Steam Assisted Gravity Drainage Steam Chamber Research Articles

Mechanisms and Operational Strategies of Multi-Lateral Steam-Assisted Gravity Drainage (SAGD) for Heterogeneous Reservoirs

As the SAGD steam chamber and production performance in heavy oil reservoirs under fluvial sedimentation environment are heavily impacted by reservoir heterogeneity, an innovative strategy was proposed in this study coupling rock dilation and multi-lateral wells in SAGD projects to break the mud barriers and achieve uniform steam chamber growth. True tri-axial experiments and numerical simulation were designed to validate the feasibility of this strategy, based on which the branches of the SAGD well pairs were designed and the operational parameters were optimized for different geologic heterogeneity conditions. The tri-axial experiment results indicate that the rock formations in the heavy oil reservoirs of the F oilfield exhibit significant shear dilation effects under low confinement pressure conditions, with a volumetric dilation capacity of up to 7%. The branches should be deployed in an interleaved manner, with a horizontal displacement of 20 m and a vertical displacement of 6 m. The optimal results are achieved when the branches intersect the interbeds, allowing for enhanced steam chamber conformance and enlarged volume. Dilation zones of 3–8 m can be created above the steam-injection horizontal wells and around the branches in the reservoir during the dilation of SAGD steam chambers. The maximum volume of dilation fluid used for hydraulic dilation is suggested to be less than 2000 m3. This strategy has been validated as being successful in the pilot SAGD well pair in the F oilfield, China, with the SAGD preheating time reduced by 50% and an incremental oil rate of 4.5 tones/day, indicating encouraging potentials in similar heavy oil reservoirs.

Dimethyl ether-steam assisted gravity drainage: Physical 2D heavy oil simulation

Steam-assisted gravity drainage (SAGD) is a mature heavy oil thermal recovery technology but inherently limited to low recovery efficiency at mid-late stage due to the increasing heat loss in the steam chamber. Here, the dimethyl ether (DME)-assisted SAGD technology and its relevant transport and interfacial properties are specifically investigated through physical 2D heavy oil simulation. A total of four-set physical experiments, with different DME injection ratios, were conducted and compared with the traditional SAGD. The experimental results indicate DME is capable of reducing the heavy oil’s viscosity and interfacial tension at high temperatures. Three stages, rising, lateral expansion, and descending stages, are determined in the SAGD steam chamber’s development; the mid-late SAGD stage is defined when the steam chamber extends to the caprock boundary. With DME additions at the mid-late SAGD stage, higher oil recovery factor and sweep efficiency could be reached and the dew point temperature of steam chamber’s lead decreases by 0.7–1.8 °C. The optimum gas–water ratio is determined to be 3:1, with the fastest oil recovery growth and the highest final cumulative oil-vapor ratio of 0.163. This study provides reliable experimental bases and valuable analyses for the DME-SAGD technology and its application in heavy oil reservoirs.

Mechanisms and Influence Factors of Downhole Electrical Heating-Assisted Steam-Assisted Gravity Drainage Production.

Approximately 70% steam-assisted gravity drainage (SAGD) wellpairs have entered into the production phase in China, while due to the fluvial sedimentation environment with strong reservoir heterogeneity, only 53% of the horizontal well section develops a steam chamber. In order to massively recover the bypassed oil and expand the steam chamber along the horizontal section, downhole electrical heating was proposed, and its mechanisms of high temperature-induced rock mechanics change and influence factors are investigated in this study using laboratory experiments and electrical steam hybrid numerical simulation. It is found that the electrical heating-assisted SAGD has four key mechanisms, namely, localized temperature elevation, development of a fixed point steam chamber, localized oil gravity drainage, and petrophysical property improvement. The influence factors include the static and operational factors, in which the permeability ratio is the primary factor for choosing SAGD wellpairs, while the steam injection rate, steam chamber operational pressure, injector and producer pressure difference, adjacent SAGD steam chamber pressure differential, heater surface temperature, and electrical heating period integrally influence the incremental production performance. Through carefully modifying the parameters, the typical SAGD wellpair steam chamber could expand from 67 to 100% along the horizontal section, with an incremental oil rate of 3–5 m3/day, and the cumulative steam/oil ratio decreases from 6.67 to 4.17. The downhole electrical heating is particularly efficient in improving steam chamber conformance in heterogeneous reservoirs and also has significant potential in similar reservoirs developed by horizontal wells.

