Gas Assisted Gravity Drainage Process Research Articles

Experimental Study on Oil-Gas Interface Stability Mechanism and Control Method of Gas-Assisted Gravity Drainage

Abstract The stability of oil-gas interface is an important factor affecting the recovery of the gas-assisted gravity drainage (GAGD). In this paper, the interface morphology characteristics of GAGD in porous media under 8 flow rates and 3 displacement dip angles were studied by using a visual physical model. The non-dimensional parameters such as capillary number and bond number were introduced to qualitatively characterize the oil gas interface morphology, and the process of gas invading the pore throat of porous media under different interface morphologies was revealed. The fractal dimension is employed for quantitative characterization of the morphology of the oil gas interface during the GAGD process, and the boundary of stable interface and unstable interface was divided. The study showed that the balance effect of gravity on the competition between capillary force and viscous force was the main reason for the stability of GAGD flooding leading edge. The stability of the oil-gas interface can be ensured by controlling the bond number to be greater than 2.75×10−4 and the capillary number to be less than 2.48×10−3. With a constant bond number, an increase in the capillary number leads to a gradual transition of the oil-gas interface from stable to unstable. Similarly, with a constant capillary number, an increase in the bond number causes the oil-gas interface to shift from stable to unstable. The fractal dimension divides the boundary between stable and non-stable displacement. When Df is greater than 1.87, the oil interface is stable, when Df is less than 1.75, the oil interface is viscous fingering, and when Df is between 1.75 and 1.87, the oil interface is mainly capillary fingering.

Experimental evaluation of Carbon Dioxide-Assisted Gravity Drainage process (CO2-AGD) to improve oil recovery in reservoirs with strong water drive

The Gas-Assisted Gravity Drainage (GAGD) process has been developed to improve and enhance oil production in both secondary and tertiary recovery stages. In the GAGD process, oil is produced through horizontal wells above the oil-water contact. Therefore, high levels of water cresting are probably happening, especially in reservoirs with active water aquifers. However, all the previous GAGD feasibility experimental studies have never been addressing the existence of active water aquifers. Consequently, the current research was conducted to test the immiscible GAGD process feasibility to improve oil recovery and minimize water cut in brown reservoirs with strong water coning tendencies.A Hele-Shaw model consists of two parallel glass plates packed with silica sand to visually discern the process performance. Specifically, the CO2-AGD process feasibility versus the Free Fall Gravity Drainage (FFGD) mechanism was investigated on the physical model with large, moderate, and without bottom water drive. The experimental results indicated that the GAGD process performance is higher performance than the FFGD in all runs because of its fast high oil recovery, especially when implemented on the reservoir without a bottom water drive. The immiscible CO2-AGD process is not only important in terms of increasing oil recovery, but it also significantly delays the gas breakthrough. The strength of the aquifer inversely impacts the gas breakthrough time as increasing the aquifer strength results in fast oil production and early gas breakthrough. Additionally, the immiscible GAGD experiments illustrated that oil recoveries in the FFGD and GAGD processes improved to 35% and 47% of OOIP for the model without bottom water drive, respectively. However, the oil recovery increased in the same FFGD and GAGD processes to 61% and 63% of OOIP for the model with a large bottom water drive, respectively. Specifically, the ultimate oil recovery during the GAGD process with a large bottom water drive was higher than the FFGD run with only 2%; while there is increasing in ultimate oil recovery for the model without the bottom water drive of about 12%. Consequently, the GAGD process is more efficient to improve oil recovery in reservoirs with limited or without bottom water drive.

