Oil-Wet Reservoir: Recognition from Well Logs

G. B. Asquith
Department of Geosciences and The Center for Applied Petrophysical and Reservoir Studies, Texas Tech University Lubbock,Texas

When we work with the problem of oil-wet versus water-wet reservoirs, there are three questions we need to answer.

1. Is there a reason why we believe that carbonate reservoirs are more often oil-wet compared to siliclastic reservoirs

Answer: The presence of polar organic compounds in crude oil and the carbonate versus siliclastic mineralogy both effect wettability of oil versus water. Because carbonates react more readily with organic acids in crude oil than quartz, carbonates reservoirs are more often oil-wet to partially oil-wet when compared to siliclastic reservoirs.

2. Why are we so often able to calculate reliable water saturations defaulting to a saturation exponent [n] value of 2.0 when a reservoir is partially oil-wet or is oil –wet?

Answer: Work by Morgan and Pirsion (1961) has illustrated that at least 40% of the grain surfaces must be oil-wet before saturation exponent [n] is >> 2.0. addition work by Mugan and Moore (1967) has illustrated that even in an oil-wet reservoir the water saturation (Sw) must be below 35% before the saturation exponent [n] is >> 2.0.

3. How can we determine from well logs if a reservoir is water-wet, oil-wet or partially oil-wet? To help answer Questions #3 the following log parameters are used: Sw(archie) a=1, m=n=2 Water-Wet with intergranular/intercrystalline porosity PHIrxo = (Rmf/Rw)^0.5 intergranular/intercrystalline porosity with no correction for the presence of hydrocarbons. Sw(ratio) = [(Rxo/Rt)/(Rmf/Rw)]^0.625 Water-Wet [Sxo = Sw^0.2 Sw/Sxo = [(Rxo/Rt)/(Rmf/Rw)]^0.5 BVW = Sw(archie) * PHInd

For example: If: Sw(archie) = Sw(ratio), PHInd = PHIsonic = PHIrxo and BVW <= 0.04 [water-wet reservoir w/ intergranular/intercrystalline porosity] However if: Sw(archie)<>PHIrxo and BVW< 0.005 [oil-wet reservoir w/ itergranular/intercrystalline porosity] In addition to the above logging parameters the application of dielectric logs and water saturations by the Lucia (1995) method will be presented.

These logging parameters and methods will be applied to the following four reservoirs: 1.) Cretaceous Glen Rose skeletal grainstone [water-wet with intergranular porosity], 2.) Mississippian Chester ooid grainstone [oil-wet with intergranular porosity], 3.) Cretaceous Caballos quartz arenite [oil-wet with intergranular porosity], and 4.) Siluro-Devonian Fusselman [partially oil-wet? with vuggy and intercrystalline porosity].

Determining if a reservoir is water-wet versus oil-wet is very important, because knowledge of the wetting phase in a reservoir dictates how we set-up the laboratory measurements of saturation exponent (n) and relative permeabilies including the value of residual oil saturation (ROS).