Formation Evaluation Through Combined Use of Core Analysis and Electrical Resistivity Logs
Authors: Osborn C.K. 
Published: December 2007

Determinations of zonal watercut behavior, water saturation distribution, and height above a point where capillary pressure is zero are made by properly combining data from routine core analysis, special core analysis and electrical resistivity logs. Pertinent rock properties are converted to equivalent values of resistivity and are plotted on transparencies for overlay and rapid computation from logarithmic resistivity logs. A statistically accurate evaluation of the productive characteristics of a formation can thus be made with a minimum of difficulty and time consumption.

INTRODUCTION

Information essential to interpretation of hydrocarbon and/or water productivity is not available from a single: measurement technique, whether it be core analysis, complete suites of electrical logs or bottom hole pressure build-up or fall-off tests1. The best features of each technique can be combined to obtain mutually consistent interpretations which result in improved evaluation of potentially productive intervals in a well.

Methods are described which combine core analysis data with electrical resistivity logs. This combination yields information required to select zones for completion, zonal producing characteristics and their possible down-dip productive limits. To accomplish this, core analysis and appropriate reservoir fluid data are converted to values of resistivity. These values are plotted on transparent overlays which are compatible with resistivity scales reported on the down hole resistivity log. The conversion of core analysis data is easily accomplished by digital computer and plotter, but it is not too complicated or time consuming to be done by hand.

THEORETICAL CONSIDERATIONS

Basic Resistivity Equations

The resistivity of a formation is governed by its porosity, pore geometry and the properties and saturations of the fluids within the pores. Relationships to describe the effects of these variables on resistivity have been reported by several workers, but those developed by Archie2 are presented as they are used in this study:Ro=Rw(1/Ï?m)=Rw(FF) (1)Rt=Ro(1/Swn)=Ro(RI) (2)

Many well completions have been, and are being, made employing gross assumptions relative to values for most of the variables in these equations. There is often a tendency to discredit the limited measured data available when interpretations prove erroneous, even though insufficient measured data is the source of error.

Basic Resistivity Equations

The resistivity of a formation is governed by its porosity, pore geometry and the properties and saturations of the fluids within the pores. Relationships to describe the effects of these variables on resistivity have been reported by several workers, but those developed by Archie2 are presented as they are used in this study:Ro=Rw(1/Ï?m)=Rw(FF) (1)Rt=Ro(1/Swn)=Ro(RI) (2)

Many well completions have been, and are being, made employing gross assumptions relative to values for most of the variables in these equations. There is often a tendency to discredit the limited measured data available when interpretations prove erroneous, even though insufficient measured data is the source of error.