Abstract
Steam-assisted gravity drainage (SAGD) is one of the steam injection techniques to exploit heavy oil and extra heavy oil resources, where the nature of steam is crucial to the production efficiency. Replacing saturated steam with superheated steam can effectively improve the steam quality at the bottom of the well and the production efficiency. In this study, based on the 2-D SAGD experiments, the recovery mechanisms of SAGD under the 220°C saturated steam and 260°C (superheated degree of 40°C) and 300°C (superheated degree of 80°C) superheated steam are compared and analyzed. The numerical model was developed based on experimental results, and the influence of steam superheated degree on the recovery degree of the SAGD process was further investigated. The physical experiment results and numerical simulation results show that the advantages of high enthalpy and large specific volume of superheated steam are significant at the horizontal expansion stage of the steam chamber stage compared to those of saturated steam. However, although the superheated steam can improve the recovery degree, the economic efficiency may decrease with the addition of superheated steam since it requires higher energy to generate the superheated steam. Thus, the SOR (steam-oil ratio) cannot appropriately describe the energy and economic efficiency when superheated steam is considered. Therefore, the cumulative FOR (fuel-oil ratio) is proposed, and the optimal superheated degree, optimal injection strategy, and its relation with the recovery mechanisms are studied. The results indicate that using superheated steam at 80°C superheated degree during the steam chamber horizontal expansion stage can increase the recovery factor around 12% and also reduce the cumulative FOR around 5.3 compared to the conventional SAGD strategy.
1. Introduction
With the increasing demand for oil resources, unconventional oil resources have gained more attention [1, 2]. Heavy oil is one of the unconventional resources which is characterized by its high viscosity at reservoir temperature. The thermal EOR (enhanced oil recovery) technology has been widely applied to develop the heavy oil reservoirs [3, 4]. In the 1980s, Butler [5] proposed the SAGD (steam-assisted gravity drainage) technique, which is considered an efficient and commercially successful technology for heavy oil development. The main mechanism of SAGD relies on the gravity force which is caused by the density difference between steam and heavy oil. During the SAGD process, the steam is injected from the bottom of the reservoir and rises to the top to form a steam chamber. The heavy oil viscosity is reduced by heat exchange with steam, and the heated oil and condensed water are produced with the aid of gravity forces. The SAGD technology is different from steam flooding. During the steam flooding process, oil viscosity is still high except which in contact with steam, resulting in a high-pressure gradient between the production well and the injection well. However, the SAGD technology mainly relies on the gravity force, where the oil around the steam chamber maintains at the heated state and thus can be easily produced. With the expansion of the steam chamber volume, the oil flowing rate is increased as well [6, 7]. One of the critical factors for SAGD is to maintain the steam chamber temperature. In order to enhance the steam quality, superheated steam can be used to replace the saturated steam.
The mechanism of superheated steam to enhance oil recovery mainly includes the following four aspects: viscosity reduction, distillation, aquathermolysis reaction, and reservoir reconstruction. The mechanism of viscosity reduction by superheated steam at high temperature has been fully studied. Most experimental results show that the viscosity reduction effect of superheated steam is better than that of ordinary saturated steam. The higher the superheated degree, the more obvious the decrease in crude oil viscosity [8–14]. In terms of the distillation, the light components account for most of the distillation products, and the heavy components are still liquid [15–21]. Butler [22] shows that the steam superheated degree and light component content are very important to steam distillation rate. Vafaei et al. [23] show that the superheated steam can greatly improve the steam distillation rate, and the higher the content of light components in crude oil components, the stronger the distillation effect. In terms of the aquathermolysis reaction, compared with ordinary steam, superheated steam of the same quality can bring more heat into the formation and provide better temperature conditions, and the aquathermolysis reaction of heavy oil is more intense. Through the experiment of heavy oil aquathermolysis reaction, Clark and Hyne [24], Muraza [25], and Wu et al. [26] show that under high-temperature conditions, after the pyrolysis reaction between heavy oil and steam, the chain breaks occur in the molecules of heavy oil components, the light hydrocarbons increase, and the heavy hydrocarbons decrease. The content of carbon atoms decreased, and the content of hydrogen atoms increased significantly. Lamoureux-Var and Lorant [27] and Liu et al. [28] believe that when the ordinary steam is injected, the aquathermolysis reaction of heavy oil is generally not very intense, so the aquathermolysis reaction is also an important mechanism for the difference between superheated steam and ordinary steam. In terms of the reservoir reconstruction, Fan [29] scanned the microstructure of oil sand before and after the action of superheated steam by using an electron microscope. The comparison of pore structure distribution shows that the rock pore surface after the action of superheated steam is flatter and smoother. When superheated steam is injected into the formation, strong scouring of reservoir rocks at high temperature will cause migration and cracking of reservoir rock mineral particles and even dissolution of soil minerals, smooth the reservoir rock pore throat channel, improve reservoir rock structure, improve reservoir rock permeability, and reduce residual oil saturation [30].
