Jawaharlal Nehru Technological University Hyderabad B.Tech IV ENHANCED OIL RECOVERY TECHNIQUES Previous Question Paper (July/August - 2022 - R18)

Enhanced Oil Recovery (EOR) refers to techniques used to increase the amount of crude oil that can be extracted from an oil field, typically applied after primary recovery (natural pressure) and secondary recovery (waterflooding or gas injection) have become inefficient. EOR processes involve injecting substances into the reservoir to alter the fluid properties, reduce interfacial tension, or improve mobility ratios to displace trapped oil.

The classification of EOR processes is generally categorized into three main types:

• Thermal Recovery: Involves introducing heat into the reservoir to reduce oil viscosity, making it easier to flow. Common methods include steam flooding, cyclic steam stimulation, and in-situ combustion.
• Gas Injection (Miscible/Immiscible): Utilizes gases like carbon dioxide (CO2), nitrogen, or natural gas. Miscible gas injection works by dissolving into the oil to reduce viscosity and swelling the oil volume, while immiscible injection primarily provides pressure support.
• Chemical Flooding: Involves injecting chemicals to improve displacement efficiency. This includes polymer flooding (to improve mobility control), surfactant flooding (to reduce interfacial tension), and alkaline flooding (to react with acidic components in the oil).

The sweep efficiency of injected gas is significantly influenced by the heterogeneity of the reservoir's pore structure. In dual-porosity systems, the distribution of gas flow between the high-permeability and low-permeability zones dictates the recovery performance.

In an inter-crystalline/inter-granular porosity system, the reservoir is characterized by a matrix where the primary storage and flow occur within the pore spaces between grains or crystals. Here, gas injection typically follows a more uniform displacement front, provided the permeability contrast between layers is low. However, if there is significant variation in grain size, gas tends to channel through higher permeability zones, leading to early breakthrough and reduced sweep efficiency in the tighter, finer-grained matrix sections.

In a fracture-matrix porosity system, the reservoir consists of a highly permeable fracture network and a low-permeability matrix. This system poses a severe challenge to sweep efficiency:

• Fracture Dominance: Injected gas preferentially enters the high-conductivity fractures, bypassing the oil stored in the matrix. This leads to rapid gas cycling and premature breakthrough at production wells.
• Matrix Imbibition/Diffusion: The sweep efficiency depends on the gas's ability to transfer from the fractures into the matrix blocks via diffusion or pressure-driven flow. If the matrix is tight, the gas remains confined to the fractures, resulting in poor volumetric sweep efficiency despite high flow rates in the fractures.

Overall, while fracture systems provide high injectivity, they drastically reduce the sweep efficiency compared to inter-granular systems unless the matrix blocks are small enough to allow for effective mass transfer between the fracture and matrix.