A comprehensive investigation of SAGD steam chamber in dual horizontal well pairs: Expansion angel and connection characteristics

The expansion behaviors of the steam chamber play a significant role in the stability of steam-assisted gravity drainage (SAGD). For Long Lake extra-heavy oil reservoir, the 3D SAGD physical model based on similarity criteria is established in this work to investigate the expansion angel and connection characteristics of the steam chamber at varying well-pairs spacing and vertical distance. The results show that expansion angles of the well spacing of 50 m, 75 m, and 100 m are 16.5° and 17.6°, 22.5° and 23.0°, 25.4° and 24.8°, and the sweep efficiency is 96.7%, 90.8%, 78.6%, respectively. The smaller the well spacing between the dual well-pairs, the smaller the final expansion angle of the steam chamber, and the greater development effects. The vertical distances of 3 m have the maximum expansion angles (24.0° and 16.7°), and the highest sweep efficiency (81.6%). The smaller vertical distance of the dual well pair is accompanied by the larger expansion angle, which is beneficial to the development and propagation of steam chambers. The vertical expansion of the steam chamber is concentrated in the early stage of development, and the lateral expansion speed increases after the steam chamber contacts the top of the reservoir. The lateral expansion velocity and width of the steam chamber are the largest (0.36 cm/min and 0.4 cm/min, 20 cm) at the well-pairs spacing of 100 m. The heavy oil recovery factors at varying well-pairs spacing are in turn, 39.4% (50 m), 37.1% (75 m), and 33.7% (100 m). Compared to 6 m and 9 m, the vertical distances of 3 m have the highest ultimate oil recovery, 35.4%. Considering the economy and exploitation effect, the dual well-pairs with 50–75 m spacing and no vertical distance is recommended to promote the successful application of the SAGD technique in heavy oil reservoirs with 20–30 m thickness.

Influence of top water on SAGD steam chamber growth in heavy oil reservoirs: An experimental study

Steam-assisted gravity drainage (SAGD) is one efficient and mature technology for recovering heavy oil and bitumen resources. The key underlying mechanism is the growth of the steam chamber after injecting steam. However, due to the complex geological environment, the thief zones exist and have a prejudicial effect on the development of the steam chamber, thus impacting the ultimate heavy-oil recovery. In this work, our objective is to investigate the effect of a top-water thief zone (i.e., water zone overlies the oil sand) on SAGD performance and further to understand the crucial mechanisms that control the heat loss during steam injection. A large-scale three-dimensional experimental apparatus is used to carry out the SAGD process with a top aquifer. Based on the similarity criterion, the field-scale model is transformed into a laboratory elemental model. To evaluate the SAGD performance quantitatively, the dynamic growth of the steam chamber is measured using the thermal detectors and the production data is recorded. The results show that the steam chamber exhibits three distinguished stages, that is, upward spread, lateral extension, and downward development in the presence of top-water zone. The bottom-water zone has less impact on the steam-chamber growth. The existence of a confined top-water zone, however, significantly affects SAGD performance, especially the lateral expansion of the steam chamber. The lateral propagation of the steam front is hindered by the top thief zone due to the heat exchange with the top water. Once the steam chamber reaches the boundary, the accumulation of energy in the water thief zone, in turn, can reduce the remaining oil saturation along the topwater–oil interface. This study provides us some key insights into the development of heavy oil resources with top thief zones when implementing SAGD technology.

