Article pubs.acs.org/EF
Combined Steam−Air Flooding Studies: Experiments, Numerical Simulation, and Field Test in the Qi-40 Block Jian Yang,†,‡ Xiangfang Li,*,† Zhangxin Chen,†,‡ Ji Tian,§ Xinguang Liu,§ and Keliu Wu‡ †
Ministry of Education Key Laboratory of Petroleum Engineering, China University of Petroleum, Beijing 102249, People’s Republic of China ‡ Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada § State Key Laboratory of Offshore Oil Exploitation, China National Offshore Oil Corporation (CNOOC), Beijing 100010, People’s Republic of China ABSTRACT: Air has the characteristics of low-temperature oxidation (LTO) as well as improving drainage energy. Therefore, it can be co-injected with steam to enhance an oil recovery factor. This paper studies combined steam−air flooding for heavy oil recovery. Four experiments, including one steam flooding experiment and three combined steam−air flooding experiments, are studied. The volume ratios of air and steam in these three combined flooding experiments are 1:2, 1:1, and 2:1, respectively. The experimental results show that combined steam−air flooding can enhance the oil recovery factor when the volume ratio of air and steam is in a certain range. Furthermore, numerical simulation results show that its temperature and oil recovery tendency are similar to those from the experiments. In the numerical simulation study, an optimal volume ratio of air and steam is 1.5:2. Finally, a field test of combined steam−air flooding in the Qi-40 block of the Liaohe oil field is studied to validate the experimental and numerical results. Results show that air can be an effective additive that enhances heavy oil recovery in a steam flooding process. In the steam−air flooding experiments, when a volume ratio of air and steam is 2:1, the oil recovery factor is the lowest. Steam flooding is the second lowest in these experiments. When a volume ratio of air and steam is 1:2, the oil recovery factor is a little more than that when the volume ratio of air and steam is 1:1. Furthermore, the outlet temperature indicates that, if too much air is injected into a sand pack, the temperature declines, and thus, air has a negative effect on oil recovery. Numerical simulations are also performed to study combined steam−air flooding. Numerical simulation results show a similar tendency as in experiments. More importantly, numerical simulation results show that, when the volume ratio of air and steam is 1.5:2, the oil recovery factor ranks the highest and heat loss is the lowest at the given conditions. Moreover, in 2010, a field test of combined steam−air was used in the Qi-40 block of the Liaohe oil field and was shown to be effective. The results of this field test also show the accuracy of experiments and numerical simulations mentioned in this paper.
1. INTRODUCTION Thermal methods are widely used in developing heavy oil reservoirs, including cyclic steam stimulation (CSS),1−3 steam flooding,3−5 and in situ combustion.6−8 Because there exist big differences between steam and oil in density and viscosity, steam override and, thus, steam breakthrough occurs easily. This will result in a decline in sweeping efficiency as well as heat utilization efficiency. A great number of researchers did a lot of research in steam flooding with additives to enhance oil recovery. Foam flooding,9−13 combined steam−CO2 flooding,14 and steam−flue gas15 were used for this purpose. Besides, alkline flooding16,17 can also greatly enhance an oil recovery factor. Although these additives can be used to increase sweeping efficiency or reduce interfacial tension, their high costs limit their application in oil fields. Air has the characteristics of both low-temperature oxidation (LTO)18 and high-temperature oxidation (HTO),19 which have been used in high-pressure air injection (HPAI)20,21 and in situ combustion,22−24 respectively. Moreover, Ivory et al.25 studied a steam−air injection process, which showed the feasibility of air in cyclic steam stimulation. Hutchinson et al.26 showed that the LTO was the main flooding mechanism that enhanced an oil recovery factor the most. Belgrave et al.27 showed an example of steam-assisted gravity drainage (SAGD) with air injection, indicating that air has the function of increasing an oil recovery factor of SAGD. Furthermore, a low cost of collecting and injecting air makes it an economic additive gas.25−30 However, a study of combined steam−air flooding is limited. Moreover, the parameters of this flooding need be investigated. As a result, we perform a set of experiments and numerical simulations to study the combined steam−air flooding method. © 2016 American Chemical Society
2. MECHANISMS OF COMBINED STEAM−AIR FLOODING 2.1. LTO. It is well-known that oxygen exists in air, and thus, air has a feature of LTO, which is used in enhancing light oil recovery.18,20,21 Ren et al.31 showed that, when the temperature is under 300 °C, LTO can play an important role in heavy oil recovery. The main fuction of LTO is to reduce the bitumen component of heavy oil and increase the aromatic hydrocarbons of heavy oil,32 which reduces the viscosity of heavy oil. Received: December 4, 2015 Revised: February 1, 2016 Published: February 17, 2016 2060
DOI: 10.1021/acs.energyfuels.5b02850 Energy Fuels 2016, 30, 2060−2065
Article
Energy & Fuels
Figure 3. Oil recovery factor of steam flooding and combined steam− air flooding. Figure 1. Compressibility coefficients of air, CO2, and natural gas (under a pressure of 6 MPa).