Characteristics of gas-oil contact and mobilization limit during gas-assisted gravity drainage process

Gravity can reduce the instability of the gas-oil contact that is caused by gas channeling in locations with low flow resistance, such as high-permeability layers, macropores, and fractures during the gas-assisted gravity drainage process. Herein, the microscopic forces during the gas-assisted gravity drainage process were analyzed and combined with the capillary model to study the occurrence boundary of gas-assisted gravity drainage process, and the characteristics of the gas-oil contact in the gas-assisted gravity drainage process was discussed. The results show that free gravity drainage occurs only in pores where a certain height of the oil column and pore radius are reached. Furthermore, the lower the oil-gas interface migration rate, the easier free gravity drainage occurs. In other scenarios, additional gas injection is required. During the gas-assisted gravity drainage process, the gas-oil contact moves down stably as a transition. The width of the transition zone and the available pore radius are related to the gas-oil contact migration rate and the oil viscosity; the smaller the gas-oil contact migration rate and the lower the oil viscosity, the smaller pore throat can be involved in mobilization. Optimizing the gas injection rate and reducing the oil viscosity can delay the gas channeling maturity time, which is beneficial for the realization of the gas-assisted gravity drainage process. Finally, a method considering micropore heterogeneity is established for determining the critical gas injection rate, while the mainstream pore throat can be involved in mobilization and the gas-oil contact can be stabilized at the same time. The method of determining the critical gas injection rate can help researchers and reservoir engineers to better understand and implement the gas-assisted gravity drainage process. Cited as: Kong, D., Gao, J., Lian, P., Zheng, R, Zhu, W., Xu, Y. Characteristics of gas-oil contact and mobilization limit during gas-assisted gravity drainage process. Advances in Geo-Energy Research, 2022, 6(2): 169-176. https://doi.org/10.46690/ager.2022.02.08

A Single-Well Gas-Assisted Gravity Drainage Enhanced Oil Recovery Process for U.S. Deepwater Gulf of Mexico Operations

The U.S. Deepwater Gulf of Mexico (DGOM) area that has some of the most prolific oil reservoirs is still awaiting the development of a viable enhanced oil recovery (EOR) process. Without it, DGOM will remain severely untapped. Exorbitant well costs, in excess of $200 million, preclude having extensive injection patterns, commonly used in EOR design frameworks. Aside from injection patterns, even operationally waterflooding has met with significant challenges because of injectivity issues in these over pressurized turbidities. The gas-assisted gravity drainage (GAGD) EOR process, that holds promise for deepwater environments because of lesser injectivity issues, among others, has been adapted in this work to overcome these limitations. A novel design in the form of a single well—gas assisted gravity drainage (SW-GAGD) process, has been demonstrated to emulate the benefits of a GAGD process in a cost-effective manner. Unlike conventional GAGD processes, which need multiple injectors and separate horizontal production wells, the SW-GAGD process just uses a single well for injection and well production. The performance of the process has been established using partially scaled visual glass models based on dimensional analyses for scale up of the process. The recovery factor has been shown to be in the range of 65–80% in the immiscible mode alone, and the process is orders of magnitude faster than natural gravity drainage. A toe-to-heel configuration of the SW-GAGD process has also been tested and for the configuration to be immune from reservoir layering, the toe of the well should ideally end at the top of the payzone. Better sweep of the payzone and consequent high recovery factor of 80% OOIP was observed, if the heel part of the bottom lateral is located in a lower permeability zone.

Pore-scale investigation of immiscible gas-assisted gravity drainage

Gas-assisted gravity drainage (GAGD) is an effective method of oil recovery that is influenced by the properties of the fluids and formations involved. In this paper, a direct numerical simulation method is employed to investigate immiscible GAGD in an oil-wet porous medium. The interface between oil and gas is tracked via the phase-field method. A series of numerical simulations are performed over a large range of values of various factors (gravity force, capillary force, viscous force, viscosity ratio, and porous medium properties) to investigate the gas flooding process in a porous medium. The results show the oil–gas interface as a transition zone that migrates during the GAGD process. Gravity improves oil–gas interfacial stability because the continuous oil film gravity-assisted hydraulic connection effect can overcome the capillary force in a small pore. The oil displacement process is dominated by gravity instead of the capillary and viscous forces when the gravitational number exceeds 500. Finally, the pore-scale dimensionless number (Npore) enables a quantitative analysis of the effects of various factors on GAGD. Npore helps predict GAGD oil recovery.

