Method for Determining Wettability of Reservoir Rocks

Abstract

Abstract A semiquantitative method for measuring the wettability of reservoir rocks has been developed. These data are needed for reservoir analysis and for interpretation of laboratory displacement studies. The wettability of a core sample is measured by the contact angle for the system oil-water-solid. These contact angles are calculated from the displacement pressure (threshold pressure) obtained by use of the centrifuge, using first oil-water and second air-oil in the same core sample. This method is based on the assumption that the air-oil and oil-water interfaces occupy similar positions in the porous medium when desaturation of the wetting phase is initiated. The assumption is also made that the contact angle for the air-oil-solid system which is close to zero in value does not change appreciably even when the contact angle for the oil-water-solid system experiences marked changes. "Apparent contact angles" for five different solids ranging from 31? to 82? have been determined, and changes in the "apparent contact angle" of a given sample with laboratory use from 33? to 53? have been observed. Introduction Reservoir rocks vary in their wettability, some being preferentially water wet while other are apparently preferentially oil wet. The degree of wettability, in theory, is measured quantitatively by the contact angle for the system solid-oil-water, but in practice this quantity is extremely difficult to determine. The need for obtaining some measure of this quantity has become obvious in recent years as a result of both laboratory and field observations. Several reservoirs such as the Bradford Sand in Pennsylvania and the Wilcox at Oklahoma City are reported to be oil wet. If these reports correctly reflect the true nature of the surface of the rock in the ground, then these reservoirs probably will not perform as predicted on the basis of a water wet rock.