schematic diagram of LTO experimental equipment 1 High pressure gas tank (air or N2); 2 Valve; 3 Four-way connection; 4 Digital-control thermo-tank; 5 Stainless steel reactor; 6 Mixture of oil, water and sand; 7 Pressure sensor; 8 Temperature sensor; 9 Data acquisition system; 10 Computer Table 1 Basic properties of oil samples used in lab experiments No
Oil type a
Density, g/ml
Viscosity, mPa·s
SARA Composition b
Source
1 Super heavy oil
0.988 (20 oC)
68198 (50 oC)
S-12.09%; A1-9.33%; R-39.91%; A2-38.67%
Du80 block, Liaohe oilfield
2 Ordinary heavy oil
0.962 (20 oC)
6505 (50 oC)
S-16.79%; A1-26.46%; R-52.83%; A2-3.92%
Biqian10 block, Henan oilfield
3 Light oil
0.907 (20 oC)
42.73 (20 oC)
S-68.69%; A1-17.06%; R-13.87%; A2-0.38%
Ming15 block, Zhongyuan oilfield
Notes: a. heavy oil classification principle: ordinary heavy oil: 50-10000 mPa·s, extra heavy oil: 10000-50000 mPa·s and super heavy oil: >50000 mPa·s; b. S - saturated hydrocarbon, A1 - aromatic hydrocarbon, R - resin, and A2 - asphaltine.
2.3. Procedures. 2.3.1. LTO experiment. During a typical air assisted steam stimulation process, the LTO reaction mainly occurs in the front of steam injection, where the reservoir temperature is usually no more than 200 oC. Therefore, the LTO experiments were conducted at different temperatures in a range of 80-170 oC. Firstly, a certain amount of crude oil, formation water and quartz sand were mixed together (70 ml quartz sand, 12.6 ml dehydrated oil, and 5.6 ml formation water, the porosity of mixture is about 40% with So of 45%, Sw of 20% and Sg of 35%), and put into the reactor (100 ml); then the reactor was placed in the digital-control thermo-tank to keep at a certain temperature, and was charged by high pressure air to 10 MPa to simulate reservoir conditions; finally, the data acquisition system was employed to record the pressure and temperature in the reactor during LTO process. Each experiment lasted for about 20 hours. At the end of the experiment, after the reactor was cooled down, the O2 and CO2 contents in the tail gas were measured using gas analyzers. The SARA compositions of crude oils before and after LTO reaction were also tested. Then an oxidation kinetic analysis was performed. 2.3.2. Coking experiment. Critical coking temperature of heavy oils may be affected by LTO process, which will improve the coking risk in heavy oil reservoirs developed by air assisted steam stimulation. Hence, a coking experiment was designed to investigate the critical coking temperature of heavy oils in air and N2 environments (the sample 1 with the greatest coking risk was used). The reactor with a mixture of heavy oil, formation water and quartz sand was heated by digital-control thermo-tank, and maintained at 250 oC for several hours, then air or N2 was injected into the reactor to improve the pressure up to 7 MPa, and stayed for 2 hours (until most of the O2 was consumed if air was injected). Subsequently, the reactor temperature was raised to 250-400 oC and kept stable for 7 hours, and finally cooled down to room temperature. The oil sand was washed using methylbenzene to judge if the solid coke (undissolved substance in methylbenzene) is produced or not, and to determine the coking temperature of heavy oil.
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3. RESULTS AND DISCUSSION 3.1. O2 Consuming Ability of Heavy Oils The LTO reaction of crude oils caused the pressure of reactor decreased. The pressure drops in the reactor at different temperatures are shown in Figure 2. Due to the small pressure drop rate and the pressure sensor with limited precision of ±0.01 MPa, the monitored pressures in the LTO reactor decline like stairs especially for the cases in a lower temperature range of 80-120 oC (see Figure 2 a, b, c). In addition, the sudden drops and fluctuations in the pressure curves especially at temperature of 170 oC (see Figure 2e) are mainly caused by the temporary instability of the pressure sensor. Contents of O2 and CO2 in the tail gas after LTO reaction were measured, as shown in Figure 3 and Figure 4. The CO content in the tail gas was neglected because of its pretty small amount. The change of pressure in the reactor during LTO process was mainly induced by O2 consumption and CO2 production. According to the pressure change in the reactor and the O2 and CO2 contents in the tail gas, the amounts of O2 consumed and CO2 produced were calculated, and the conversion ratio of O2 to CO2 (RCO2/O2 = produced CO2/consumed O2) was further obtained [15](see Figure 5). Based on the LTO theory, the maximum RCO2/O2 should be no more than 0.67 (assuming 1.5 O2 can be converted to 1 CO2). In this study, the measured RCO2/O2 is usually smaller than 0.50. It makes the total amount of gas in the reactor reduced, and leads to the reactor pressure decline during LTO process. According to the amounts of O2 consumed during effective LTO times (5-20 hours until the pressure reached stable for each experiment), the O2 consuming rates of crude oils at different temperatures were calculated (with errors of 0.24-0.99% analyzed using error transfer formula based on the measured data, see Figure 6). The SARA compositions of crude oils before and after LTO reaction at 120 oC are presented in Figure 7. Due to an abnormal large temperature increase observed in the LTO experiment of light oil at 170 oC (an instantaneous HTO reaction may occur), the related data is not drawn in these figures.
Figure 2 Pressure drop in reactor during LTO at different temperatures
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Figure3 CO2 content in tail gas
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Figure 4 O2 content in tail gas
Figure 5 Conversion ratio of O2 to CO2
Figure 7 SARA composition before and after LTO (120 oC)
Figure 6 O2 consuming rate
Figure 8 Oxidation rate of pure components
From Figures 2-6, it can be seen that no matter for heavy oil or for light oil, the degree of LTO reaction will be deepened if the experimental temperature is increased (from 80 oC to 170 oC). As the experimental temperature rises, a larger pressure drop in the reactor can be achieved in a shorter time. More O2 is consumed and more CO2 is generated. O2 consuming rate is improved along with a larger RCO2/O2. It can also be seen that the pressure drop rule and O2 consuming ability of different oils during LTO process are different from each other [16-19]. Under the same experimental conditions, the crude oil with higher viscosity usually has a larger pressure drop during LTO (