Numerical Simulation of Immiscible CO2-Assisted Gravity Drainage Process to Enhance Oil Recovery

The Gas Assisted Gravity Drainage (GAGD) process has become one of the most important processes to enhance oil recovery in both secondary and tertiary recovery stages and through immiscible and miscible modes. Its advantages came from the ability to provide gravity-stable oil displacement for improving oil recovery, when compared with conventional gas injection methods such as Continuous Gas Injection (CGI) and Water – Alternative Gas (WAG).
 Vertical injectors for CO2 gas were placed at the top of the reservoir to form a gas cap which drives the oil towards the horizontal oil producing wells which are located above the oil-water-contact. The GAGD process was developed and tested in vertical wells to increase oil recovery in reservoirs with bottom water drive and strong water coning tendencies. Many physical and simulation models of GAGD performance were studied at ambient and reservoir conditions to investigate the effects of this method to enhance the recovery of oil and to examine the most effective parameters that control the GAGD process.
 A prototype 2D simulation model based on the scaled physical model was built for CO2-assisted gravity drainage in different statement scenarios. The effects of gas injection rate, gas injection pressure and oil production rate on the performance of immiscible CO2-assisted gravity drainage-enhanced oil recovery were investigated. The results revealed that the ultimate oil recovery increases considerably with increasing oil production rates. Increasing gas injection rate improves the performance of the process while high pressure gas injection leads to less effective gravity mediated recovery.

Potential application of the CO<SUB align="right">2-assisted gravity drainage process in a mature oil field: insights from reservoir-scale EOR evaluation

The gas-assisted gravity drainage (GAGD) process was implemented through continuous and cyclic immiscible injection to enhance the recovery of bypassed oil in the upper sandstone reservoir in the South Rumaila oil field, located in Southern Iraq. A compositional simulation model was constructed for the carbon dioxide (CO2) flooding evaluation through the GAGD process implementations. After achieving history matching, 20 vertical CO2 injectors and 11 horizontal oil producers were placed in top reservoir and oil zone, respectively. The immiscible CO2-GAGD performance was evaluated for 10 years of future prediction. The continuous and cyclic immiscible GAGD cases resulted in reaching recovery factor of 14.5% and 21.3% given the remaining oil, respectively. However, the recovery factor given the remaining oil was 7.6% through primary production by the end of prediction period. Additionally, the obtained amount of oil in 10 years primary production can be produced in only one year by the continuous case and in 8 months by the cyclic case. Consequently, the optimal implementation of the GAGD process is by adopting the cyclic CO2 flooding that efficiently enhance the recovery of bypassed oil. [Received: December 30, 2017; Accepted: December 17, 2018]

From Coreflooding and Scaled Physical Model Experiments to Field-Scale Enhanced Oil Recovery Evaluations: Comprehensive Review of the Gas-Assisted Gravity Drainage Process

The gas-assisted gravity drainage (GAGD) process has been suggested to improve oil recovery in both secondary and tertiary stages through immiscible and miscible injection modes. In contrast to con...

Robust Optimization of Cyclic CO2 flooding through the Gas-Assisted Gravity Drainage process under geological uncertainties

The purpose of this research is to determine an estimate for actual optimal oil recovery through cyclic Gas-Assisted Gravity Drainage (GAGD) process in a heterogeneous sandstone reservoir under geological uncertainties. A robust optimization approach was adopted to determine the optimal durations of gas injection, soaking, and oil production under geological uncertainties. 100 stochastic reservoir realizations of the 3D permeability and porosity distributions were created to honor geological constraints. Ranking was applied through quantifying of reservoir oil response to select P10, P50, and, P90 that represent the overall reservoir uncertainty. A compositional reservoir flow simulation was used for the GAGD process performance evaluation. Approximately 200 simulation jobs were created, including the aforementioned durations and geological uncertainty parameters, through Design of Experiments (DoE). The Latin Hypercube Sampling was adopted to create these 200 simulation jobs that then evaluated by the compositional reservoir simulation to calculate the cumulative oil production by the end of 10 prediction years. The robust optimization approach was then applied to select the true optimal solution of the highest oil recovery by taking into account the geological uncertainties in permeability, porosity, and anisotropy models. The nominal optimization of one single realization was also adopted for the comparison. The robust optimization has shown its feasibility to increase oil production through the cyclic GAGD process from 4.535 to 4.62547 billion barrels. However, the nominal optimization case increased oil production to 5.9726 billion barrels. The presented robust optimization workflow under geological uncertainties resulted in higher oil recovery and net present value than nominal realization optimization, with providing degrees of freedom for the decision-maker to significantly reduce the project risk. It was specifically concluded that the robust optimal solution represents the most economically feasible solution to obtain the highest NPV through the GAGD process for a range $(30–80) per barrel oil prices. However, the base case and nominal solution (no geological uncertainties) were not economical when the oil price declines to be less than 36 and 32, respectively.

