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An experimental and numerical study of a steam chamber and production characteristics of SAGD considering multiple barrier layers
Journal of Petroleum Science and Engineering 180 (2019) 716–726
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An experimental and numerical study of a steam chamber and production characteristics of SAGD considering multiple barrier layers
T
Shijun Huanga,b, Lijie Yangb,∗, Yun Xiab, Mengge Dub, Yanwei Yangb a b
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China Department of Petroleum Engineering, China University of Petroleum, Beijing, 102249, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Steam-assisted gravity drainage (SAGD) Multiple barrier layers Steam chamber Production characteristics Combinations of barrier layers
Steam-assisted gravity drainage (SAGD) is an efficient technology that has been used to develop oil sand resources, and it has been successfully and maturely applied in Canada. However, oilfield production has demonstrated that reservoir heterogeneity has a serious impact on SAGD development, among which the most drastic impact is caused by multiple barrier layers in the reservoir. These barrier layers can severely impede the development of a steam chamber and the drainage of oil. Hence, it is important to investigate their influence mechanism. In this study, oil samples from the Long Lake oil field were used in laboratory experiments to study the development of the steam chamber and the residual oil distribution under the effect of multiple barrier layers. Then, a theoretical numerical simulation model was established to describe the fluid flow more precisely. In addition, the production characteristics of SAGD under the influence of different numbers of barrier layers were analyzed by comparing the development of the steam chamber. Finally, the impacts of multiple barrier layers with different modes of combinations were studied for their effects on SAGD production. The results indicated that for impermeable barrier layers of the same length, the steam primarily developed upward after flowing around the first barrier layer. After the steam chamber reached the top of the reservoir, the steam began to enter into the interbedded zone. Based on an unchanged first barrier layer, the number of barrier layers had little influence on the overall shape of the steam chamber and SAGD production, which confirmed that the first barrier layer played a dominant role in the influence of multiple barrier layers. In addition, characteristic points (P1, P2, P3), which corresponded to changes in the stage of steam chamber development, were established and used to evaluate the effect of the barrier layers. Different combinations of barrier layers were realized by changing the relative properties of the first barrier layer. The results showed that the longer the length of the first barrier layer, the closer it was to the steam injection well and the lower the permeability; hence, the more obvious its hysteresis effect on the SAGD process.
1. Introduction In recent years, due to the increasing demand for oil and a reduction in conventional oil production, more attention has been paid to heavy oil resources. It is estimated that the reserves of heavy oil are approximately six times that of conventional crude oil (Owen et al., 2010). Due to the sensitivity between oil viscosity and temperature, thermal methods, a class of enhanced oil recovery (EOR) methods, have achieved great advances in the development of heavy oil. Thermal methods mainly include cyclic steam stimulation (CSS), steam flooding (SF) and in-situ combustion (ISC), hot water flooding, and steam-assisted gravity drainage (SAGD) (Shi et al., 2017; Doranehgard et al., 2018; Li et al., 2018). In the methods that use steam as a heat carrying substance, cyclic steam stimulation is widely used for its easy operation ∗
and high early yields (Chen et al., 2019). Nevertheless, due to the limited heating radius and diminishing yield per cycle, its recovery factors are only 20%–40% of the original oil-in-place (OOIP) (Santos et al., 2014). Steam flooding is often used as a replacement during the late stage of CSS, and it can reach recovery factors of up to 60% OOIP (Huang et al., 2019). However, steam channeling easily occurs during the later stage of production due to long-term steam flooding. SAGD technology, first proposed and developed by Butler (Butler et al., 1981a, b), has been widely and maturely applied to the exploitation of oil sands in Canada (Tian et al., 2017; Baghernezhad et al., 2019). The method involves drilling a pair of parallel horizontal wells near the bottom of a reservoir and steam is injected into the formation through the upper well. Under the action of gravity, the heated crude oil and condensate flow to the lower well. With high recovery factors of 55%
Corresponding author. E-mail addresses: [email protected], [email protected] (L. Yang).
https://doi.org/10.1016/j.petrol.2019.05.062 Received 3 February 2019; Received in revised form 23 May 2019; Accepted 26 May 2019 Available online 05 June 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
Journal of Petroleum Science and Engineering 180 (2019) 716–726
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showed a relatively minimal difference in recovery. Dang et al. (2013) claimed that only long, continuous shale interlayers (larger than 50 m) would affect the SAGD performance. Furthermore, in terms of the location of the interlayer relative to the well, Shin et al. (2009), Huang et al. (2016), and Wang et al. (2018) agreed that the existence of the barrier layer between the producer and injector had a significant impact on SAGD production. Kim et al. (2017) studied the influence mechanism of a single interlayer. He was the first to associate the inflection point on the productivity curve with a steam chamber development stage. With progress in mathematical calculation methods, several scholars have used proxy models to predict the interlayer size and SAGD production under the influence of an interlayer (Zheng et al., 2016). In addition, some novel approaches, such as the use of the artificial intelligence method, have been used to analyze production data (Ma et al., 2016; Nguyen et al., 2018). Although scholars have conducted a lot of research on the effect of the barrier layer, the influence mechanism of multiple barrier layers has attracted little attention. In addition, it is not sufficient to measure the impact of the barrier layer using only some productivity indicators (such as oil recovery). In fact, the effect of the barrier layer on the shape of the steam chamber is the most direct and intuitive. In this study, the effects of multiple barrier layers on SAGD production are investigated primarily from the perspective of steam chamber development and production characteristics. First, based on existing experimental devices, two groups of physical SAGD simulations are conducted. Then, a theoretical numerical simulation is established for further study, and the flow of steam and oil is described more precisely. In addition, some characteristic points on the productivity curve are used to stage the development of the steam chamber. Finally, the effects of multiple barrier layers with different combinations on the development of the steam chamber and SAGD production are analyzed in detail.