Assessment of SAGD heat transfer mechanism via noncondensable gas CO-INJECTION

The favored process for extracting heavy oil from reservoirs using twin horizontal wells is called steam-assisted gravity drainage (SAGD). During this application, steam is injected into the reservoir from above well and when heated, heavy oil is mobilized under gravity to the producer lying below. Steam build-up inside the pay zone is called the steam chamber. The main problems facing SAGD are the thermal efficiency and the thermal conductivity of the heat flux at the uppermost part of the steam chamber. Moreover, the role of solution gas and injection of gas in SAGD steam chambers remains unclear, particularly issues involving heat transfer. The objective of the study is to analyze the variation of heat transfer behavior by conduction and convection on heavy oil production using heat fluxes via the co-injection of noncondensable gas (NCG) during SAGD. To evaluate this phenomenon, the field verified SAGD analytical model was modified and used to analyze the experimental results of steam with/without NCG addition at comparable volumetric injection ratios. The outcomes of the modified heat transfer study conducted during SAGD on co-injection of NCG showed that an adequately large concentration at the uppermost part of the chamber decreased the heat flow at the edge. Besides, conduction was the primary heat transfer method for steam injection only. In the modified model, there was no obvious convective heat transfer below 125 °C for steam alone and below 150 °C for cases with NCG addition. However, the convective heat flux was more dominant after 230 °C and 240 °C for the NCG addition to steam at 1:1.29 and 1:4.41 volumetric ratio cases respectively. For the conductive and convective heat fluxes, the modified model fit with the steam only case. However, the results of the addition of NCG to steam at both runs showed a tendency to decrease at higher temperatures. On the contrary, the interaction between NCG and steam at the edge of the chamber influences the flow of heat resulting in both a decreased transfer of heat to the overburden as well as to the sides of the depletion chamber and a low recovery mechanism that produces the steam and gas push (SAGP) mechanism.

Predicting Geomechanical Dynamics of Steam-Assisted Gravity-Drainage Process. Part II: Modified Cam-Clay Model

SummaryIn Part I of this study (Irani 2018), the geomechanical effects in the reservoir associated with steam-assisted gravity drainage (SAGD) steam chamber growth was evaluated on the basis of two core assumptions: reservoir yield behavior follows that of the Mohr-Coulomb (MC) dilative behavior, and the reservoir stress response follows that of a drained sand. In Part I, it was shown that although the dilative model nicely described the shearing and the sheared zone thickness at the front of the SAGD steam chamber, it could not predict the displacements associated with cold dilation in SAGD reservoirs, in which cold dilation refers to vertical displacement created in the zone ahead of the heated zone caused by isotropic unloading generated by the pore pressure increase and the increase in far-field horizontal stress. In cold dilation, the stresses do not reach the critical state line (CSL), which defines the yield surface and should, therefore, be analyzed considering elastic behavior. A modified Cam-Clay (MCC) model, however, can be used to describe the behavior of the oil sand in the cold dilation zone before reaching the CSL. In this study and as an extension to the results presented in Part I, strains developed in the reservoir during SAGD operation are calculated using an MCC model, and the associated oil rate enhancement and displacements are evaluated. The vertical strains and displacements are compared with measured values from the extensive monitoring program conducted at the Underground Test Facility (UTF) in the late 1980s. Two aspects of geomechanical effects are compared between the cap models (Part II) and dilative models (Part I): first, prediction of the sheared zone thickness and its effect on SAGD production enhancement, and second, prediction of vertical and horizontal displacements. It is shown that consideration of the material model effects on production rates are negligible for both models and that the MCC model can predict displacements in both the heated and cold zones of the reservoir reasonably accurately. Although dilative constitutive models can be used to predict horizontal and vertical displacements in the heated zone quite accurately, they lack the ability to predict the response in the “cold dilation zone.” Another main advantage of using an MCC model is that the MCC model provides a better description of a stress path and how the reservoir mobility can affect reservoir dilation, especially in the cold dilation zone.

