WIRELINE LOGGING

Wireline logging isperformed with asonde or probelowered into theborehole or well,usually after thedrillstring has beenwithdrawn.

Openhole loggingis based onmeasurements ofthe formation'selectrical, nuclear

and acoustical properties. Other openholewireline services include formation sampling,fluid sampling and pressure measurements.

Cased-hole logging includes measurement ofnuclear, acoustical and magnetic properties.Other cased-hole wireline tools includeperforator guns and various production logs.

CALIPER LOG

Caliper logs are required toassist in the quantitativeinterpretation of many otherlogs that are sensitive toborehole diameter and wallroughness (rugosity).

Compensated logs such asdensity (FDC) and neutron(CNL) are corrected for thesefactors.

The caliper shows wheredeviations occur from thenominal drill bit diameter.

The deflections are towardssmaller radius where mudcake has accumulated inporous formations and theoversize excursions wherecaving has taken place.

Shales and coals arelithologies that tend to cave.The absence of mud cakeadjacent to a porous bedmay indicate a tight sand orpossible overpressure.

ELECTRICAL LOGS

There are a large number of electrical sondes.They are used to measure electrical propertiesin three different frequency ranges:

1. DC voltages that appear spontaneouslyin wells and boreholes (SP)

2. Strata and fluid resistivities (at low tomedium frequencies 10 Hz to 20 kHz).

3. Dielectric constants (at high frequencies>10 MHz and up to 1 GHz).

We will discuss in detail only SP and a fewresistivity logs.

Dielectric logs give good results in low orvariable salinity formation waters whereresistivity methods have poor performance.Dielectric logs can also be used where lowgravity oils are not displaced. UHF logs have avery shallow investigation range; VHF logsinvestigate deeper.

SP LOGS

Spontaneous potentials (SPs) are naturallyoccurring potentials within the earth. Inwellbores, they are measured between asingle downhole electrode and a referenceelectrode in a mud pit on the surface. SPsarise because of differences in salinitybetween mud filtrate and formation fluids.

The potential read for shales normally variesvery little with depth. SP is measured relativeto this base line zero called the shale line.Negative deflections, measured in millivolts(mv), to the left of the shale line occur oppositesands and reach a maximum in clean porousbrine-saturated sands called the clean sandline.

The SP is positive if the mud filtrate is saltierthan the formation water, and negative if theformation water is more saline than the mudfiltrate. Positive SPs occur in freshwater sandsnear the surface.

SP INTERPRETATION

The main use of the SP log isto differentiate between tight,electrically conductive beds(shales) and more permeable,electrically resistive beds(sandstone and carbonatereservoirs).

The magnitude of the SPdeflection depends on theresistivity contrast betweenthe mud filtrate and theformation brine.

Permeable beds typicallydeflect the SP curve to the left(more negative SP).

The rightmost deflection ischaracteristic of shale. Theleftmost deflections indicateclean sands (no fines) orcarbonates. Baseline shifts inSP curves are caused bychanges in formation fluidsalinity. SP curves are alsosensitive to bed thickness, thedeflection being attenuated inthin beds.

RESISTIVITY LOGS

"Conventional" resistivity logs were made bymeans of electrodes in contact with theformation through the drilling mud. There wereseveral sondes capable of measuring todifferent distances (Short Normal, LongNormal and Lateral).

Conventional logs gave good results in softformations with fresh mud but the quality ofresults declines in hard formations andcarbonates. They have been largelyabandoned in favour of modern vertically orspherically focused logs and induction tools.

Guard logs or Laterlogs produce results thatare much less dependent on mud resistivitythan conventional logs. These sondes haveexcellent vertical resolution to identify thinporous layers. The short guard or Laterlog 8(LL8) is usually combined with the dualinduction log.

INDUCTION LOGS

Induction sondes are designed to measureresistivity in wells drilled with non-conductivemud. In addition, they are focused to minimizethe effect of the borehole and the invadedzone. Induction logs measure conductivityrather than resistivity.

The DIL (Dual Induction Laterlog) systemconsists of a deep investigation inductionsonde (ILd), a medium range induction sonde(ILm), a Laterlog 8 (LL8) and an SP electrode.

The three focused resistivity readings can beused to accurately determine the trueformation resistivity, Rt, even if the invadedzone is extensive.

The formation resistivity is necessary in orderto calculate porosity and fluid saturationsusing other logs.

MICRORESISTIVITY LOGS

Microresistivity logs are recorded on a smallvolume near a well filled with conductive mud.The aim is to determine the flushed zoneresistivity, Rxo, and the exact thickness ofbeds.

