1964 Paper B

THE EFFECT OF FRACTIONAL WETTABILITY ON THE ARCHIE SATURATION EXPONENT

W. B. Morgan, Petroleum Engineer, Sunray DX Oil Company, Shreveport, Louisiana; S. J. Pirson, Professor of Petroleum Engineering, The University of Texas

ABSTRACT

The determination of fluid saturations from electric well logs uses a calculation procedure which is highly sensitive to a parameter known as the Archie saturation exponent. For several years it has been known that this parameter could be changed for a sandstone of fixed pore geometry by artificially altering its wettability. Because of the difficulties inherent in determining the wettability of a consolidated sandstone, no quantitative relationship could be ascertained at that time between the saturation exponent and the matrix wettability. The junior author conceived a method of surmounting this obstacle and proposed it to the senior author as a Master of Science thesis problem.

The concept of fractional wettability visualizes that a fraction of the matrix surface is oil-wet and the remainder water-wet. An alternate concept would have the matrix of uniform wettability, with its relative wettability determined by the relative affinity of the matrix for oil versus water. An experimental study was made based on the first concept, using mixtures of naturally water-wet glass beads and silicone-treated oil-wet observed.

Mixtures covering the entire wettability spectrum in ten per cent increments were used to prepare unconsolidated packs in capillary pressure resistivity cells. Each pack was saturated with brine and the brine displaced with oil. Resistivity measurements were made at progressive stages of desaturation and the saturation exponent for each pack was determined. An approximately linear relationship between the saturation exponent and fractional oil wettability was observed.