# Roles of Flue Gas in Promoting Steam Flow and Heat Transfer in Multithermal Fluid Flooding

Authors: Zhuangzhuang Wang; Zhaomin Li
Journal: Mathematical Problems in Engineering
 (2019)

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## Abstract

Multithermal fluid technology is becoming an important method in the field of heavy oil development. However, because of insufficient investigation on the heat transfer for the multithermal fluid, some development phenomena and characteristics still cannot be well explained. In order to determine the effect of flue gas on the thermal swept scope, multithermal fluid flooding experiments were carried out through 1D sandpack. The temperatures along the sandpack were measured. On this basis, steam heat transfer simulation experiments were conducted and the heat transfer coefficients were calculated. The mechanism of flue gas on steam heat transfer was analyzed. The results show that at the same heat injection conditions, the thermal swept scope for the multithermal fluid flooding was larger than that for the steam flooding. With the increase of flue gas proportion in the multithermal fluid, the heat transfer coefficient decreased and the condensation pattern was transformed from drop condensation to film condensation gradually. The flue gas can form gas film on the surface of the cold body and inhibit the heat transfer between steam and the cold body. Because of the inhibiting effect of flue gas on steam heat transfer, flue gas can reduce the heat transferred to the rock matrix in flooding and thus promote steam to carry more heat further. Meanwhile, flue gas can accelerate the flow of steam in porous media, which also leads to the expansion of the thermal swept scope for the multithermal fluid flooding.


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## Body

## 1. Introduction

Heavy oil reserve is mainly developed by thermal recovery method. The common development technologies include steam flooding, cycle steam stimulation, and steam assisted gravity drainage. Steam is the most common injection media in thermal recovery. But steam as heat carrier has some disadvantages, such as severe heat loss, large occupied area by steam generator, and large emission of flue gas [1–5]. Multithermal fluid is a new heat carrier which has gradually emerged in recent years. The multithermal fluid is a high temperature mixture of steam, hot water, and flue gas. It is produced by a generator in which fuel oil and air combust and water is heated into steam. The mechanism of the multithermal fluid generator is same as the rock engine essentially [6–10]. The multithermal fluid generator is lowered to the down hole with the pipes, so the steam, hot water, and the combustion products can be injected into the reservoir directly. Compared with conventional steam injection, this new technology can not only reduce the heat loss in the injection process but also decrease the greenhouse gas emission. Therefore, multithermal fluid has a good application prospect.At present, the multithermal fluid technology has been applied in oilfield. In Bohai Oilfield, China, due to the limitation of the offshore platform area, the conventional technology based on steam injection is unsuitable and the multithermal fluid technology was introduced [11–13]. The field results show high oil recovery and good economic benefit. After several years of development, the multithermal fluid technology has become the principle means for offshore heavy oil development. In order to reduce the greenhouse gas emission and improve the thermal efficiency of injected steam, some onshore heavy oilfield also used the multithermal fluid technology [6, 7]. The field results show the multithermal fluid technology can reduce the steam consumption and greenhouse gas emission but also increase the oil production rate. The success of the multithermal fluid technology in oilfield greatly impels its development.Lots of researches have been conducted around the enhanced oil recovery (EOR) mechanism of the multithermal fluid [14–19]. It is commonly believed that the multithermal fluid has the advantages of steam and flue gas at the same time. Steam is the main component of multithermal fluid. Steam provides the most part of the heat, and its function is to heat heavy oil and decrease the viscosity. The main components of flue gas are nitrogen (N2) and carbon dioxide (CO2). N2 has a good compressibility, so it can supplement formation energy and improve the oil production rate [20, 21]. CO2 has a high solubility in heavy oil. When CO2 dissolves into the heavy oil, it can decrease the viscosity of oil remarkably [22–25]. Meanwhile, CO2 also can decrease the oil-gas interfacial tension and extract the light component of heavy oil [24]. Besides, it is likely for flue gas to accumulate at the top of the reservoir, so the flue gas also can act as a gas cap to improve the oil displacement rate. And due to the high heat transfer resistance of the gas cap, it is believed that the gas cap can reduce the heat loss to the cap rock and improve the steam swept efficiency horizontally [25].However, the investigation on the heat transfer of multithermal fluid is few. Some phenomena and characteristics in multithermal fluid development still cannot be explained reasonably. Some researchers found from the field results that the temperature above the reservoir increased after the flue gas injection. This result is contradictory with the heat insulation effect of gas cap. Therefore, the effect of flue gas on the heat transfer of multithermal fluid should be investigated.In this work, steam flooding experiment and multithermal fluid flooding experiments were conducted to study the effect of flue gas on the thermal swept scope firstly with 1D sandpack. In order to analyze the effect of flue gas on the steam heat transfer, an experimental setup was designed to simulate the steam flow and heat transfer. The heat transfer coefficient was calculated and condensation pattern was observed. On this basis, the mechanism of flue gas on the steam heat transfer in flow was analyzed, and the impact of flue gas on expanding thermal swept scope in multithermal fluid flooding was explained.


