SummarySteam-assisted Gravity Drainage Research Articles

Mechanics of SAGD Efficiency Improvement Using Combination of Chemicals: An Experimental Analysis through 2D Visual Models

Summary Steam-assisted gravity drainage (SAGD) is (and will be) a dominating method for in-situ recovery of heavy oil and bitumen in Canada. Its efficiency, however, has been a chronic problem due to the excess use of water and energy causing inflated costs. Solutions are needed to both maintain the production, especially at the late stages, and reduce the amount of steam. One method to improve efficiency is injection of chemicals with steam. This study addresses this problem by focusing on three critical aspects of SAGD: sweep improvement (faster and larger chamber growth), better displacement efficiency (lower residual oil), and reducing the amount of steam (lower steam/oil ratio and lowered steam temperature). By addressing these issues, we provide answers for the proper formulation of chemical blends, optimal injection strategies (continuous or slug), and the right time to introduce chemicals (beginning, midstream, or mature phase). The efficiency of a single chemical additive in recovery improvement is limited because it serves for only one of the mechanisms previously listed. For a better performance, blending chemicals with different functionalities was proposed in this study. Additives showing the highest microscopic oil displacement efficiency (heptane, Novelfroth® 190) and the highest areal sweep efficiency EA [LTS-18, silicon dioxide (SiO2) nanoparticle, Tween™ 80, deep eutectic solvent (DES) 11] in our previous studies were selected. Eleven different combinations of these six chemical additives were tested for different injection strategies, and the SAGD process was visualized on Hele-Shaw cells filled with heavy oil. The interaction between different chemical additives was determined by analyzing emulsification, wettability alteration, and the growth and shape of the steam chamber. The optimal formulation and ideal injection strategy were selected by analyzing ultimate EA, microscopic oil displacement efficiency, ultimate oil recovery, energy and water consumption, and the price of chemical additives. The interaction rule of combined chemical additives and their contribution to the recovery during SAGD were clarified, and the optimal injection strategies (best blends and proper time to introduce chemicals) were identified to provide a reference for chemical selection for field applications. This is a critical attempt to reduce the steam/oil ratio (particularly the amount of steam used) in SAGD applications, especially at mature stages.

Review of Temperature Dependence of Thermal Properties in Steam-Assisted Gravity-Drainage Process: Mathematical Formulation

Summary Steam-assisted gravity drainage (SAGD) is the preferred thermal-recovery method used to produce bitumen from Athabasca deposits in Alberta, Canada. In SAGD, heat energy is transferred from steam to the reservoir, which reduces bitumen viscosity, and enables the bitumen to flow toward the horizontal production well under gravity forces. Conduction is the main heat-transfer mechanism at the edge of the steam chamber. It is controlled by reservoir thermal conductivity and heat capacity. Both factors are temperature dependent and neglecting such dependency can cause errors that should be evaluated in one study. In this study an analytical SAGD model is discussed that includes temperature dependency in the prediction of the temperature evolution ahead of the interface, oil-production rates, and steam/oil ratio (SOR). Such a model is used to evaluate the importance of temperature dependency in different operational conditions. The results of the presented analytical model reveal that the oil rate can be calculated ignoring temperature dependency, suggesting thermal conductivity and specific heat calculated at a dimensionless temperature ratio between 0.75 to 0.8 and between 0.37 to 0.41, respectively. For both laterally expanding and angularly expanding reservoirs, the SOR is independent of the thermal conductivity. Unlike the effect of temperature-dependent thermal conductivity on SOR, temperature dependency on heat capacity significantly affects SOR. Such errors are larger for higher injection pressures (hence, higher steam-chamber temperatures). This study provides a modified Butler analytical SAGD model that combines independently calculated modification factors for thermal conductivity and heat capacity into a modification factor (CT). Interestingly, the modification factor is identical with the TANDRAIN factor that is suggested by Butler to reduce the overestimated rates from the original theory. Such a finding is important because it addresses modifications such as in TANDRAIN and LINDRAIN estimations that are a result of ignoring the temperature dependency in analytical formulation.

