Wettability Literature Survey-Part 6: The Effects of Wettability on Waterflooding

Summary. The wettability of a core will strongly affect its waterflood behavior and relative permeability because wettability is a major factor controlling the location, flow, and distribution of fluids in a porous medium. When a strongly water-wet system is waterflooded, recovery at water breakthrough is high, with little additional oil production after breakthrough. Conversely, water breakthrough occurs much earlier in strongly oil-wet systems, with most of the oil recovered during a long period of simultaneous oil and water production. Waterfloods are less efficient in oil-wet systems compared with water-wet ones because more water must be injected to recover a given amount of oil. This paper examines the effects of wettability on waterflooding, including the effects on the breakthrough and residual oil saturations (ROS's) and the changes in waterflood behavior caused by core cleaning. Also covered are waterfloods in heterogeneously wetted systems. Waterfloods in fractionally wetted sandpacks, where the size of the individual water-wet and oil-wet surfaces are on the order of a single pore, behave like waterfloods in uniformly wetted systems. In a mixed-wettability system, the continuous oil-wet paths in the larger pores alter the relative permeability curves and allow the system to be waterflooded to a very low ROS after the injection of many PV's of water. Introduction This paper is the sixth in a series of literature surveys covering the effects of wettability on core analysis. Wettability has been shown to affect waterflood behavior, relative permeability, capillary pressure, irreducible water saturation (IWS), ROS, dispersion, simulated tertiary recovery, and electrical properties. Earlier but less complete reviews covering the effects of wettability on waterflooding and relative permeability can be found in Refs. 6 through 17. Waterflooding is a frequently used secondary recovery method in which water is injected into the reservoir, displacing the oil in front of it. Assuming that the reservoir is initially at IWS, only oil is produced until breakthrough, the time when water first appears at the production well. After breakthrough, increasing amounts of water and decreasing amounts of oil are produced. The process continues until the WOR is so high that the well becomes uneconomical to produce. Waterfloods in water-wet and oil-wet systems have long been known to behave very differently. For uniformly wetted systems, it is generally recognized that a water-flood in a water-wet reservoir is more efficient than one in an oil-wet reservoir. An example of the effect of wettability on waterflood performance calculations is shown in Fig. 1. Steady-state oil/water relative permeabilities were measured in an outcrop Torpedo sandstone using a mild NaCl brine and a 1.7-cp [ 1.7-mPa-s] refined mineral oil. The wettability of the system was controlled by adding either (1) various amounts of barium dinonyl naphthalene sulfonate to the oil, which made the system more oil-wet, or (2) Orvus K TM liquid (a detergent) to the brine to achieve a strongly water-wet system with a contact angle of O degrees through the brine. Wettability was monitored by contact-angle measurements on a quartz crystal. The measured relative permeability curves were used to calculate field performance, assuming a single 20-acre [8-ha] five-spot with homogeneous properties. Oil and water viscosities were assumed to be 1.74 and 0.35 cp [1.74 and 0.35 mPa-s], respectively. The calculated waterflood results are shown in Fig. 1, where water breakthrough is the point at which each curve first becomes nonlinear. Fig. 1 demonstrates that earlier water break-through and less efficient oil recovery occur as the system becomes more oil-wet. For example, 8 % less oil will be produced at a WOR of 25 if the contact angle is 138 degrees [2.4 rad], rather than 47 degrees [0.82 rad]. Waterflood recovery is controlled by the oil and water relative permeabilities of a system and by the water/oil viscosity ratio. In laboratory-scale experiments, inlet and outlet end effects can also affect the recovery. The effects of relative permeabilities and viscosity ratio on waterflooding are demonstrated by the fractional flow equation. If we neglect capillary effects and assume a horizontal system, the simplified form of the fractional flow equation (e.g., see Craig) is (1) where fw = fractional flow of water, Sw = water saturation, = oil and water viscosities, respectively, cp, and = oil and water relative permeabilities, respectively. Eq. 1 shows that the fractional flow of water at a given saturation is increased when the water/oil viscosity ratio is decreased. Decreasing the water/oil viscosity ratio will cause earlier breakthrough and less efficient oil production. Similar effects will occur when the water/oil relative permeability ratio is increased. The oil and water relative permeabilities are explicit functions of the water saturation. They are also affected by pore geometry, wettability, fluid distribution, and saturation history. Water-Wet Systems. As discussed by Anderson, wettability has a strong effect on relative permeability. As the core becomes more oil-wet, the water relative permeability increases and the oil relative permeability decreases. The water will flow more easily in com-parison with the oil during a waterflood, causing progressivelearlier breakthrough and less efficient recovery. Wettability affects relative permeability and waterflood behavior because it is a major factor controlling the location, flow and spatial distribution of fluids in the core. Craigs and Raza et al. have given good summaries of the effects of wettability on the distribution of oil and water in a core. Consider a strongly water-wet rock initially at the IWS. Water, the wetting phase, will occupy the small pores and form a thin film over all the rock surfaces. Oil, the nonwetting phase, will occupy the centers of the larger Pores. This fluid distribution occurs because it is most energetically favorable. Any oil placed in the small pores would be displaced into the center of the large pores by spontaneous water imbibition, because this would lower the energy of the system. JPT P. 1605^