Injection Rate Controlled Capillary Imbibition Transfer in Fractured Systems

Abstract

Abstract In this paper, the effects of injection rate and fracture configuration on capillary imbibition transfer in fractured porous media were studied and analysed. Displacement experiments were conducted on artificially fractured rock samples and the experimental data were evaluated using a numerical simulator. The limiting value of injection rate and fracture capillary number were defined for an efficient capillary imbibition transfer. Some of the core flood experiments were enhanced with the fluid distribution measurements using CT-scan. Also, experiments on 2-D glass bead samples were conducted to scrutinize the influence of the fracture configuration on the matrix saturation distribution at different injection rates. Visualization studies on 2-D glass bead and CT scan of linear core flood experiments revealed that as the injection rate is increased, fracture pattern begins to become an effective parameter on the matrix saturation distribution. As the rate is lowered, however, the system begins to behave like a homogeneous system showing a frontal displacement regardless of the fracture configuration. Introduction In fractured porous media, viscous displacement occurs in fracture network due to its higher conductivity compared to matrix while an exchange of fluids occurs between these two media. Displacement process differs from homogeneous porous media because of this transfer phenomenon. The fluid transfer is due to capillary imbibition if the matrix is water-wet and water is injected through the fracture. Although the different aspects of drainage and forced imbibition (wetting phase displacing non-wetting phase by viscous forces) in homogeneous and fractured porous media have been studied in the past, the physics of capillary (spontaneous) imbibition, especially under the influence of the flow in fracture, has not been completely clarified yet. Depending on matrix permeability and flow rate in the fracture, the imbibition transfer may occur by capillary forces and/or viscous forces (forced imbibition) if the system is saturated with a non-wetting fluid and wetting phase is injected through the fracture. Recent experimental studies to understand the drainage and forced imbibition in homogeneous and fractured porous media were concentrated on the pore level modeling using etched glass models as porous media. In this type of experimentation, fracture/matrix permeability (conductivity) ratio is low because of the model preparation restrictions of the etched glass. That's why, core scale experimentation is preferable to create much higher fracture/matrix permeability ratio so that the imbibition transfer occurs only by capillary imbibition. Previous studies related to the core flood experiments on fractured rock samples are a few in literature. Kleppe and Morse and Kazemi and Merill studied the effects of injection rate on the recovery performance for high permeability matrix element (Berea Sandstone) and numerically verified the results. P. 343^