Experiments and Modelling of Water Injection in Water-wet Fractured Porous Media

Abstract

Abstract For predicting the performance of water injection in naturally fractured reservoirs, scale-up of the recovery data from immersing an oil-saturated core into water is commonly used. Oil recovery from some of the naturally fractured reservoirs of the North Sea has been better than what was predicted using the immersion laboratory experiments. In the field, the matrix blocks do not become surrounded by water at once; they experience an advancing fracture-water level (FWL). In this paper, the results of experiments of water injection in fractured porous media comprising a number of water-wet matrix blocks are reported for the first time. The blocks experienced an advancing fracture-water level (FWL). Immersion-type experiments were performed for comparison; the dominant recovery mechanism changed from co-current to counter-current imbibitions when the boundary conditions changed from advancing FWL to immersion-type. We performed single block experiments of co-current and counter-current imbibitions and found that co-current imbibitions led to more efficient recovery. Kansas chalk and Berea sandstone were investigated. A column of three blocks of Berea sandstone (Φ = 0.22, k = 0.62 µm2, pore volume (PV) = 8,800 ? 10−6 m3) and a stack of 12 blocks (four rows and three columns) of an outcrop Kansas chalk (Φ = 0.30, k = 0.002 - 0.005 µm2, PV = 13,900 ? 10−6 m3) were used. Breakthrough recoveries were 0.2 - 0.4 for the Berea and 0.2 - 0.6 of PV for the chalk experiments. Corresponding ultimate recoveries were around 0.5 and 0.65 of PV; oil recovery from low permeability chalk was better than that of high permeability Berea. Fracture apertures in all the above experiments were in the range of 150 - 200 µm. An approximate mathematical model was developed for counter-current imbibition. It was found that the late-time matrix-fracture transfer function simplifies to an exponential function. Hence, the physical significance of the empirical transfer function of Aronofsky et al. was demonstrated. The exponential transfer function was incorporated in a model, which was used to match the water injection experiments performed on a stack of very low permeability Austin chalk (Φ = 0.05, k = 0.00001 - 0.00005 µm2, PV = 287 ? 10−6 m3). These experiments were dominated by counter-current imbibition. Introduction Water injection is known as an important method for oil recovery from some fractured reservoirs. In water-wet fractured reservoirs, the capillary pressure contrast between the fracture and the matrix media provides the main driving force for water imbibition which can be an efficient recovery mechanism(1). Field application of water injection in fractured reservoirs has been implemented since the early fifties(2). Many issues, however, remain unresolved in the understanding of this process. Since the early studies, it was understood that recovery behaviour from a block totally covered by water is different than the same block in contact with water from some faces and with oil from other faces(2). However, the majority of studies have centred on immersion-type boundary conditions(3). Intuitively, if a block is surrounded by water, oil is forced to flow in the opposite direction of water flow, hence by counter-current imbibition.