Figure 4. Comparison of the outlet temperature of steam flooding and combined steam−air flooding. thermostatic oven, a six-way valve, a precision pressure gauge, and several measuring cylinders (10−100 mL). The experimental model is a sand-packed model. The average permeability is 2−2.5 D. The porosity of this model is about 30%. The experimental apparatus is shown in Figure 2. Four valves are connected with the six-way valve, named 1−4. 3.2. Procedure of the Experiment. (1) Empty the steam generator and add silicon oil into it. Then, inject water into the steam generator. (2) Keep the temperature of the thermostatic oven in 50 °C to simulate the reservoir temperature. (3) Perform the experiment of steam flooding. Before steam flooding, keep all four valves closed. Inject water into the steam generator. The injection rate is 2 mL/min. Open valve 1, which is connected to the steam observation port. At first, hot water will flow from the steam observation port. When water seldom flows from the steam observation port, it indicates that steam is generated. Then, close valve 1 and open valves 2 and 3. Then, steam begins to displace oil in the sand pack. A set of measuring cylinders are placed at the outlet of the sand pack to measure the liquid production, oil production, and water production. Stop the experiment when the water cut reaches 98%. Set aside for a few minutes after oil and water are separated. When the experiment is finished, the temperature at the outlet of the sand pack is measured. During the whole steam flooding process, valve 4 is kept closed. (4) Perform the experiments of combined steam−air flooding. Before combined steam−air flooding, keep all four valves closed. Inject water into the steam generator. Open valve 1 to observe steam, as shown in procedure 3. When steam is
Figure 2. Experimental apparatus of steam flooding and combined steam−air flooding.
In addition, LTO increases the temperature of heavy oil, which has a positive impact on heavy oil recovery.33 2.2. Enhancing Drainage Energy. In comparison to other gases, which are always used as injection gas or additive gas, such as CO2 and natural gas, air has a much greater compressibility coefficient,34 even in high temperatures. In comparison to other gases, air contains more energy, and if the reservoir pressure drops, more oil is extracted. The compressibility coefficients of air, CO2, and natural gas are shown in Figure 1.
3. EXPERIMENTAL SECTION To study the role air plays in combined steam−air flooding, we perform a set of experiments. These experiments are based on the dehydrated crude oil and formation water in the Qi-40 block in the Liaohe oil field. 3.1. Experimental Apparatus. The experimental apparatus mainly consists of a steam generator, an injection pump, an air container that is filled with air, a mass flow controller, a sand pack (25 × 600 mm), a
Table 1. Parameters of the Sand-Packing Model model parameter
experiment 1
experiment 2
experiment 3
experiment 4
volume of sand pack (cm3) saturation volume (water) (mL) saturation volume (oil) (mL) porosity oil saturation permeability (D)
294.52 97.5 71 0.331 0.72 2.1
294.52 98.6 69 0.298 0.70 2.3
294.52 97.8 68 0.308 0.71 2.5
294.52 98.6 68 0.28 0.69 2.1
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DOI: 10.1021/acs.energyfuels.5b02850 Energy Fuels 2016, 30, 2060−2065
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Figure 5. Numerical simulation model of the Qi-40 block in the Liaohe oil field.
