The Determination of Qv From Membrane Potential Measurements on Shaly Sands
Authors: Thomas E.C. 
Published: December 2007

This technique determines the value of cation-exchange capacity per unit pore volume (Qv) of a shaly sand formation sample by using it as a membrane in an electrochemical cell. This new technique is nondestructive, eliminates prior core analyses, uses small samples, and provides representative values of Qv for nonisotropic and homogeneous samples.

Introduction

The electric log is the oil industry's most used tool for in-situ detection of hydrocarbon accumulations. However, quantitative measurement of oil in place is not simple, because calculation of the oil saturation of a reservoir from electric log responses is complicated by the presence of clay minerals in the pore network. The complexity is twofold.

First, the clay geometry probably influences the conduction of electric current. To illustrate, some microphotographs of shaly sandstones as observed by scanning electron microscopy (SEM) are shown in Fig. 1. The use of SEM photographs to study clays in sedimentry rocks has enhanced our understanding of pore geometries. Most clay coatings on sand grains are less than 5 microns thick and are usually less than 2 microns. This thickness of coating shows up only as a thin, brown-black edge on sand grains in the usual thinsection photograph and, thus, provides little detail. However, the depth of field of provides little detail. However, the depth of field of SEM is large even at high magnifications, thus providing an exceptional view of the rock surfaces. Notice that the clay platelets usually assume one of four orientations:

Type 1. The basal plane surface of the clay platelets is parallel to the rock surface. (The example shown in Fig. 1 is illite, but any clay type can assume this orientation, particularly when the clays are detrital, pore-filling material.) pore-filling material.) Type 2. The basal plane surface of the clay platelets is normal to the rock surface. (The example shown in Fig. 1 is chlorite and montmorillonite with some kaolinite. This orientation is often assumed by authigenic material.)

Type 3. The day platelets stack into elongated "tubes" or "books" with their basal planes normal to the rock surface and parallel to adjoining platelets. (The example shown in Fig. 1 is kaolinite and is the principal clay type found in this authigenic configuration.)

Type 4. The clay platelets grow in webs or wisps across the pores or over previously deposited clay crystals. (The example shown in Fig. 1 is chlorite, but other clay types are found in this authigenic configuration.)

When viewing a Type 2 clay geometry, it is not difficult to see that even when the pore center is filled with hydrocarbons the water in the clay "forest" on the sand grains affords significant volume for electrical conduction. Thus, this configuration may produce lowresistivity, shaly, pay sands. And to further compound the problem, if the clay pay sands. And to further compound the problem, if the clay mineral in a given shaly sand is exclusively chlorite, it will not appear "shaly" on the gamma ray log because the chlorite lattice contains no potassium and, hence, may have very low gamma ray activity.

The second complication is that clay minerals have the ability to conduct electricity through ion-exchange reactions.

JPT

P. 1087