Pore-Level Investigation of the Influence of Wettability and Production Rate on the Recovery of Waterflood Residual Oil with a Gas Assisted Gravity Drainage Process

Gas assisted gravity drainage (GAGD) is an oil recovery mechanism that can be implemented after waterflood to enhance the recovery of oil. The performance of postwaterflood GAGD is affected by a variety of parameters that determine the balance between capillary, gravitational, and viscous forces. In this research, the influence of the wettability, heterogeneities, and production rate on the recovery of oil have been studied at the pore level to recognize phenomena affecting mechanisms of oil recovery visualizing interfaces in a newly designed micromodel that contains a coarse pore network covered by fine capillaries. Experimental results show that regions with high oil saturation (oil-bank) were formed ahead of the gas front in both oil-wet and water-wet micromodels when the production rate was low. Under oil-wet conditions, the size of the oil-bank was greater, and the recovery of oil initiated prior to a gas breakthrough. Under water-wet conditions, the flow of the residual oil after a gas breakthrough ...

Optimizing the location of the gas injection well during gas assisted gravity drainage in a fractured carbonate reservoir using artificial intelligence

Gas assisted gravity drainage (GAGD) is a novel subdivision of gas injection method. In this method the injection wells are located in the upper bed of the oil zone, and the production wells are drilled at the bottom bed of the oil zone. Reservoir simulation is among the decision tools for investigating production rate and selecting the best scenarios for developing the oil and gas fields. Selecting the location of the injection wells for reaching the optimized pressure and production rate is one of the most significant challenges during the injection process. Recent experiences have shown that artificial intelligence (AI) is a reliable solution for taking the mentioned decision appropriately and in a least possible time. This study is attributed to the investigation of applying the artificial neural network (ANN) as an artificial intelligence method and a potent predictor for choosing the most proper location for injection in a GAGD process in a fractured carbonate reservoir. The results of this investigation clearly show the efficiency of the ANN as a powerful tool for optimizing the location of the injection wells in a GAGD process. The comparison between the results of ANN and black oil simulator indicated that the predictions obtained from the ANN is highly reliable. In fact the production flow rate and pressure can be obtained in every possible location of the injection well.

Gas-assisted gravity drainage process for improved oil recovery in Bao Den fractured basement reservoir

Gas injection has been widely used for Improved Oil Recovery (IOR)/ Enhanced Oil Recovery (EOR) processes in oil reservoirs. Unlike the conventional gas injection (CGI) modes of CGI and Water Alternating Gas (WAG), the Gas-Assisted Gravity Drainage (GAGD) process takes advantage of the natural segregation of reservoir fluids to provide gravity stable oil displacement. It has been proved that GAGD Process results in better sweep efficiency and higher microscopic displacement to recover the bypassed oil from un-swept regions in the reservoir. Therefore, dry gas has been considered for injection in fractured basement reservoir, Bao Den (BD) oil field located in Cuu Long basin through the GAGD process application. This field, with a 5-year production history, has nine production wells and is surrounded by a strong active edge aquifer from the North-West and the South East flanks. The depth of basement granite top is about 2,800 mTVDss with a vertical oil column of 1,500m. The pilot GAGD project has been designed to test an isolated domain in the BD fractured basement reservoir where there is favorable reservoir conditions to implement GAGD. Both reservoir simulation and Lab test have been run and confirmed the feasibility and the benefit of GAGD project in the selected area.The Dry gas will be periodically injected through existing wellwith high water cut production that located in the isolated area. As the injected gas rises to the top to form a gas zone pushing GOC (gas oil contact) downward, and may push WOC (water oil contact) to lower part of this producer (or even away from bottom of the well bore) could lower down water cut when switch this well back to production mode. The matched reservoir model with reservoir and fluid properties have been used to implement sensitivity analysis, the result indicated that there is significantly oil incremental and water cut reduction by GAGDapplication. Many different scenarios have run to find the optimal reservoir performance through GAGD process. Among these runs, the optimal scenario, which has distinct target, requires high levels of gas injection rate to attain the maximum cumulative oil production.