On Temperature-Falloff Interpretation in the Circulation Phase of the Steam-Assisted-Gravity-Drainage Process

Summary Steam-assisted gravity drainage (SAGD) is the preferred method to extract bitumen from Athabasca oil-sand reservoirs in western Canada. Bitumen at reservoir conditions is immobile because of high viscosity, and its saturation is typically large, which limits the injectivity of steam at in-situ conditions. In current industry practice, steam is circulated within injection and production wells. In theory, wells should be converted to SAGD production mode after a period when bitumen is mobile and communication is established between the injector and the producer. Operators use temperature-falloff data to predict successful conversion time. But temperature-falloff data are evaluated qualitatively, and there is not an analytical/numerical framework in which one can use such data. Although the bitumen heating sounds simple, approach wells are failing after steam injection because of steam breakthrough or sand production. Most of these wells are periodically returned to circulation/bullheading to ramp up production rates and heal the hot spots. Most of such failures are associated with early conversion to full SAGD, which shows the need to formulate an analytical/numerical framework to predict the right timing for conversion to full SAGD. In this presentation, the time of flight (ToF) is effectively used to convert spatial variations of temperature into time response of temperature variation at the well sandface. ToF defines the time an oil droplet needs to travel through a medium—more specifically, from its current location to the well sandface. By solving the heat transfer and Darcy's law simultaneously, the ToF is converted to a relationship of the temperature vs. time profile at the producer. This approach has been applied to SAGD well pairs with different geology, and the temperature-falloff trends are presented.

Influence of Pressure Difference Between Reservoir and Production Well on Steam-Chamber Propagation and Reservoir-Production Performance

Summary Steam-assisted gravity drainage (SAGD) is the most-effective thermal recovery method to exploit oil sand. The driving force of gravity is generally acknowledged as the most-significant driving mechanism in the SAGD process. However, an increasing number of field cases have shown that pressure difference might play an important role in the process. The objective of this paper is to simulate the effects of injector/producer-pressure difference on steam-chamber evolution and SAGD production performance. A series of 2D numerical simulations was conducted using the MacKay River and Dover reservoirs in western Canada to investigate the influence of pressure difference on SAGD recovery. Meanwhile, the effects of pressure difference on oil-production rate, stable production time, and steam-chamber development were studied in detail. Moreover, by combining Darcy's law and heat conduction along with a mass balance in the reservoir, a modified mathematical model considering the effects of pressure difference is established to predict the SAGD production performance. Finally, the proposed model is validated by comparing calculated cumulative oil production and oil-production rate with the results from numerical and experimental simulations. The results indicate that the oil production first increases rapidly and then slows down when a certain pressure difference is reached. The pressure difference has strong effects on steam-chamber-rising/expansion stages. However, at the expansion stage, lower pressure difference can achieve the same effect as high pressure difference. In addition, it is shown that the steam-chamber-expansion angle is a function of pressure difference. Using this finding, a new mathematical model is established considering the modification of the expansion angle, which (Butler 1991) treated as a constant. With the proposed model, production performance such as cumulative oil production and oil-production rate can be predicted. The steam-chamber shape is redefined at the rising stage, changing from a fan-like shape to a hexagonal shape, but not the single fan-like shape defined by (Butler 1991). This shape redefinition can clearly explain why the greatest oil-production rate does not occur when the steam chamber reaches the caprock. Literature surveys show few studies on how pressure difference influences steam-chamber development and SAGD recovery. The current paper provides a modified SAGD production model and an entirely new scope for SAGD enhanced oil recovery (EOR) that makes the pressure difference a new optimizable factor in the field.

An Investigation Into Propagation Behavior of the Steam Chamber During Expanding-Solvent SAGP (ES-SAGP)

Summary Steam-assisted gravity drainage (SAGD) and steam and gas push (SAGP) are used commercially to recover bitumen from oil sands, but for thin heavy-oil reservoirs, the recovery is lower because of larger heat losses through caprock and poorer oil mobility under reservoir conditions. A new enhanced-oil-recovery (EOR) method, expanding-solvent SAGP (ES-SAGP), is introduced to develop thin heavy-oil reservoirs. In ES-SAGP, noncondensate gas and vaporizable solvent are injected with steam into the steam chamber during SAGD. We used a 3D physical simulation scale to research the effectiveness of ES-SAGP and to analyze the propagation mechanisms of the steam chamber during ES-SAGP. Under the same experimental conditions, we conducted a contrast analysis between SAGP and ES-SAGP to study the expanding characteristics of the steam chamber, the sweep efficiency of the steam chamber, and the ultimate oil recovery. The experimental results show that the steam chamber gradually becomes an ellipse shape during SAGP. However, during ES-SAGP, noncondensate gas and a vaporizable solvent gather at the reservoir top to decrease heat losses, and oil viscosity near the condensate layer of the steam chamber is largely decreased by hot steam and by solvent, making the boundary of the steam chamber vertical and gradually a similar, rectangular shape. As in SAGD, during ES-SAGP, the expansion mechanism of the steam chamber can be divided into three stages: the ascent stage, the horizontal-expansion stage, and the descent stage. In the ascent stage, the time needed is shorter during ES-SAGP than during SAGP. However, the other two stages take more time during nitrogen, solvent, and steam injection to enlarge the cross-sectional area of the bottom of the steam chamber. For the conditions in our experiments, when the instantaneous oil/steam ratio is lower than 0.1, the corresponding oil recovery is 51.11%, which is 7.04% higher than in SAGP. Therefore, during ES-SAGP, not only is the volume of the steam chamber sharply enlarged, but the sweep efficiency and the ultimate oil recovery are also remarkably improved.

