Study of foam generation and propagation in fully characterized physical-model fracture

Abstract

Foam greatly reduces gas mobility for gas enhanced-oil-recovery (EOR) projects. It substantially increases both the effective viscosity of gas and gas trapping. Numerous studies have been conducted to understand foam rheology in rock matrix both theoretically and experimentally. The knowledge of foam flow in fractured porous media is incomplete, however. This study aims to contribute to the understating of foam generation and propagation in a fully characterized physical-model fracture. We investigate foam-generation mechanisms and the propagation of pre-generated foam. Gas mobility was greatly reduced as a result of in-situ foam generation. Foam-generation mechanisms similar to those seen in 3D porous media were observed on this model fracture. Foam was generated predominantly by capillary snap-off and lamella division. Lamella division was observed at high gas fractional flow at two different superficial velocities. Fracture wall roughness played an important role in foam generation. In the case of pre-generated foam, two very distinct bubble sizes were injected: fine-textured bubbles much smaller than the roughness scale and coarse-textured foam with bubbles much larger than the roughness scale. The first case did not show any significant change in bubble size as foam propagated through the model fracture, while in the second case, the fracture played a role in reducing bubble size. Inter-bubble diffusion did not regulate bubble size in our apparatus because residence time in the fracture is relatively short. We cannot confirm that foam reached local equilibrium in our experiments, but we believe that local equilibrium lies between the cases of in-situ- and pre-generated foams.