Optimized well configuration of gas-assisted gravity drainage process

CO2 flooding has been widespread as one of the effective enhanced oil recovery methods by oil swelling and viscosity reduction. In spite of the efficiencies, the improvement of oil recovery falls short of expectation, especially for immiscible flooding because unswept zone by injected CO2 still exists at the bottom of the reservoir owing to gravity overriding phenomenon. Gas-assisted gravity drainage (GAGD) has been applied to use CO2 gravity overriding conversely and improve carbon capture sequestration efficiency. During GAGD, the area of swept zone by injected CO2 is affected greatly by injector length and producer well perforation design. The producer well perforation just below the injectors inhales CO2 downwardly, which interrupts gas spread from the top. Moreover, CO2 out of the injector far away from the top is affected by the downward pull of the producer more than buoyancy to the top; so, gas does not spread throughout the wide range of the area. The unswept zone existed below the injectors at the end of production can be solved by new perforations in the producer under the injectors. Results obtained would give not only the strength of GAGD but also a suggestion for an effective well length and perforation design of GAGD process.

Capillary, viscous and gravity forces in gas-assisted gravity drainage

The capillary, viscous and gravity forces were analyzed by performing numerical simulations of gas assisted gravity drainage (GAGD) process. The analysis was conducted using three dimensionless groups: capillary number (the ratio between the viscous and the capillary forces), bond number (the ratio between the gravity and the capillary forces) and gravity number (the ratio between the gravity and the viscous forces). The effect of each number on the oil recovery factor was evaluated. The results that were obtained from the numerical models were also compared with data that were obtained from the physical and visual models, core floods and gravity drainage field projects. The simulation results indicated that before the gas breakthrough, higher oil recoveries were obtained at lower capillary and bond numbers and higher gravity number. In addition, after the gas breakthrough, higher oil recoveries were obtained at higher capillary and bond numbers and lower gravity number. The numerical models are consistent with the reported results in the literature. In conclusion, a numerical simulation with dimensionless groups can be a useful tool to predict the performance of the gas-assisted gravity drainage process. • The capillary, viscous and gravity forces are analyzed in GAGD process. • The analysis was conducted using three dimensionless groups. • A 2D numerical simulation of GAGD process is proposed. • Comparison with other models of GAGD process is also presented.

Scaling Analysis and Modeling of Immiscible Forced Gravity Drainage Process

Scaling study of fluids displacement leads to proper understanding of pore-to-field scale flow mechanisms and correct evaluation of effectiveness of various recovery methods. Scaling study of immiscible forced gravity drainage, or gas assisted gravity drainage (GAGD), at laboratory scale and reservoir scale is considered here. Inspectional analysis (IA) is used to determine dimensionless scaling groups that characterize the fluid displacement and production mechanisms. It is found that scaling immiscible GAGD displacement in a homogeneous reservoir needs matching of five dimensionless scaling groups. For heterogeneous reservoirs, Dykstra-Parson coefficient which represents the permeability heterogeneity is also required. It is shown that none of the dimensionless groups can individually correlate the efficiency of the process. Hence, a new combined dimensionless group in reservoir scale which incorporates all the dominant forces is derived. The model is evaluated and verified by comparing its predictions with experimental results and extensive field simulations figures. The model is found reliable for fast oil recovery prediction of GAGD process after 2 pore volume injection in homogeneous and heterogeneous reservoirs and proposing their optimal production plan.