Approximate Physics-Discrete Simulation of the Steam-Chamber Evolution in Steam-Assisted Gravity Drainage

SummarySteam-assisted gravity drainage (SAGD) is a thermal-recovery process to produce bitumen from oil sands. In this technology, steam injected in the reservoir creates a constantly evolving steam chamber while heated bitumen drains to a production well. Understanding the geometry and the rate of growth of the steam chamber is necessary to manage an economically successful SAGD project. This work introduces an approximate physics-discrete simulator (APDS) to model the steam-chamber evolution. The algorithm is formulated and implemented using graph theory, simplified porous-media flow equations, heat-transfer concepts, and ideas from discrete simulation. The APDS predicts the steam-chamber evolution in heterogeneous reservoirs and is computationally efficient enough to be applied over multiple geostatistical realizations to support decisions in the presence of geological uncertainty. The APDS is expected to be useful for selecting well-pair locations and operational strategies, 4D-seismic integration in SAGD-reservoir characterization, and caprock-integrity assessment.

Real-Time Production Optimization of Steam-Assisted-Gravity-Drainage Reservoirs Using Adaptive and Gain-Scheduled Model-Predictive Control: An Application to a Field Model

Summary Steam-assisted gravity drainage (SAGD) is a thermal-recovery process to produce bitumen from deep oil-sands deposits. The efficiency of the SAGD operation depends on developing a uniform steam chamber and maintaining an optimal subcool (difference in saturation and actual temperature) along the length of the horizontal well pair. Heterogeneity in reservoir properties might lead to suboptimal subcool levels without the application of closed-loop control. Recently, model-predictive control (MPC) has been proposed for real-time feedback control of SAGD well pairs based on real-time production, temperature, and pressure data along with other well and surface constraint information; however, reservoir dynamics has been represented using extremely simplified and unrealistic models. Because SAGD is a complex, spatially distributed, nonlinear process, an MPC framework with models that account for nonlinearity over an extended control period is required to achieve optimized subcool and steam conformance. In this research, two novel work flows are proposed to handle nonlinear reservoir dynamics in MPC. The first approach is adaptive MPC, and includes continuous re-estimation of the model at each control interval. This allows the evolution of the coefficients of a fixed-model structure such that the updated system-identification model in the MPC controller reflects current reservoir dynamics adequately. Another approach, gain-scheduled MPC, decomposes the subcool-control problem in a parallel manner, and uses a bank of multiple controllers rather than only one controller. This ensures effective control of the nonlinear reservoir system even in adverse control situations by using appropriate variations in input parameters based on the operating region. The work flows are implemented using a history-matched numerical model of a reservoir in northern Alberta. Steam-injection rates and liquid-production rate are considered input variables in MPC, constrained to available surface facilities. The well pair is divided into multiple sections, and the subcool of each section is considered an output variable. Results are compared with actual field data (in which no control algorithm is used), and are analyzed on the basis of two criteria: (1) Do all subcools track the set point while maintaining stability in input variables? and (2) Does the net present value (NPV) of oil improve with adaptive and gain-scheduled MPC? In general, we conclude that both adaptive and gain-scheduled MPC provide superior tracking of subcool set points and, hence, better steam conformance caused by adequate representation of reservoir dynamics by re-estimation of coefficients and multiple controllers, respectively. In addition, the results indicate stability in input parameters and improvement in economic performance. NPV is improved by 23.69 and 10.36% in case of adaptive and gain-scheduled MPC, respectively. The proposed work flows can improve the NPV of an SAGD reservoir by optimizing the well-operational parameters while considering constraints of surface facilities and minimizing environmental footprint.

On Subcool Control in the SAGD Producers—Part III: Efficiency of Subcool Trapping in the Nsolv Process

Summary Steam-assisted gravity drainage (SAGD) is currently the preferred thermal-recovery method used to produce bitumen from Athabasca deposits in Alberta, Canada. SAGD is, however, an energy-intensive process with large amounts of greenhouse-gas (GHG) emissions and required water treatment. One option to reduce emissions and water usage is with solvent-based techniques, such as the Nsolv™ process. Suncor and Nsolv have been working together on a bitumen-extraction solvent-technology (BEST) field demonstration at Suncor's Dover test site. The solvent-injection–produced-oil ratio (SvOR) is among the key performance indicators (KPIs) of the BEST facility. Solvent breakthrough caused by inefficiency of thermodynamic trapping, such as subcool trapping, contributes to SvOR and affects the facility's economic and artificial-lift efficiencies. Subcool is the temperature difference between the injected butane (at saturated condition) and produced fluids (mixture of butane condensate, upgraded bitumen, and formation water). One of the unknowns in this process is the efficiency of thermodynamic trapping. On the basis of field results, it is shown that, like the SAGD process, liquid-pool depletion presents a critical control on the performance of the recovery process. Although the produced fluids are depleted from the liquid pool at the base of the chamber, because liquid-butane viscosity dependency on temperature is not strong and its viscosity changes slightly with temperature, thermodynamic trapping (or subcool trapping) is not efficient. Stability analysis of BEST data suggests that vapor breakthrough is part of the process but can be minimized by operating at temperatures greater than the second subcool limit. The second subcool limit (or Nsolv optimal reservoir subcool) will vary over time, and it is shown to occur at slightly less than 4°C after liquid-pool development.