Analysis of Athabasca Oil Sands Investigates SAGD Performance Variability

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 195348, “Steam-Assisted Gravity Drainage Performance Variability—Analysis of Actual Production Data for 28 Athabasca Oil Sands Well Pairs,” by Quang T. Doan, Vincano; S.M. Farouq Ali, SPE, University of Houston; and Thomas B. Tan, SPE, Petrostudies Consultants, prepared for the 2019 SPE Western Regional Meeting, San Jose, California, 23–26 April. The paper has not been peer reviewed. Steam-assisted gravity drainage (SAGD) performance in bitumen-recovery projects in Alberta is affected by geological deposits, reservoir quality, and operational experience. The authors reviewed and analyzed actual field production and injection data for 28 Athabasca oil-sands-deposit SAGD well pairs (WPs). Based on analysis of field-production data, a numerical model was built and calibrated against production data from two of the poorer-performing WPs among the 28 studied. Agreement between simulated and actual cumulative oil and steam/oil ratio (SOR) was within 10% after 6 years of operations on a first-iteration basis. Problem and Investigation Methodology A survey of the literature reveals that some aspects of the SAGD process, particularly with regard to the behavior of gas, still largely are not understood. While successful history-matching simulations of SAGD WPs exist for several different projects, these rely on dead-oil pressure/volume/temperature treatment and exclude any findings and discussion on gas production. Such history-matched models likely would need to be modified significantly to be useful for modeling the wind-down stage of SAGD operations. Recent numerical studies modeling the generation of aquathermolysis gases focused on the injection of noncondensable gas into a mature SAGD steam chamber. In preparation for a series of numerical studies on aspects of SAGD performance, a reservoir model simple enough to be relatively adaptable for different geological settings was built. Such a model had to be calibrated rigorously with actual production data to provide a high degree of confidence in its results and predictive capability in appropriate contexts. The investigation in the complete paper is aimed at addressing several fundamental aspects of SAGD operations, including the effects of reservoir heterogeneities and the behavior of gas. The first part of the complete paper is focused on reviewing and analyzing actual field production data spanning more than 1,700 days from 28 SAGD WPs from four different pads (A, B, C, and D) of the Jackfish 1 project; this synopsis will not include that extensive data. Jackfish 1 Project. Jackfish 1 consists of 42 WPs divided into six pads (including the 28 WPs in Pads A, B, C, and D analyzed for the authors’ study) and is part of the geological oil-sands trend of the Athabasca oil-sands deposit. Jackfish 1 has a nameplate capacity of 35,000 BOPD, with a designed SOR of 2.7. Consistently exceeding 90% of its nameplate capacity since first steam, it is commonly considered to be a successful SAGD project. The results of data analysis for the WPs studied indicated the following: Considerable variance exists in the recovery performance of SAGD WPs on the same pad. Considering the small drainage area of the 7-WP pad (800×800 m), such variances bear important implications for the planning and execution of SAGD projects (particularly greenfield development but also with regard to brownfield expansion). The authors stress that forecasting project productivity should not be based on a single-WP, deterministic simulation model without any consideration for distributions in reservoir geology and parameters affecting SAGD performance. This assessment is equally applicable in the estimation of reserves.

Graph-based structural controllability and observability of steam assisted gravity drainage pressure chamber: A data driven approach

Abstract Distributed parameter processes are challenging when it comes to modeling and control. Steam assisted gravity drainage (SAGD), used for in-situ extraction and recovery of oil sands bitumen, is a large scale distributed parameter process. The analysis of control relevant properties like controllability and observability enables to address the problem of control of steam chamber growth and sensor placement. We present a data driven and computationally affordable technique to assess the controllability and observability of the SAGD steam chamber dynamics in a structural perspective by exploiting the underlying interaction amongst different regions of the reservoir. A reservoir simulator is used to gather the data, and density-based clustering combined with Granger causality is used to develop a directed graph through which the structural controllability and observability of the SAGD process is characterized. This paper presents a detailed procedure and results for the sensor and actuator locations for partial and full controllability and observability to validate the discussed approach using the data acquired from the CMG-STARS simulator.

Cleaner production from Steam-Assisted Gravity Drainage using seismic-based automated control

Besides cost, one major challenge facing the recovery of bitumen from the Canadian oil sands is the amount of greenhouse gas (GHG) emitted to the environment from the Steam-Assisted Gravity Drainage (SAGD) process. The situation is worse in reservoirs that are considered difficult-to-produce due to their geology. This has placed social pressures on oil sands operators to find cleaner, more environmentally friendly ways to produce oil from these reservoirs. The research documented here examines the combined use of flow control devices, specifically outflow control devices (OCDs) in the steam injection wells, and automated control to improve SAGD steam chamber conformance thereby raising the recovery process' efficiency and lowering its environmental impact. The combination of process control and flow control devices in the steam injection well is novel and offers a new method to reduce environmental impact of the process. The results show that there is an increase of the cumulative oil produced by up to 10.7% and 6.16% when compared to the base and OCDs without control cases, respectively, after 6 years of operation. There has also been a reduction of up to 11.7% and 6.5% of the cumulative steam-to-oil ratio, and thus the GHG emissions to produced oil ratio, compared to the base and OCDs without control cases, respectively. The results confirm that cleaner production is possible for SAGD by using seismic-based control together with OCDs in SAGD operations.