## 2. Experiment

### 2.1. Experimental Material

Multithermal fluid was simulated by the mixture of steam and flue gas in the work. The steam was produced by steam generator with distilled water. The flue gas was prepared with N2 and CO2 in the proportion of 4:1. The purities of N2 and CO2 were both 99.9%.


### 2.2. Experimental Apparatus

The schematic of the experimental setup is shown in Figure1. The setup can be separated into three parts: injection system, flooding experiment system, and heat transfer simulation system. In the injection system, the steam and the flue gas were mixed in the six-way valve to form multithermal fluid and then were injected into flooding experiment system and heat transfer simulation system.Figure 1
Schematic diagram of the experiment.In the flooding experiment system, the sandpack tube is 60.0cm long with the diameter of 2.54cm, which was located in a thermotank. Three thermocouples were distributed on the sandpack tube evenly. The distances of the three thermocouples from the entrance were 5cm, 30cm, and 55cm, respectively. The measured temperature data was recorded by the computer.In the heat transfer system, the core parts were the observation cell and the measuring block. The observation cell was 100mm long, 20mm wide and 300mm high, as shown in Figure2. There were electric heating plates attached on the outer surface of the observation cell to control its temperature. The measuring block that was made of copper was embedded on the wall of the observation cell as a condenser, as shown in Figure 3. Five K-thermocouples with an accuracy of 0.1 K were distributed on the measuring block to measure its temperature.Figure 2
A snapshot of the heat transfer simulation system.Figure 3
Schematic diagram of the heat transfer from the steam to the measuring block.


### 2.3. Experimental Procedures

#### 2.3.1. Multithermal Fluid Flooding Experiment

(1) Four sandpacks were prepared. The specific parameters are shown in Table1. (2) Put the sandpack in the thermotank. The temperatures of the thermotank and the steam generator were set to 60°C and 250°C, respectively. (3) When the temperature of the thermotank and the steam generator reached stability, start to inject the steam and flue gas into the sandpack. The steam was injected at the rate of 1.5mL/min (water equivalent) and at the temperature of 108°C. (4) The temperatures of sandpack were recorded by the computer during the experiment.Table 1
Sandpack parameters and experimental scheme.
No. Experimental scheme Porosity (%) Permeability (mD) Flue gas injection rate at standard condition (mL/min) 1 Steam flooding 32.58 3155 0 2 Multithermal fluid flooding 32.81 3250 5 3 Multithermal fluid flooding 31.06 3050 10 4 Multithermal fluid flooding 32.26 3130 20


#### 2.3.2. Heat Transfer Simulation Experiment

(1) Test the gas-tightness of the observation cell and then set its temperature to 100°C. (2) Start the circulation of the cooling liquid. (3) After the temperatures of the cooling liquid and the observation cell reached stability, inject steam and flue gas into the observation cell from the upper inlet. The injection temperature and rate of steam were 108°C and 10mL/min (water equivalent). The injection rate of flue gas depended on the gas-water ratio. Four experiments with the gas-water ratio of 0, 0.5, 1.0, and 2.0 were conducted. (4) During the experiment, record the condensation phenomena and the temperatures of the measuring block. (6) Change the temperature of the cooling liquid and repeat the steps (3)-(4).