The superheated SAGD technique has been extensively studied. Badea and Daripa [31] studied the recovery efficiency of the superheated SAGD and conventional SAGD; the results show that the superheated steam can lower the steam-oil ratio. Fredman [32] conducted numerical simulation on the huff and puff preheating stage of SAGD using superheated steam. The results show that the cumulative oil production increases with the increase in the superheated degree, which was mainly due to the steam chamber expansion and reservoir temperature increment. Wu et al. [33] also studied the mechanisms of superheated steam enhanced oil recovery. They stated that the greater heat enthalpy and larger specific volume and latent heat of vaporization are the main characteristics for superheated steam, which result in higher production and economic performance. Also, the superheated steam is often used to replace the steam in the late stage of SAGD. There are many studies similar to the above scholars, most of which are based on the numerical simulation experiments. The conclusions are that superheated steam can significantly reduce steam consumption and improve economic benefits.
The economic benefits of superheated steam are also widely studied. Gates and Larter [34] proposed a thermal efficiency parameter for the steam injection technique, which is the ratio of theoretical SOR to actual SOR. The theoretical SOR is the ratio of the equivalent amount of steam required to raise the temperature from reservoir temperature to steam temperature to the oil volume. It was stated that the average thermal efficiency of actual SAGD projects is about 30%. Alharthy et al. [35] used energy gain to evaluate the recovery efficiency for the SAGD process, where the energy gain is defined as the ratio of the energy produced to the energy injected. The concept of energy gain is more capable of reflecting the energy efficiency of the SAGD process than SOR. Pinto et al. [36] also used the ratio of the energy produced to the energy injected to evaluate the SAGD performance. They also claimed that using energy gain can better reflect the energy efficiency of the SAGD process. Wang [37] used the ratio of the difference between injected steam energy and produced water energy to the injected steam energy. In order to evaluate the energy efficiency for the superheated SAGD process, the ratio of the energy produced to the energy injected is adopted rather than SOR. Although the studies have proven that superheated steam has a stronger potential to improve heavy oil recovery compared with saturated steam, whether superheated steam has an economic advantage compared with saturated steam is worthier of attention and answer. The SOR is often used as a key economic indicator for the steam recovery process. However, it cannot reflect the actual energy efficiency when it comes to the superheated steam. For instance, although the SOR value of superheated steam is lower than that of saturated steam because superheated steam recovers more oil than saturated steam, the energy consumption required to produce superheated steam is also higher than that required to produce saturated steam. Therefore, Liang et al. [38] and Yuan et al. [39] used the cumulative enthalpy of injected steam to represent the cost instead of the volume of injected steam. They proposed the cumulative enthalpy-oil ratio (EOR) to evaluate the cost of superheated steam. However, using cumulative EOR is not very intuitive to evaluate the energy efficiency and cannot be very convenient to help the on-site decision-making; it is necessary to use another indicator to evaluate the energy efficiency of superheated steam recovery.
Based on the 2-D SAGD experiments, this paper compares and analyzes the recovery mechanisms of SAGD under 220°C saturated steam and 260°C (superheated degree of 40°C) and 300°C (superheated degree of 80°C) superheated steam. The numerical model was developed which was verified based on experimental results, and the influence of superheat degrees on SAGD and detailed recovery mechanisms were further investigated. The cumulative fuel-oil ratio (FOR) is proposed to investigate the optimal superheat degree and its relation with the recovery mechanisms. Also, the optimal superheated steam and saturated steam combination strategy is investigated, and the superheated steam enhancing recovery mechanisms for the SAGD technique is proposed.