The measuring device is mounted on a padheld against the well wall. The Microlog (ML),Microlaterlog (MLL), Proximity Log (PL) andMicro Spherically Focused Log (MSFL) aremicroresistivity sondes. The sondes arevariously affected by factors such as mud cakethickness and the extent of the invaded zone.

Microresistivity logs are not used for correlationbut because they focus on very small volumes,they provide a means for the very precisedelineation of lithological boundaries.Microresistivity is used to estimate porosityassuming the flushed zone is saturated withmud filtrate.

RESISTIVITY LOG INTERPRETATION

The set of threefocused logs shownare produced by theDIL system. Allthree sondes showa similar responsein shales.

The ILd log tends toapproximate, Rt, theformation resistivity.

Where the threelogs deflect to theright, severe mudfiltrate invasion of areservoir pay zoneis indicated.

The shallow guard(LL8) is mostaffected by mudfiltrate invasion anddeflects right in allporous formations.Conglomerates withoil saturation mayhave lower Rt thansands due to lowerirreducible water.

ELECTRICAL LOG INTERPRETATION

RADIATION

Nuclear logs record radioactivity that may beeither naturally emitted or induced by particlebombardment.

Atomic mass is made up by positively chargedprotons and neutrons. Negative electronsbalance the proton charge.

Radioactive materials emit alpha, beta andgamma radiation. Only gamma radiation hassufficient penetrating power to be used in welllogging.

Only neutrons are used to excite atoms bybombardment in well logging. They have highpenetrating power and are only significantlyabsorbed by hydrogen atoms.

This is an important property for well loggingsince the hydrogen atoms in formation fluids(hydrocarbons and water) are very effective inslowing neutrons.

NUCLEAR LOGS

There are a very large number of nuclear welllogs. The more common basic logs are:

1. Conventional Natural Gamma Ray (GR)2. Spectral Gamma Ray (SGR)3. Formation Density Compensated (FDC)4. Photoelectric Effect or Litholog (PE)5. Compensated Neutron (CNL)6. Sidewall Neutron Porosity (SNP)

Sophisticated nuclear logs now detectparticular and measure individual elementcompositions for C, 0, Cl, H, Si, Ca, Fe, and S.These logs include neutron and gammaspectroscopy and nuclear magnetic resonance(NML) logs.

Examples of the response of natural gammaray (GR), photoelectric effect (PE), and bothcompensated density (FDC) and neutron(CNL) logs will be discussed.

NATURAL GAMMA-RAY LOG

Natural radiation is due to disintegration of nuclei inthe subsurface. Potassium, Thorium and Uraniumare the major decay series that contribute to naturalradiation.

In the conventional gamma sonde, a scintillationcounter indiscriminately detects total disintegrationsfrom all sources in a radial region close to the hole(150-250 mm).

Because K, Th, U tend to be concentrated in shalesand are low or absent in clean sandstones andcarbonates, the gamma response is similar to theSP log.

Openhole and cased-hole gamma logs can also becorrelated and used to precisely locate pay zonesfor perforation.

Gamma-ray logs can yield an approximatequantitative estimate of clay content or shaliness.

GAMMA LOG INTERPRETATION

The GR and SPlogs show strongcorrelation. Bothdeflect to the rightfor shales and tothe left for cleansands.

Notice that the GRresponse is muchless sensitive tobed thickness andthat coals producealmost no gammaresponse.

Because GR is notsensitive to bedthickness and canbe run as a cased-hole log, it is usedto delineate zonesfor perforation.

SP is ineffective insalt mud and non-conductive mud.GR is unaffectedand is valuable inthese situations.

SPECTRAL GAMMA LOG

Spectral gamma logs record individualresponses for K-, Th- and U-bearing minerals.The detectors record radiation in severalenergy windows as GR-K, GR-U, GR-TH.

The main applications of spectral gamma logsare:

1. Clay content evaluation - spectral logswill distinguish between clays and otherradioactive minerals such as phosphate.

2. Clay type identification - ratios such asTh:K are used to distinguish particularclay minerals.

3. Source rock potential - there is anempirical relationship between U:Kratios and organic carbon in shales.

Spectral gamma sondes also provide a totalGR count that is equivalent to a conventionalgamma log.

DIFFUSED GAMMA-RAY LOGS

A gamma source is used to bombard theformation and the scattered energy returningto the wellbore is measured. The source ispressed onto the borehole wall by a pad. Twodetectors are used at different distances fromthe source so that a correction for the effect ofmud cake can be made.

Gamma rays react with matter in three ways:

1. Photoelectric absorption occurs for lowenergy gamma rays. The absorptiondepends on the atomic number of thenucleus and is the basis for thelitholog (PE).