Figure 8. Average reservoir pressure of steam flooding and combined steam−air flooding.
Table 2. Parameters of the Simulation Model parameter length (m) width (m) thickness (m) well pattern permeability (mD) porosity initial oil saturation well spacing (m) steam temperature (°C) reservoir pressure (MPa) steam quality
140 140 28 inverted nine spot 2062 0.315 0.75 70 235 3 0.6
Figure 9. Field pilot of combined steam−air flooding. generated, close valve 1 and then open valves 2−4 to conduct combined steam−air flooding. Three experiments of combined steam−air flooding are conducted. These three experiments have a different air− steam volume ratio, ranging from 1:2 to 2:1. In all three experiments, the injection rate of steam is 2 mL/min. In every experiment, liquid production, oil production, and water production are all measured by measuring cylinders. Stop the experiments when the water cut reaches 98%. Set the measuring cylinders aside for a few minutes after oil and water are separated. After the experiments are finished, temperatures at the outlet of the sand pack are measured. The parameters are listed below in Table 1. Because the table shows that the parameters of every experiment are nearly the same, the four experiments are comparable. 3.3. Experimental Results and Discussion. In the steam flooding experiment, the oil recovery factor is 62.11%. When the
Figure 6. Numerical simulation study of the recovery factor of steam flooding and combined steam−air flooding.
Table 3. Parameters of the Field Pilot parameter well spacing (m) thickness (m) well pattern average permeability (mD) average porosity initial oil saturation steam temperature (°C) average reservoir pressure (MPa) steam quality
Figure 7. Average reservoir temperature of steam flooding and combined steam−air flooding. 2062
70−100 20 inverted nine spot 2100 0.3 0.7−0.75 245 3.6 0.6 DOI: 10.1021/acs.energyfuels.5b02850 Energy Fuels 2016, 30, 2060−2065
Article
Energy & Fuels
Figure 10. Field pilot of combined steam−air flooding in the Qi-40 block. volume ratio of air and steam is 0.5:1, the oil recovery factor is 68.67%, which is almost 2.8% higher than that when the volume ratio of air and steam is 1:1. However, when the volume ratio of air and steam reaches 2:1, the recovery factor of combined steam−air flooding is 57.36%, which is the lowest among all experimental results. The results are shown in Figure 3. According to the results, when the volume ratios of air and steam are 0.5:1 and 1:1, steam plays the predominant role in oil recovery. Air, however, is an additive gas, which can enhance the oil recovery factor by decreasing oil viscosity, increasing oil mobility, and facilitating a LTO procedure. Hence, in these conditions, the combined steam−air flooding can effectively enhance oil recovery. However, when the volume ratio of air and steam reaches 2:1, the air that has a relatively low temperature plays a negative role in steam flooding. The more the air that exists in the combined gas system, the lower the temperature of combined flooding. Therefore, this is the reason that the oil recovery factor of combined steam−air flooding is lower than that of steam flooding when the ratio of air and steam is 2:1. According to the temperature measured at the outlet of a sand pack, when the volume ratios of air and steam are 1:1 and 0.5:1, the temperature of the outlet has little difference from that in steam flooding (Figure 4). When the volume ratio of air and steam is 2:1, the temperature of the outlet of the sand pack is obviously lower than that in steam flooding. Thus, in this condition, the steam chamber is damaged by air because the temperature is low. As a result, steam flooding no longer exists in the sand pack and hot water flooding takes place instead. Therefore, air has a negative effect when too much air exists in a combined steam−air system.
air has the characteristics of enhancing the oil recovery factor effectively. However, when more cold air is injected, the temperature of the system delines and, thus, low temperature becomes a dominant factor, which has a negative effect on oil recovery in the later period. As a result, when the volume ratios of air and steam are 1.5:1 and 2:1, oil recovery is less than that of steam flooding at the end. Figure 7 shows that, when the volume ratio of air and steam exceeds 1:1, the temperature declines obviously. As mentioned above, too much air exists in this combined system. As a result, air has a negative influence on oil recovery. 4.2.2. Reservoir Pressure. Because air has a high compressibility coefficient, air can make up for the reservoir pressure when too much oil is produced during a steam flooding process. The reservoir pressure comparisons of steam flooding and combined steam−air flooding are shown in Figure 8. Figure 8 shows that, when the volume ratio of air and steam is 0.75:1, the reservoir pressure is the highest. When too much air is injected in the reservoir, a low temperature may have a negative effect on enhancing reservoir energy, which will, in turn, lead to a lower reservoir pressure and, thus, result in a lower oil recovery factor.
