T.N. Nasr

Novel Expanding Solvent-SAGD Process "ES-SAGD"

The steam assisted gravity drainage (SAGD) process has been successfully tested in field pilots, and commercial applications are currently underway by a number of oil companies. The process yields higher oil rates and faster reservoir depletion, as compared to other in situ oil recovery processes. Current developments of the SAGD process are aimed at improving oil rates, improving oil-to-steam ratios OSR, reducing energy, and minimizing water disposal requirements. In addition to SAGD, progress has been made in the development of solvent injection process. These processes result in lower oil rates and energy, requirements as compared to SAGD. At the present time, limited field results are available for the solvent processes to allow for adequate evaluation of field performance. A novel approach for combining the benefits of steam and solvents in the recovery of heavy oil and bitumen has been undertaken at the Alberta Research Council (ARC). A newly patented Expanding Solvent SAGD ES-SAGD process has been developed. The process has been successfully field-tested and resulted in improved oil rates improved OSR, and lower energy and water requirements as compared to SAGD. The paper discusses the concept and laboratory testing of the ES-SAGD process.

Simulating the ES-SAGD Process With Solvent Mixture in Athabasca Reservoirs

The ES-SAGD process was developed to improve the energy and oil drainage efficiency of the SAGD process. The idea of the ES-SAGD process is to co-inject solvent with steam and the co-injected solvent mixes with the bitumen to further reduce the viscosity of the heated bitumen along the boundary of the steam chamber thus enhances the oil recovery. Practically, the co-injected solvent will be a solvent mixture (such as diluent /naphtha) because of its availability and reduced cost than a pure hydrocarbon. This paper reports the results of an ES-SAGD lab test conducted with steam and diluent co-injection using Athabasca bitumen. To simulate the ES-SAGD test, a pseudo-component scheme to represent the complex solvent mixture in the numerical model is derived, based on the diluent composition and measured PVT data. The behaviours and ef- fects of the co-injected solvent in the ES-SAGD process are analyzed through detailed history matching of the ES-SAGD test. Numerical sensitivity analyses are also performed to investigate the effects of some key parameters in the numerical approach.

Laboratory Experimental Testing and Development of an Efficient Low Pressure ES-SAGD Process

Certain Athabasca reservoirs have low pressures because they have been depleted due to production of overlying gas. Other reservoirs are naturally occurring low pressure shallow bitumen reservoirs. Hence, there is a need to develop or investigate recovery processes under which such low pressure reservoirs can be developed. As a result of this, experiments were initiated to extend the Expanding Solvent-SAGD (ES-SAGD) process application to low pressure Athabasca reservoirs in order to evaluate oil recovery from such reservoirs. The goal of these experiments is to develop a low pressure ES-SAGD process with better performance than, or comparable performance to, that of the high pressure SAGD process. This paper describes five sets of laboratory experiments examining recovery processes, which includes a low pressure (500 kPag +/- 50 kPag) SAGD experiment, a propane-SAGD experiment, multi-component ES-SAGD (at low and high concentrations) experiments and a high pressure (2,100 kPag +/- 50 kPag) SAGD experiment. The results of these experiments are presented and analyzed in order to evaluate the performance of low pressure ES-SAGD in comparison to SAGD (at low and high pressure) and propane-SAGD at low pressure. The processes were assessed for recovery, recovery time, heat loss, steam chamber growth and energy efficiency. The principal conclusion is that the low pressure multi-component ES-SAGD at the right concentration (mostly at low concentration) is fairly competitive with SAGD at a high pressure. The energy consumption in the steam or steam/solvent zone per oil recovered (ECDZ) for low pressure multi-component ES-SAGD experiments is much lower than the low pressure and high pressure SAGD tests. The propane-SAGD test recovery is very low, even at higher energy consumption, than that of the ES-SAGD experiment at low concentration. The work presented in this paper shows that the application of a multi-component ES-SAGD process in the field at low pressure is a practical option. It also shows that bitumen/heavy oil reservoirs that would have remained untapped due to low reservoir pressure could be produced at lower energy consumption per oil recovered if a low pressure ES-SAGD process at low concentration of the diluents is employed in the recovery of the oil.

