Characterization of Wettability From Spontaneous Imbibition Measurements
Authors: Ma S.M. , Zhang X. , Morrow N.R. , Zhou X. 
Published: March 2010

Of the many methods of characterizing wettability of a porous medium, the most commonly used are the Amott test and the USBM test. The Amott test does not discriminate adequately between systems that give high values of wettability index to water and are collectively described as very strongly water-wet. The USBM test does not recognize systems which achieve residual oil saturation by spontaneous imbibition. For these conditions the measurement of imbibitions rate provides a useful characterization of wettability.

Methods of interpreting spontaneous imbibition data are reviewed and a new method of quantifying wettability from rate of imbibition is proposed. Capillary pressure is the driving force in spontaneous imbibition. The area under an imbibition curve is related to the work of displacement that results from a decrease in surface free energy. Imbibition rate data can thus be compensated for differences in interfacial tension, viscosity, pore structure, and sample size. Wettability, the remaining key factor in determining the capillary driving force and the related imbibition rate, can be used to plot the difference in saturation as a function of time. These data are then used to obtain pseudo imbibition capillary pressure curves and a wettability index based on the relative areas under these curves is defined as the relative pseudo work of imbibition. The method is applied to two crude oil/brine/rock systems. The method is compared with the Amott wettability index for different wettability states in cores with crude oil. Correlations of wettability indices with waterflood recoveries are presented.

Introduction

Reservoir wettability is a critical parameter in many types of oil recovery processes. Recovery of oil by spontaneous imbibition of water into the matrix of fractured reservoirs is a frequently cited example. Many methods of characterizing wettability have been proposed. The Amott test(1) and USBM test(2) are the most commonly used methods of characterizing wettability of oil/brine/rock systems. Both tests depend on capillary pressure and microscopic displacement efficiency. In application of either test, it is common to first displace water by oil to reach an initial water saturation, Swi. This step in the test is important because the wettability index, in addition to core treatment such as aging time and temperature, is strongly dependent on the value of the initial water saturation(3). The next step is to immerse the sample in water and measure the oil recovered by spontaneous imbibition. The corresponding increase in water saturation is ΔSws.

In the Amott test, the sample is then subjected to what is known as forced displacement either by centrifuging, as originally proposed by Amott(1), or by waterflooding(4). If the increase in water saturation by forced displacement is ΔSwf, the Amott wettability index to water, Iw, is then defined as

Equation 1 (available in full paper)

If nearly all of the oil recovery occurs by spontaneous imbibition, Iw is close to 1.0 and the system is described as very strongly water-wet. Systems are increasingly less water-wet as for smaller values of Iw.