Comparisons Between Log And Capillary Pressure Data To Estimate Reservoir Wetting
Authors: Holmes Michael , Tippie Douglas B. 
Published: December 2007

From log analysis, saturation gradients above the oil-water contact can be derived at reservoir conditions. Additionally, average values of irreducible water saturation, again at reservoir conditions, are available from logs. Measurements of capillary pressures in the laboratory can be made on samples treated so as to be highly water-wet. Variations in capillary pressure can be converted to an equivalent height above the oil-water contact, assuming the reservoir also is highly water-wet.

Comparisons between log-derived and capillary pressure-derived data for oil-water systems are used pressure-derived data for oil-water systems are used to estimate in-situ reservoir wetting. If the two sets of data agree, then the reservoir is water-wet. If the log data indicate lower capillarity than that calculated from capillary pressure data, then values of, the wetting angle, can be derived. The method only applies when is less than 90 degrees (i.e., to basically water-wet systems).

Results are presented that suggest that is 0 degrees for one sandstone reservoir (highly water-wet),is about 40 degrees for a second sandstone reservoir, and is about 50 degrees for a carbonate reservoir. Other rock property measurements on the same reservoirs are consistent with this interpretation. Irreducible water saturations for the water-wet sandstones are apparently lower than would be predicted under capillary equilibrium.

Introduction

Reservoir performance analyses require the knowledge of saturation-dependent functions. One important property that controls reservoir behavior is wetting behavior of the saturating fluids. Relative permeability is strongly affected by wetting. Studies have been directed toward changing reservoir wetting to optimize recoveries.

Methods of calculating wetting include measurement of threshold pressures, determining area under capillary pressure curves, and by direct measurement of contact angles using core surfaces and reservoir fluids. The major problem is that measurements at surface conditions do not necessarily represent in-situ conditions.

The method presented here has the advantage of comparing in-situ measurements (from logs) where wetting is unknown with surface measurements (from capillary pressures) where wetting is known. From comparison of the two measurements, an estimate of in-situ reservoir wetting is made.

CAPILLARITY AND WETTING

Capillary forces are responsible for saturation distributions (oil, gas, and water) in reservoir rocks. Capillary pressures (Pc) can be related to pore entry size (r) by the equation pore entry size (r) by the equation2 cosPc =------------ ................................(1)r

where

Pc = capillary pressure, psi= interfacial tension between the saturating phases, dynes/cm= contact (or wetting) angle, degreesr = pore entry size

Laboratory measurements of capillary pressures are made routinely, largely for the determination of irreducible water saturation, Swi. Fig. 1a shows a typical experimentally derived capillary pressure curve. Values of capillary pressures will vary from one fluid system to the next, dependent upon the magnitude of interfacial tension () and wetting angle ():

Pc cos1 1 1------- = ---------------------,..............(2)Pc cos2 2 2

where subscripts 1 and 2 refer to different saturating fluids.