Investigation of Post-Abandonment Surface Subsidence in Steam-Assisted-Gravity-Drainage Operations

Summary Steam-assisted gravity drainage (SAGD) has been extensively applied in thermal recovery from oil sands reservoirs in the Athabasca region of Northern Alberta, Canada. As the steam chambers associated with SAGD well pairs become mature, a form of abandonment is often applied that may include pressure maintenance in the depleted zone. Quantification of potential surface subsidence associated with SAGD abandonment becomes critical especially when the mature wells are in proximity to future developments. In addition, induced shear stresses should be estimated to fulfill well-integrity requirements. In the context of this case study, first, the development of a static geomechanical model (SGM) derived from a fine-tuned geomodel realization is discussed, which forms the basis for the iteratively coupled simulation model. The calibration work flow of the coupled reservoir/geomechanical simulation model to historical heave data is then reviewed and the effects of different parameters on calibration quality are investigated. Finally, the estimation of subsidence and the induced shear stresses in the nearby wells are discussed, and the magnitude of residual heave is quantified. The results of this study show that only a fraction (up to 38%) of surface heave is reversible (in form of subsidence) during the abandonment phase. Therefore, the magnitude of the surface subsidence and the associated shear stresses are small. The modeling study has also shown that a small magnitude of subsidence may be recorded even 10 years after abandonment. However, more than 50% of the surface subsidence is observed in the first 2 years after abandonment. Other important findings of this study include documenting the effects of thief-zone interaction and pseudoundrained loading as they relate to irreversibility of surface heave; documenting the effects of various geomechanical parameters on the quality of calibration against the historical heave data; observation of the relative effects of the isotropic unloading, thermal expansion, and shear dilatancy on the magnitude of heave; and quantification of incremental, yet small, shear stresses along the nearby horizontal wells.

Experimental Study on Geomechanical Dilation of Oil Sands During Cold–Water Injection

Summary Steam-assisted-gravity-drainage (SAGD) processes become effective only after thermal and hydraulic communication between an injection and production well has been established during the startup operation of the well pair. Conventional steam-circulation startup operations typically take 2 to 3 months to achieve interwell communication, but reductions in the startup time can have a favorable impact on project economics. Enhancement of interwell permeability using fluid-injection (water, or steam, or solvent) strategies to promote geomechanical dilation of the oil sands has been proposed as a startup technique. These fluid-injection processes will produce complex interactions of thermal, geomechanical, and multiple-phase flow behavior in the interwell formation region. Understanding better the role that these interactions play in establishing well-pair communication will provide opportunities to improve SAGD recovery performance. A triaxial experimental program has been designed and executed to explore whether cold-water injection would be sufficient to induce enhancements in effective permeability to water from geomechanical dilation mechanisms. Sample preparation techniques were modified to allow the preparation of reconstituted, very dense water-wet/bitumen sand specimens with different fluid saturations and almost identical porosities. Reclaimed/cleaned tailings sand from oil-sands mining operations was used to prepare artificial specimens, which are representative of McMurray Formation oil sands. A water-wet or bitumen sand core plug was then tested in an environmental chamber to simulate reservoir boundary conditions in terms of stress state, temperature, and pore pressure. A set of experiments was carried out in a triaxial cell under either initial isotropic or initial anisotropic stress state. Experimental results highlight the promising potential to dramatically enhance effective permeability to water and porosity in the dilated zone using cold-water injection at modest levels of stress anisotropy. The experimental results also provide support for the development of numerical models used in predicting SAGD startup performance and proactive utilization of the dilation as startup process for in-situ oil-sands development.

