Foam Generation and Rheology in a Variety of Model Fractures

Abstract

Gas is used in petroleum reservoirs to displace oil for enhanced oil recovery. The microscopic displacement efficiency of gas is very good, but at the reservoir scale the process suffers from poor sweep efficiency, especially in naturally fractured reservoirs. Foam can improve the sweep. There have been considerable scientific contributions toward understanding foam flow in nonfractured porous media, with relatively little work on foam flow in fractured porous media. We investigate foam-generation mechanisms in five fully characterized glass models of fractures with different apertures and correlation lengths of the aperture distribution. We also study the rheology of the in situ-generated foam by varying the superficial velocities of the gas and surfactant solution. We compare the measured pressure gradient against the fracture attributes, aperture, and the correlation length of the aperture. We also compare foam texture as a function of position within the fracture as the generated foam propagates through the fracture. Gas mobility was greatly reduced as a result of in situ foam generation in our model fractures. Foam was generated predominantly by capillary snap-off and lamella division. The measured mobility reduction depends on fracture attributes. Fracture-wall roughness, represented by both the hydraulic aperture and the correlation length of the aperture, plays an important role in foam generation and mobility. The average bubble size increases as the aperture increases, which results in a significant decrease in pressure gradient. Two model fractures show the same two foam-flow regimes central to the understanding of foam in nonfractured porous media: a low-quality regime where pressure gradient is independent of liquid velocity and a high-quality regime where pressure gradient is independent of gas velocity. The mechanisms thought to be behind these two regimes in nonfractured porous media do not apply to these experiments, however.