Influence Of Shale Conductivities On Electric Conductivity Of Low Permeability Rocks
Authors: Volk L.J. , Carroll H.B. , Raible _ 
Published: December 2007

The correlations among the physical properties of core plugs taken from the Mesaverde formation in the Unita basin (Utah) were examined. Porosity, permeability, surface area and cation-exchange capacity were determined. The effects of the KCl-based drilling mud on electric logs were of particularconcern because of the relatively high invasion profile. There is some evidence that the particular counterion may need to be considered. Although few samples were available and variability was limited, the Waxman-Smits model or the'dual-water' model of Clavier, Coats and Dumanoir were applied equally well. Byeither model, the resistivity formation factor was above that for a clean sand, even though the clay content of the core plugs seemed to be low.

Introduction

A major objective of the Department of Energy's Western Gas Sands Project(WGSP) is to provide accurate reservoir evaluation so that completion and stimulation experiments can be adequately evaluated. Objectives of this research conducted at the Bartlesville Energy Technology Center were to measure the electrical effects of shaliness on a sample of rocks from a specific type of reservoir and to provide data for estimating water saturations from loganalysis.

A principal aim of well logging is to provide quantitative information about porosity and water saturation of the permeable formations penetrated by the borehole. Drilling procedures in some areas of the Unita basin call forsalt-based (KCl) mud systems. These systems are necessary because of imbibition of water by the Wasatch expandable layer clays upon prolonged exposure to waters of low salinity, resulting in borehole problems 1.

Resistivity logging technology, in turn, is limited because of the tendency for current to travel through the highly conductive and column. The deep invasion profiles encountered here also aggravate the problem.

For clean sands, the relationships between measured physical quantities and porosity or saturation are known2. However, western gas sands are not typical because permeabilities are low (less than 0.1 millidarcy),porosities are low (less than 10%) and clay or shale content is high (greaterthan 5%). Water resistivities have a wide variation but generally they are high(greater than 2 ohm-meters). The small grain size of the rock matrix implies a high internal surface area. The high matrix surface area and the presence of clay require that the effect of surface or interface conductivity be considered when evaluating electric logs.

However, the presence of clay minerals greatly complicate loginterpretation, particularly the electrical resistivity and SP logs. This can affect evaluation of hydrocarbon-bearing formations.

The conductance and electrochemical behavior of shaly sands and the irrelation to low interpretation have been studied by man workers3–15. Generally, laboratory research has been conducted with NaCl electrolytes to confirm the theoretical models. However, in the Unita basin, drilling has been conducted with KCl-base muds. The formation has a deepinvasion profile with a KCl-base fluid, which would effect resistivity logs. This study was, therefore, initiated to determine if there is a significant deviation in the parameters of shaly sand models when KCl electrolyte systems are used in laboratory studies.

Literature Review

The use of electrical resistivity logs as a means of estimating porosity is based upon the work of Archie2, whose equation ha traditionally been used as a basis for estimating hydrocarbon saturation in reservoir rock. This equation is adequate for nonshaly or clean formations having typical porosity and permeability. However, in shaly sands, there are anomalies not explained by this theory.