On Subcool Control in Steam-Assisted-Gravity-Drainage Producers— Part I: Stability Envelopes

Summary Steam-assisted-gravity-drainage (SAGD) industry experience indicates that the majority of producer workovers occur because of liners or electrical submersible pumps (ESPs), and both failures appear to result from inefficient “steam-trap control.” Thermodynamic steam-trap control, also termed “subcool control,” is a typical operation strategy for most SAGD wells. Simply, subcool (or reservoir subcool vs. pump subcool) is the temperature difference between the steam chamber (or injected steam) and the produced fluid. The main objective is to keep subcool higher than a set value that varies between 0 to 40° and even higher values. This study presents a method to calculate the liquid-pool level from the temperature profile in observation wells, and liquid-pool shrinkage as a function of time. Unfortunately, it is not practical to monitor the liquid level by having observation wells for every SAGD well pair. For this reason, the algebraic equation for liquid-pool depletion on the basis of wellbore-drawdown, subcool, and emulsion productivity is generated. By use of this equation, the envelopes are suggested to differentiate three different regimes: “stable production,” “liquid-pool depletion,” and “steam-breakthrough limit.” Gas lift operations such as the MacKay River thermal project suggested that envelopes for constant wellbore drawdown are not practical. Therefore, the steam-breakthrough limit is defined for constant rate, which is more consistent in gas lift operations. In this study, the steam-breakthrough limit is validated for operation data from the MacKay River. This study provides a new insight into how factors such as production rate and wellbore drawdown can compromise subcool control and cause steam breakthrough, and how liquid-pool depletion may result in uncontrolled steam coning at long time. As a part of this study, a minimum-subcool concept (or target reservoir subcool) is presented as a function of skin and pressure drawdown. It is shown that the minimum subcool is highly dependent on the maturity of steam-chamber and underburden heat loss especially for zero-skin producers. The results of this work emphasize that the target subcool on the producer should increase slightly with chamber maturity, considering that the skin is nonzero for most SAGD producers.

Analytical Treatment of Steam-Assisted Gravity Drainage: Old and New

Summary Steam-assisted gravity drainage (SAGD) is a widely tested method for producing bitumen from oil sands (tar sands). Several analytical treatments of the basic process have been reported. In a typical model, the focus is on bitumen drainage ahead of an advancing heat front. In a few cases, a steady expression for bitumen-drainage rate is obtained. This has been modified by several investigators to include other effects. In all cases, the bitumen rate is obtained with no recourse to the steam-injection rate, which is worked out after the fact. The treatment of time dependence, in a few models, is tenuous, building it in partly by use of experimental data. In this work, the SAGD process is considered to develop during two stages: steam-chamber rise (or unsteady stage) and sideways-expansion (or steady stage). The sideways-expansion phase is modeled by two different approaches: constant volumetric displacement (CVD) and constant heat injection (CHI). In the transient-steam-chamber-rise stage of SAGD, initially there is no heat ahead of the rising front, but as the front rises with time, heat accumulates ahead of the front. In the sideways-spreading stage, there is a dynamic equilibrium situation. The accumulated heat ahead of the front plays a crucial role in this phase of SAGD modeling to find the advancing-front velocity. There is a reciprocal relation between the advancing-front velocity and the amount of stored heat ahead of the front. Higher front velocity leads to lower heat accumulation ahead of the front for mobilizing oil ahead and making it drain. By considering the equilibrium situation for thermal-recovery methods with a dominant-gravity-drainage driving force, the advancing-front velocity is responsible for heat accumulation ahead of the front, and, in turn, this heated oil drains away and is responsible for advancing the front. Therefore, the key point in the modeling is to determine the advancing-front movement that satisfies heat and mass balances over the system under equilibrium. In the CVD model, we postulate that the front movement is such that the steam-chamber growth is constant; that is, the oil-production rate is constant over time. In this work, it is shown that to obtain a constant oil-production rate from a mass balance, the injected heat has to be increased to compensate for the heat loss to the overburden and the growing accumulated heat ahead of the front caused by interface extension and decreasing front velocity. In the CHI model, heat is injected at a constant rate into the system, which provides heat for the growing steam-chamber size, increasing heat loss to the overburden, and heat flow by conduction ahead of the front. In this model, we are computing the front velocity that satisfies heat balance and mass balance for a constant heat-injection rate. Decreasing steam-chamber velocity with time from this model leads to decreasing oil-production rate. The modeling of the SAGD process in this work is different from that in previous works because it is believed that the steam-chamber velocity is the key point in SAGD modeling. In the CVD model, a constant maximum steam-chamber velocity is derived that gives a constant oil-production rate with a better agreement with field data. In the CHI approach, steam-chamber velocity, and hence the oil-production rate, is decreasing with time (strongly affected by increasing heat loss to the overburden).

