Pore-scale mechanisms of immiscible and miscible gas injection in fractured carbonates

Abstract

Oil recovery in fractured carbonates is low because of fracture-matrix permeability contrast, oil/mixed-wettability, and high capillary entry pressure. Gas injection can enhance oil recovery because gas-oil interfacial tension is less than water-oil interfacial tension and diminishes to zero towards miscibility. Nevertheless, pore-scale mechanisms of gas-oil displacement and diffusion within fractured systems are still poorly understood. We created a fractured system using a 1.1-cm long and 0.5-cm diameter Texas cream limestone core (matrix) immersed in sandpack (fracture). Immiscible (methane) and miscible (ethane) gas injections were conducted and monitored with X-ray computed microtomography at 10 µm resolution for oil saturation/concentration evolution. During immiscible displacements, matrix oil saturation change was cyclic instead of monotonic due to oil imbibition and vugs (10–300 µm) remained 100% oil saturated when the ultimate oil recovery was 10% in matrix and 78% in fracture. During miscible displacements, oil concentration monotonically decreased and vugs became 100% gas-filled at 50% overall matrix oil concentration. The ultimate oil recovery was 69% in matrix and 100% in fracture. The effective ethane diffusivity in fracture and matrix were estimated to be 5.07 × 10−5 and 1.70 × 10−6 cm2/s, respectively. Oil in vugs can be recovered only after invading intergranular pores (1–6 µm) connecting vugs. Fracture permeability determines the maximum pressure in the gas phase which must overcome the matrix capillary pressure to invade the matrix under immiscible conditions. Finally, spatial heterogeneity of effective diffusivity can exist in carbonates and create gravitationally-unstable oil concentration profiles both in lab and field scales under miscible conditions.