OOIP (Santos et al., 2014), SAGD has two distinct advantages. One is that its main oil-displacement mechanism is gravity drainage, and the other is that it effectively utilizes the disadvantage of steam overlap. Many scholars have comprehensively studied and continue to investigate SAGD using different methods. The most important aspect of this method in the early stage is how to characterize the production capacity. Butler first proposed his classic formula based on Darcy's law for estimating the oil production rate (Butler et al., 1981a, b). Reis (1993) further modified the productivity formula based on the assumption that the shape of the steam chamber is an inverted triangle. Later, Wei et al. (2014) thought the shape of the steam chamber was related to the steam injection rate, and a low steam injection rate would lead to a convex parabola. By combining experimental and mathematical models, Liu et al. (2018) considered the advantages and disadvantages of different steam chamber shapes in terms of the production rate and heat consumption. It could be seen that the development of the steam chamber is an important indicator that can be used to evaluate SAGD production. With the wide applications of SAGD technology in the oil field, researchers hope to make it more efficient. The problems have been focused on primarily two aspects: (i) how to optimize the operational parameters, and (ii) the influence of geological factors. For the former, Gates et al. (2006) thought that a high injection pressure should be applied in the early stage, and this would allow the steam chamber to reach the top quickly and reduce heat loss. A thorough study regarding the injection-production parameters was conducted by Siavashi et al. (2017). The results showed that a high steam injection rate would bring more energy to the formation and promote the development of a steam chamber, but the SOR also rose. In addition, the heating range can be expanded by increasing the fluid production rate appropriately. With additional studies, the injection side became more diverse. Yuan et al. (2018) adopted an experimental study and numerical simulation method to further investigate nitrogen-assisted SAGD (NA-SAGD). The results showed that nitrogen can effectively maintain the pressure and reduce heat loss. Solvent-aided SAGD (SASAGD) was introduced for lower greenhouse gas emissions and lower steam usage (Liu et al., 2017). The impact of geological factors including water sand, fractures, and barrier layers is also significant. When there is bottom water in a formation, it can affect heat transfer in case of water coning or steam loss to bottom water (Masih et al., 2012). An effective solution is to maintain the steam chamber pressure above the aquifer pressure, but this can also lead to low oil production (Qin et al., 2014). Moreover, the effect of the top water is greater than that of the bottom water due to more severe heat loss (Law et al., 2003a, b). In fractured reservoirs, the drainage of the initial mobile water contained in a fracture makes the steam chamber primarily develop in an upward direction (Ali et al., 2012). In reality, according to the actual production process in an oil field, the existence of a barrier layer plays a more important role in how geological factors affect oil production. The barrier layer not only can affect the effective flow of oil and gas but also has a great impact on heat and mass transfer. As early as 1992, several 2D experiments were performed by Yang et al. (1992) to study the effect of horizontal layers with different steam injection positions on the SAGD production. The results showed that the effect of the layer is not significant with top steam injection, but heated oil could not be produced with bottom injection due to the steam pressure. Pooladi et al. (2002) investigated the influence of different continuities of the shale layer on SAGD in a reservoir with gas cap and top water. Due to the great distance between the shale layer and the steam injection well, the influence of the different schemes was not significant. Ipek et al. (2008) designed thirty schemes to study the influence of the shale interlayer in the reservoir on SAGD. One of his conclusions was that the higher the proportion of shale, the lower the production. Fatemi et al. (2012) investigated impact of the geometrical properties of the impermeable interlayers (such as density, location, and dispersion) on the SAGD process, but he only
2. Experiments 2.1. Experimental apparatus This experiment primarily investigated the development of the steam chamber on section. To weaken the influence of an uneven distribution of steam along the long horizontal well (Ong et al., 1990; Huang et al., 2018), a visualized two-dimensional physical model (30 cm in length, 25 cm in width, and 8 cm in depth) was developed, and the similarity criterion and parameters for the experiment were established (Huang et al., 2016; Liu et al., 2018). Fig. 1 shows the photograph and schematic diagram of the model. The production process of the SAGD was simulated by placing the experimental equipment on the vertical. The outside of the model was covered with an insulating ceramic wool with heaters in it, which was used to simulate the original reservoir temperature and reduce heat loss during the experiment. In front of the model was a transparent glass plate for visualization. Previous studies have shown that the shape of the steam chamber is symmetrical, so the horizontal well group was placed on the left side of the model to better observe changes in the shape of the steam chamber. Temperature sensors and pressure sensors were installed at the ports of the injector and producer to monitor operating conditions during the experiment. In addition, 68 temperature measuring points were arranged in the device to record the development of the steam chamber. As shown in Fig. 2, the experimental apparatus included three parts: the oil driving system, the reservoir model system, and the data collection system. 2.2. Design of the experiment Oil samples from the Long Lake oil field were used in this experiment. The content of saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene in the oil was 19.87%, 47.32%, 7.68%, and 717
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Fig. 1. The photograph and schematic of the model.
2.3.1. The pre-experiment preparation Sand packing: According to the established scheme, the four Perspex plates (barrier layers) were fixed with super glue. Then, considering the degree of compaction of the model, 40-mesh glass beads were chosen to fill into the model and the permeability was 240 D (Huang et al., 2016). Since the model would later be placed vertically, the glass beads were filled evenly and compactly. Airtightness test: After sealing the model, the model was pressed with N2, and the pressure of the model was stabilized at 0.5 MPa for 0.5 h to observe whether or not the pressure changed. Saturated water process: Deionized water was injected into the model from the lower pipeline and the water injection speed was 4 ml/ min. Then, the amount of deionized water injected into the model was recorded to measure the porosity. By assuming the density of water was 1 g/cm3, the porosity of model was calculated to be 0.415. Saturated oil process: This step was performed at 80 °C to increase the mobility of the oil. The oil injection speed was set at 3 ml/min for full saturation. When the model no longer produced water, the process was finished.
25.13% respectively. The oil viscosity at 80 °C was 560 cP, and the oil density was 1016 kg/m3. Most of the barrier layers in the Long Lake oilfield are impermeable and show parallel characteristics in the longitudinal direction. Perspex plates were set in the displacement model to simulate the impermeable barrier layers. The size of the Perspex plates was 12 cm × 1 cm × 8 cm and they were placed along the y direction of the model. In addition, the thickness of 1 cm was consistent with the actual barrier layer thickness of 1–2 m. Two experimental schemes were designed, as shown in Fig. 3. Pattern A was a homogeneous formation that was used as the control for comparison. Four parallel barrier layers were placed above the steam injection well in Pattern B. Table 1 shows the conditions of the different patterns.
2.3. Experimental procedures and control conditions The actual production of SAGD primarily includes two stages: the preheating stage and the formal production stage. Therefore, the physical simulation process was divided into three parts: pre-experiment preparation, preheating, and the formal production.
2.3.2. SAGD experiment To simulate the preheating process, a heating pipe with diameter of
Fig. 2. Schematic of the experimental apparatus. 718
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Fig. 3. Schematic of the experimental schemes.
influence of a single barrier layer that had been previously studied (Huang et al., 2016). Due to the blocking effect, the steam accumulated below the first barrier layer and heated the interbedded zone due to heat conduction. However, after the steam flowed around the first barrier layer, the development of the steam chamber began to differ. A certain shape was formed after flowing around the first barrier layer. In addition, since the length of each barrier layer was equal, the steam chamber remained very regular during the upward development. Because of the effect of multiple barrier layers, the heat conduction of the steam was weakened, and a low temperature zone formed in the interbedded zone. After the steam chamber reached the top of the model, the steam began to enter the interbedded zone during the second lateral expansion process.
Table 1 Conditions of different patterns. Scheme
Formation condition
Formation texture
Barrier texture
A B
No barrier layer Four barrier layers
Glass beads Glass beads
– Perspex plates
3 mm was wrapped around the outside of the producer and injector. Prior to the formal production, high temperature steam was injected into the heating pipe to heat the area near the wells. The process was switched to the formal production stage until the temperature of the formation between the two wells reached 70 °C. During the formal production stage, steam was injected into the upper well, and heated oil and condensate water flowed into the lower well. The temperature of the injected steam was maintained at 115–120 °C. To control for steam breakthrough, the rate of steam injection was constantly adjusted to ensure that the temperature difference between the injector and producer was 5–35 °C (Huang et al., 2016).
2.4.2. The distribution of the residual oil Fig. 5 shows the location of the residual oil of pattern B. The shape of the low oil saturation area is basically the same as that of the steam chamber. Due to the gravitational difference, most of the oil remained below the steam chamber. The oil between the barrier layers could barely flow during the early stage. As the steam entered the interbedded zone and heated the oil, the oil had a sharply decreased viscosity and discharged at the edge of the zone. However, most of the oil was trapped in the zone due to the hindrance of multiple barrier layers.