2. SAGD Physical Experiment Study
2.1. Physical Experiment System
The SAGD physical experiments are carried out through a 2-D SAGD system. The maximum internal temperature tolerance is 450°C. The size of the model is , and the maximum working pressure of the model is 25000 kPa. The physical simulation system is shown in Figure 1, and the model is displayed in Figure 2.
(a) The appearance of the model
(b) The interior of the model and the location of the wells
Thermal insulation materials are coated on the inner surface of the model to reduce heat loss. The quartz sand is used to simulate the reservoir rock. The pressure and temperature monitoring probes are placed evenly in the model, which connect to the computer to show the real-time temperature and pressure distribution. The steam generator in Figure 1 can produce the saturated steam; then, the saturated steam will be heated to superheated steam by the steam heater. When steam reaches the experimental condition, it will be injected into the SAGD physical simulation model. The heavy oil used in the experiments is from Xinjiang, China, and the API is 10. Figure 3 shows the sample of oil sand filled in the physical experiment.
2.2. Experimental Method
The experiment is based on a SAGD well group in Xinjiang, and according to the SAGD similarity rule, the laboratory-scale parameters are obtained, which are shown in Table 1.
Table 2 shows the experimental schemes. The operating pressure was 2350 kPa, and the steam temperature was 220°C (saturated steam), 260°C (superheated degree of 40°C), and 300°C (superheated degree of 80°C), respectively.
2.3. Experimental Procedures
The experimental procedures are as follows: (1)Material preparation. Prepare quartz sand and crude oil according to the experimental design. Check all devices and equipment to ensure they are in good condition(2)Model filling. Fill the model with mixed oil sand at a designed ratio and compact. When the oil sand is filled, fill the clay into the model to simulate the overburden rock(3)Check the air tightness of the model. Pressurize the model with nitrogen, stabilize the pressure at 5000 kPa and maintain it for more than 24 h, and use a surfactant to detect whether there is air leakage at each outlet port of the model(4)Model initialization. Put the model in the oven and heat the model to make the whole model temperature reach 90°C(5)Experimental operation. During the experiment, the monitoring system will monitor the temperature and pressure of the model body, steam generator outlet, and thermostat in real time. The production fluid collection system will collect and measure the produced liquid. The experiment will stop when the SOR is 10
3. Experimental Results of SAGD Physical Simulation
3.1. 220°C Saturated SAGD
Figures 4–7 show the temperature distribution, the pressure distribution, and the production performance of the 220°C saturated SAGD experiment. Based on the results of the experiment, the SAGD process can be divided into three stages: rising stage, stable production stage, and falling stage. In the rising stage, the steam rapidly moves upward due to density differences and the entire steam chamber has a small contact area with crude oil, so the oil is produced mainly from the area between the injection well and the production well, which corresponds to the fact that the average water cut is about 50% and the recovery degree is 11.1%. In the stable production stage, the steam chamber reaches the overburden layer when the peak oil production rate was reached. And then the steam chamber begins to horizontally expand, which lasted about 35 min. The average water cut is 81%, and the average SOR is 4.6. Moreover, the average oil production rate is 8 cm3/min, and the recovery degree is 18.3%. In the falling stage, the steam chamber expands to the both sides of the boundary. With the injection of steam, the steam chamber slowly falls downward along the boundary. The average water cut increases to around 90%, and the oil production rate decreases rapidly. The recovery degree is 8.9%. The final recovery degree of this experiment is 38.3%, and the cumulative SOR is 4.3. The SOR continues to rise during the experiment, and it eventually rises to 10 at the end of this experiment.