5. FIELD PILOT In the year 2010, a combined steam−air flooding test was conducted in one well pattern in the Qi-40 block of the Liaohe oil field, shown in Figure 9. The average permeability and porosity of this pilot field are about 2.1 D and 30%, respectively (Table 3). Before combined steam−air flooding was conducted in the Qi-40 block, steam flooding kept being used and the oil production rate kept declining. The steam injection rate of this pilot is kept constant, which is 120 m3/day. As seen in Figure 10, during combined steam−air flooding, the air injection rate is almost 85 m3/day, which means that the volume ratio of air and steam is almost 0.75:1. After the combined steam−air flooding was conducted on April 27, 2010, the oil production rate increased from 17 to almost 25 m3/day. It shows that the combined steam−air flooding can effectively increase oil recovery. Therefore, the field plot shows the accuracy of both the experiments and the numerical simulations in the paper.
4. NUMERICAL SIMULATION 4.1. Simulation Model. To perform further research on combined steam−air flooding in enhancing oil recovery, this paper conducts a numerical study on this flooding process. The model is based on the geological characteristics of the Qi-40 block in the Liaohe oil field, whose average porosity is 30% and average permeability is 2.1 D. The model is shown in Figure 5. The parameters of this model are listed in Table 2. 4.2. Parameter Study on Steam Flooding and Combined Steam−Air Flooding. 4.2.1. Recovery Factor. Figure 6 shows the simulation results of a CMG simulator of combined steam−air flooding. This figure indicates that air can effectively enhance the oil recovery factor. At the beginning of the oil production procedure, the oil recovery factors of combined steam−air flooding are all higher than that of steam flooding, which indicates that, when the temperature is high, 2063
DOI: 10.1021/acs.energyfuels.5b02850 Energy Fuels 2016, 30, 2060−2065
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6. CONCLUSION (1) Experiments show that, when the volume ratios of air and steam are 1:2 and 1:1, the oil recovery factors of combined steam−air flooding are higher than that of steam flooding. When the volume ratio of air and steam is 2:1, the oil recovery factor of this flooding is lower than that of steam flooding. It is because too much injected air lowers the temperature and has a negative effect on steam chambers. Therefore, hot water flooding exists in a sand pack instead. As a result, the oil recovery factor decreases rapidly. Moreover, temperatures of the sandpack outlet also show this. (2) Results of numerical simulation are similar to those of experiments. As seen, when the volume ratio of air and steam is 1.5:2, the reservoir pressure and oil recovery are kept the highest. As a result, this numerical simulation study shows that, under the conditions of the Qi-40 block, the optimal volume ratio of air and steam is 1.5:2. (3) The field pilot in the Qi-40 block shows that, when the volume ratio of air and steam is almost 1.5:2, the combined steam−air flooding can effectively increase the oil production rate, from 17 to almost 25 m3/day. The field test also shows the accuracy of the experiments and numerical simulations in this paper.
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APPENDIX
A comparison of parameters of the experiment, numerical simulation, and field pilot is shown in Table 4. Table 4. Comparison of Parameters of the Experiment, Numerical Simulation, and Field Pilot experiment
numerical simulation
field pilot
2200 0.3 0.75 3 235
2062 0.315 0.75 3 235
2100 0.3 0.7−0.75 3.2 237
permeability (mD) porosity initial oil saturation reservoir pressure (MPa) steam temperature (°C)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This paper was sponsored by the National Natural Science Foundation Project (50974128), the Key Laboratory for Petroleum Engineering of the Ministry of Education, and the China University of Petroleum Foundation. The authors recognize the support of the Ministry of Education Key Laboratory of Petroleum Engineering in China at the China University of Petroleum (Beijing) for the permission to publish this paper.
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