2.4. Results and discussion 2.4.1. The development of the steam chamber Fig. 4 shows the temperature field for different patterns at different times. Under the condition of pattern A, the steam chamber primarily developed upward during the early stage, and then began to expand laterally after reaching the top of the model. This pattern was consistent with the development characteristics of steam chambers during SAGD production in homogeneous reservoirs (Butler et al., 1981; Wei et al., 2014). In the case of multiple barrier layers, the steam chamber was first affected by the first barrier layer, which is consistent with the
3. Numerical simulation Some problems arose in the study of the effects of multiple barrier layers on SAGD production using the physical model. First, steam is severely affected by barrier layers, and it can easily enter the production well. Once a high-permeability channel is formed, it could lead to an early end of the experiment. Also, it is difficult to describe the variation characteristics of the steam chamber in the experimental device with limited temperature points. Finally, it is difficult to study the influencing factors due to the large variety of combinations of multiple barrier layers. To solve the above-mentioned problems, a numerical simulation method with the help of CMG was used. 3.1. Simulation model The numerical simulator used in this study was STARS. A theoretical numerical simulation model was established based on the average reservoir parameters of the Long Lake oil field. The porosity and permeability of the simulation model was homogeneous. The grid number of the model was set at 38 × 5 × 27 (length × width × height) and a single grid size was 1 m × 1 m × 1 m. The basic parameters are shown in Table 2. A rheometer (Anton Paar MCR 302) was used to measure the viscosity−temperature curve of the Long Lake oil under atmospheric pressure, as shown in Fig. 6. Fig. 7 shows the oil−water relative permeability curve.
Fig. 4. Temperature field for different patterns at different times. 719
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Fig. 5. The residual oil saturation distribution of pattern B. Table 2 Main parameters of the simulation model. Parameter
Value
Reservoir Reservoir (m) Reservoir temperature (°C) Oil saturation (%) Water saturation (%) Reservoir thickness (m) Lateral boundary (m) Horizontal permeability (D) Vertical permeability (D) Permeability of impermeable barrier layer (mD) Porosity (%)
38 × 5 × 27 25.7 83 17 27 76 5 2 0.001 32
Thermal properties Rock heat capacity [kJ/(m3°C)] Rock thermal conductivity [kJ/(m day°C)] Water thermal conductivity [kJ/(m day°C)] Oil thermal conductivity [kJ/(m day°C)] Overburden thermal conductivity [kJ/(m day°C)]
2390 460 53.5 11.8 460
Fig. 7. The relative permeability curve of oil and water.
3.2. The effects of different numbers of multiple barrier layers
Steam injection parameters Steam temperature (°C) Injection pressure (kPa) Steam quality Preheating time (day)
3.2.1. Simulation scheme The first scheme was designed based on the distribution of the barrier layers in mode B. To better observe the fluid motion, a vector diagram of the steam and oil flow was introduced into the field patterns. In addition, to investigate the influence rule of multiple barrier layers, two comparison schemes were added: pattern C reduced one barrier layer on the basis of B, while pattern D increased one barrier layer. According to the phenomena that occurred during the experiment, the first barrier layer was deliberately not changed. Finally, pattern A in the experiment was added as the control group to better compare the production characteristics of SAGD under the influence of multiple barrier layers.
280 2800 0.95 100
3.2.2. Results and discussion 3.2.2.1. Temperature and oil saturation distribution. As depicted in Fig. 8, when the steam was blocked by the first barrier layer, it began to expand laterally along the direction of the barrier layer. After reaching the end of the barrier layer, the steam began to flow around, and then the steam chamber began to develop upward for a second time. Before reaching the top of the model, a small amount of steam entered the interbedded zone, which is basically consistent with the phenomenon shown in Fig. 4. After the second lateral expansion of the steam chamber, it can be clearly seen that the steam entered the interbedded zone along the lower edge of the barrier layers. In addition,
Fig. 6. The viscosity−temperature curve of the Long Lake oil.
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3.2.2.2. Production characteristic. Fig. 10 shows the production characteristics of the different patterns. In agreement with previous studies, the production of SAGD was divided into three stages under the homogeneous model, as shown in pattern A. However, under the influence of multiple barrier layers, this stage division method was no longer applicable. A typical feature is that there was basically no stable production period. By comparing patterns B, C, and D, it can be found that the number of barrier layers primarily affected the second peak in production. However, its impact was not significant throughout the entire SAGD production process. This also confirmed the dominance of the first barrier layer in the impact of multiple barrier layers. By comparing the inflection points in the curve with the development of the steam chamber, some characteristic points can be found that are related to the development of the steam chamber. They are the first peak of production (P1) that means that the steam chamber was severely hampered by the first barrier layer and production began to decline significantly. The first low point of production (P2) represents that steam chamber began to develop upward for a second time. The second peak of production (P3) represents the steam chamber was already in the descending phase. In addition, the corresponding time of the above three characteristic points was established. In combination with field practice, t2, t3 and P3 are crucial. The parameter t2 directly affects the deployment time of adjacent wells, which relates to connectivity of steam chambers between the well groups. The parameters t3 and P3 are critical to the economics and potential of SAGD production.
Fig. 8. Temperature profile of different patterns.
the amount of steam that entered the zone between every two barrier layers was also different. As the steam entered the interbedded zone, the temperature began to rise rapidly. By comparing the development of the steam chamber under the three patterns, it was found that the overall shape was similar and largely depended on the first barrier layer. The difference was primarily reflected in the secondary lateral expansion, and the shape of the inner side of the steam chamber varied with the number of barrier layers. Fig. 9 shows the oil saturation profile of different patterns. It can be seen that the oil flow primarily occurred at the edge of the steam chamber, and some oil flow occurred in the steam chamber. Before the steam chamber reached the top, a small amount of oil was discharged from the edge of the interbedded zone due to the heating effect of the upward developing steam and steam below the first barrier layer. As the steam entered the interbedded zone, the drainage area became larger. Consistent with the steam chamber, the difference in oil flow was primarily reflected in the interbedded zone. When the spacing between two barrier layers was larger, the area where the oil drainage occurred was larger.
3.3. Different combinations of multiple barrier layers 3.3.1. The design of the scheme Due to the different distribution and properties, the combinations of multiple barrier layers have diversified characteristics. Considering their influence mode, it is reasonable to use the first barrier layer to characterize the influence of different combinations. Based on the established numerical simulation model, the number of barrier layers was first determined to be three to give the parameter a wider range of values. Then, the effects of changes in the following properties were studied: Relative location: This is the ratio of the distance between the first barrier layer and the injector to the reservoir thickness. Provided that other conditions were invariant, the position of the two upper barrier layers were fixed, and there was only change in the position of the first barrier layer. A total of six schemes from mode A to F were set. Mode A represented the furthest distance (0.37), and mode F represented the nearest (0.11). Relative size: This is the ratio of the first barrier layer length to well spacing. Provided that the other conditions were invariant, the size of the two upper barrier layers were fixed, and there was only a change in the size of the first barrier layer. A total of six schemes from mode G to L were set. Mode G represented the shortest length (0.13), and mode L represented the longest (0.79). Permeability: Under the condition that the position and size of the barrier layers remained unchanged, the two upper barrier layers were set as impermeable (0.001 mD), and only the permeability of the first barrier changed. A total of six schemes from mode G to L were set. The first barrier layer in mode M was impermeable (0.001 mD), and the permeability of the first barrier layer in mode R was 100 mD. Table 3 shows the specific conditions of the first barrier layer in modes A−R. The development of the steam chamber and oil production rate under the different schemes were compared in detail. 3.3.2. Results and discussion 3.3.2.1. Relative position of the first barrier layer. Fig. 11 shows the temperature profiles of modes B, D, and E at different times. By comparing the three modes, it can be found that when the first barrier layer was far away from the injector, the entire multiple
Fig. 9. Oil saturation profile of different patterns. 721
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Fig. 10. Oil production map of different patterns.