(a) Temperature distribution at the initial stage (220°C)
(b) Temperature distribution at the maximum oil production rate (220°C)
(c) Temperature distribution when the SOR is 10 (220°C)
3.2. 260°C Superheated SAGD
It can be observed from Figure 8 that the final recovery degree of 260°C superheated SAGD is about 44.8%, which is about 6.5% higher than that of 220°C saturated SAGD and the SOR is lower. Obviously, the SAGD process is enhanced significantly when the superheated steam is used. Figures 8–11 show the temperature distribution, the pressure distribution, and the production performance of the 260°C superheated SAGD experiment. It can be seen that the steam chamber development pattern for 220°C saturated SAGD and 260°C superheated SAGD is similar, which can also be divided into three stages. As shown in Figure 9(b), the steam chamber is larger and the temperature of the steam chamber is higher, which is caused by the fact that superheated steam carries more heat and has a higher temperature. Moreover, it can be proven from Figures 9(c) and 10 that superheated steam exists in the steam chamber. Due to the promotion of the steam chamber by superheated steam, the maximum oil production rate of 260°C superheated SAGD is higher, which reaches 37 cm3/min. Also, it is observed from Figure 12 that 260°C superheated SAGD has a higher average oil production rate and a lower average water cut during the stable production stage.
(a) Temperature distribution at the end of preheating (260°C)
(b) Temperature distribution at the maximum oil production rate (260°C)
(c) Temperature distribution when the SOR is 10 (260°C)
3.3. 300°C Superheated SAGD
It can be found from Figure 13 that the final recovery degree of 300°C superheated SAGD is 49.6%, which is 4.8% higher than that of 260°C superheated SAGD. And, it is worth noting that the production time of 300°C superheated SAGD is significantly longer. Distinctly, the increase in superheated degree of the superheated steam improves the SAGD production performance. Figures 14–17 show the temperature distribution, the pressure distribution, and the production performance of the 300°C superheated SAGD experiment. It can be seen that the average temperature of the steam chamber is only about 280°C at the end of the experiment, which is due to the heat loss of the superheated steam which is significantly higher than that of 220°C saturated SAGD. Compared with 220°C saturated SAGD and 260°C superheated SAGD, the steam chamber of 300°C superheated SAGD is much larger and the temperature in the center of the steam chamber is higher. What is more, based on Figures 14(c) and 15, it can be proven that the superheated degree in the steam chamber of 300°C superheated SAGD is higher than that of 260°C superheated SAGD. And it can be seen from Figures 16 and 17 that the stable production stage lasts about 107 min, which is much longer than that of the two previous experiments. So, it is obvious that the increase in the superheated degree has a great potential to improve the degree of reservoir exploitation.
(a) Temperature distribution at the end of preheating (300°C)
(b) Temperature distribution at the maximum oil production rate (300°C)
(c) Temperature distribution when the SOR is 10 (300°C)
3.4. Summary of Physics Experiment
Figure 18 shows the recovery degrees at different stages for SAGD tests. It shows that the horizontal expansion stage of the steam chamber in the SAGD process contributes the most to the final recovery degree. Also, the superheated steam has an obvious improvement in the recovery degree. When the 220°C saturated steam is converted to the 260°C superheated steam, the recovery degree at the horizontal expansion stage increased by 6.4%. When the 300°C superheated steam is used, the recovery degree at the horizontal expansion stage is 10.4% higher than that of the 220°C saturated steam. This is because when the steam chamber rises to the top, as the steam chamber expands, the pressure of the steam chamber decreases, and the advantages of large enthalpy and large specific volume of superheated steam are fully utilized. However, Figure 18 shows that as the steam temperature rises, the recovery degree decreases during the falling stage. This is because the superheated steam can advance the peak of oil production, a large amount of crude oil has been extracted in the previous stages, and reservoir is depleted later. Table 3 shows the cumulative steam injection, cumulative oil production, and cumulative SOR of the three experiments. It shows that the cumulative oil production increases with the increase in superheated degree, while the cumulative SOR is also gradually increasing. This is because the heat loss of the physical simulation experiment is more than that of the actual oil reservoir, which caused a higher cumulative SOR. In particular, in the superheated steam experiments, the temperature of the steam chamber is higher; the heat loss is more than that of the saturated steam experiment. Although the advantages of superheated steam are also reflected in the results of laboratory experiments, the advantages of superheated steam in actual oil reservoirs will be more obvious.
