8.5 Enhanced oil recovery

Enhanced oil recovery (EOR) techniques boost oil production from existing reservoirs by altering fluid properties or rock-fluid interactions. Applied after primary and secondary recovery, EOR aims to extract the remaining 50-70% of oil left in place, requiring a deep understanding of microscopic and macroscopic displacement mechanisms.

Chemical, thermal, gas injection, and microbial methods are common EOR approaches. Each targets specific reservoir conditions and oil properties to improve microscopic and macroscopic displacement efficiency. Successful EOR implementation involves careful screening, modeling, field testing, and monitoring to maximize oil recovery while minimizing environmental impact.

Fundamentals of enhanced oil recovery

• Enhanced oil recovery (EOR) aims to increase oil production from existing reservoirs by altering the properties of the reservoir fluids or the rock-fluid interactions
• EOR techniques are applied after primary and secondary recovery methods have been exhausted, typically when the oil recovery factor is between 30-50%
• The success of EOR depends on a thorough understanding of the microscopic and macroscopic displacement mechanisms in the reservoir

Primary vs secondary recovery

Top images from around the web for Primary vs secondary recovery

• 

  Methods in Oil Recovery Processes and Reservoir Simulation View original

  Is this image relevant?

• 

  Enhanced Oil Recovery by Injecting Oleic Acid as a Surfactant into the Porous Medium View original

  Is this image relevant?

• 

  Frontiers | Corrosion Control in CO2 Enhanced Oil Recovery From a Perspective of Multiphase Fluids View original

  Is this image relevant?

• 

  Methods in Oil Recovery Processes and Reservoir Simulation View original

  Is this image relevant?

• 

  Enhanced Oil Recovery by Injecting Oleic Acid as a Surfactant into the Porous Medium View original

  Is this image relevant?

1 of 3

Top images from around the web for Primary vs secondary recovery

• 

  Methods in Oil Recovery Processes and Reservoir Simulation View original

  Is this image relevant?

• 

  Enhanced Oil Recovery by Injecting Oleic Acid as a Surfactant into the Porous Medium View original

  Is this image relevant?

• 

  Frontiers | Corrosion Control in CO2 Enhanced Oil Recovery From a Perspective of Multiphase Fluids View original

  Is this image relevant?

• 

  Methods in Oil Recovery Processes and Reservoir Simulation View original

  Is this image relevant?

• 

  Enhanced Oil Recovery by Injecting Oleic Acid as a Surfactant into the Porous Medium View original

  Is this image relevant?

1 of 3

• Primary recovery relies on natural reservoir energy (pressure depletion, gravity drainage, or gas cap expansion) to drive oil to the production wells
• Secondary recovery involves the injection of water or gas to maintain reservoir pressure and displace oil towards the production wells
• Primary and secondary recovery methods typically recover only 20-40% of the original oil in place (OOIP)

Tertiary recovery techniques

• Tertiary recovery, also known as enhanced oil recovery, targets the remaining oil left behind after primary and secondary recovery
• EOR methods aim to improve the microscopic and macroscopic displacement efficiency by altering the properties of the reservoir fluids or the rock-fluid interactions
• Common EOR techniques include chemical flooding, thermal recovery, gas injection, and microbial enhanced oil recovery

Microscopic vs macroscopic displacement

• Microscopic displacement refers to the mobilization of oil at the pore scale, which is influenced by factors such as interfacial tension, wettability, and capillary forces
• Macroscopic displacement describes the sweep efficiency of the injected fluids, which is affected by reservoir heterogeneity, mobility ratio, and gravity segregation
• Successful EOR requires optimization of both microscopic and macroscopic displacement mechanisms to maximize oil recovery

Chemical enhanced oil recovery

• Chemical EOR involves the injection of chemicals into the reservoir to improve oil recovery by altering the properties of the reservoir fluids and the rock-fluid interactions
• The main chemical EOR methods are surfactant flooding, polymer flooding, and alkaline flooding
• Chemical EOR can be applied in a wide range of reservoir conditions, but the selection of the appropriate chemical formulation is crucial for the success of the process

Surfactant flooding

• Surfactants are amphiphilic molecules that reduce the interfacial tension (IFT) between oil and water, allowing for the mobilization of trapped oil droplets
• Ultra-low IFT (< 0.001 mN/m) is required for the formation of microemulsions, which can significantly improve the microscopic displacement efficiency
• Surfactant formulations must be tailored to the specific reservoir conditions (temperature, salinity, and crude oil composition) to ensure optimal performance

