Wettability Literature Survey- Part 1: Rock/Oil/Brine Interactions Part 1: Rock/Oil/Brine Interactions and the Effects of Core Handling on Wettability
Wettability is a major factor controlling the location, flow, and distribution of fluids in a reservoir. The wettability of a core will affect almost all types of core analyses, including capillary pressure, relative permeability, waterflood behavior, electrical properties, and simulated tertiary recovery. The most accurate results are obtained when native- or restored-state cores are run with native crude oil and brine at reservoir temperature and pressure. Such conditions provide cores that have the same wettability as the reservoir.
The wettability of originally water-wet reservoir rock can be altered by the adsorption of polar compounds and/or the deposition of organic material that was originally in the crude oil. The degree of alteration is determined by the interaction of the oil constituents, the mineral surface, and the brine chemistry. The procedures for obtaining native-state, cleaned, and restored-state cores are discussed, as well as the effects of coring, preservation, and experimental conditions on wettability. Also reviewed are methods for artificially controlling the wettability during laboratory experiments.
This paper is the first of a series of literature surveys covering the effects of wettability on core analysis. Changes in wettability have been shown to affect capillary pressure, relative permeability, waterflood behavior, dispersion of tracers, simulated tertiary recovery, irreducible water saturation (IWS), residual oil saturation (ROS), and electrical properties. For core analysis to predict the behavior of a reservoir accurately, the wettability of a core must be the same as the wettability of the undisturbed reservoir rock. A serious problem occurs because many aspects of core handling can drastically affect wettability.
Water-Wet, Oil-Wet, and Neutrally Wet. Wettability is defined as "the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids. "In a rock/oil/brine system, it is a measure of the preference that the rock has for either the oil or water. When the rock is water-wet, there is a tendency for water to occupy the small pores and to contact the majority of the rock surface. Similarly, in an oil-wet system, the rock is preferentially in contact with the oil; the location of the two fluids is reversed from the water-wet case, and oil will occupy the small pores and contact the majority of the rock surface. It is important to note, however, that the term wettability is used for the wetting preference of the rock and does not necessarily refer to preference of the rock and does not necessarily refer to the fluid that is in contact with the rock at any given time.
For example, consider a clean sandstone core that is saturated with a refined oil. Even though the rock surface is coated with oil, the sandstone core is still preferentially water-wet. This wetting preference can be preferentially water-wet. This wetting preference can be demonstrated by allowing water to imbibe into the core. The water will displace the oil from the rock surface, indicating that the rock surface "prefers" to be in contact with water rather than oil. Similarly, a core saturated with water is oil-wet if oil will imbibe into the core and displace water from the rock surface. Depending on the specific interactions of rock, oil, and brine, the wettability of a system can range from strongly water-wet to strongly oil-wet. When the rock has no strong preference for either oil or water, the system is said to be of neutral (or intermediate) wettability. Besides strong and neutral wettability, a third type is fractional wettability, where different areas of the core have different wetting preferences.
The wettability of the rock/fluid system is important because it is a major factor controlling the location, flow, and distribution of fluids in a reservoir. In general, one of the fluids in a porous medium of uniform wettability that contains at least two immiscible fluids will be the wetting fluid. When the system is in equilibrium, the wetting fluid will completely occupy the smallest pores and be in contact with a majority of the rock surface (assuming, of course, that the saturation of the wetting fluid is sufficiently high). The nonwetting fluid will occupy the centers of the larger pores and form globules that extend over several pores.
In the remainder of this survey, the terms wetting and nonwetting fluid will be used in addition to water-wet and oil-wet. This will help us to draw conclusions about a system with the opposite wettability. The behavior of oil in a water-wet system is very similar to the behavior of water in an oil-wet one.