Wettability Profile of a Thick Carbonate Reservoir by the New Rise in Core Wettability Characterization Method

Ghedan, Shawkat G. (The Petroleum Institute) | Canbaz, Celal Hakan (The Petroleum Institute) | Boyd, Douglas A. (ZADCO Petroleum Co) | Mani, George M. (Core Laboratories) | Haggag, Marwan Khamis (ADCO Producing Co. Inc.)


The evaluation of the reservoir wettability is fundamental to the understanding of fluid flow in porous media. Wettability has direct impact on saturation end points and the shape of capillary pressure and relative permeability curves. This will, in turn, directly affect the relative movement of the reservoir fluids and eventually affect the displacement efficiency of oil by the different injected fluids. Having reliable wettability data deems very necessary for reliable predictions of expected oil
recoveries under different development options.

The Amott and the USBM tests are the most commonly used methods for quantifying reservoir wettability. Combination of the two methods is also in use. In addition to the elaborate experimental effort and time required for these methods, the USBM method does not recognize very strongly water or oil wet systems, while Amott method fails to distinguish between important degrees of strong water and oil-wetness.

This paper describes a new technique, the Rise in Core (RIC), wettability characterization method based on a modified form of the Washburn equation. It enables relatively quick, accurate measurements of wettability in terms of contact angle and not wettability index as the other methods do. The method is easy to use and requires no complex equipment. During the RIC experiments, core samples saturated with one reservoir fluid are subjected to imbibitions from a second reservoir fluid. As the imbibition process takes place, the core samples weight changes continuously due to adjustments in relative saturations of the two fluids. Monitoring the square of the core mass change with time using a high precision balance, the acquired data is analyzed with modified Washburn equation to determine the cores wettability.

For the sake of assurance, RIC wettability measurements were compared to ambient conditions modified Amott-USBM measurements for a thick limestone oil reservoir using core plug pairs from different heights above the free water level. The results compare well. The RIC technique proved to be much simpler to construct, much faster to perform and much easier to analyze and interpret than traditional methods. Moreover, the method gives the operator a chance to evaluate the uncertainty in the wettability data.

1. Wettability
Wettability is the relative preference of a solid surface to be coated by a certain fluid in a system1. In a rock/oil/brine system, it is a measure of the preference that the rock surface has for either the oil or water2. When the rock is water-wet, there is a tendency for water 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, oil will contact the majority of the rock surface2. When the rock surface does not display a preference for oil or water contact, intermediate wettabiltiy exists. Cases where water coates the surface of small pores while oil coates the surface of large pores are fractional or mixed wet if continous paths of oil and water wet rock are present.