Polymer flooding

• Polymers are high molecular weight compounds that increase the viscosity of the injected water, improving the mobility ratio and reducing viscous fingering
• Commonly used polymers include partially hydrolyzed polyacrylamide (HPAM) and biopolymers such as xanthan gum
• Polymer flooding enhances the macroscopic sweep efficiency by providing a more uniform displacement front and reducing the effects of reservoir heterogeneity

Alkaline flooding

• Alkaline flooding involves the injection of high pH solutions (sodium hydroxide, sodium carbonate, or sodium silicate) to generate in-situ surfactants by reacting with the acidic components of the crude oil
• The generated surfactants reduce the IFT and alter the wettability of the reservoir rock, promoting the mobilization of residual oil
• Alkaline flooding is most effective in reservoirs with high acid number crudes and low salinity formation water

Synergistic effects of chemical EOR

• Combinations of surfactants, polymers, and alkalis can lead to synergistic effects that enhance oil recovery beyond the individual contributions of each chemical
• Alkaline-surfactant-polymer (ASP) flooding is a promising chemical EOR method that exploits the benefits of all three chemical agents
• The design of an optimal chemical formulation requires careful consideration of the interactions between the chemicals, the reservoir fluids, and the rock surface

Thermal enhanced oil recovery

• Thermal EOR methods involve the injection of heat into the reservoir to reduce the viscosity of heavy oils, improve mobility, and enhance oil recovery
• The main thermal EOR techniques are steam injection, cyclic steam stimulation, in-situ combustion, and electromagnetic heating
• Thermal EOR is most suitable for heavy oil reservoirs with high viscosity and low API gravity (< 20° API)

Steam injection

• Steam injection involves the continuous injection of high-temperature steam into the reservoir to heat the oil and reduce its viscosity
• The injected steam forms a steam zone that expands and displaces the heated oil towards the production wells
• Steam injection is effective in reservoirs with good vertical permeability and sufficient reservoir thickness to accommodate the steam zone

Cyclic steam stimulation

• Cyclic steam stimulation, also known as "huff and puff," is a three-stage process that involves steam injection, soaking, and production from the same well
• During the injection phase, steam is injected into the reservoir to heat the oil and reduce its viscosity
• The well is then shut-in for a soaking period to allow the heat to dissipate and the oil to mobilize
• Finally, the well is put back on production to recover the heated oil

In-situ combustion

• In-situ combustion is a thermal EOR method that involves the ignition and propagation of a combustion front through the reservoir
• Air or oxygen is injected into the reservoir to initiate and sustain the combustion of a small fraction of the oil in place
• The generated heat reduces the viscosity of the oil ahead of the combustion front, while the combustion gases help to drive the oil towards the production wells

Electromagnetic heating

• Electromagnetic heating uses high-frequency electromagnetic waves to heat the reservoir and reduce the viscosity of the oil
• The two main types of electromagnetic heating are microwave heating and radio frequency heating
• Electromagnetic heating can be applied in reservoirs with low permeability or high heterogeneity, where conventional thermal EOR methods may be less effective

Gas injection enhanced oil recovery

• Gas injection EOR involves the injection of gases into the reservoir to improve oil recovery through various mechanisms, such as oil swelling, viscosity reduction, and miscible displacement
• The main gases used in gas injection EOR are carbon dioxide, nitrogen, and hydrocarbon gases
• The success of gas injection EOR depends on the miscibility of the injected gas with the reservoir oil, which is influenced by factors such as reservoir pressure, temperature, and oil composition

Carbon dioxide injection

• Carbon dioxide (CO2) injection is a widely used gas injection EOR method that exploits the high solubility of CO2 in oil
• When CO2 dissolves in oil, it causes oil swelling, viscosity reduction, and interfacial tension reduction, leading to improved oil mobility and recovery
• CO2 injection can be either miscible or immiscible, depending on the reservoir pressure and temperature conditions

Nitrogen injection

• Nitrogen (N2) injection is a gas injection EOR method that is often used in high-pressure reservoirs where CO2 injection may not be feasible
• N2 is less soluble in oil compared to CO2, but it can still improve oil recovery through mechanisms such as pressure maintenance and gravity drainage
• N2 injection is typically used in deep, high-pressure reservoirs with light oils

Hydrocarbon gas injection

• Hydrocarbon gas injection involves the injection of natural gas or other hydrocarbon gases (methane, ethane, or propane) into the reservoir to improve oil recovery
• Hydrocarbon gases can achieve miscibility with the reservoir oil at lower pressures compared to CO2 or N2, making them suitable for shallow reservoirs with low fracture gradients
• The injected hydrocarbon gases can be recovered and reused, reducing the overall cost of the EOR process