Steam-Assisted Gravity-Drainage Performance With Temperature-Dependent Properties—A Semianalytical Approach

Summary Steam-assisted gravity drainage (SAGD) is one of the successful in-situ thermal-recovery methods for oil-sands production. In this paper, we provide a simple semianalytical model that can accurately analyze an SAGD project with variable properties. In particular, we investigate the effect of temperature-dependent properties such as thermal conductivity, heat capacity, and rock density on SAGD performance. The proposed model sequentially solves the transient nonlinear heat-transfer equation coupled with the continuity equation with Kirchhoff's transformation and the heat integral method (HIM). A criterion for timestep selection is defined on the basis of the Courant and Péclet numbers to guarantee the stability of the sequential technique. The results illustrate that the temperature-dependent physical properties affect temperature distributions ahead of steam chamber which consequently have a significant impact on the cumulative oil production and oil-production rate. Moreover, the results show that the temperature profile ahead of the steam chamber changes with time and space, and a 2D transient assumption for SAGD modeling is necessary. The semianalytical model runs in a small fraction of numerical-simulator runtime, yet it provides reasonable results. Thus, it has the potential to be used as a tool for quick SAGD evaluations.

Asphaltene Precipitation During Bitumen Extraction With Expanding-Solvent Steam-Assisted Gravity Drainage: Effects on Pore-Scale Displacement

Summary Steam-assisted gravity drainage (SAGD) is a proved enhanced-oil-recovery technique for oil-sand extraction. However, the environmental and the economic challenges associated with steam generation limit the application of this technology. To address these issues, we have investigated the effectiveness of expanding-solvent-SAGD (ES-SAGD) over base SAGD on a bitumen sample (8.8 °API). Experimental studies are conducted with a 2D physical model. Different strategies for solvent injection are tested (coinjection and cyclic injection) to examine the impact of the deposition of the asphaltene fraction of the bitumen on porous media and the behavior of the asphaltene fraction in produced oil. Toluene is used as asphaltene-soluble solvent, and n-hexane is selected as asphaltene-insoluble. Steam-chamber development is monitored with temperature profiles from 47 separate positions. The oil rate, recovery factor, and the produced-oil quality are evaluated together. The effectiveness of SAGD and ES-SAGD is discussed by considering the role of asphaltenes and their interactions with clays in both produced- and residual-oil samples. This study reveals that coinjection of hydrocarbon solvents with steam enhances the steam-chamber development with higher oil-production rate. Moreover, ES-SAGD results in recovery of more-upgraded oil and has a lesser environmental impact. We observe that the selections of solvent type and injection strategy are the most crucial parameters for the design of a hybrid SAGD process, and solvent cost and toxicity can be minimized with the recycling of solvent for continuous injection of solvents. High-energy consumption for steam generation during the SAGD process can be reduced by coinjection of proper solvent type with steam at a proper injection strategy. Our study reveals that the ES-SAGD process has environmental and economic benefits that are preferable to those of the base SAGD. However, some solvents can cause undesirable effects because of asphaltene destabilization and precipitation in production or transportation lines. The results of this work show that not only asphaltenes but also the other fractions of oil, along with the reservoir-clay type and the clay amount, affect the ES-SAGD performance.

Overview of Performance and Analytical Modeling Techniques for Electromagnetic Heating and Applications to Steam-Assisted-Gravity-Drainage Process Startup

Summary Steam-assisted gravity drainage is the method of choice to extract bitumen from Athabasca oil-sand reservoirs in Western Canada. Under reservoir conditions, bitumen is immobile because of high viscosity, and its typically high level of saturation limits the injectivity of steam. In current industry practice, steam is circulated within injection and production wells. Operators keep the steam circulating until mobile bitumen breaks through the producer and communication is established between the injector and the producer. The “startup” phase is a time-consuming process taking three or more months with no oil production. A variety of processes could be used to minimize the length of the startup phase, such as electromagnetic (EM) heating in either the induction (medium frequency) or radio-frequency ranges. Knowledge of the size of the hot zone formed by steam circulation and of the benefits of simultaneous EM-heating techniques increases understanding of the startup process and helps to minimize startup duration. The aim of the present work is to introduce an analytical model to predict startup duration for steam circulation with and without EM heating. Results reveal that resistive (electrothermal) heating with/without brine injection cannot be a preferable method for mobilizing the bitumen in startup phase. Induction slightly decreases startup time at frequencies smaller than 10 kHz, and at 100 kHz it can reduce startup time to less than two months.

