Pore-level foam generation, propagation, and sweep efficiency were visualized using a two-dimensional sandstone pore structure etched in a silicon wafer with accurate representation of grain shape, grain size and aspect ratios. In situ foam generation occurred by snap-off in the interior of the porous network (rectilinear snap-off) and at permeability discontinuities. Lamella creation by the two snap-off mechanisms identified here resulted in different foam textures. During foam injection for enhanced oil recovery, microvisual data revealed that the aqueous phase advanced as film flow along water-wet grains whereas discontinuous gas bubbles were located in the center of pores. Individual gas bubbles were mobilized by lamella displacement. Experimental results showed enhanced sweep efficiency in terms of greater pore occupancy by gas and larger contact area with displaced fluid for foam injection compared to continuous gas injection.

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Foam generation for gas mobility reduction in porous media is a well-known method and frequently used in field applications. Application of foam in fractured reservoirs has hitherto not been widely implemented, mainly because foam generation and transport in fractured systems are not clearly understood. In this laboratory work, we experimentally evaluate foam generation in a network of fractures within fractured carbonate slabs. Foam is consistently generated by snap-off in the rough-walled, calcite fracture network during surfactant-alternating-gas (SAG) injection and coinjection of gas and surfactant solution over a range of gas fractional flows. Boundary conditions are systematically changed including gas fractional flow, total flow rate, and liquid rates. Local sweep efficiency is evaluated through visualization of the propagation front and compared for pure gas injection, SAG injection, and coinjection. Foam as a mobility-control agent resulted in significantly improved areal sweep and delayed gas breakthrough. Gas-mobility reduction factors varied from approximately 200 to more than 1,000, consistent with observations of improved areal sweep. A shear-thinning foam flow behavior was observed in the fracture networks over a range of gas fractional flows.

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Here we show for the first time the combined positron emission tomography (PET) and computed tomography (CT) imaging of flow processes within porous rocks to quantify the development in local fluid saturations. The coupling between local rock structure and displacement fronts is demonstrated in exploratory experiments using this novel approach. We also compare quantification of 3‐D temporal and spatial water saturations in two similar CO2 storage tests in sandstone imaged separately with PET and CT. The applicability of each visualization technique is evaluated for a range of displacement processes, and the favorable implementation of combining PET/CT for laboratory core analysis is discussed. We learn that the signal‐to‐noise ratio (SNR) is over an order of magnitude higher for PET compared with CT for the studied processes.