In summary, physical experiments prove that the superheated steam has the ability to enhance oil recovery and show that the contribution of superheated steam to enhancing oil recovery is mainly at the horizontal expansion stage of the steam chamber. However, due to the limitation of physical experimental equipment and huge heat loss, the amount of steam injected in the superheated steam experiments is too large. The cumulative SOR cannot truly reflect the effect of superheated steam in the actual reservoir. Therefore, the following study will use CMG to establish a numerical model to study the steam chamber development and energy efficiency of superheated SAGD and select the best steam injection scheme for superheated SAGD.
4. SAGD Numerical Simulation Experiment
4.1. Establishment of the Numerical Model
Based on the physical simulation experiments, a two-dimensional numerical simulation model was established through the STARS module of CMG. The grid design of the numerical model is , the grid size is , the porosity is 0.32, and the oil saturation is 0.8. The model is a homogeneous model, and the permeability in all directions is equal. Figure 19 shows the - view of the model, and the green dots indicate the locations of the wells. The distance between the wells is 1.5 cm. Table 4 shows the physical property parameter of the numerical model. The physical property parameters of the rock and fluids are shown in Table 5, and Figure 20 shows the physical property curve of the fluid.
(a) Permeability curve
(b) Viscosity vs. temperature curve of the oil
4.2. The Effect of the Superheated Degree on SAGD
In this section, the 220°C saturated SAGD experiments and the superheated SAGD experiments with superheated degrees of 20°C, 40°C, 60°C, 80°C, and 100°C were set to study the effect of the superheated degree on SAGD. The steam chamber development and production performance under different superheated degrees were investigated. Moreover, the effect of the superheated degree on the cumulative SOR was obtained.
Figure 21 shows the cumulative SOR under different superheated degrees. It is observed that the cumulative SOR decreased with the increase in the superheated degree. In particular, the cumulative SOR decreases greatly from 220°C saturated SAGD to 320°C superheated SAGD, which is reduced by 10.0%. Besides, Figures 22–25 show the temperature distribution and the production performance of the numerical simulation experiments. In the rising stage, it can be seen that the increase in the superheated degree does not improve the SAGD performance, which corresponds to the same average oil production rate, steam chamber volume, and steam injection rate of the saturated SAGD and the superheated SAGD. However, in the horizontal expansion stage, it is obvious that the average oil production rate of superheated SAGD is significantly higher than that of saturated SAGD. Among them, the oil production rate of 320°C superheated SAGD reaches 21.5 cm3/min at the highest, and the average oil production rate of 220°C saturated SAGD is 15.5 cm3/min. It can be seen from Figure 24 that the volume of the steam chamber in the superheated SAGD is significantly larger than that of the saturated SAGD. Nevertheless, Figure 25 shows that the average steam injection rate of saturated SAGD is 36 cm3/min, while the minimum steam injection rate of superheated SAGD is 27.5 cm3/min, which indicates that the use of superheated steam can significantly reduce the amount of steam injection. This is caused by the fact that the same volume of superheated steam can carry more heat. Obviously, it can be found that the superheated steam can greatly improve SAGD performance during the horizontal expansion stage, and this improvement continues to the falling stage.
(a) Rising stage (8th minute)
(b) Horizontal expansion stage (23th minute)
(c) Falling stage (42th minute)
In a word, numerical simulation results show that the use of superheated steam can greatly reduce the cumulative SOR. Moreover, the increase in the superheated degree does not improve the SAGD performance during the rising stage. However, in the horizontal expansion stage, the superheated steam can greatly save the steam injection and improve the SAGD performance significantly. The SAGD performance improves more obviously with the increase in the superheated degree. In the falling stage, the superheated steam can also improve the SAGD performance, but the rate of steam injection is higher, which is due to the more heat loss of the superheated steam.
4.3. Optimization of the Steam Injection Strategy for Superheated SAGD
In this section, numerical simulation experiments are set to optimize the steam injection strategy for superheated SAGD. So, the purpose of this experiment is to study at which stage superheated steam can be used to obtain the greatest economic benefit. As shown in Table 6, case 1 and case 4 can show the effect of the superheated steam only used during the horizontal expansion stage, case 2 and case 3 can show the effect of the superheated steam only used during the rising stage, and case 3 and case 4 can show the effect of the superheated steam only used during the falling stage. Moreover, the effect of the superheated steam used during the horizontal expansion stage and the falling stage can be observed from case 1 and case 3. All tests were performed under 320°C superheated steam. The simulation will stop when the SOR is 10.