## 2.1. Experimental Material

Multithermal fluid was simulated by the mixture of steam and flue gas in the work. The steam was produced by steam generator with distilled water. The flue gas was prepared with N2 and CO2 in the proportion of 4:1. The purities of N2 and CO2 were both 99.9%.


## 2.2. Experimental Apparatus

The schematic of the experimental setup is shown in Figure1. The setup can be separated into three parts: injection system, flooding experiment system, and heat transfer simulation system. In the injection system, the steam and the flue gas were mixed in the six-way valve to form multithermal fluid and then were injected into flooding experiment system and heat transfer simulation system.Figure 1
Schematic diagram of the experiment.In the flooding experiment system, the sandpack tube is 60.0cm long with the diameter of 2.54cm, which was located in a thermotank. Three thermocouples were distributed on the sandpack tube evenly. The distances of the three thermocouples from the entrance were 5cm, 30cm, and 55cm, respectively. The measured temperature data was recorded by the computer.In the heat transfer system, the core parts were the observation cell and the measuring block. The observation cell was 100mm long, 20mm wide and 300mm high, as shown in Figure2. There were electric heating plates attached on the outer surface of the observation cell to control its temperature. The measuring block that was made of copper was embedded on the wall of the observation cell as a condenser, as shown in Figure 3. Five K-thermocouples with an accuracy of 0.1 K were distributed on the measuring block to measure its temperature.Figure 2
A snapshot of the heat transfer simulation system.Figure 3
Schematic diagram of the heat transfer from the steam to the measuring block.


## 2.3. Experimental Procedures

### 2.3.1. Multithermal Fluid Flooding Experiment

(1) Four sandpacks were prepared. The specific parameters are shown in Table1. (2) Put the sandpack in the thermotank. The temperatures of the thermotank and the steam generator were set to 60°C and 250°C, respectively. (3) When the temperature of the thermotank and the steam generator reached stability, start to inject the steam and flue gas into the sandpack. The steam was injected at the rate of 1.5mL/min (water equivalent) and at the temperature of 108°C. (4) The temperatures of sandpack were recorded by the computer during the experiment.Table 1
Sandpack parameters and experimental scheme.
No. Experimental scheme Porosity (%) Permeability (mD) Flue gas injection rate at standard condition (mL/min) 1 Steam flooding 32.58 3155 0 2 Multithermal fluid flooding 32.81 3250 5 3 Multithermal fluid flooding 31.06 3050 10 4 Multithermal fluid flooding 32.26 3130 20


### 2.3.2. Heat Transfer Simulation Experiment

(1) Test the gas-tightness of the observation cell and then set its temperature to 100°C. (2) Start the circulation of the cooling liquid. (3) After the temperatures of the cooling liquid and the observation cell reached stability, inject steam and flue gas into the observation cell from the upper inlet. The injection temperature and rate of steam were 108°C and 10mL/min (water equivalent). The injection rate of flue gas depended on the gas-water ratio. Four experiments with the gas-water ratio of 0, 0.5, 1.0, and 2.0 were conducted. (4) During the experiment, record the condensation phenomena and the temperatures of the measuring block. (6) Change the temperature of the cooling liquid and repeat the steps (3)-(4).


## 2.3.1. Multithermal Fluid Flooding Experiment

(1) Four sandpacks were prepared. The specific parameters are shown in Table1. (2) Put the sandpack in the thermotank. The temperatures of the thermotank and the steam generator were set to 60°C and 250°C, respectively. (3) When the temperature of the thermotank and the steam generator reached stability, start to inject the steam and flue gas into the sandpack. The steam was injected at the rate of 1.5mL/min (water equivalent) and at the temperature of 108°C. (4) The temperatures of sandpack were recorded by the computer during the experiment.Table 1
Sandpack parameters and experimental scheme.
No. Experimental scheme Porosity (%) Permeability (mD) Flue gas injection rate at standard condition (mL/min) 1 Steam flooding 32.58 3155 0 2 Multithermal fluid flooding 32.81 3250 5 3 Multithermal fluid flooding 31.06 3050 10 4 Multithermal fluid flooding 32.26 3130 20


