Induction logging tools were designed to work in oil-based drilling mud as laterolog does not works in the absence of conductive medium.In their simplest form induction tools are made of minimum two coils with one acting as transmitter and other acting as receivers mounted on mandrel co-axially.Modern induction tools operates at varied range of frequency from tens to hundreds of kilohertz.In typical induction tool coil separates from 1 to 10ft apart and each coil has hundreds or more turns depending upon design.The alternating current generated by time varying magnetic field induces eddy current in surrounding formations and these eddy current setup secondary magnetic field.These secondary magnetic field induces voltage in receivers and magnitude of this voltage is function of rocks conductivity.induction tool measure conductivity which is than converted to resistivity .Conductivity measured by induction tool is measured in milliSiemens per meter (mS/m).
Modern induction tools in order to measure resistivity at different depth of investigations (10in,20in,30in,60in and 90in) and different vertical resolution (1ft,2ft and 4ft) combines multiple arrays of receivers at varying distances from transmitter ranging from 6in to 80in.
Figure:- A basic two coil induction logging tool configuration
Main application of Induction tools is to provide resistivity measurement at different depth on investigations in oil based drilling mud system,resistivity measurement from induction tools can be used for below purposes:-
Laterolog tools are designed to record resistivity measurement in boreholes drilled with high salinity drilling mud or surrounded by highly resistive formations.Laterolog contains focusing electrodes on both sides of central electrode.Function of focusing electrodes are to force current to flow in lateral direction.The current forced into formation along with voltage measurement between reference electrodes determines formation resistivity.Focussed resistivity measurement is usually done at three depth of investigations.
There are two basic types of focused electrode devices in industry they are three electrode system and other also known as LL3 other one is 7 to 13 electrode system also known as LL7 or dual laterolog.Depth of investigation of Laterolog varies from 90in (deep) to 18in.
Resisitivity log is used for water saturation estimation in reservoir ,basic principle behind water saturation calculation using resistivity log is most of the rock matrix and hydrocarbon in pores of the rock is having infinite resistivity whereas saline formation water in pores of the rock has low resistivity due to presence of salt in water.In simple words adding saline water will decrease the overall resistivity of formation and adding hydrocarbon will increase the overall resistivity of formation for same lithology.
Drilling results into invasion of drilling fluid in formation,which brings the problem of getting true formation resistivity.To solve this problem resistivity tools measure resistivity at several depth of investigations.By using the resistivity in progressively increasing depth of investigation,uninvaded zone true formation resistivity can be calculated using available charts.
Micro spherically focused log (MSFL)
MSFL tool is used to measure resistivity of flushed zone,its has pad mounted electrodes to force current into the formation.The pad of MSFL on which electrode is mounted also works as caliper arm.The depth of investigation of MSFL tool is few inches from borehole wall.MSFL tool being a padded device measures flushed zone resistivity (Rxo) ,and hence all the applications related to MSFL tool are related to flushed zone evaluation.
Applications of MSFL tool are as follow:-