An Experimental Study of Waterflooding From a Two-Dimensional Layered Sand Model

Summary The effect of flow rate on oil recovery by waterflooding is studied with a two-dimensional (2D), layered sand model that allows visual observation. The model consists of three communicating layers of equal thickness of water-wet sand with the permeability ratio of 2 :4 : 1 from top to bottom. Three white mineral oils of viscosity 15, 30, and 150 cp [15, 30, and 150 mPa · s] have been used for immiscible liquid/liquid displacement under constant pressure drops covering a range of flow rates. The changes in flow regimes in the various layers are observed with respect to the variations in pressure drop across the model. The effects of capillary imbibition, gravity segregation, and viscous pressure gradient are observed and an attempt is made to calculate the instantaneous crossflow. Crossflow of oil from the tight layers to the most-permeable layer increases the intermediate oil recovery for a given volume of water injected. Oil recovery increases somewhat with a decrease in flow rate and considerably with a decrease in oil viscosity. Slow flow rates produce higher recovery but may not be feasible in the field for economic reasons. For M > 1, viscous crossflow contributes very slightly to oil recovery until the time of breakthrough of water from the most-permeable layer. After that, the viscous crossflow increases and creates a secondary oil stream close to the interfaces of the most-permeable layer. The additional oil is produced until the least-permeable layer also breaks through. A sequence of slow flow rate followed by a fast flow rate is shown to improve oil recovery for a given volume of water injected, and this occurs within a reasonable experimental time frame. The slow flow rate is set from a criterion of stable displacement in the most-permeable layer based on stability analysis, and the fast flow rate is based on the maximum pressure the model can take. The effect of variation in interfacial tension (IFT) on the displacement behavior was also studied. IFT is reduced from 35 to 4 and 0.5 dynes/cm [35 to 4 and 0.5 mN/m], respectively, by addition of two surfactants to the injected flood water. These IFT ranges are not sufficiently low to increase oil recovery from interfacial effects. The two surfactants influenced the recovery behavior differently.