Wettability Literature Survey-Part 3: The Effects of Wettability on the Electrical Properties of Porous Media
Authors: Anderson William G. 
Published: December 2007

This paper examines the effects of wettability on the Archie saturation exponent and the formation factor, which are determined experimentally in cores. These parameters are important in the investigation of the hydrocarbon saturation of a formation by use of resistivity data obtained from well logging. The Archie saturation exponent, n, typically has a value of about in water-wet formations and cleaned cores, while in native-state, non-water-wet cores and formations it is generally larger than 2. In uniformly oil-wet cores with low brine saturations, n can reach values of 10 or more. The exponent is higher in oil-wet cores at low saturations because a portion of the brine is trapped or isolated in dendritic fingers where it is unable to contribute to electrical conductivity. If a cleaned water-wet core is used to measure n and the reservoir is actually oil-wet, interstitial water will be underestimated during logging. No definite conclusions can be drawn about the effects of wettability on the formation factor. However, the wettability of clays in a core is likely to affect this parameter.

Introduction

This paper is the third in a series on the effects of wettability on core analysis. Changes in the wettability of the core have been shown to affect electrical properties. capillary pressure, waterflood behavior, relative permeability, dispersion, tertiary recovery, irreducible water permeability, dispersion, tertiary recovery, irreducible water saturation, and residual oil saturation. For core analysis to predict the behavior of a reservoir, the wettability of the core must be the same as the wettability of the undisturbed reservoir rock.

In the first report, the various kinds of wettability, such as mixed wettability, were discussed. That paper also defined native-state, cleaned, and restored-state cores and gave the procedures necessary to obtain each type. Note that a restored-state core has been cleaned and then aged with native crude oil and brine at reservoir temperature until the native wettability is restored. This definition is used in the majority of the more recent literature. Be aware, however, that in some papers, particularly older ones, the term "restored state" is used for what are actually "cleaned" cores (e.g., see Craig ).

Wettability and saturation history are important factors in the determination of the electrical resistivity of a porous medium because they control the location and distribution of fluids. The electrical resistivity of a core is determined by the lengths and cross-sectional areas of the conducting paths through the brine. Large resistivity is caused by small cross-sectional areas and long conduction paths. First, consider a 100% brine-saturated core. The resistivity of the core is much higher than the resistivity of an equivalent volume of brine because the nonconductive rock reduces the cross-sectional area through which the current can flow. At the same time, the rock increases the length of the conducting paths.

The resistivity of the core is increased further by any hydrocarbon saturation in the core because hydrocarbons are also nonconductive. The increase will depend on the saturation, wettability, and saturation history, the factors that control the location and distribution of the oil and water in the rock. In a water-wet rock, the brine occupies the small pores and forms a continuous film on the rock surfaces. In an oil-wet rock, the brine is located in the centers of the larger pores. This difference in brine distribution caused by the wettability becomes very important as the brine saturation is lowered. Generally, almost all of the brine in the water-wet rock remains continuous, so the resistivity increases because of the decrease in the cross-sectional area that can conduct flow. In an oil-wet rock, a portion of the brine will lose electrical continuity as the saturation is lowered, so the electrical resistivity will increase at a faster rate.

Effects of Wettability on Resistivity and the Archie Saturation Exponent

The hydrocarbon saturation of a formation is often estimated from resistivity data obtained by well logging. The empirically determined Archie saturation equation is often used: (1)

where

S = brine saturation in the porous medium, R = resistivity of the porous medium at saturation S, and R = resistivity of the 100% brine-saturated formation.

JPT

P. 1371