development of steam chamber during the early stage, more steam entered into the interbedded zone, and the entire cavity was more complete. But in mode E, because the first barrier layer was too close to the steam injection well, the steam chamber was subject to greater resistance during the early development process. This led to a lag in the development during the middle and late stages. In addition, there were still large areas in the interbedded zone that did not heat. Fig. 12 shows the oil production rate and the cumulative steam-tooil ratio (CSOR) for the different modes. The results reveal that the closer the first barrier layer was to the steam injection well, the lower the oil production and the higher the CSOR were before the P3 was reached. This indicated that the multiple barrier layers had a greater impact during the earlier and middle stages, while the change in P3 was not obvious. In terms of production time, because the length of the first barrier layer in each mode was the same, and the rate of steam chamber upward development was relatively fast, there was little difference at t1 and t2. The difference was primarily reflected in t3. The closer the barrier layer was to the injector, the later P3 will be reached. Additionally, the time (t2) for the steam to flow around the first barrier
Table 3 Influencing factors of the first barrier layer. Mode
Position
Size
Permeability (mD)
Mode
Position
Size
Permeability (mD)
A B C D E F G H I
0.37 0.33 0.26 0.22 0.15 0.11 0.15 0.15 0.15
0.39 0.39 0.39 0.39 0.39 0.39 0.13 0.26 0.39
0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001
J K L M N O P Q R
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.52 0.65 0.79 0.39 0.39 0.39 0.39 0.39 0.39
0.001 0.001 0.001 0.001 20 40 60 80 100
barrier layers were distributed in the upper part of the model, and the steam chamber was relatively complete in the longitudinal direction. The closer the distance between the first barrier layer and the injector, the inclination of the outer leading edge of the steam chamber became greater, and the shape became flatter. In mode B, due to the good
Fig. 11. Temperature profiles of mode B, D, and E at different times. 722
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Fig. 12. Oil production rate and cumulative steam-to-oil ratio (CSOR) for different modes.
layer is predictable using the CSOR curve.
comparison of the three modes shows that the longer the length of the first barrier layer was, the development of the whole steam chamber was more lagged. In mode G, the length of the first barrier layer was shorter than the others, and the steam could easily bypass the first barrier layer and continue to develop upward. In the lateral expansion, unlike in mode I, the steam first fully heated the area between the first and second barrier layer, and the steam chamber developed better later. Due to the long length of the first barrier layer in mode K, the steam just reached the top at the same time, and the oil in the interbedded zone was basically unused. Fig. 14 shows the oil production rate and CSOR for the different modes. It can be seen that since the length of the first barrier layer in model G was short, the oil production rate during the early stage did not decrease significantly. However, as the length of the first barrier layer increased, the CSOR was higher after t2, and P3 was lower. In terms of production time, the longer the barrier layers, the later t2 and t3 appeared.
3.3.2.2. Relative size of the first barrier layer. Fig. 13 shows the temperature profiles of modes G, I, and K at different times. The
3.3.2.3. Permeability of the first barrier layer. Fig. 15 shows the temperature profiles of modes M, O, and R at different times. It can be seen from a comparison of the three modes that with an increase in the permeability of the first barrier layer, the steam passed more easily through the barrier layer, and the lower part of the chamber also developed better. The permeability of the first barrier layer in mode O
Fig. 13. Temperature profiles of mode G I and K at different times. 723
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Fig. 14. Oil production rate and cumulative steam-to-oil ratio (CSOR) for different modes.
was 20 mD, and its influence was quite obvious during the early stage. But in mode R, the steam easily rose through the first barrier layer. Fig. 16 shows the oil production rate for the different modes. It can be seen that with an increase in the permeability of the first barrier layer, the oil production was higher and the CSOR was lower before reaching P3. However, P3 did not change significantly. In terms of production time, t and t3 appeared earlier. 4. Conclusions On the basis of these studies, the following conclusions were made: (1) SAGD experiments were conducted under the influence of multiple barrier layers. The results of the temperature and residual oil profiles revealed that for impermeable barrier layers with the same length, the steam primarily developed upward after flowing around the first barrier layer, and the shape of the steam chamber remained very regular. During the secondary lateral expansion, steam began to enter the interbedded zone, but due to the blocking effect of the barrier layers, a large amount of oil in the interbedded zone had difficulty flowing. (2) Due to defects in the experiment, a theoretical numerical simulation
Fig. 15. The temperature profiles of mode M, O, and R at different times.
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Fig. 16. Oil production rate and cumulative steam-to-oil ratio (CSOR) for different modes.
● From the point of view of characteristic points, the relative position of the first barrier layer primarily affected t3. The relative size directly affected t2, and the longer the length of the first barrier layer, the lower P3. The higher the permeability of the first barrier layer, the earlier t3 appeared. In addition, at time t2, the CSOR curve changed significantly. These points can be used to more thoroughly understand the properties of barrier layers.
was conducted to study the influence of the multiple barrier layers in more detail. By introducing a vector diagram, it was found that steam entered the interbedded zone along the lower edge of barrier layers and presented as a non-uniform distribution. (3) Under the condition that the first barrier layer remained unchanged, the number of multiple barrier layers had an impact on the development of the steam chamber and SAGD production. However, this effect was not significant, which confirmed that the first barrier layer played a dominant role in the influence of multiple barrier layers. (4) Several feature points (P1, P2, P3), which corresponded to the development of a steam chamber, were used to describe the production characteristics of SAGD under the influence of multiple barrier layers. (5) The multiplicity of the multiple barrier layer combinations was characterized by a change in the properties and location of the first barrier layer. ● From the point of view of the steam chamber, the results showed that the longer the length of the first barrier layer, the closer it was to the steam injection well, and the lower the permeability. Therefore, the more obvious its hysteresis effect on the development of the steam chamber.
Acknowledgements The authors acknowledge the project named Mechanism and Characterization of Exploitation with Complex Solvent-Superheated Steam, which is provided by the National Natural Science Foundation of China of China (NO. U1762102), and this work was supported in part by a grant from National Science and Technology Major Projects of China (2016ZX05031-003-005).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.05.062. 725
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Ali, F., Hamed, A., Nawi, D.M., 2012. Impact of reservoir heterogeneity on Steam Assisted Gravity Drainage in heavy oil fractured reservoirs. Energy Explor. Exploit. 30 (4), 553–566. Baghernezhad, D., Siavashi, M., 2019. Optimal scenario design of steam-assisted gravity drainage to enhance oil recovery with temperature and rate control. Energy 166, 610–623. Butler, R.M., Stephens, D.J., 1981. The gravity drainage of steam-heated heavy oil to parallel horizontal wells. J. Can. Pet. Technol. 20 (2) 36-36. Butler, R.M., Mcnab, G.S., Lo, H.Y., 1981. Theoretical studies on the gravity drainage of heavy oil during in-situ steam heating. Can. J. Chem. Eng. 59 (4), 455–460. Chen, F., Liu, H., Dong, X., 2019. A New Analytical Model to Predict Oil Production for Cyclic Steam Stimulation of Horizontal Wells. Society of Petroleum Engineershttps:// doi.org/10.2118/195291-MS. Dang, T.Q.C., Chen, Z., Nguyen, T.B.N., 2013. Numerical simulation of SAGD recovery process in presence of shale barriers, thief zones, and fracture system. Petrol. Sci. Technol. 31 (14), 1454–1470. Doranehgard, M.H., Siavashi, M., 2018. The effect of temperature dependent relative permeability on heavy oil recovery during hot water injection process using streamline-based simulation. Appl. Therm. Eng.: Design, processes, equipment, economics 129, 106–116. Fatemi, S.M., 2012. The effect of geometrical properties of reservoir shale barriers on the performance of steam-assisted gravity drainage (SAGD). Energy Sources, Part A Recovery, Util. Environ. Eff. 34 (23), 2178–2191. Gates, I.D., Chakrabarty, N., 2006. Optimization of steam assisted gravity drainage in McMurray reservoir. J. Can. Pet. Technol. 45 (5), 54–62. Huang, S.J., Xiong, H., Wei, S.L., 2016. Physical simulation of the interlayer effect on SAGD production in mackay river oil sands. Fuel 183, 373–385. Huang, S.J., Xia, Y., Xiong, H., 2018. A three-dimensional approach to model steam chamber expansion and production performance of SAGD process. Int. J. Heat Mass Transf. 127, 29–38. Huang, S., Chen, X., Liu, H., 2019. Experimental and numerical study of steam-chamber evolution during solvent-enhanced steam flooding in thin heavy-oil reservoirs. J. Pet. Sci. Eng. 172, 776–786. Ipek, G., Frauenfeld, T., Yuan, J.Y., 2008. Numerical Study of Shale Issues in SAGD. Petroleum Society of Canadahttps://doi.org/10.2118/2008-150. Kim, M., Shin, H., 2017. Development and Field Application of Proxy Models for Predicting the Shale Barrier Size Using SAGD Production Data. Society of Petroleum Engineershttps://doi.org/10.2118/186925-MS. Law, D.H.S., Nasr, T.N., Good, W.K., 2003a. Lab-Scale Numerical Simulation of SAGD Process in the Presence of Top Thief Zones: A Mechanistic Study. Petroleum Society of Canadahttps://doi.org/10.2118/03-03-02. Law, D.H.S., Nasr, T.N., Good, W.K., 2003b. Field-Scale Numerical Simulation of SAGD Process with Top-Water Thief Zone. Petroleum Society of Canadahttps://doi.org/10. 2118/03-08-01. Li, X.L., Shi, L.,X., Li, H.Z., 2018. Experimental study on viscosity reducers for SAGD in developing extra-heavy oil reservoirs. J. Pet. Sci. Eng. 166, 25–32. Liu, H., Cheng, L., Yang, D., 2017. A Heat and Mass Transfer Coupling Model for Transition Interface of Expanding Solvent Steam Assisted Gravity Drainage. Special Topics & Reviews in Porous Media.