Miscible vs immiscible displacement

• Miscible displacement occurs when the injected gas forms a single phase with the reservoir oil, eliminating the interfacial tension and capillary forces that trap residual oil
• Immiscible displacement involves the injection of a gas that does not form a single phase with the reservoir oil, but still improves oil recovery through mechanisms such as oil swelling and viscosity reduction
• Miscible displacement is more efficient than immiscible displacement, but it requires higher reservoir pressures and is more sensitive to reservoir heterogeneity

Microbial enhanced oil recovery

• Microbial enhanced oil recovery (MEOR) involves the injection of microorganisms or their metabolic products into the reservoir to improve oil recovery
• MEOR can enhance oil recovery through various mechanisms, such as the production of biosurfactants, biopolymers, and gases, as well as the alteration of reservoir properties
• MEOR is a potentially cost-effective and environmentally friendly alternative to conventional EOR methods, but its success depends on the ability of the microorganisms to thrive in the harsh reservoir environment

Microbial metabolic products

• Microorganisms can produce a wide range of metabolic products that can enhance oil recovery, such as biosurfactants, biopolymers, and gases (methane, carbon dioxide, or hydrogen)
• Biosurfactants reduce the interfacial tension between oil and water, promoting the mobilization of trapped oil droplets
• Biopolymers increase the viscosity of the injected water, improving the mobility ratio and sweep efficiency

Alteration of reservoir properties

• Microorganisms can alter the properties of the reservoir rock and fluids through various mechanisms, such as the degradation of heavy oil components, the modification of rock wettability, and the selective plugging of high-permeability zones
• The degradation of heavy oil components by microorganisms can reduce the viscosity of the oil and improve its mobility
• The modification of rock wettability from oil-wet to water-wet can enhance the spontaneous imbibition of water and the displacement of oil from the pore spaces

Advantages and limitations of MEOR

• MEOR has several advantages over conventional EOR methods, such as lower capital and operating costs, reduced environmental impact, and the ability to target specific reservoir zones
• However, MEOR also has some limitations, such as the difficulty in controlling the growth and activity of the microorganisms in the reservoir, the potential for microbial plugging of the pore spaces, and the sensitivity of the microorganisms to reservoir conditions (temperature, salinity, and pH)
• The success of MEOR depends on the careful selection and adaptation of the microorganisms to the specific reservoir environment, as well as the optimization of the injection strategy and monitoring techniques

Screening criteria for EOR methods

• The selection of an appropriate EOR method for a given reservoir depends on various screening criteria, such as reservoir characteristics, fluid properties, and economic considerations
• Screening criteria are used to identify the most suitable EOR method for a specific reservoir and to prioritize the potential EOR candidates for further evaluation
• The screening process involves the comparison of the reservoir and fluid properties with the established screening criteria for each EOR method, as well as the assessment of the technical and economic feasibility of the EOR project

Reservoir characteristics

• Reservoir characteristics, such as depth, temperature, permeability, and heterogeneity, play a crucial role in the selection of an EOR method
• For example, thermal EOR methods are most suitable for shallow, thick reservoirs with high permeability and low heterogeneity, while chemical EOR methods can be applied in a wider range of reservoir conditions
• The presence of natural fractures or high-permeability zones in the reservoir can impact the efficiency of EOR methods by causing channeling or early breakthrough of the injected fluids

Fluid properties

• The properties of the reservoir fluids, such as oil viscosity, API gravity, and composition, are important factors in the selection of an EOR method
• For instance, thermal EOR methods are most effective for heavy oils with high viscosity and low API gravity, while gas injection EOR methods are more suitable for light oils with low viscosity
• The composition of the reservoir fluids, particularly the presence of asphaltenes or other heavy components, can affect the compatibility and effectiveness of the injected chemicals or gases

Economic considerations

• The economic viability of an EOR project depends on various factors, such as the oil price, the cost of the EOR agents, the infrastructure requirements, and the expected incremental oil recovery
• The selection of an EOR method must take into account the capital and operating costs associated with the project, as well as the potential revenue generated from the incremental oil production
• The economic evaluation of an EOR project involves the estimation of key financial metrics, such as the net present value (NPV), the internal rate of return (IRR), and the payback period, based on the projected cash flows and the discount rate