## 2.3.2. Heat Transfer Simulation Experiment

(1) Test the gas-tightness of the observation cell and then set its temperature to 100°C. (2) Start the circulation of the cooling liquid. (3) After the temperatures of the cooling liquid and the observation cell reached stability, inject steam and flue gas into the observation cell from the upper inlet. The injection temperature and rate of steam were 108°C and 10mL/min (water equivalent). The injection rate of flue gas depended on the gas-water ratio. Four experiments with the gas-water ratio of 0, 0.5, 1.0, and 2.0 were conducted. (4) During the experiment, record the condensation phenomena and the temperatures of the measuring block. (6) Change the temperature of the cooling liquid and repeat the steps (3)-(4).


## 3. Calculation Method for Heat Transfer Coefficient

When high temperature steam contacts with a low temperature body, heat will be transferred from steam to the cold body and part of the steam will condense on the surface. The heat transfer intensity in the process can be characterized by the heat transfer coefficient [26–30]. The greater the heat transfer coefficient is, the more heat will be transferred in the same time.The heat transfer process in the experiment can be schematically shown in Figure3. Because heat insulation treatment was done on the surface of the measuring block except the contact surface with steam and cooling water, it can be considered that the heat was only transferred in the y direction, as shown in Figure 3.The heat transfer coefficient is calculated in four steps. Firstly, based on the five measured temperatures of the measuring block, we get its temperature distribution through linear fitting. The linear distribution of temperature on the measuring block is showed in (1):(1)Ty=ay+b(2)Fa,b=∑i=1nayi+b-Ti2(3)∂F∂a=∂F∂b=0Secondly, we calculate the surface subcooling through (4):(4)∆T=T-v-T0where ∆T is surface subcooling, K; T-v is the mean temperature of the steam, K; T(0) is the surface temperature of the measuring block, K.Thirdly, the heat flux between the steam and the measuring block is calculated through(5)q=λ∂Ty∂y,y=0where q is heat flux, W/m2; λ is thermal conductivity of the measuring block, W/(m·K).Finally, we can calculate the condensation heat transfer coefficient through(6)h=q∆Twhere h is condensation heat transfer coefficient, W/m2·K; q is heat flux, W/m2; ∆T is surface subcooling, K.


## 4. Results and Discussions

### 4.1. Effect of Flue Gas on Thermal Swept Scope for Multithermal Fluid Flooding

Figure4 shows the changes of temperatures with the time under different experimental schemes. We can see at the position 5cm from the entrance that the temperatures were almost the same. But at the positions 30cm and 55cm from the entrance, the temperature for steam flooding was lower than that for multithermal fluid flooding. With the increase of the distance from the entrance, the temperature difference increased gradually. Furthermore, with the increase of flue gas injection rate in multithermal fluid flooding, the improvement also increased gradually. Figure 5 compares the temperature distribution along the sandpack under different experimental schemes. It is evident that the multithermal fluid flooding could expand the thermal swept scope significantly. The flooding experiments indicate, at the same heat injection condition, the addition of flue gas can promote steam to transfer more heat further.Figure 4
Changes of temperatures with the time under different experimental schemes. (a) is at the position 5cm from the entrance. (b) is at the position 30cm from the entrance. (c) is at the position 55cm from the entrance.
(a) (b) (c)Figure 5
Temperature distribution along the sandpack under different experimental schemes.