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
An experimental and numerical study of a steam chamber and production characteristics of SAGD considering multiple barrier layers
T
Shijun Huanga,b, Lijie Yangb,∗, Yun Xiab, Mengge Dub, Yanwei Yangb a b
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China Department of Petroleum Engineering, China University of Petroleum, Beijing, 102249, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Steam-assisted gravity drainage (SAGD) Multiple barrier layers Steam chamber Production characteristics Combinations of barrier layers
Steam-assisted gravity drainage (SAGD) is an efficient technology that has been used to develop oil sand resources, and it has been successfully and maturely applied in Canada. However, oilfield production has demonstrated that reservoir heterogeneity has a serious impact on SAGD development, among which the most drastic impact is caused by multiple barrier layers in the reservoir. These barrier layers can severely impede the development of a steam chamber and the drainage of oil. Hence, it is important to investigate their influence mechanism. In this study, oil samples from the Long Lake oil field were used in laboratory experiments to study the development of the steam chamber and the residual oil distribution under the effect of multiple barrier layers. Then, a theoretical numerical simulation model was established to describe the fluid flow more precisely. In addition, the production characteristics of SAGD under the influence of different numbers of barrier layers were analyzed by comparing the development of the steam chamber. Finally, the impacts of multiple barrier layers with different modes of combinations were studied for their effects on SAGD production. The results indicated that for impermeable barrier layers of the same length, the steam primarily developed upward after flowing around the first barrier layer. After the steam chamber reached the top of the reservoir, the steam began to enter into the interbedded zone. Based on an unchanged first barrier layer, the number of barrier layers had little influence on the overall shape of the steam chamber and SAGD production, which confirmed that the first barrier layer played a dominant role in the influence of multiple barrier layers. In addition, characteristic points (P1, P2, P3), which corresponded to changes in the stage of steam chamber development, were established and used to evaluate the effect of the barrier layers. Different combinations of barrier layers were realized by changing the relative properties of the first barrier layer. The results showed that the longer the length of the first barrier layer, the closer it was to the steam injection well and the lower the permeability; hence, the more obvious its hysteresis effect on the SAGD process.
1. Introduction In recent years, due to the increasing demand for oil and a reduction in conventional oil production, more attention has been paid to heavy oil resources. It is estimated that the reserves of heavy oil are approximately six times that of conventional crude oil (Owen et al., 2010). Due to the sensitivity between oil viscosity and temperature, thermal methods, a class of enhanced oil recovery (EOR) methods, have achieved great advances in the development of heavy oil. Thermal methods mainly include cyclic steam stimulation (CSS), steam flooding (SF) and in-situ combustion (ISC), hot water flooding, and steam-assisted gravity drainage (SAGD) (Shi et al., 2017; Doranehgard et al., 2018; Li et al., 2018). In the methods that use steam as a heat carrying substance, cyclic steam stimulation is widely used for its easy operation ∗
and high early yields (Chen et al., 2019). Nevertheless, due to the limited heating radius and diminishing yield per cycle, its recovery factors are only 20%–40% of the original oil-in-place (OOIP) (Santos et al., 2014). Steam flooding is often used as a replacement during the late stage of CSS, and it can reach recovery factors of up to 60% OOIP (Huang et al., 2019). However, steam channeling easily occurs during the later stage of production due to long-term steam flooding. SAGD technology, first proposed and developed by Butler (Butler et al., 1981a, b), has been widely and maturely applied to the exploitation of oil sands in Canada (Tian et al., 2017; Baghernezhad et al., 2019). The method involves drilling a pair of parallel horizontal wells near the bottom of a reservoir and steam is injected into the formation through the upper well. Under the action of gravity, the heated crude oil and condensate flow to the lower well. With high recovery factors of 55%
Corresponding author. E-mail addresses: [email protected], [email protected] (L. Yang).
https://doi.org/10.1016/j.petrol.2019.05.062 Received 3 February 2019; Received in revised form 23 May 2019; Accepted 26 May 2019 Available online 05 June 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
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showed a relatively minimal difference in recovery. Dang et al. (2013) claimed that only long, continuous shale interlayers (larger than 50 m) would affect the SAGD performance. Furthermore, in terms of the location of the interlayer relative to the well, Shin et al. (2009), Huang et al. (2016), and Wang et al. (2018) agreed that the existence of the barrier layer between the producer and injector had a significant impact on SAGD production. Kim et al. (2017) studied the influence mechanism of a single interlayer. He was the first to associate the inflection point on the productivity curve with a steam chamber development stage. With progress in mathematical calculation methods, several scholars have used proxy models to predict the interlayer size and SAGD production under the influence of an interlayer (Zheng et al., 2016). In addition, some novel approaches, such as the use of the artificial intelligence method, have been used to analyze production data (Ma et al., 2016; Nguyen et al., 2018). Although scholars have conducted a lot of research on the effect of the barrier layer, the influence mechanism of multiple barrier layers has attracted little attention. In addition, it is not sufficient to measure the impact of the barrier layer using only some productivity indicators (such as oil recovery). In fact, the effect of the barrier layer on the shape of the steam chamber is the most direct and intuitive. In this study, the effects of multiple barrier layers on SAGD production are investigated primarily from the perspective of steam chamber development and production characteristics. First, based on existing experimental devices, two groups of physical SAGD simulations are conducted. Then, a theoretical numerical simulation is established for further study, and the flow of steam and oil is described more precisely. In addition, some characteristic points on the productivity curve are used to stage the development of the steam chamber. Finally, the effects of multiple barrier layers with different combinations on the development of the steam chamber and SAGD production are analyzed in detail.
