Pore-Scale Oil Displacement by Gas and Foam in Fractured Media at Immiscible and Near-Miscible Conditions

Abstract:

Fractures exist in oil reservoirs due to either tectonic forces or hydraulic fracturing. The permeability contrast between matrix and fracture is typically a few orders of magnitude and this contrast makes the sweep efficiency and thus the ultimate oil recovery low, especially for low permeability formations. Two common strategies of enhancing oil recovery in fractured media are (1) decreasing the interfacial tension (IFT) between oil and injection fluid, e.g., gas injection towards gas-oil miscibility, and (2) diverting flow from fracture to matrix by decreasing mobility in fracture, e.g. foam injection.

In this work, we conducted gas, a mixture of methane and ethane, injection and then foam injection to recover oil in fractured low-permeability carbonates from immiscible conditions to near-miscible conditions. The change of saturations within the system was quantitatively monitored in-situ with a calibrated micro-CT scanner at a resolution of 10 micron. During gas injection, we observe a drastic increase in oil recovery at both fracture and matrix as the gas composition changes from immiscible to near-miscible. For both immiscible and near-miscible conditions, we observe that the oil saturation in matrix decreased from the matrix-fracture interface to the inside of matrix by following a diffusion-type profile. During foam injection, foam breaks at the fracture-matrix interface, during which gas and water enters matrix and recovers additional oil compared to gas injection.