### 4.2. Effect of Flue Gas on the Steam Heat Transfer in Flow

#### 4.2.1. Condensation Pattern

Figure6 compares the observed condensation phenomena at different gas-water ratios. We can see the condensate showed different forms. For pure steam, steam mainly condensed in the form of droplet, as shown in Figure 6(a). When the gas-water ratio was 0.5, a small amount of water film appeared. At the gas-water ratio of 2 and 5, most of the condensate was in the form of film. The condensation pattern in Figure 6(a) can be called drop condensation, while the condensation pattern in Figures 6(c) and 6(d) can be called film condensation. This indicates, with the increase of gas proportion in the steam-flue gas mixture, the condensation pattern was transformed from drop condensation to film condensation gradually. Meanwhile, we also found the drop frequency of condensate was different. For the pure steam, the condensate droplet fell off frequently and then new droplet formed soon. For the film condensation, condensate film formed and then fell slowly. This indicates that the addition of flue gas makes it less easy for steam to condense.Figure 6
Condensation phenomena of steam on the surface of measuring block under different gas-water ratios. (a) is at the condition of pure steam. (b) is at the gas-water ratio of 0.5. (c) is at the gas-water ratio of 1. (d) is at the gas-water ratio of 2.
(a) (b) (c) (d)


#### 4.2.2. Heat Transfer Coefficient

Figure7 shows the changes of heat transfer coefficient under different gas-water ratios. We can see the heat transfer coefficient for the pure steam was higher than that for the steam-flue gas mixture at the same subcooling. Besides, with the increase of gas proportion in the steam-gas mixture, the heat transfer coefficient decreased. This indicates flue gas can inhibit the heat transfer of steam. Because of the decline of heat transfer after flue gas addition, less heat was transferred from steam to the copper block at the same condition. Steam can remain in the vapor state with more heat. So it is less likely for steam to condense in the presence of flue gas, and its condensation pattern is film condensation.Figure 7
Changes of heat transfer coefficient with surface subcooling at different gas-water ratios.


#### 4.2.3. Mechanism of Flue Gas on Steam Heat Transfer in Flow

Figure8 schematically describes the effect of flue gas on steam condensation and heat transfer. For the pure steam, as shown in Figure 8(a), steam contacts with the cold body directly. Steam transfers the heat and then condenses. However, for the steam-flue gas mixture, both steam and flue gas contact with the cold body. The steam can condense and then fall off, while flue gas accumulates on the surface of cold body due to its incondensability. Macroscopically, the accumulated flue gas can be regarded as a gas film, as shown in Figure 8(b). The gas film that located between steam and cold body can increase the heat transfer resistance and hinder the move of steam to the cold body. Therefore, the addition of flue gas is unfavorable to the heat transfer between steam and cold body.Figure 8
Schematic diagram of steam condensation with or without flue gas.


### 4.3. Roles of Flue Gas in Expanding Thermal Swept Scope for Multithermal Fluid Flooding

Flue gas plays two roles in multithermal fluid flooding, which result in the expansion of thermal swept scope:(1) Flue gas has a better flow ability in porous media compared with steam. Its flow resistance is low. When steam and flue gas flow together, gas fingering usually happens, even gas channeling. The flow of flue gas will open a way for the subsequent steam and decrease the flow resistance. Subsequent steam can flow quickly and decrease heat loss in the flow process. So more heat can be carried by steam further.(2) Flue gas can inhibit the heat transfer between steam and rock matrix. As is well-known, when steam flows in the porous media, a part of lateral heat is transferred to the rock matrix and the left heat is carried further, as shown in Figure9. For the pure steam, its heat transfer coefficient is high. An intense heat transfer happens in the pure steam flow. Quantities of lateral heat can be transferred to the rock matrix, resulting in less heat transferred further. For the multithermal fluid, gas film can form on the surface of porous media. This can increase the heat transfer resistance and decrease the heat transfer efficiency. The amount of heat transferred to the rock matrix will decrease. As a result, steam can remain in the vapor state and flow further with more heat transferred. So multithermal fluid can expand the thermal swept scope compared with steam.Figure 9
Schematic diagram of flow and heat transfer for pure steam and multithermal fluid.


## 4.1. Effect of Flue Gas on Thermal Swept Scope for Multithermal Fluid Flooding

Figure4 shows the changes of temperatures with the time under different experimental schemes. We can see at the position 5cm from the entrance that the temperatures were almost the same. But at the positions 30cm and 55cm from the entrance, the temperature for steam flooding was lower than that for multithermal fluid flooding. With the increase of the distance from the entrance, the temperature difference increased gradually. Furthermore, with the increase of flue gas injection rate in multithermal fluid flooding, the improvement also increased gradually. Figure 5 compares the temperature distribution along the sandpack under different experimental schemes. It is evident that the multithermal fluid flooding could expand the thermal swept scope significantly. The flooding experiments indicate, at the same heat injection condition, the addition of flue gas can promote steam to transfer more heat further.Figure 4
Changes of temperatures with the time under different experimental schemes. (a) is at the position 5cm from the entrance. (b) is at the position 30cm from the entrance. (c) is at the position 55cm from the entrance.
(a) (b) (c)Figure 5
Temperature distribution along the sandpack under different experimental schemes.