Field-Scale Numerical Simulation of SAGD Process With Top-Water Thief Zone

There is a major concern that the existence of thief zones, such as top water and/or a gas cap overlying the oil sand deposit, has a detrimental effect on the oil recovery in the application of the steam-assisted gravity drainage (SAGD) process. The objective of this numerical study is to investigate SAGD performance in the Athabasca oil sands in the presence of a top water zone. The reservoir model, STARS, developed by the Computer Modelling Group (CMG) Ltd., has been previously validated based on a 3D SAGD laboratory experiment with top water that was conducted at the Alberta Research Council (ARC). It is believed that the numerical simulation captured the major mechanism of oil movement from the pay zone into the top water zone, as was observed in the experiment. In the field-scale simulation, SAGD performance in the presence of confined and non-confined top water zones was investigated. The operating strategies under the conditions of non-depleted top water/non-depleted pay zones and depleted top water/non-depleted pay zones were considered. Numerical findings indicated that: (1) there is a detrimental effect of a top water zone on SAGD performance; (2) plugging of a top water zone with oil was not observed in this study for a top water thickness of 8 m; and, (3) operating conditions that lead to a higher pressure difference between the steam chamber and the top water, either by depletion of the top water zone pressure or a higher steam injection pressure, results in a more detrimental effect on the SAGD performance.

SAGD Operating Strategies

The steam assisted gravity drainage (SAGD) process has emerged as an effective technology for the recovery of oil from oil sands deposits that are deeply buried for surface mining. In general, the process involves drilling paired horizontal wells, one well above the other and separated by a distance, near the bottom of the oil-bearing formation. The top well is used to inject steam into the oil sands, heating up the oil and allowing it to drain, under the action of gravity, into the bottom well. The paper first provides a review of the technological advances made in SAGD operating strategies. In addition to the successful field testing at the Underground Test Facility (UTF), the SAGD process has been recently extended to other types of reservoirs. These include reservoirs with lower permeability as compared to that at the UTF site and reservoirs with bottom water transition zones. Challenges facing application of the process in such types of reservoirs will be discussed. Concepts to overcome the challenges will be presented in terms of laboratory studies that have been carried out. The concepts tested include: the use of mechanical means such as drilling vertical drainage channels between paired horizontal wells; combining a vertical well with paired horizontal wells and potential for solvent and gaseous additives in steam (to improve SOR and reduce water requirements). Results provide valuable observations for field seale development of a more robust and economically viable process.

SAGD Wind-Down: Lab Test and Simulation

As SAGD moves from pilot test to commercial operation, a number of issues need to be dealt with. These include diagnosing and solving operational problems and improving energy efficiency. One of the methods of improving energy efficiency is to prolong oil production after steam injection stops by using the energy remaining in place. The results of a laboratory experiment and corresponding numerical history matching are reported in this paper. The study showed that the hot chamber continued its expansion after steam injection was stopped and a gas injection was initiated. The continuous expanding period represented the most productive period in the gas injection wind-down process. A total of 12.5% of OOIP was recovered during wind-down. Successful history matching of both the oil production curve and temperature profiles at different times demonstrated that the numerical simulation could handle the gas/steam mixing phenomena. Gas concentration profiles from numerical simulation indicated that gas was concentrated at the region where oil saturation was experiencing big changes.

SAGD Application in Gas Cap and Top Water Oil Reservoirs

The paper presents experimental results on the impact of top water and gas caps on SAGD performance. The effect of the top thief zone on oil drainage rates and potential oil and steam loss into the top zone were measured. The study involved the use of a large scale high-pressure/high temperature experimental facility for injecting steam into an oil sand pack and measuring oil drainage rates and development of temperature ahead of the steam chamber. Numerical modelling was conducted to predict field scale performance using the CMGs STARS simulator. An elemental experimental approach was used in the study to simulate a generic reservoir in the Athabasca region with a pay zone thickness of 50 m and an overlying thief zone thickness of 8 m. In this approach, a reservoir element was selected close to the oil/top thief zone interface. The element was located ahead of an advancing steam front. In order to set the initial conditions of the laboratory element to be similar to those in the field, field scale numerical simulation was conducted to determine the temperature distribution in the element. The field scale temperature profile was established in the laboratory elemental model to represent the elements initial temperature before the start of steam injection during the experiments. The paper discusses the results from the study and highlights the potential implications of the top thief zone on SAGD applications. In addition, differences between gas cap and top water thief zones on impacting the thermal and recovery efficiency of the SAGD process are demonstrated.