Evolution characteristics of SAGD steam chamber and its impacts on heavy oil production and heat consumption

Currently, Steam-Assisted Gravity Drainage (SAGD) is the most successful commercialized method used to produce bitumen from oil sands and heavy oil reservoirs. Precise description of the steam chamber evolution is important for evaluating the economic effectiveness and Greenhouse Gas (GHG) emission of the SAGD process. In this study, the properties of MacKay River Oil Sands were used in laboratory experiments to compare the chamber evolution and production performance of SAGD under different permeability distributions. Then, a mathematical model was established to predict the steam chamber evolution and the closely related oil production and heat consumption in a heterogeneous formation. Next, the calculated production performance and steam chamber evolution were compared with measured experimental data to verify the accuracy of the model. Finally, the chamber evolution characteristics and their impacts on SAGD oil production and heat consumption are discussed in this paper for formations with different permeability distributions. The results indicate that horizontal permeability controls the evolution of steam chamber such that higher horizontal permeability may cause an obvious convex shape of the chamber edge, whereas vertical permeability has little effect on the chamber shape despite significant influence on the oil production in the early stage of SAGD. Moreover, a convex-shaped chamber interface indicates a higher production rate in the spreading stage and a lower rate in the depleting stage. In addition, this study shows that to minimize the heat consumption of the SAGD process, so that GHG emission can be curbed, a concave-like chamber shape is favorable in the early spreading stage, whereas a convex shape is better in the late spreading stage and depleting stage.

Optimization of Fast-steam-assisted gravity drainage for the energy-efficient operations at a heterogeneous oil-sands reservoir

This paper discusses the energy-efficient operation of Fast-steam-assisted gravity drainage wellpad system in the presence of reservoir heterogeneity, different well constraints, and lateral flux communication between adjacent steam chambers. Fast-steam-assisted gravity drainage incorporates cyclic steam stimulation in an unrecovered area between steam-assisted gravity drainage wellpairs, and the well constraints of the wellpad system (including the injection pressure and steam injection rate at the injectors, bottom hole pressure, surface liquid rate, and steam rate at the producers) are simultaneously optimized to accomplish the minimum cumulative steam-to-oil ratio for a given bitumen recovery constraint. The higher injection pressures of the cyclic steam stimulation can result in greater efficiency by pushing the diluted fluid mixture to the steam-assisted gravity drainage producers through the cross-over zone between the steam chambers. At an early stage, a greater amount of steam should be injected through the cyclic steam stimulation work, and at the late stage, a lower injection pressure is needed to use the latent heat. The positive effects of the cyclic steam stimulation at the edges of the steam-assisted gravity drainage steam chambers are concentrated at localized flow paths where the lateral flux transport occurs due to spatial heterogeneity. A sensitivity analysis shows that the injection pressure and the steam rate produced for the steam-assisted gravity drainage wellpairs influence the energy efficiency of the entire thermal operation when compared to other configurations.

Comment on: “Feasibility of electromagnetic methods to detect and image steam-assisted gravity drainage steam chambers” (S. G. R. Devriese and D. W. Oldenburg, Geophysics, 81, no. 4, E227–E244)

