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 lime, 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 lime. 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) behavior in water-wet, intermediate-wet and oil-wet systems. Possible explanations are offered for the discrepancies in and poor reproducibility of the other conventional contact angle measurement techniques.

This new technique not only overcomes the uncertainty of the drop-volume alteration methods but also the limitation of atmospheric pressure experiments of the Wilhelmy plate technique. This simple technique enables easier and accurate study of the effect of various reservoir parameters such as the mineralogy, solid surface roughness, solvent addition, temperature, pressure, brine chemistry and oil omposition on wettability. Of all the procedures published in the literature so for, the DDDC technique presented in this paper appears to be the solitary one that includes the effects of adhesion in a quantitative and reproducible manner in characterizing reservoir wettability through contact angles.

Conventional techniques of wettability characterization

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).

Anderson (October, 1986) provides several references that acknowledge the significant role of polar components in crude oils (such as resins and asphaltenes) in establishing the native wettability state. In wettability tests conducted under ambient conditions stock-tank crude oils have to be used which have lost their light ends. 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 band, 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.

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