Abstract

The preferential wettability (water versus oil-wet) of a reservoir is of extreme importance in: 1) the calculation of water saturations, 2) the determination of multiphase flow properties, and 3) the calculation recoverable oil reserves. Therefore, it is important to be able to determine from well logs when a reservoir is water versus oil-wet, so that laboratory core analysis can be used to determine saturation exponent and relative permeabilities.

When sandstones like the Delaware Mountain Group are dominantly water-wet both Archie (m=n=2) and Ratio water saturations should be approximately equal. Also, in dominantly water-wet sandstones resistivity derived porosity should be approximately equal to porosity calculated from porosity logs. However, as a sandstone becomes more oil-wet Archie (m=n=2) water saturations will be much less than Ratio water saturations because Sxo is less than Sw^0.2 due to high ROS, and in strongly oil-wet reservoirs saturation exponent (n) and cementation exponent (m) may be greater than 2.0. Concomitantly, resistivity porosity in a more oil-wet reservoir will be much less than porosity calculated from porosity logs, because of higher residual oil saturations in more oil-wet reservoirs.

Comparing cross plots of Archie versus Ratio water saturations and resistivity versus density porosities from the Bell Canyon and Brushy Canyon formations with similar cross plots from a well documented oil-wet sandstone reveals that the Delaware sands are strongly water-wet to moderately oil-wet. Using core derived Swirr values and modified Jones (1945) equations, Kro and Krw values for both water-wet and partially oil-wet Delaware reservoirs can be calculated. Relative permeability curves based on these Kro and Krw values exhibit a shift in the Sw value where Kro=Krw from 63% (water-wet) to 55% (30% oil-wet). This small shift in the Sw value where Kro=Krw has profound effect on reservoir production. For example, at a water saturation of 60% a water-wet Delaware reservoir has Kro > Krw. However, in a 30% oil-wet reservoir Krw >> Kro. These possible changes in wettability may be the reason oil versus water production is so difficult to predict in the Delaware sands.