SAGD application in gas cap and top water oil reservoirs

The paper presents experimental results on the impact of top water and gas caps on SAGD performance. The effect of the top thief zone on oil drainage rates and potential oil and steam loss into the top zone were measured. The study involved the use of a large scale high-pressure/high temperature experimental facility for injecting steam into an oil sand pack and measuring oil drainage rates and development of temperature ahead of the steam chamber. Numerical modelling was conducted to predict field scale performance using the CMGs STARS simulator. An elemental experimental approach was used in the study to simulate a generic reservoir in the Athabasca region with a pay zone thickness of 50 m and an overlying thief zone thickness of 8 m. In this approach, a reservoir element was selected close to the oil/top thief zone interface. The element was located ahead of an advancing steam front. In order to set the initial conditions of the laboratory element to be similar to those in the field, field scale numerical simulation was conducted to determine the temperature distribution in the element. The field scale temperature profile was established in the laboratory elemental model to represent the elements initial temperature before the start of steam injection during the experiments. The paper discusses the results from the study and highlights the potential implications of the top thief zone on SAGD applications. In addition, differences between gas cap and top water thief zones on impacting the thermal and recovery efficiency of the SAGD process are demonstrated.

Impacts of Initial Gas-to-Oil Ratio (GOR) on SAGD Operations

This study addresses the important role of initial gas-to-oil ratio (GOR) in steam assisted gravity drainage (SAGD) operations. A numerical model using CMG’s STARS was validated through history matching of laboratory experiments conducted at the Alberta Research Council. The impacts of initial GOR on process performance were then studied using field scale numerical simulations. The results indicate that high initial GOR may have beneficial effects, namely, reduction of oil viscosity, and improvement of the oil-to-steam ratio (OSR). A detrimental impact, however, is also shown as the gas impedes the rate of steam chamber growth, and hence reduces oil production rates. Further analysis indicated that this is because of a “dynamic vacuum” effect due to steam condensation at the front of the steam chamber. This dynamic vacuum effect dominates the diffusion process and creates a gas-rich zone at the front of the steam chamber, thereby resisting further growth of the steam chamber and slowing oil production. The same effect occurred when noncondensable gas was co-injected with steam in either live oil or dead oil reservoirs.

Generalized Phase Mobilities In Gravity Drainage Processes

Adequate numerical prediction and forecasting of reservoir performance rely on the knowledge of relative permeabilities. In gravity driven processes the flow is complex, consisting of co-current and counter-current flows. This work describes the characteristics of gravity driven flow and provides generalized permeabilities (or mobilities) for such flow. The results can be used in improved numerical simulation of gravity drainage processes. BACKGROUND The. standard multiphase relative permeability approach used in existing reservoir simulators is sufficient for describing steady state processes for either coor countercurrent flow. However, in the case of complex dueedimensional flow that combines both coand counter-current components, such as that in gravity drainage processes, transport coefficients have more complex form and should be addressed through matrix formulation. Furthermore, in the case of non-steady state counter-current flow in gravity drainage, the effective mobility becomes a more complex ftmction of steady state relative permeabilities and the staudard approach no longer represents the physics of the process accurately. During the last decade, new matrix formulation of phase mobilities has been extensively discussed in the literature. The theory is mature and its implementation is long overdue. The new momentum equations are a natural extension of the standard formulation with proper representation of crossinfhrences between flowing phases. The theory provides improved flow description as compared to the standard relative permeability concept in modelhng complicated nonsteady state processes such as gravity drainage. This is due to: 1. correct description of multiphase flows, 2. easy incorporation into existing multiphase simulators, 3. being a natural extension of existing standard relative permeability theory.