What are the various enhanced oil recovery (EOR) methods employed in reservoir engineering, and how do they improve hydrocarbon recovery?

Enhanced Oil Recovery (EOR) methods are employed in reservoir engineering to improve hydrocarbon recovery from reservoirs beyond the primary and secondary recovery stages. These methods aim to mobilize and displace trapped oil, increase the sweep efficiency, and maximize oil production. Here is an in-depth discussion of various EOR methods and their impact on hydrocarbon recovery:

1. Thermal EOR Methods:
a) Steam Injection: In this method, steam is injected into the reservoir to heat the oil, reduce its viscosity, and improve its mobility. The heated oil flows more easily, enabling better displacement and recovery.
b) In-Situ Combustion: This method involves igniting a portion of the reservoir to create a combustion front that propagates through the reservoir, heating the oil and reducing its viscosity. The combustion gases and pressure assist in displacing the oil towards production wells.
2. Chemical EOR Methods:
a) Polymer Flooding: Polymers are injected into the reservoir to increase the viscosity of injected water. This helps improve sweep efficiency by reducing channeling and improving the displacement of oil.
b) Surfactant Flooding: Surfactants are used to reduce the interfacial tension between the injected water and oil, enhancing oil recovery by mobilizing trapped oil and improving displacement efficiency.
c) Alkaline Flooding: Alkaline chemicals are injected to increase the pH of the reservoir, which helps in emulsifying and solubilizing trapped oil, reducing interfacial tension, and improving oil recovery.
d) CO2 Flooding: Carbon dioxide (CO2) is injected into the reservoir, where it mixes with oil and swells it, reducing its viscosity and enhancing its mobility. CO2 flooding is commonly used in mature oil fields and can also help in CO2 sequestration.
3. Gas EOR Methods:
a) Gas Injection: Various gases, such as natural gas, nitrogen, or flue gas, are injected into the reservoir to maintain reservoir pressure, displace oil, and improve sweep efficiency. Gas injection methods include gas flooding, gas-assisted gravity drainage (GAGD), and immiscible gas displacement.
b) Huff and Puff: This cyclic gas injection method involves injecting gas into the reservoir, followed by a soaking period and subsequent production. It helps in improving oil mobility, reducing oil viscosity, and displacing oil towards production wells.
4. Microbial EOR:
Microorganisms or their by-products are introduced into the reservoir to enhance oil recovery. Microbes can alter the reservoir conditions, produce acids or gases that improve oil mobility, or degrade oil components, making it easier to displace.
5. Electromagnetic EOR:
Electromagnetic techniques, such as electromagnetic heating and electromagnetic pulse, are employed to heat the reservoir and reduce oil viscosity, enhancing oil mobility and improving recovery.

Each EOR method has its own applicability and effectiveness, depending on the reservoir characteristics, fluid properties, and operational constraints. Reservoir engineers analyze reservoir data, conduct laboratory experiments, and use reservoir simulation models to assess the feasibility and potential benefits of different EOR methods. The choice of the EOR method depends on factors such as reservoir depth, temperature, oil viscosity, rock permeability, reservoir heterogeneity, and economic considerations.

EOR methods improve hydrocarbon recovery by increasing the sweep efficiency, improving displacement of oil, reducing interfacial tension, altering fluid properties, and mobilizing trapped oil. These methods help access and recover additional oil that is not recoverable by primary and secondary recovery methods. Implementing EOR techniques can significantly increase the ultimate recovery factor of a reservoir, extend its economic life, and maximize hydrocarbon production.