Devriese and Oldenburg (2016), abbreviated below as DO16, propose a synthetic case study aimed at demonstrating the advantages and limitations in using galvanometric- and induction-based resistivity methods to monitor the growth of steam-assisted gravity drainage steam chambers. They first simulate a simplistic case and then they apply their strategy to a more realistic geometry inspired from a case study operated by Imperial Oil. The geophysical work is based on a set of petrophysical relationships and classical regularization methods for the inverse problem. The petrophysical model used to connect the underlying physics of the problem to the geophysical observable is far from being correct, raising some questions regarding the validity of the input parameters, hence conclusions reached in the paper. The choices for the regularizers and inversion schemes are also problematic as discussed below. I start first with the petrophysical model used in DO16. Despite the claims made in DO16, equation 11 is not Archie’s law for the resistivity formation factor and cannot be found in the seminal paper of Archie (1942). It is a mix of the first and second Archie’s law with a tortuosity added to the equation without any justification. Archie’s law is an equation connecting the intrinsic formation factor to the porosity through a power law relationship. The formation factor can also be defined as the ratio of the tortuosity by the connected porosity (Pride, 1994). In their equation 11, we have both factors used concomitantly, which is not correct (see the discussion in Appendix A of Revil, 2013). Then, DO16 propose to use a Waxman and Smits (1968) equation, see their equations 12–15, to take into account the surface conductivity. Their form of the Waxman and Smits equation suffers four problems:

Feasibility of electromagnetic methods to detect and image steam-assisted gravity drainage steam chambers

We have investigated the use of electric and electromagnetic (EM) methods to monitor the growth of steam-assisted gravity drainage (SAGD) steam chambers. SAGD has proven to be a successful method for extracting bitumen from the Athabasca oil sands in Alberta, Canada. However, complexity and heterogeneity within the reservoir could impede steam chamber growth, thereby limiting oil recovery and increase production costs. Using seismic data collected over an existing SAGD project, we have generated a synthetic steam chamber and modeled it as a conductive body within the bitumen-rich McMurray Formation. Simulated data from standard crosswell electrical surveys, when inverted in three dimensions, show existence of the chamber but lack the resolution necessary to determine the shape and size. By expanding to EM surveys, our ability to recover and resolve the steam chamber is significantly enhanced. We use a simplified survey design procedure to design a variety of field surveys that include surface and borehole transmitters operating in the frequency or time domain. Each survey is inverted in three dimensions, and the results are compared. Importantly, despite the shielding effects of the highly conductive cap rock over the McMurray Formation, we have determined that it is possible to electromagnetically excite the steam chamber using a large-loop surface transmitter. This motivates a synthetic example, constructed using the geology and resistivity logging data of a future SAGD site, where we simulate data from single and multiple surface loop transmitters. We have found that even when measurements are restricted to the vertical component of the electric field in standard observation wells, if multiple transmitters are used, the inversion recovers three steam chambers and discerns an area of limited steam growth that results from a blockage in the reservoir. The effectiveness of the survey shows that this EM methodology is worthy of future investigation and field deployment.

Drained/Undrained-Zones Boundary in Steam-Assisted-Gravity-Drainage Process

Summary Li et al. (2004) described three zones at the edge of steam chambers on the basis of drainage conditions: drained, partially drained, and undrained. In the drained zone, the pore pressure is controlled by injection pressure, and fluid mobility within this region is sufficient to drain additional pore pressures because of shear dilation and pore-fluid thermal expansion. The undrained zone lies beyond the partially drained zone and extends to virgin reservoir far beyond the chamber. In this zone shearing behaves under undrained conditions; by this, Li et al. (2004) mean no volume change occurs but shear lead to changes in pore pressure. Li et al. (2004) proposed that the boundaries of these zones are dependent on bitumen viscosity, which relates to the temperature distribution beyond the steam interface. Because drained/undrained conditions affect the geomechanics at the edge of the chamber, we investigate whether the assumption of Li et al. (2004) that there is no volume change within the sheared zone is correct and is supported by field data. Here, we establish the physics associated with the undrained zone at the edge of steam-assisted gravity-drainage steam chamber and explore the pressure front vs. temperature front of different oil-sand field projects. The results reveal that the drained zone governed by pressure-front advancement is greater in extent than the sheared zone. The thermodynamics of the undrained zone are discussed to derive a new theory for mechanothermal phenomena at the edge of the chamber. The results from the theory show that the drained zone extends beyond the temperature front and thus, from a geomechanical point of view, the system solely consists of the drained and partially drained zones.