OOIP (Santos et al., 2014), SAGD has two distinct advantages. One is that its main oil-displacement mechanism is gravity drainage, and the other is that it effectively utilizes the disadvantage of steam overlap. Many scholars have comprehensively studied and continue to investigate SAGD using different methods. The most important aspect of this method in the early stage is how to characterize the production capacity. Butler first proposed his classic formula based on Darcy's law for estimating the oil production rate (Butler et al., 1981a, b). Reis (1993) further modified the productivity formula based on the assumption that the shape of the steam chamber is an inverted triangle. Later, Wei et al. (2014) thought the shape of the steam chamber was related to the steam injection rate, and a low steam injection rate would lead to a convex parabola. By combining experimental and mathematical models, Liu et al. (2018) considered the advantages and disadvantages of different steam chamber shapes in terms of the production rate and heat consumption. It could be seen that the development of the steam chamber is an important indicator that can be used to evaluate SAGD production. With the wide applications of SAGD technology in the oil field, researchers hope to make it more efficient. The problems have been focused on primarily two aspects: (i) how to optimize the operational parameters, and (ii) the influence of geological factors. For the former, Gates et al. (2006) thought that a high injection pressure should be applied in the early stage, and this would allow the steam chamber to reach the top quickly and reduce heat loss. A thorough study regarding the injection-production parameters was conducted by Siavashi et al. (2017). The results showed that a high steam injection rate would bring more energy to the formation and promote the development of a steam chamber, but the SOR also rose. In addition, the heating range can be expanded by increasing the fluid production rate appropriately. With additional studies, the injection side became more diverse. Yuan et al. (2018) adopted an experimental study and numerical simulation method to further investigate nitrogen-assisted SAGD (NA-SAGD). The results showed that nitrogen can effectively maintain the pressure and reduce heat loss. Solvent-aided SAGD (SASAGD) was introduced for lower greenhouse gas emissions and lower steam usage (Liu et al., 2017). The impact of geological factors including water sand, fractures, and barrier layers is also significant. When there is bottom water in a formation, it can affect heat transfer in case of water coning or steam loss to bottom water (Masih et al., 2012). An effective solution is to maintain the steam chamber pressure above the aquifer pressure, but this can also lead to low oil production (Qin et al., 2014). Moreover, the effect of the top water is greater than that of the bottom water due to more severe heat loss (Law et al., 2003a, b). In fractured reservoirs, the drainage of the initial mobile water contained in a fracture makes the steam chamber primarily develop in an upward direction (Ali et al., 2012). In reality, according to the actual production process in an oil field, the existence of a barrier layer plays a more important role in how geological factors affect oil production. The barrier layer not only can affect the effective flow of oil and gas but also has a great impact on heat and mass transfer. As early as 1992, several 2D experiments were performed by Yang et al. (1992) to study the effect of horizontal layers with different steam injection positions on the SAGD production. The results showed that the effect of the layer is not significant with top steam injection, but heated oil could not be produced with bottom injection due to the steam pressure. Pooladi et al. (2002) investigated the influence of different continuities of the shale layer on SAGD in a reservoir with gas cap and top water. Due to the great distance between the shale layer and the steam injection well, the influence of the different schemes was not significant. Ipek et al. (2008) designed thirty schemes to study the influence of the shale interlayer in the reservoir on SAGD. One of his conclusions was that the higher the proportion of shale, the lower the production. Fatemi et al. (2012) investigated impact of the geometrical properties of the impermeable interlayers (such as density, location, and dispersion) on the SAGD process, but he only
2. Experiments 2.1. Experimental apparatus This experiment primarily investigated the development of the steam chamber on section. To weaken the influence of an uneven distribution of steam along the long horizontal well (Ong et al., 1990; Huang et al., 2018), a visualized two-dimensional physical model (30 cm in length, 25 cm in width, and 8 cm in depth) was developed, and the similarity criterion and parameters for the experiment were established (Huang et al., 2016; Liu et al., 2018). Fig. 1 shows the photograph and schematic diagram of the model. The production process of the SAGD was simulated by placing the experimental equipment on the vertical. The outside of the model was covered with an insulating ceramic wool with heaters in it, which was used to simulate the original reservoir temperature and reduce heat loss during the experiment. In front of the model was a transparent glass plate for visualization. Previous studies have shown that the shape of the steam chamber is symmetrical, so the horizontal well group was placed on the left side of the model to better observe changes in the shape of the steam chamber. Temperature sensors and pressure sensors were installed at the ports of the injector and producer to monitor operating conditions during the experiment. In addition, 68 temperature measuring points were arranged in the device to record the development of the steam chamber. As shown in Fig. 2, the experimental apparatus included three parts: the oil driving system, the reservoir model system, and the data collection system. 2.2. Design of the experiment Oil samples from the Long Lake oil field were used in this experiment. The content of saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene in the oil was 19.87%, 47.32%, 7.68%, and 717
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Fig. 1. The photograph and schematic of the model.
2.3.1. The pre-experiment preparation Sand packing: According to the established scheme, the four Perspex plates (barrier layers) were fixed with super glue. Then, considering the degree of compaction of the model, 40-mesh glass beads were chosen to fill into the model and the permeability was 240 D (Huang et al., 2016). Since the model would later be placed vertically, the glass beads were filled evenly and compactly. Airtightness test: After sealing the model, the model was pressed with N2, and the pressure of the model was stabilized at 0.5 MPa for 0.5 h to observe whether or not the pressure changed. Saturated water process: Deionized water was injected into the model from the lower pipeline and the water injection speed was 4 ml/ min. Then, the amount of deionized water injected into the model was recorded to measure the porosity. By assuming the density of water was 1 g/cm3, the porosity of model was calculated to be 0.415. Saturated oil process: This step was performed at 80 °C to increase the mobility of the oil. The oil injection speed was set at 3 ml/min for full saturation. When the model no longer produced water, the process was finished.
25.13% respectively. The oil viscosity at 80 °C was 560 cP, and the oil density was 1016 kg/m3. Most of the barrier layers in the Long Lake oilfield are impermeable and show parallel characteristics in the longitudinal direction. Perspex plates were set in the displacement model to simulate the impermeable barrier layers. The size of the Perspex plates was 12 cm × 1 cm × 8 cm and they were placed along the y direction of the model. In addition, the thickness of 1 cm was consistent with the actual barrier layer thickness of 1–2 m. Two experimental schemes were designed, as shown in Fig. 3. Pattern A was a homogeneous formation that was used as the control for comparison. Four parallel barrier layers were placed above the steam injection well in Pattern B. Table 1 shows the conditions of the different patterns.
2.3. Experimental procedures and control conditions The actual production of SAGD primarily includes two stages: the preheating stage and the formal production stage. Therefore, the physical simulation process was divided into three parts: pre-experiment preparation, preheating, and the formal production.
2.3.2. SAGD experiment To simulate the preheating process, a heating pipe with diameter of
Fig. 2. Schematic of the experimental apparatus. 718
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Fig. 3. Schematic of the experimental schemes.
influence of a single barrier layer that had been previously studied (Huang et al., 2016). Due to the blocking effect, the steam accumulated below the first barrier layer and heated the interbedded zone due to heat conduction. However, after the steam flowed around the first barrier layer, the development of the steam chamber began to differ. A certain shape was formed after flowing around the first barrier layer. In addition, since the length of each barrier layer was equal, the steam chamber remained very regular during the upward development. Because of the effect of multiple barrier layers, the heat conduction of the steam was weakened, and a low temperature zone formed in the interbedded zone. After the steam chamber reached the top of the model, the steam began to enter the interbedded zone during the second lateral expansion process.
