JNTU B.Tech IV ENHANCED OIL RECOVERY TECHNIQUES Previous Question Paper (March - 2021 - R15)

Gas injection in carbonate formations is significantly influenced by the complex, heterogeneous nature of the porous system. Carbonates typically exhibit multi-modal pore size distributions, including interparticle, vuggy, and fracture porosity, which dictate the efficiency of gas displacement.

The effects of these porous systems are categorized as follows:

• Pore Size Distribution and Heterogeneity: Carbonates often contain a mix of micropores (matrix) and macropores (vugs). Gas tends to bypass the tighter matrix pores due to high capillary entry pressures, leading to early gas breakthrough through larger, more permeable vuggy networks. This results in poor sweep efficiency.
• Vug Connectivity: Vugs can be isolated or interconnected. Interconnected vugs act as high-conductivity channels (thief zones) that accelerate gas fingering. Isolated vugs may trap oil, as gas cannot easily penetrate the surrounding low-permeability matrix to mobilize the oil held within them.
• Fracture Networks: Naturally fractured carbonates dominate the flow dynamics. Gas injection in these systems is characterized by rapid gas channeling through fractures, leaving the oil in the matrix largely unrecovered. The process relies heavily on the rate of matrix-to-fracture oil transfer, which is governed by gravity drainage and capillary imbibition.
• Wettability Alterations: Carbonate surfaces are often oil-wet or mixed-wet. Gas injection can alter the wettability toward more water-wet or gas-wet states depending on the gas composition (e.g., CO2). A shift toward gas-wetness can increase the relative permeability of gas, further worsening channeling, whereas CO2-induced wettability changes can sometimes improve oil recovery by reducing capillary forces.
• Capillary Pressure Effects: In carbonate systems, the high capillary entry pressure of the matrix often prevents gas from entering the smaller pores. This creates a significant capillary barrier, necessitating higher injection pressures or the use of miscible gases to reduce interfacial tension (IFT) and allow gas to penetrate the matrix pores.

To mitigate these effects, techniques such as Water-Alternating-Gas (WAG) injection are employed to control mobility and improve sweep efficiency by blocking high-permeability channels with water while the gas accesses the matrix.