Understanding the Thermo-Hydromechanical Pressurization in Two-Phase (Steam/Water) Flow and Its Application in Low-Permeability Caprock Formations in Steam-Assisted-Gravity-Drainage Projects

SummarySteam-assisted gravity drainage (SAGD) is one successful thermal-recovery technique applied in Alberta oil-sand reservoirs. When considering in-situ production from bitumen reservoirs, one must reduce viscosity for the bitumen to flow toward the production well. Steam injection is currently the most promising thermal-recovery method. Although steamflooding has proved to be a commercially viable way to extract bitumen from bitumen reservoirs, caprock integrity and the risk of losing steam containment can be challenging operational problems. Because permeability is low in Albertan thermal-project caprock formations, heating greatly increases the pressure on any water trapped in pores as a result of water thermal expansion. This water also sees a great increase in volume as it flashes to steam, causing a large effective-stress reduction. After this condition is established, pore-pressure increases can lead to caprock shear failure or tensile fracturing, and to subsequent caprock-integrity failure or potential casing failure. It is typically believed that low-permeability caprocks impede the transmission of pore pressure from reservoirs, making them more resistant to shear failure (Collins 2005, 2007). In considering the “thermo-hydromechanical pressurization” physics, low-permeability caprocks are not always more resistant. As the steam chamber rises into the caprock, the heated pore fluids may flash to steam. Consequently, there is a vapor region between the steam-chamber interface penetrated into the caprock and the water region within the caprock which is still at a subcritical state.This study develops equations for fluid-mass and thermal-energy conservation, evaluating the thermo-hydromechanical pressurization in low-permeability caprocks and the flow of steam and water after steam starts to be injected as part of the SAGD process. Calculations are made for both short-term and long-term responses, and evaluated thermal pressurization is compared for caprocks with different stiffness states and with different permeabilities. One can conclude that the stiffer and less permeable the caprock, the greater the thermo-hydromechanical pressurization; and that the application of SAGD can lead to high pore pressure and potentially to caprock shear, and to subsequent steam release to the surface or potential casing failure.

Understanding the Convection Heat-Transfer Mechanism in the Steam-Assisted-Gravity-Drainage Process

Summary Steam-assisted gravity drainage (SAGD) is the preferred method to extract bitumen from Athabasca oil-sand reservoirs in western Canada. In SAGD, steam, injected outward from a horizontal injection well, loses its latent heat when it contacts the cold bitumen at the edge of a steam chamber. Consequently, the viscosity of the bitumen falls several orders of magnitude, enabling it to flow under gravity toward a horizontal production well directly below to the injection well. It is commonly believed that conduction is the dominant heat-transfer mechanism at the edge of the chamber. Heat transfer by convection is not considered in classic SAGD mathematical models such as the one derived by Butler. Researchers such as Butler and Stephens (1981), Reis (1992), Akin (2005), Liang (2005), Nukhaev et al. (2006), and Azad and Chalaturnyk (2010) considered the conduction from steam to cold reservoir to be the only heat-transfer component. Farouq-Ali (1997), Edmunds (1999a, b), Ito and Suzuki (1996, 1999), Ito et al. (1998), Sharma and Gates (2011), and Irani and Ghannadi (2013) questioned the assumption that thermal conduction dominates heat transfer at the edge of a SAGD chamber. Sharma and Gates (2011) and Irani and Ghannadi (2013) studied convective flux from condensate flow at the edge of an SAGD steam chamber. Irani and Ghannadi (2013) derived a new formulation that solves the energy balance and pressure-driven condensate flow normal to the steam-chamber interface into the cold bitumen reservoir and concluded that the assumption of conduction-dominated heat transfer is valid; however, all previous analyses do not include convective heat transfer arising from draining bitumen and condensate. Although a few researchers have studied convective flux from condensate flow at the edge of an SAGD steam chamber (e.g., Sharma and Gates 2011; Irani and Ghannadi 2013), there is a lack of understanding of bitumen and condensate drainage parallel to the edge of the chamber and of its effect on transverse heat transfer into the oil sand beyond the chamber. In this study, the relative roles of convective heat flux both parallel and normal to the edge of a steam chamber are examined. The results suggest that the convective heat flux associated with flow parallel to the chamber edge is minor compared with that normal to the edge.

On the Stability of the Edge of a Steam-Assisted-Gravity-Drainage Steam Chamber

Summary Steam-assisted gravity drainage (SAGD) is a successful thermal-recovery technique applied in oil-sand reservoirs in which the viscosity of the oil (bitumen) is typically in the hundreds of thousands to millions of centipoise. For the in-situ production from bitumen reservoirs, bitumen viscosity must be reduced to achieve the mobility required to flow toward the production well. Many factors influence the efficiency and rate at which bitumen is mobilized. The controlling feature of steam-based recovery processes is heat transfer from the steam chamber to the formation—the greater the heat flux, the larger the oil volume heated, and the higher the oil-drainage rate. Previous studies have demonstrated that instability at the steam-chamber edge can enhance heat transfer there by creating limited-amplitude steam fingers that enlarge the heat-transfer area, thus leading to greater thermal efficiency of the recovery process. This, in turn, increases oil production. At this point, stability studies have focused on the instability between steam and oil at the edge of the chamber—none has examined the case between steam condensate and oil. In the research documented here, the stability between steam condensate and bitumen at the edge of the chamber is explored. Here, a steam-pressure diffusion equation at the moving chamber interface is derived. The perturbations of the pressure and condensate velocity are substituted into the pressure diffusion equation and Darcy's law to realize a linear-stability equation governing the growth of disturbances at the interface. The results show that the stability is controlled by moving-interface velocity and reservoir water-phase hydraulic diffusivity.