Based on Figure 26, the ratio of the volume of fuel consumed to the volume of steam produced in each case was calculated, and the results are shown in Figure 27. It is obvious that using cumulative SOR to evaluate the economic benefits of each case is not applicable, because the production costs of saturated steam and superheated steam are not the same. It can be found from Figure 27 that the cumulative FOR can better reflect the actual economic benefits. Based on the cumulative FOR, two key conclusions can be drawn. First, the use of the superheated steam does not greatly improve economic efficiency as shown by cumulative SOR. Compared with case 1, the cumulative SOR of case 2 is reduced by 10.0%, but the cumulative FOR is only reduced by 1.6%. Second, the cumulative FOR of case 4 is the lowest, indicating that the economic benefits of using superheated steam only during the horizontal expansion stage are higher than those of using superheated steam all the time.
Figures 28 and 29 show the oil production rate and the steam chamber volume of different cases. It can be observed that the average oil production rates during the horizontal expansion stage of case 3 and case 4 increased rapidly after injecting 320°C superheated steam during the horizontal expansion stage, increasing from 14.6 cm3/min to 19.5 cm3/min. Figure 30 shows that the steam chamber is fully expanded due to the high enthalpy and large specific volume of the superheated steam after injection of superheated steam. At the end of the horizontal expansion stage, the volume of the steam chamber of case 1 is only 290 cm3, and the volume of the steam chamber of case 3 and case 4 is both 360 cm3. In the falling stage, case 3 used 320°C superheated steam at this stage and case 4 used 220°C saturated steam. Apparently, it can be found from Figures 28 and 29 that the use of superheated steam at the falling stage has little effect on SAGD production.
To sum up, the best use time of superheated steam is during the horizontal expansion stage of the steam chamber, which corresponds to the fact that the volume of the steam chamber increases rapidly and the advantage of superheated steam is significant. Moreover, the superheated degree needs to be determined by the FOR again.
Next, the superheated degree of the steam used during the horizontal expansion stage is optimized by using cumulative FOR. Table 7 shows the optimal schemes of the superheated degree. It can be found from Figure 30 that as the temperature of the superheated steam rises, the cumulative FOR first decreases and then rises, which is due to the fact that the steam chamber becomes larger and larger with the increase in superheated degree during the horizontal expansion stage, and more oil can be produced. However, the increase in the superheated degree increases the cost per unit of steam, so the cumulative FOR rises later. The cumulative FOR of case 8 is 248.3, which is the lowest. Therefore, based on the cumulative FOR, case 8 is the most cost-effective scheme. The cumulative FOR of case 8 is 248.3, which is the lowest.
In summary, due to the high enthalpy and large specific volume of the superheated steam, the best use time of superheated steam is during the horizontal expansion stage of the steam chamber, which corresponds to the fact that the volume of the steam chamber increases rapidly and the advantage of superheated steam is significant. Compared with the traditional cumulative SOR, the cumulative FOR can better reflect the actual economic benefits of SAGD production and provide suggestions for the formulation of making SAGD program schemes.
5. Conclusions
(1)The superheated steam can significantly enhance the recovery factor compared to the saturated steam, especially during the steam chamber horizontal expansion stage. Also, the recovery factor increases with the increment of superheat degree; when superheated degree 80°C is used, around 10.4% recovery degree increment is reached(2)Although the superheated steam can improve the recovery degree, the economic efficiency may decrease with the addition of superheated steam. Therefore, it is suggested to use the FOR to optimize the injection strategy, where the FOR can more directly show the energy cost in the production process and optimize the steam injection strategy(3)Compared to the conventional SAGD operation, the optimized superheated SAGD strategy can increase recovery degree around 12% and reduce FOR around 5.3
Data Availability
The data of the research results of this paper are reflected in this paper.
Conflicts of Interest
The authors declare no conflict of interest regarding the publication of this manuscript.