Table 1 Conditions of different patterns. Scheme
Formation condition
Formation texture
Barrier texture
A B
No barrier layer Four barrier layers
Glass beads Glass beads
– Perspex plates
3 mm was wrapped around the outside of the producer and injector. Prior to the formal production, high temperature steam was injected into the heating pipe to heat the area near the wells. The process was switched to the formal production stage until the temperature of the formation between the two wells reached 70 °C. During the formal production stage, steam was injected into the upper well, and heated oil and condensate water flowed into the lower well. The temperature of the injected steam was maintained at 115–120 °C. To control for steam breakthrough, the rate of steam injection was constantly adjusted to ensure that the temperature difference between the injector and producer was 5–35 °C (Huang et al., 2016).
2.4.2. The distribution of the residual oil Fig. 5 shows the location of the residual oil of pattern B. The shape of the low oil saturation area is basically the same as that of the steam chamber. Due to the gravitational difference, most of the oil remained below the steam chamber. The oil between the barrier layers could barely flow during the early stage. As the steam entered the interbedded zone and heated the oil, the oil had a sharply decreased viscosity and discharged at the edge of the zone. However, most of the oil was trapped in the zone due to the hindrance of multiple barrier layers.
2.4. Results and discussion 2.4.1. The development of the steam chamber Fig. 4 shows the temperature field for different patterns at different times. Under the condition of pattern A, the steam chamber primarily developed upward during the early stage, and then began to expand laterally after reaching the top of the model. This pattern was consistent with the development characteristics of steam chambers during SAGD production in homogeneous reservoirs (Butler et al., 1981; Wei et al., 2014). In the case of multiple barrier layers, the steam chamber was first affected by the first barrier layer, which is consistent with the
3. Numerical simulation Some problems arose in the study of the effects of multiple barrier layers on SAGD production using the physical model. First, steam is severely affected by barrier layers, and it can easily enter the production well. Once a high-permeability channel is formed, it could lead to an early end of the experiment. Also, it is difficult to describe the variation characteristics of the steam chamber in the experimental device with limited temperature points. Finally, it is difficult to study the influencing factors due to the large variety of combinations of multiple barrier layers. To solve the above-mentioned problems, a numerical simulation method with the help of CMG was used. 3.1. Simulation model The numerical simulator used in this study was STARS. A theoretical numerical simulation model was established based on the average reservoir parameters of the Long Lake oil field. The porosity and permeability of the simulation model was homogeneous. The grid number of the model was set at 38 × 5 × 27 (length × width × height) and a single grid size was 1 m × 1 m × 1 m. The basic parameters are shown in Table 2. A rheometer (Anton Paar MCR 302) was used to measure the viscosity−temperature curve of the Long Lake oil under atmospheric pressure, as shown in Fig. 6. Fig. 7 shows the oil−water relative permeability curve.
Fig. 4. Temperature field for different patterns at different times. 719
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Fig. 5. The residual oil saturation distribution of pattern B. Table 2 Main parameters of the simulation model. Parameter
Value
Reservoir Reservoir (m) Reservoir temperature (°C) Oil saturation (%) Water saturation (%) Reservoir thickness (m) Lateral boundary (m) Horizontal permeability (D) Vertical permeability (D) Permeability of impermeable barrier layer (mD) Porosity (%)
38 × 5 × 27 25.7 83 17 27 76 5 2 0.001 32
Thermal properties Rock heat capacity [kJ/(m3°C)] Rock thermal conductivity [kJ/(m day°C)] Water thermal conductivity [kJ/(m day°C)] Oil thermal conductivity [kJ/(m day°C)] Overburden thermal conductivity [kJ/(m day°C)]
2390 460 53.5 11.8 460
Fig. 7. The relative permeability curve of oil and water.
3.2. The effects of different numbers of multiple barrier layers
Steam injection parameters Steam temperature (°C) Injection pressure (kPa) Steam quality Preheating time (day)
3.2.1. Simulation scheme The first scheme was designed based on the distribution of the barrier layers in mode B. To better observe the fluid motion, a vector diagram of the steam and oil flow was introduced into the field patterns. In addition, to investigate the influence rule of multiple barrier layers, two comparison schemes were added: pattern C reduced one barrier layer on the basis of B, while pattern D increased one barrier layer. According to the phenomena that occurred during the experiment, the first barrier layer was deliberately not changed. Finally, pattern A in the experiment was added as the control group to better compare the production characteristics of SAGD under the influence of multiple barrier layers.
280 2800 0.95 100
3.2.2. Results and discussion 3.2.2.1. Temperature and oil saturation distribution. As depicted in Fig. 8, when the steam was blocked by the first barrier layer, it began to expand laterally along the direction of the barrier layer. After reaching the end of the barrier layer, the steam began to flow around, and then the steam chamber began to develop upward for a second time. Before reaching the top of the model, a small amount of steam entered the interbedded zone, which is basically consistent with the phenomenon shown in Fig. 4. After the second lateral expansion of the steam chamber, it can be clearly seen that the steam entered the interbedded zone along the lower edge of the barrier layers. In addition,
Fig. 6. The viscosity−temperature curve of the Long Lake oil.
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3.2.2.2. Production characteristic. Fig. 10 shows the production characteristics of the different patterns. In agreement with previous studies, the production of SAGD was divided into three stages under the homogeneous model, as shown in pattern A. However, under the influence of multiple barrier layers, this stage division method was no longer applicable. A typical feature is that there was basically no stable production period. By comparing patterns B, C, and D, it can be found that the number of barrier layers primarily affected the second peak in production. However, its impact was not significant throughout the entire SAGD production process. This also confirmed the dominance of the first barrier layer in the impact of multiple barrier layers. By comparing the inflection points in the curve with the development of the steam chamber, some characteristic points can be found that are related to the development of the steam chamber. They are the first peak of production (P1) that means that the steam chamber was severely hampered by the first barrier layer and production began to decline significantly. The first low point of production (P2) represents that steam chamber began to develop upward for a second time. The second peak of production (P3) represents the steam chamber was already in the descending phase. In addition, the corresponding time of the above three characteristic points was established. In combination with field practice, t2, t3 and P3 are crucial. The parameter t2 directly affects the deployment time of adjacent wells, which relates to connectivity of steam chambers between the well groups. The parameters t3 and P3 are critical to the economics and potential of SAGD production.
Fig. 8. Temperature profile of different patterns.
the amount of steam that entered the zone between every two barrier layers was also different. As the steam entered the interbedded zone, the temperature began to rise rapidly. By comparing the development of the steam chamber under the three patterns, it was found that the overall shape was similar and largely depended on the first barrier layer. The difference was primarily reflected in the secondary lateral expansion, and the shape of the inner side of the steam chamber varied with the number of barrier layers. Fig. 9 shows the oil saturation profile of different patterns. It can be seen that the oil flow primarily occurred at the edge of the steam chamber, and some oil flow occurred in the steam chamber. Before the steam chamber reached the top, a small amount of oil was discharged from the edge of the interbedded zone due to the heating effect of the upward developing steam and steam below the first barrier layer. As the steam entered the interbedded zone, the drainage area became larger. Consistent with the steam chamber, the difference in oil flow was primarily reflected in the interbedded zone. When the spacing between two barrier layers was larger, the area where the oil drainage occurred was larger.