## 4.2. Effect of Flue Gas on the Steam Heat Transfer in Flow

### 4.2.1. Condensation Pattern

Figure6 compares the observed condensation phenomena at different gas-water ratios. We can see the condensate showed different forms. For pure steam, steam mainly condensed in the form of droplet, as shown in Figure 6(a). When the gas-water ratio was 0.5, a small amount of water film appeared. At the gas-water ratio of 2 and 5, most of the condensate was in the form of film. The condensation pattern in Figure 6(a) can be called drop condensation, while the condensation pattern in Figures 6(c) and 6(d) can be called film condensation. This indicates, with the increase of gas proportion in the steam-flue gas mixture, the condensation pattern was transformed from drop condensation to film condensation gradually. Meanwhile, we also found the drop frequency of condensate was different. For the pure steam, the condensate droplet fell off frequently and then new droplet formed soon. For the film condensation, condensate film formed and then fell slowly. This indicates that the addition of flue gas makes it less easy for steam to condense.Figure 6
Condensation phenomena of steam on the surface of measuring block under different gas-water ratios. (a) is at the condition of pure steam. (b) is at the gas-water ratio of 0.5. (c) is at the gas-water ratio of 1. (d) is at the gas-water ratio of 2.
(a) (b) (c) (d)


### 4.2.2. Heat Transfer Coefficient

Figure7 shows the changes of heat transfer coefficient under different gas-water ratios. We can see the heat transfer coefficient for the pure steam was higher than that for the steam-flue gas mixture at the same subcooling. Besides, with the increase of gas proportion in the steam-gas mixture, the heat transfer coefficient decreased. This indicates flue gas can inhibit the heat transfer of steam. Because of the decline of heat transfer after flue gas addition, less heat was transferred from steam to the copper block at the same condition. Steam can remain in the vapor state with more heat. So it is less likely for steam to condense in the presence of flue gas, and its condensation pattern is film condensation.Figure 7
Changes of heat transfer coefficient with surface subcooling at different gas-water ratios.


### 4.2.3. Mechanism of Flue Gas on Steam Heat Transfer in Flow

Figure8 schematically describes the effect of flue gas on steam condensation and heat transfer. For the pure steam, as shown in Figure 8(a), steam contacts with the cold body directly. Steam transfers the heat and then condenses. However, for the steam-flue gas mixture, both steam and flue gas contact with the cold body. The steam can condense and then fall off, while flue gas accumulates on the surface of cold body due to its incondensability. Macroscopically, the accumulated flue gas can be regarded as a gas film, as shown in Figure 8(b). The gas film that located between steam and cold body can increase the heat transfer resistance and hinder the move of steam to the cold body. Therefore, the addition of flue gas is unfavorable to the heat transfer between steam and cold body.Figure 8
Schematic diagram of steam condensation with or without flue gas.


## 4.2.1. Condensation Pattern

Figure6 compares the observed condensation phenomena at different gas-water ratios. We can see the condensate showed different forms. For pure steam, steam mainly condensed in the form of droplet, as shown in Figure 6(a). When the gas-water ratio was 0.5, a small amount of water film appeared. At the gas-water ratio of 2 and 5, most of the condensate was in the form of film. The condensation pattern in Figure 6(a) can be called drop condensation, while the condensation pattern in Figures 6(c) and 6(d) can be called film condensation. This indicates, with the increase of gas proportion in the steam-flue gas mixture, the condensation pattern was transformed from drop condensation to film condensation gradually. Meanwhile, we also found the drop frequency of condensate was different. For the pure steam, the condensate droplet fell off frequently and then new droplet formed soon. For the film condensation, condensate film formed and then fell slowly. This indicates that the addition of flue gas makes it less easy for steam to condense.Figure 6
Condensation phenomena of steam on the surface of measuring block under different gas-water ratios. (a) is at the condition of pure steam. (b) is at the gas-water ratio of 0.5. (c) is at the gas-water ratio of 1. (d) is at the gas-water ratio of 2.
(a) (b) (c) (d)


