Impacts From Fractures On Oil Recovery Mechanisms In Carbonate Rocks At Oil-Wet And Water-Wet Conditions - Visualizing Fluid Flow Across Fractures With MRI

Abstract

Abstract The fracture/matrix transfer and fluid flow behavior in fractured carbonate rock was experimentally investigated using magnetic resonance imaging (MRI). Viscous oil-water displacements in stacked carbonate core plugs were investigated at wettability conditions ranging from strongly water-wet to moderately oil-wet. The impact of wettability and was investigated in a series of flooding experiments. The objective was to determine the impacts on fluid flow from different types of fractures at various wettability conditions. A general-purpose commercial core analysis simulator was used to simulate the flood experiments and to perform a parameter sensitivity study. The results demonstrated how capillary continuity across open fractures may be obtained when wetting phase bridges were established. A viscous component over the open fractures was provided when the wetting preference between the injected fluid and the rock surface allowed the formation of stable wetting phase bridges. The combination of high spatial resolution imaging and rapid data acquisition revealed how the transport mechanisms for oil and water were governed by the wetting affinity between the rock surface and the fluids in the fracture; both at moderately water wet conditions and at moderately oil wet conditions. Introduction Production of oil from naturally fractured reservoirs is commonly governed by co- and counter-current imbibition of water. Imbibition is dependent on wettability due to the controlling capillary forces, and waterflooding fractured reservoirs have been successful in many water-wet reservoirs. Extensive waterflooding over several years in the oil-wet field Ghaba North in Oman, however, resulted in very low oil recovery (around 2 %) as most of the oil was produced from the fractures only. Fractures generally exhibit a relatively small volume of the total porosity in fractured reservoirs (typically 1–3 %), but the fracture network is important for fluid flow due to much higher permeability and the augmentation of accessible surface in which imbibition may occur. In water-wet reservoirs, oil is produced from the matrix to the fracture system by capillary imbibition of water with subsequent oil expulsion. Capillary continuity between isolated matrix blocks is in general recognized as favorable in fractured reservoirs dominated by gravity drainage. Capillary continuity across fractures in preferentially oil-wet reservoirs may increase ultimate recovery during gas assisted gravity drainage. Capillary continuity in preferentially water-wet reservoirs increases the height of the continuous matrix column and reduces the amount of capillary trapped oil. For oil recovery in fractured reservoirs produced by viscous fluid displacement, establishing stable wetting phase bridges may contribute to added viscous pressure components over isolated matrix blocks, and thus may increase the oil recovery above the spontaneous imbibition potential. Several authors 1–3 have shown experimentally that vertical capillary continuity across fractures becomes important when gravity is the driving force. Saidi 4 (1987) introduced the idea of capillary continuity through stable liquid bridges. Labastie 5 (1990) found that the permeability of the fractured material influenced the ultimate recovery of the gravity drainage; increased permeability lead to increased oil recovery. Stones et al.6 (1992) investigated the effect of overburden pressure and the size of the contact area of the porous material across the fracture. They concluded that the size of the contact area controls the transmissibility of oil, and therefore the ability of the fracture to transport liquids across the fracture. O'Meara Jr. et al.7 (1992) investigated the film drainage along coreholder end-pieces in centrifuge capillary pressure measurements, where they argued that if the conductivity of the film was large enough, the assumption of zero capillary pressure at the outlet end of the plug could be disregarded. Firoozabadi and Markeset 8 (1994) observed capillary continuity between isolated matrix blocks by liquid film drainage along non-porous spacers placed inside the fracture, and by liquid bridging forming inside the fracture. They concluded that the film flow and the degree of fracture liquid transmissibility controlled the rate of drainage across a stacked matrix blocks.