2. Compton scattering occurs over theentire energy spectrum and is the basisof the density log (FDC).The intensityat the diffused energy at the boreholewall is proportional to the bulk density.

3. Electron-positron pairs are produced atrelatively high energy.

LITHOLOGS

The litholog sonde records low energy gammaradiation arriving at the detector. The photoelectricabsorption factor depends on the atomic number ofthe atoms in the formation and the PE log issensitive to the composition of mineral phases.

Because photoelectric effect is only slightly affectedby porosity and variations in fluid content, the PElog provides a direct indication of lithology.

Photoelectric absorption factors (Pe) for commonsedimentary minerals are as follows:

Mineral Pe

Quartz 1.81Kaolinite 1.83Montmorillonite 2.04Dolomite 3.14Illite 3.45Halite 4.65Anhydrite 5.05Calcite 5.08Chlorite 6.30

LITHOLOG INTERPRETATION

The photoelectric absorptionfactor is typically about 2 forsandstones and 4 for shales.

In the simple clastic section,coal shows a very low PEfactor since carbon has arelatively low atomic number.

Notice the increase in the PEfactor in the calcareous unitswhere the log values areinfluenced by the calcite (Pe ≈5) and the lower deflection forthe dolomitic bed (Pe ≈ 3).

In common with the GR log,PE is not strongly affected bybed thickness so the thincoals and sandstone units arerelatively well resolved.

Small quantities of siderite,pyrite and especially baritecan produced very high PElog values.

DENSITY LOG

If the grain density and the density of the mud filtrateare known, density logs give direct estimates ofporosity (n). Mud filtrate has a density from 1.0 to1.1 Mg/m3.

n = (ρρρρg - ρρρρb) / (ρρρρg - ρρρρf)

It is usual to calculate two porosities, one using thegrain density of quartz (2.65 Mg/m3) and anotherusing the density of calcite (2.71 Mg/m3). Dolomitehas an even higher density (2.85 Mg/m3). Shalegrain densities are in the range 2.4 to 2.6 Mg/m3.

Assume the density log (FDC) indicates a bulk density of2.2 Mg/m3 with a mud filtrate density of 1.1 Mg/m3, thenfor sand and lime:

nsand = (2.65 - 2.2) / (2.65 - 1.1) = 0.45 / 1.55 = 0.290nlime = (2.71 - 2.2) / (2.71 - 1.1) = 0.51 / 1.61 = 0.317

If the formation is gas-saturated (porosity calculatedfrom density logs give anomalously high values since ρf

for gas is 0.1 to 0.3 Mg/m3 and 1.0 to 1.1 was assumed.

NEUTRON LOGS

Fast neutrons are emitted by a source in thesonde and travel through the formation wherethey are slowed mainly by collision withhydrogen atoms. Slow neutrons are capturedby atoms with the emission of a gamma ray.Various logs detect:

1. Capture gamma rays2. Slow (thermal) neutrons3. Partly slowed (epithermal) neutrons

The Compensated Neutron Log (CNL) tool hastwo detector spacings and is sensitive to slowneutrons. The Dual Porosity CNL tool has twosets of detectors for both thermal andepithermal neutrons. CNL logs can be run inliquid-filled openholes and cased-holes.

In addition, there are several single-detector,pad-type neutron tools that use epithermaldetectors. These include the Sidewall NeutronPorosity (SNP).

POROSITY LOGS

Neutron logs respond to hydrogen ion contentand hence to the fluids occupying porosity.Since both oil and water have roughly thesame hydrogen ion content per unit volume,calibrations for oil and water saturation arevery similar.

Hydrocarbon gas has a much lower hydrogenion content per unit volume and neutronporosity logs underestimate gas filled porosity.

On the other hand, porosities derived fromdensity logs overestimate gas filled porosity.The density and neutron porosity exhibit adiagnostic cross-over in gas-saturatedformations.

Because neutron porosity logs respond tohydrogen ions, minerals containing H in thestructure (e.g. gypsum, clays) can beresponsible anomalously high apparentporosities.

POROSITY LOG INTERPRETATION

The porosity logs shown are calibrated forsandstone based on density (FDC) andneutron (CNL) logs. The SP log is also shownto indicate lithology.

At 1020m, the porosity is overestimated wherethe shale "wash out" has occurred.

Between 1040 and 1058m, the density andneutron logs show separation as shalinessincreases.

At about 1105m, the two porosity logs cross-over indicating a clean gas zone in thesandstone reservoir.

At 1130m, a very low porosity is indicated in atight sandstone layer.

Where the two porosity logs agree at 1135 to1142m, a clean sandstone is indicated.

POROSITY LOG INTERPRETATION