## 4.2.2. Heat Transfer Coefficient

Figure7 shows the changes of heat transfer coefficient under different gas-water ratios. We can see the heat transfer coefficient for the pure steam was higher than that for the steam-flue gas mixture at the same subcooling. Besides, with the increase of gas proportion in the steam-gas mixture, the heat transfer coefficient decreased. This indicates flue gas can inhibit the heat transfer of steam. Because of the decline of heat transfer after flue gas addition, less heat was transferred from steam to the copper block at the same condition. Steam can remain in the vapor state with more heat. So it is less likely for steam to condense in the presence of flue gas, and its condensation pattern is film condensation.Figure 7
Changes of heat transfer coefficient with surface subcooling at different gas-water ratios.


## 4.2.3. Mechanism of Flue Gas on Steam Heat Transfer in Flow

Figure8 schematically describes the effect of flue gas on steam condensation and heat transfer. For the pure steam, as shown in Figure 8(a), steam contacts with the cold body directly. Steam transfers the heat and then condenses. However, for the steam-flue gas mixture, both steam and flue gas contact with the cold body. The steam can condense and then fall off, while flue gas accumulates on the surface of cold body due to its incondensability. Macroscopically, the accumulated flue gas can be regarded as a gas film, as shown in Figure 8(b). The gas film that located between steam and cold body can increase the heat transfer resistance and hinder the move of steam to the cold body. Therefore, the addition of flue gas is unfavorable to the heat transfer between steam and cold body.Figure 8
Schematic diagram of steam condensation with or without flue gas.


## 4.3. Roles of Flue Gas in Expanding Thermal Swept Scope for Multithermal Fluid Flooding

Flue gas plays two roles in multithermal fluid flooding, which result in the expansion of thermal swept scope:(1) Flue gas has a better flow ability in porous media compared with steam. Its flow resistance is low. When steam and flue gas flow together, gas fingering usually happens, even gas channeling. The flow of flue gas will open a way for the subsequent steam and decrease the flow resistance. Subsequent steam can flow quickly and decrease heat loss in the flow process. So more heat can be carried by steam further.(2) Flue gas can inhibit the heat transfer between steam and rock matrix. As is well-known, when steam flows in the porous media, a part of lateral heat is transferred to the rock matrix and the left heat is carried further, as shown in Figure9. For the pure steam, its heat transfer coefficient is high. An intense heat transfer happens in the pure steam flow. Quantities of lateral heat can be transferred to the rock matrix, resulting in less heat transferred further. For the multithermal fluid, gas film can form on the surface of porous media. This can increase the heat transfer resistance and decrease the heat transfer efficiency. The amount of heat transferred to the rock matrix will decrease. As a result, steam can remain in the vapor state and flow further with more heat transferred. So multithermal fluid can expand the thermal swept scope compared with steam.Figure 9
Schematic diagram of flow and heat transfer for pure steam and multithermal fluid.


## 5. Conclusions

Based on the experiments conducted in this work and keeping in mind the limitations and assumptions made, the following conclusions are drawn:(1) Flue gas can inhibit the heat transfer between steam and cold body. With the increase of gas proportion in the steam-flue gas mixture, the heat transfer coefficient decreases, and the condensation pattern was transformed from drop condensation to film condensation. This is because gas film forming on the surface of the cold body increases the heat transfer resistance.(2) At the same heat injection condition, the thermal swept scope for multithermal fluid flooding is much larger than that for steam flooding. Flue gas plays two roles in expanding the thermal swept scope. The flow of flue gas can open a way for subsequent steam and accelerate steam flow. Meanwhile, flue gas can inhibit the heat transfer between steam and rock and thus promote steam to carry more heat further.



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