A Test for the Wettability of Carbonate Rocks
Authors: Duyvis E.M. , Smits L.J.M. 
Published: December 2007

Direct imbibition experiments to test carbonate-rock wettability are occasionally prevented by high viscosity of the oil or rigid films between oil and water. The oil must then be removed from the rock before the imbibition test. A new extraction procedure was tested on limestones born Middle East reservoirs. Samples were taken from rubber-sleeve cores under nitrogen in a polythene glove bag to avoid formation of surface-active compounds through oxidation of crude oil. Conventional Soxhlet extraction of crude oil made water-wet carbonate rock oil-wet. Obviously the hot, dry solvent removes the water before the oil is completely extracted; the oil then contacts the rock surface, making it oil-wet. The extraction procedure was therefore modified so that cold and water-saturated chloroform reached the sample. To remove the oil effectively, the material was crushed and then stirred vigorously during extraction. Fig. 1 shows the extraction apparatus. The chloroform in the extraction thimble was kept saturated with water by the initial addition of some water to the boiling vessel. The vapor from this vessel is then richer in water than cold, water-saturated chloroform. The alundum thimble was made oil-wet (by dimethyl dichlorosilane allowing the solvent to pass through. Blank tests with water-wet and oil-wet samples showed a 1-week test to be appropriate for the extraction. The samples were dried and the wettability was determined by imbibition. A small amount of the sample was placed as a ridge in a hollow of a test plate and was wetted with toluene. By placing plate and was wetted with toluene. By placing water and toluene on either side of the ridge, we could determine whether water displaces toluene from the sample. This can be detected easily because sample material wetted with water is much lighter than that wetted with toluene. If water was indeed imbibed the sample was water-wet. Those samples in which water was not imbibed were tested as follows:

the material was mixed with water a edge was again formed in a hollow; and water and oil were placed on either side to determine whether or not toluene displaced water.

So far, we have never observed this spontaneous imbibition. We therefore mixed the fluids and the sample and observed whether the grains were now wetted by toluene (darkening of the grain surface). If so, the sample was called oil-wet. A sample showing no imbibition in either case was neutral. The reliability of the procedure was verified by subjecting limestone core samples to both dry Soxhlet extraction and our wet extraction. The parts of samples from the dry extraction were parts of samples from the dry extraction were oil-wet, and those from the wet extraction were water-wet. Thus, either the samples were originally water-wet and became oil-wet by dry extraction, or they were originally oil-wet and became water-wet through wet extraction. The oil-wet samples could not be made water-wet by subsequent prolonged wet extraction. Thus the original samples must have been water-wet. Wet extraction does change an oil-wet condition to neutral, but never to water-wet. Therefore, a sample found to be water-wet was water-wet before extraction, and a sample found to be neutral was either oil-wet or neutral before extraction.

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