Understanding the Heat-Transfer Mechanism in the Steam-Assisted Gravity-Drainage (SAGD) Process and Comparing the Conduction and Convection Flux in Bitumen Reservoirs

Summary SAGD is one successful thermal recovery technique applied in the Athabasca and Peace River reservoirs in central and northern Alberta, Canada. In SAGD, steam is injected into a horizontal injection well and is forced outward, losing its latent heat when it comes into contact with the cold bitumen at the edge of a depletion chamber. As a consequence, the viscosity of the bitumen falls several orders of magnitude, its mobility rises several orders of magnitude, and then it flows under gravity toward a horizontal production well located several meters below and parallel to the injection well. Heat-transfer mechanisms are pivotal to the SAGD process. Though heat energy is transferred from steam to reservoir by conduction and convection, heat transfer by convection is not considered in the classic SAGD mathematical models such as Butler?s. Researchers such as Butler and Stephens (1981), Reis (1992), Akin (2005), Liang (2005), Nukhaev et al. (2006), and Azad and Chalaturnyk (2010) considered conduction from steam to cold reservoir to be the only heat-transfer component. However, because the heat capacity of water is typically two to five times that of bitumen, convection caused by the mobile condensate flow in the reservoir may contradict these studies. Farouq-Ali (1997) was the first to criticize the assumption that there is only a thermal conduction mechanism in the SAGD process. He pointed out that with so much condensate flowing, convection would be expected to be the dominant heat-transfer mechanism, which can be plausible at high temperatures. In response, Edmunds (1999a) stated that on the basis of the associated change in enthalpy, the heat transfer into a cold reservoir because of convection is probably less than 5% of that because of conduction. Ito (1999) challenged Edmunds (1999a) statement, on the basis of Ito and Suzuki (1996, 1999) and Ito et al. (1998), pointing out that "this number, 5%; i.e., ratio between convection to conduction presented by Edmunds (1999a) is unrealistically low, (and) it should be in the range of 50%. This study examined the relative roles of convective and conductive heat transfer at the edge of SAGD steam chambers. In summary, the mathematical model developed in this study considered both conduction and convection, and the resultant output from the model is reasonably consistent with published field data. This study supports the idea that although convection can dominate near the chamber edge in high-water-saturation reservoirs, in bitumen-rich reservoirs, its contribution to heat transfer is less than 1% and can be neglected.

Simulation of Noncondensable Gases in SAGD-Steam Chambers

Summary Cenovus Energy has been developing the Foster Creek and Christina Lake projects using the steam-assisted gravity-drainage (SAGD) process successfully. The SAGD process at both these projects has been operated at well above the initial reservoir pressure for extended periods of time and this has been simulated adequately using dead-oil models, which omit solution gas from the simulations. As we move into later stages in the life of the more mature well pairs at these projects, it is important to understand the role of noncondensable gases on the development of the steam chambers better in order to optimize the methane-coinjection, steam rampdown, and, ultimately, blowdown phases of operations. Cenovus is also testing reduced-pressure SAGD and solvent-aided processes (SAPs) at these projects, and simulations indicate that noncondensable gas will play a significant role in these processes. Hence, understanding the flow behaviour of noncondensable gases in SAGD steam chambers could have far reaching consequences for lowering the energy intensity and associated costs, and reducing the environmental impact of bitumen production while potentially increasing reserves. This paper presents the results of some recent simulations which are improving our understanding of the role of solution gas in SAGD and the impact of noncondensable gases on steam chamber development.

Understanding the Heat-Transfer Mechanism in the Steam-Assisted Gravity-Drainage (SAGD) Process and Comparing the Conduction and Convection Flux in Bitumen Reservoirs

