A New Technique For Reservoir Wettability Characterization
Authors: Rao D.N. , Girard M.G. 
Published: March 2010

Adhesion of crude oil to the reservoir rock surface influences oil recovery in primary, secondary and tertiary operations. The various wettability measurement methods reported in the literature either do not account for the effects of adhesion on measured contact angles or they only indicate the presence or absence of adhesion without measuring contact angles. This paper reveals, for the first time, a new technique that includes the effects of adhesion on measured contact angles under reservoir conditions of pressure and temperature.

In the new technique, the buoyancy forces are used to drain the water film between the crude oil drop and mineral crystal surface to obtain adhesion equilibrium before measuring advancing and receding contact angles with respect to aging time. This is accomplished by initiating the experiment with two separate crude oil drops on two parallel crystal surfaces and by mingling the two drops after a certain pre-determined aging time. This method has been given the name of Dual-drop-dual- crystal (DDDC) technique.

Photographs and results of measured advancing and receding contact angles are presented to demonstrate the three-phase-contact-line (TPCL) behaviour in water-wet, intermediate-wet and oil-wet systems. Possible explanations are offered for the discrepancies in and poor reproducibility of the conventional contact angle measurement techniques.

Of all the procedures published so far, the DDDC technique presented in this paper is the solitary one that includes the effect of adhesion in a quantitative and reproducible manner in characterizing reservoir wettability through contact angles.

Conventional Techniques of Wettability Characterization

Background

The various techniques used in characterizing reservoir wettability have been reviewed by Anderson (November 1986). Although no single accepted method exists, three quantitative methods are generally used: contact angle measurement, the Amott test and the USBM method. The Amott and USBM tests are widely used by the oil industry, although they are usually performed under ambient conditions using refined oil or stock-tank reservoir oil (Cuiec, 1990). In wettability tests conducted under ambient conditions, stock-tank crude oils have to be used which have lost their light ends. Anderson (October 1986) notes that the use of dead crude at ambient or reservoir pressure may change the wettability because the properties of the crude are altered, and that the effects of pressure are not known at present. It would be premature to assume that the light ends play no significant role in establishing wettability. Therefore the relevance of any type of test done under ambient conditions to the characterization of reservoir wettability is at best questionable.

Contact angle measurements, on the other hand, can easily be made under reservoir conditions of temperature and pressure using live reservoir crude oils and simulated brines that match the reservoir brines in composition and pH. However, the contact angle technique has received some criticism due to the use of pure polished mineral surfaces to represent the reservoir rock surface. Recently, studies have been reported in which reservoir core samples have been directly used in contact angle tests (Sayyouh et al. 1990 and 1991).