Predicting the Method of Oil Recovery in the Gas-assisted Gravity Drainage Process

Gas-assisted gravity drainage (GAGD), solving the problem of low volume sweep coefficient for continuous gas injection (CGI) or water-alternating-gas injection (WAG) and increasing the ultimate recovery to a great extent, is a latest method for enhancing oil recovery. However, no effective predicting methods of oil recovery for GAGD exists. The oil recovery is determined by a set of parameters, such as heterogeneity of the formation, wettability, oil and gas density, oil and gas viscosity, viscous force, capillary force, gravity, gas injection rate, and other controllable parameters. Based on the research of producing mechanism of GAGD and dimensional analysis method, the authors analyzed four dimensionless varies, capillary number, bond number, gravity number, and gravity drainage number, and evaluated its influence on oil recovery. Considering the effect of wettability, viscosity ratio, and the difference between oil density and gas density, gravity number and gravity drainage number are modified. Relationship of gravity drainage number with recovery is re-established by experiments and case data. Finally a new predicting method of oil recovery for GAGD is set up. Application shows that this method is far more suitable for GAGD and its predictive result is more reasonable.

Flow Regime Characterization of the Gas-assisted Gravity Drainage Process

Flow regime characterization is helpful in designing efficient gas injection programs in commercial floods. Lenormand et al.'s (1988) phase-diagram is commonly used for flow regime identification of gravity drainage. Lenormand's phase-diagram was developed using horizontal micro-model displacement experiments, whereas the gas-assisted gravity drainage process is vertical displacement, so Lenormand et al.'s plot is not applicable for gas-assisted gravity drainage floods. Inspection of the recovery plots against dimensionless numbers and physical interpretation of dimensionless groups guide flow regime identification. Oil recovery of the gas-assisted gravity drainage process was calculated by using a black oil simulator at different injection rates and aspect ratios. Flow regime of the gas-assisted gravity drainage process was developed based on gravity number and capillary number at different aspect ratios.

The Scaling of the Gas-assisted Gravity Drainage Process Using Dimensionless Groups

Scaling of the gas-assisted gravity drainage process using inspectional analysis leads to a better understanding of the process. Seven scaling groups were derived by using inspectional analysis for the gas-assisted gravity drainage process and were reduced to five independent dimensionless groups. Based on these five dimensionless groups, a new dimensionless group was proposed for better representing all factors and forces that affect the process. The newly proposed dimensionless group was derived from experimental data in the literature and was validated by using reservoir simulation data. This number that is derived from experimental data in the core scale can predict oil recovery in reservoir scale by some modification in gravity number.

Gas Assisted Gravity Drainage by CO2 Injection

The gas assisted gravity drainage process is designed to take advantage of gravity between the injected gas and reservoir crude oil due to the difference in their densities. In this experimental study, a 2-D Hele-Shaw physical model with dimensions of 15 × 66 × 3 cm was packed with uniform and different sand packs to conduct visual experiments of CO2 injection. These experiments were designed to imitate the dimensionless parameters observed in some field projects. The results show high Bond number lead to high oil recovery because of large absolute permeability. Additionally, the reduction rate of CO2 injection from 50 to 20 cc/min tends to decrease the amount of Capillary number and then increase oil recovery. Comparison between free and forced gravity drainage applying forced gravity drainage was found to be essential with regard to oil recovery.

A Gas-assisted Gravity Drainage Process in Naturally Fractured Reservoirs

Water alternating gas injection or simultaneous water and gas injection have been proposed as a good candidate to minimize gravity segregation and provide more efficient enhanced oil recovery performance in comparison to conventional continuous gas injection. However, water alternating gas-based processes can cause some difficulties with an increase in water saturation (e.g., water shielding) including diminished gas injectivity (Rao et al., 2004). As an effective alternative for water alternating gas injection, gas-assisted gravity drainage was developed for conventional reservoirs (U.S. Patent 2006/0289157) that takes advantage of the natural segregation of gas from liquid hydrocarbon during injection. The gas-assisted gravity drainage process consists of placing a horizontal well near the bottom of an oil column and injecting gas through existing vertical wells. Gravity segregation of injected gas will cause liquid hydrocarbon and water to drain down to the aforementioned horizontal well. Application of gas-assisted gravity drainage for enhanced oil recovery in a naturally fractured reservoir is evaluated in this article based on some facts and figures.