Summary SAGD is one successful thermal recovery technique applied in the Athabasca and Peace River reservoirs in central and northern Alberta, Canada. In SAGD, steam is injected into a horizontal injection well and is forced outward, losing its latent heat when it comes into contact with the cold bitumen at the edge of a depletion chamber. As a consequence, the viscosity of the bitumen falls several orders of magnitude, its mobility rises several orders of magnitude, and then it flows under gravity toward a horizontal production well located several meters below and parallel to the injection well. Heat-transfer mechanisms are pivotal to the SAGD process. Though heat energy is transferred from steam to reservoir by conduction and convection, heat transfer by convection is not considered in the classic SAGD mathematical models such as Butler's. Researchers such as Butler and Stephens (1981), Reis (1992), Akin (2005), Liang (2005), Nukhaev et al. (2006), and Azad and Chalaturnyk (2010) considered conduction from steam to cold reservoir to be the only heat-transfer component. However, because the heat capacity of water is typically two to five times that of bitumen, convection caused by the mobile condensate flow in the reservoir may contradict these studies. Farouq-Ali (1997) was the first to criticize the assumption that there is only a thermal conduction mechanism in the SAGD process. He pointed out that with so much condensate flowing, convection would be expected to be the dominant heat-transfer mechanism, which can be plausible at high temperatures. In response, Edmunds (1999a) stated that on the basis of the associated change in enthalpy, the heat transfer into a cold reservoir because of convection is probably less than 5% of that because of conduction. Ito (1999) challenged Edmunds (1999a) statement, on the basis of Ito and Suzuki (1996, 1999) and Ito et al. (1998), pointing out that “this number, 5%; i.e., ratio between convection to conduction presented by Edmunds (1999a) is unrealistically low, (and) it should be in the range of 50%.” This study examined the relative roles of convective and conductive heat transfer at the edge of SAGD steam chambers. In summary, the mathematical model developed in this study considered both conduction and convection, and the resultant output from the model is reasonably consistent with published field data. This study supports the idea that although convection can dominate near the chamber edge in high-water-saturation reservoirs, in bitumen-rich reservoirs, its contribution to heat transfer is less than 1% and can be neglected.

Oil production rate predictions for steam assisted gravity drainage based on high-pressure experiments

Dual-well steam assisted gravity drainage (SAGD) has significant potential for extra-heavy oil recovery. China is conducting two dual-well SAGD pilot projects in the Fengcheng extra-heavy oil reservoir. Quick, direct predictions of the oil production rate by algebraic models rather than complex numerical models are of great importance for designing and adjusting the SAGD operations. A low-pressure scaled physical simulation was previously used to develop two separate theoretical models corresponding to the two different growth stages observed in the SAGD steam chambers, which are the steam chamber rising stage and the steam chamber spreading stage. A high-pressure scaled model experiment is presented here for one dual-well SAGD pattern to further improve the prediction models to reasonably predict oil production rates for full production. Parameters that significantly affect the oil recovery during SAGD were scaled for the model size based on the reservoir characteristics of the Fengcheng reservoir in China. Experimental results show the relationship between the evolution of the steam chamber and the oil production rate during the entire production stage. High-pressure scaled model test was used to improve the gravity drainage models by modifying empirical factors for the rising model and the depletion model. A new division of the SAGD production regime was developed based on the relationship between the oil production rate and the evolution of steam chamber. A method was developed to couple the rising and depletion models to predict oil production rates during the SAGD production, especially during the transition period. The method was validated with experiment data and field data from the literature. The model was then used to predict the oil production rate in the Fengcheng reservoir in China and the Athabasca reservoir in Canada.

The Effect of Geometrical Properties of Reservoir Shale Barriers on the Performance of Steam-assisted Gravity Drainage (SAGD)

Many bitumen reservoirs contain shale layers of varying thickness, lateral extent, and frequency. These shale layers, depending on their size, vertical and horizontal locations, and continuity throughout the reservoir, may act as a flow barrier and severely reduce vertical permeability of the pay zone and slow down the steam-assisted gravity drainage steam chamber development. Therefore, to improve productivity in these reservoirs, understanding of the effects of reservoir heterogeneities has become necessary. This work presents numerical investigation of the effects of shale barriers on steam-assisted gravity drainage performance when applied to produce mobile heavy oil. The most concern of the work is upon how different geometrical properties of these shale barriers, such as vertical and horizontal position, density, discontinuity, and dispersion through the reservoir, affect the steam-assisted gravity drainage performance and steam chamber development in above wells region. Simulation results showed that the performance of steam-assisted gravity drainage is lower in the case of presence of continued shale layers, their longer extension, and their higher density (number of shale layers per unit volume). As the vertical distance between the continued shale layer and injector decreases, the recovery factor will be less. Distribution scheme of the layers also affected the recovery performance and production rates, since higher was the produced oil in the case of scattered layers as compared to the case of stacked shale layers on top of each other. It was found that the effect of the aforementioned parameters on the ultimate oil recovery factor and production rates was less in the case of presence of discontinuities in the shale barriers.