3.3. Different combinations of multiple barrier layers 3.3.1. The design of the scheme Due to the different distribution and properties, the combinations of multiple barrier layers have diversified characteristics. Considering their influence mode, it is reasonable to use the first barrier layer to characterize the influence of different combinations. Based on the established numerical simulation model, the number of barrier layers was first determined to be three to give the parameter a wider range of values. Then, the effects of changes in the following properties were studied: Relative location: This is the ratio of the distance between the first barrier layer and the injector to the reservoir thickness. Provided that other conditions were invariant, the position of the two upper barrier layers were fixed, and there was only change in the position of the first barrier layer. A total of six schemes from mode A to F were set. Mode A represented the furthest distance (0.37), and mode F represented the nearest (0.11). Relative size: This is the ratio of the first barrier layer length to well spacing. Provided that the other conditions were invariant, the size of the two upper barrier layers were fixed, and there was only a change in the size of the first barrier layer. A total of six schemes from mode G to L were set. Mode G represented the shortest length (0.13), and mode L represented the longest (0.79). Permeability: Under the condition that the position and size of the barrier layers remained unchanged, the two upper barrier layers were set as impermeable (0.001 mD), and only the permeability of the first barrier changed. A total of six schemes from mode G to L were set. The first barrier layer in mode M was impermeable (0.001 mD), and the permeability of the first barrier layer in mode R was 100 mD. Table 3 shows the specific conditions of the first barrier layer in modes A−R. The development of the steam chamber and oil production rate under the different schemes were compared in detail. 3.3.2. Results and discussion 3.3.2.1. Relative position of the first barrier layer. Fig. 11 shows the temperature profiles of modes B, D, and E at different times. By comparing the three modes, it can be found that when the first barrier layer was far away from the injector, the entire multiple
Fig. 9. Oil saturation profile of different patterns. 721
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Fig. 10. Oil production map of different patterns.
development of steam chamber during the early stage, more steam entered into the interbedded zone, and the entire cavity was more complete. But in mode E, because the first barrier layer was too close to the steam injection well, the steam chamber was subject to greater resistance during the early development process. This led to a lag in the development during the middle and late stages. In addition, there were still large areas in the interbedded zone that did not heat. Fig. 12 shows the oil production rate and the cumulative steam-tooil ratio (CSOR) for the different modes. The results reveal that the closer the first barrier layer was to the steam injection well, the lower the oil production and the higher the CSOR were before the P3 was reached. This indicated that the multiple barrier layers had a greater impact during the earlier and middle stages, while the change in P3 was not obvious. In terms of production time, because the length of the first barrier layer in each mode was the same, and the rate of steam chamber upward development was relatively fast, there was little difference at t1 and t2. The difference was primarily reflected in t3. The closer the barrier layer was to the injector, the later P3 will be reached. Additionally, the time (t2) for the steam to flow around the first barrier
Table 3 Influencing factors of the first barrier layer. Mode
Position
Size
Permeability (mD)
Mode
Position
Size
Permeability (mD)
A B C D E F G H I
0.37 0.33 0.26 0.22 0.15 0.11 0.15 0.15 0.15
0.39 0.39 0.39 0.39 0.39 0.39 0.13 0.26 0.39
0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001
J K L M N O P Q R
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.52 0.65 0.79 0.39 0.39 0.39 0.39 0.39 0.39
0.001 0.001 0.001 0.001 20 40 60 80 100
barrier layers were distributed in the upper part of the model, and the steam chamber was relatively complete in the longitudinal direction. The closer the distance between the first barrier layer and the injector, the inclination of the outer leading edge of the steam chamber became greater, and the shape became flatter. In mode B, due to the good
Fig. 11. Temperature profiles of mode B, D, and E at different times. 722
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Fig. 12. Oil production rate and cumulative steam-to-oil ratio (CSOR) for different modes.
layer is predictable using the CSOR curve.
comparison of the three modes shows that the longer the length of the first barrier layer was, the development of the whole steam chamber was more lagged. In mode G, the length of the first barrier layer was shorter than the others, and the steam could easily bypass the first barrier layer and continue to develop upward. In the lateral expansion, unlike in mode I, the steam first fully heated the area between the first and second barrier layer, and the steam chamber developed better later. Due to the long length of the first barrier layer in mode K, the steam just reached the top at the same time, and the oil in the interbedded zone was basically unused. Fig. 14 shows the oil production rate and CSOR for the different modes. It can be seen that since the length of the first barrier layer in model G was short, the oil production rate during the early stage did not decrease significantly. However, as the length of the first barrier layer increased, the CSOR was higher after t2, and P3 was lower. In terms of production time, the longer the barrier layers, the later t2 and t3 appeared.
3.3.2.2. Relative size of the first barrier layer. Fig. 13 shows the temperature profiles of modes G, I, and K at different times. The
3.3.2.3. Permeability of the first barrier layer. Fig. 15 shows the temperature profiles of modes M, O, and R at different times. It can be seen from a comparison of the three modes that with an increase in the permeability of the first barrier layer, the steam passed more easily through the barrier layer, and the lower part of the chamber also developed better. The permeability of the first barrier layer in mode O
Fig. 13. Temperature profiles of mode G I and K at different times. 723
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Fig. 14. Oil production rate and cumulative steam-to-oil ratio (CSOR) for different modes.
was 20 mD, and its influence was quite obvious during the early stage. But in mode R, the steam easily rose through the first barrier layer. Fig. 16 shows the oil production rate for the different modes. It can be seen that with an increase in the permeability of the first barrier layer, the oil production was higher and the CSOR was lower before reaching P3. However, P3 did not change significantly. In terms of production time, t and t3 appeared earlier. 4. Conclusions On the basis of these studies, the following conclusions were made: (1) SAGD experiments were conducted under the influence of multiple barrier layers. The results of the temperature and residual oil profiles revealed that for impermeable barrier layers with the same length, the steam primarily developed upward after flowing around the first barrier layer, and the shape of the steam chamber remained very regular. During the secondary lateral expansion, steam began to enter the interbedded zone, but due to the blocking effect of the barrier layers, a large amount of oil in the interbedded zone had difficulty flowing. (2) Due to defects in the experiment, a theoretical numerical simulation
Fig. 15. The temperature profiles of mode M, O, and R at different times.
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Fig. 16. Oil production rate and cumulative steam-to-oil ratio (CSOR) for different modes.
● From the point of view of characteristic points, the relative position of the first barrier layer primarily affected t3. The relative size directly affected t2, and the longer the length of the first barrier layer, the lower P3. The higher the permeability of the first barrier layer, the earlier t3 appeared. In addition, at time t2, the CSOR curve changed significantly. These points can be used to more thoroughly understand the properties of barrier layers.
was conducted to study the influence of the multiple barrier layers in more detail. By introducing a vector diagram, it was found that steam entered the interbedded zone along the lower edge of barrier layers and presented as a non-uniform distribution. (3) Under the condition that the first barrier layer remained unchanged, the number of multiple barrier layers had an impact on the development of the steam chamber and SAGD production. However, this effect was not significant, which confirmed that the first barrier layer played a dominant role in the influence of multiple barrier layers. (4) Several feature points (P1, P2, P3), which corresponded to the development of a steam chamber, were used to describe the production characteristics of SAGD under the influence of multiple barrier layers. (5) The multiplicity of the multiple barrier layer combinations was characterized by a change in the properties and location of the first barrier layer. ● From the point of view of the steam chamber, the results showed that the longer the length of the first barrier layer, the closer it was to the steam injection well, and the lower the permeability. Therefore, the more obvious its hysteresis effect on the development of the steam chamber.
Acknowledgements The authors acknowledge the project named Mechanism and Characterization of Exploitation with Complex Solvent-Superheated Steam, which is provided by the National Natural Science Foundation of China of China (NO. U1762102), and this work was supported in part by a grant from National Science and Technology Major Projects of China (2016ZX05031-003-005).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.05.062. 725
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