Discussion on the Effects of Temperature on Thermal Properties in the Steam-Assisted-Gravity-Drainage (SAGD) Process. Part 1: Thermal Conductivity

Summary Steam-assisted gravity drainage (SAGD) is the preferred thermal-recovery method used to produce bitumen from Athabasca deposits in Alberta, Canada. In SAGD, steam injected into a horizontal injection well is forced into the reservoir, losing its latent heat when it comes into contact with cold bitumen at the edge of a depletion chamber. Heat energy is transferred from steam to reservoir, resulting in reduced bitumen viscosity that enables the bitumen to flow toward the horizontal production well under gravity forces. Conduction is the main heat-transfer mechanism at the edge of the steam chamber in SAGD, and reservoir thermal conductivity is a key parameter in conductive-heat transfer. Conductive-heat transfer occurs at higher rates across reservoirs with higher thermal conductivity, which in turn affects the temperature profile ahead of the steam interface. Consequently, a reservoir with higher thermal conductivity will result in higher reservoir-heating rates, which lead to higher oil rates. However, when oil-sand reservoirs are heated from reservoir temperature to steam-chamber temperature, the thermal conductivity can decrease up to 25%, which affects the temperature profile and conductive heating at the edge of the steam-saturated zone known as the steam chamber. This study provides an analytical model that includes a temperature-dependent thermal-conductivity value. This novel approach is the first of its kind to incorporate a temperature-dependent thermal-conductivity value within an analytical SAGD model to predict temperature front, oil production, and steam/oil ratio (SOR). Furthermore, if Butler's (1985) model is used, the results reveal that the arithmetic average thermal-conductivity values at reservoir and steam temperatures could be used for temperature-profile prediction, which would result in an error of less than 1% for the range of SAGD applications. The results of this study suggest that the minimum error for oil rates depends on the viscosity/temperature correlation. The optimum thermal conductivity should be calculated at the temperature that gives dimensionless temperatures [i.e., (T−Tr)/(Tst−Tr)] varying between 0.75 to 0.85 for m-values [Butler-suggested power constants (Butler 1985, 1991; Butler and Stephens 1981)] between 3 and 5.6. This study also investigates the effect of including temperature-dependent thermal conductivity on SOR variation and suggests that for both laterally expanding and angularly expanding reservoirs the SOR is independent of the thermal conductivity.

Understanding the Steam-Hammer Mechanism in Steam-Assisted-Gravity-Drainage Wells

Summary Steam-assisted gravity drainage (SAGD) is one of the successful thermal-recovery techniques applied in Alberta oil-sands reservoirs. When considering in-situ production from bitumen reservoirs, viscosity reduction is necessary to mobilize bitumen, thereby flowing toward the production well. Steam injection is currently the most effective thermal-recovery method. Although steamflooding is commercially viable, condensation-induced water hammer (CIWH) resulting from rapid steam-pocket condensation can be a challenging operational problem. In steamflooding, steam is injected through a well down to the reservoir, warming it to temperatures of 150 to 270°C (302 to 518°F) to liquefy the bitumen inside the reservoir (Garnier et al. 2008; Xie and Zahacy 2011). The liquefied bitumen then drains to a lower well through which it is produced to the surface. In this process, steam pockets can become entrapped in subcooled condensate inside either the injection or the production tubing, causing a rapid collapse of the steam pocket. This type of rapid condensation is commonly referred to as "steam hammer." In this study, three different scenarios are explored to better understand steam-hammer situations in SAGD wells. These scenarios are at injectors or producers during the startup phase (or circulation phase), in the injection tubing during the injection phase, and in the production tubing during the injection phase. Modeling each of these scenarios indicates that a steam-hammer occurrence is likely in two of the three scenarios, but that its incidence can be mitigated. The likely scenarios for a steam-hammer occurrence are in either the injection or the production tubing during the startup phase, and in the injection tubing during the injection phase. Steam-hammer occurrences during the circulation period can be controlled by lowering the injection pressure and controlling water drainage into the reservoir. Flow shocks that occur as a result of countercurrent flow limiting (CCFL) are very likely to take place in the injection tubing during the injection phase but can be controlled by injecting at a higher steam quality. The least likely scenario for a steam-hammer occurrence is in the production tubing during the injection phase. This is because the produced (or breakthrough) steam temperature would need to be more than 20°C higher than the produced-liquid temperature to start a water-hammer condition.