Transport of CO_2-Foaming Agents During CO_2-Foam Processes in Fractured Chalk Rock

摘要

The coinjection of carbon dioxide (CO_2) and a CO_2-foaming agent to form stable CO_2 foam has been found to improve the sweep efficiency during CO_2-foam processes in carbonate reservoirs. However, only a few studies of CO_2-foam transport in fractured rock have been reported. In fractured chalk reservoirs with low matrix permeability, the aqueous CO_2-foaming-agent solution will flow mainly through the fractures. The total retention of the CO_2-foam-ing agent in the reservoir will depend on how much of the matrix is contacted by the CO_2-foaming-agent solution during the project period and, therefore, on its transport rate into the matrix.rnThis paper presents results from a series of static and flow-through experiments carried out to investigate the transport and retention phenomena of CO_2-foaming agents in fractured chalk models at 55°C. Fractured chalk models with 100% water-saturation and residual-oil saturation after waterflooding were used. In the static experiments, the fractured model was created by transferring core plugs with different diameters into steel cells with an annulus space around the plugs. The fracture volume was filled with foaming-agent solutions with different initial concentrations. The experiments were carried out in parallel, with liquid samples regularly taken out from the fracture above the plugs and analyzed for the foaming-agent concentration. The experiments were monitored until the concentrations in the fractures reached a plateau.rnAt specific and constant concentrations of the foaming agent in the fractures, the plugs were demounted and samples drilled out along the whole lengths of the plugs from the outer, middle, and center portions. These samples were analyzed for foaming-agent concentration to determine how much of it had penetrated the matrix.rnResults indicate that the transport of the foaming-agent decreases toward the center of the plugs with 100% water-saturation and residual-oil saturation after waterflooding. Modeling of the static experiments using the Computer Modelling Group (CMG)'s commercial reservoir simulator STARS was also carried out to determine the transport rate for the foaming agent. A good history match between experimental and modeling results was obtained. In the flow-through experiments, the fractured model was created by drilling a concentric hole through the center of the plug. The hole, simulating an artificial fracture, was filled with glass beads of different dimensions. Fractured models with different effective permeability were flooded with equal volumes of the foaming-agent solution. Results show that the transport of CO_2-foaming agent into the matrix is slower in the fractured models than in the homogeneous models with viscous flooding of the rock.