Modeling and simulation of EOR processes

• Modeling and simulation play a critical role in the design, optimization, and performance prediction of EOR processes
• Numerical reservoir simulation is the most commonly used tool for modeling EOR processes, as it allows for the integration of complex reservoir and fluid properties, as well as the simulation of various EOR mechanisms
• The accuracy and reliability of EOR modeling and simulation depend on the quality of the input data, the robustness of the mathematical models, and the validation of the simulation results against field data

Numerical reservoir simulation

• Numerical reservoir simulation involves the discretization of the reservoir into a grid of cells and the solution of the governing equations (mass, momentum, and energy conservation) for each cell using finite difference or finite element methods
• EOR processes are typically modeled using compositional reservoir simulators, which can handle the complex phase behavior and chemical reactions involved in EOR processes
• The simulation of EOR processes requires the incorporation of specialized models for the relevant EOR mechanisms, such as the interfacial tension reduction in chemical EOR or the heat transfer in thermal EOR

Upscaling and homogenization

• Upscaling and homogenization are techniques used to bridge the gap between the small-scale heterogeneities of the reservoir and the large-scale grid blocks used in reservoir simulation
• Upscaling involves the computation of effective properties (permeability, porosity, and relative permeability) for each grid block based on the fine-scale heterogeneities of the reservoir
• Homogenization is a mathematical technique used to derive effective equations that describe the macroscopic behavior of the reservoir based on the microscopic properties and processes

Optimization of EOR strategies

• The optimization of EOR strategies aims to determine the best combination of EOR design parameters (injection rates, well placement, and chemical formulation) that maximizes the oil recovery or the economic performance of the project
• EOR optimization can be performed using various techniques, such as sensitivity analysis, design of experiments, and mathematical optimization algorithms
• The optimization of EOR strategies requires the integration of reservoir simulation, economic evaluation, and risk assessment to identify the most robust and profitable EOR design under uncertainty

Field implementation and monitoring

• The successful implementation of an EOR project requires careful planning, execution, and monitoring of the field operations
• Field implementation involves the design and construction of the surface facilities, the drilling and completion of the injection and production wells, and the procurement and handling of the EOR agents
• Monitoring and surveillance are essential for assessing the performance of the EOR project, identifying potential issues or opportunities, and optimizing the EOR strategy based on the field data

Pilot tests and field trials

• Pilot tests and field trials are small-scale EOR projects conducted to evaluate the technical and economic feasibility of the EOR method under real field conditions
• Pilot tests are typically designed to test the injectivity, compatibility, and effectiveness of the EOR agents in a limited area of the reservoir
• Field trials are larger-scale projects that aim to demonstrate the scalability and reproducibility of the EOR method, as well as to refine the EOR design parameters based on the field data

Monitoring and surveillance techniques

• Monitoring and surveillance techniques are used to collect and analyze field data during the implementation of an EOR project
• Common monitoring techniques include production logging, tracer tests, pressure transient analysis, and time-lapse seismic surveys
• Surveillance techniques involve the regular sampling and analysis of the produced fluids and the injected EOR agents to assess their quality and compatibility

Production forecasting and economics

• Production forecasting is the process of predicting the future oil production profile of an EOR project based on the reservoir simulation results and the field data
• The production forecast is used to estimate the incremental oil recovery and the economic performance of the EOR project over time
• The economic evaluation of an EOR project involves the estimation of the capital and operating costs, the revenue generated from the incremental oil production, and the calculation of the financial metrics (NPV, IRR, and payback period) based on the production forecast and the economic assumptions

Environmental aspects of EOR

• The implementation of EOR projects must consider the potential environmental impacts and the compliance with the relevant health, safety, and environmental (HSE) regulations
• The main environmental aspects of EOR include greenhouse gas emissions, produced water management, and the handling and disposal of the EOR chemicals
• The environmental performance of an EOR project can be improved through the adoption of best practices, the use of environmentally friendly EOR agents, and the implementation of effective monitoring and mitigation measures

Greenhouse gas sequestration

• Greenhouse gas sequestration, also known as carbon capture and storage (CCS), involves the capture of carbon dioxide (CO2) from industrial sources and its injection into depleted oil reservoirs or deep saline aquifers for long-term storage
• CO2-EOR projects can provide a synergistic opportunity for greenhouse gas sequestration, as the injected CO2 can be permanently stored in the reservoir while enhancing oil recovery
• The successful implementation of CO2-EOR and CCS requires the assessment of the storage capacity, the integrity, and the long-term stability of the reservoir, as well as the monitoring of the CO2 plume migration and potential leakage

Produced water management

• Produced water is the water that is co-produced with oil and gas during the production phase of an EOR project
• The management of produced water is a critical environmental aspect of EOR, as the water may contain various contaminants, such as oil,