Wireline Logging

Resistivity & SP Tools

Overview

• Formation: Reservoir quality• Fluid: Oil, Gas or Water? • Borehole: Environmental factors

Formation Properties

• Radioactivity• Porosity• Grain density• Pore Size• Stratigraphy• Bedding, Dip• Sonic Velocity• Seismic Velocity

Fluid Properties

• Salinity (Resistivity)• Density• Saturation• Pressure• Temperature• Viscosity• Mobility• Bubble Point

Borehole Properties

• Depth• Caliper• Spontaneous Potential• Temperature• Cable Tension• Deviation, Azimuth

Basic Resistivity

• Resistivity (Ohms) = Voltage / Current• Archie’s Equation:– (a/porositym x Rw / Rt)1/n

Formation Resistivity

h

Rs

Rt

Rw

Sw

Rxo

Rmf

Sxo

Di

Adjacent Bed

VirginZone

InvadedZone

tmc - mud cake thickness

Rmc

Adjacent Bed Rm

Mud Invasion

Resistivity Logging

• Electrode type tools (resistivity)– Laterolog– Micro Speherically Focused (MSFL)

• Induction Tools (conductivity)– Dual Induction/HRI– Array Induction Tools

• Conductivity = 1/Resistivity

Electrode Type Tools• Force (AC) current between two (or more)

electrodes.• Measure voltage between two intermediate

electrodes.• Force current to flow in zone of interest by using

‘bucking’ currents.• Do not work in non conductive muds such as OBM.• Can give extremely high resolution and depth of

investigation.

Laterolog Principle

Bucking Currents

Dual Laterolog Tools

• Two depths of investigation – LLD and LLS• Deep current returns to a surface electrode.• Insulated bridle to give a far deep voltage

reference.• Normally run together with MSFL to give Rxo.

MSFL Principle

MSFL Tool

• Pad based tool, similar to density.• Pad is kept in contact by a caliper.• Pad uses a spherical pattern of bucking

currents to focus measure current.• Can be affected by hole rugosity.• Pad is rubber and can be damaged.

Induction Principles

Dual Induction Tools• Two coil arrays emit a high frequency signal to excite the

formation.• The formation current generates a weak seconday signal

which is detected by receiver coils.• Strong direct signal from transmitter is (approx) 90 deg out of

phase with formation signal.• Formation signal is focused by combination of coils in array

and by source receiver spacing.• Reading will be compromised by highly conductive (saline)

water based muds.• Normally poorer vertical resolution and depth of investigation

than laterolog tools.

Array Induction tools• Large number of simpler coils at many different

spacings.• The data from all the coils at different frequencies

and phases is used at surface to compute focused Resistivity readings.

• Depths of investigation and vertical resolution become software selectable.

• Can be focussed as close as 10” horizontally and 12” vertically.

• More sensitive to erratic tool motion.

SP Physics

Spontaneous Potential

• One of the first wireline measurements.• Can be used to identify sand, shale, Vcl• Can be used to derive Rw• Based on the contrast between Rmf and Rw.• Chlorine ions are more mobile than Sodium (NaCl)

and migrate from the lower resistivity zone to the higher.

• This creates a polarisation.

SP Operational Limitations

• Only works in conductive muds.• If Rmf > Rw looks similar to GR.• In opposite case polarity is reversed.• Requires a downhole electrode (laterolog

bridle) and surface electode.• Can be compromised by electrical noise

(welding etc) or magnetised cable drums.

SP Log

Resistivity Test 11. Which three of the following are normally measured in the

invaded zone: Rw, Sxo, Rxo, Rt, Rmf, Sw?2. In a fresh mud system where Rmf>Rw the deep resistivity

would normally be (HIGHER/LOWER) than the shallow?3. What is the definition of conductivity?4. Name the tool types that are normally referred to as

‘electrode’ type tools.5. On electrode type tools why do we not normally measure

voltage at the same electrodes as the current flows to/from?

Resistivity Test 2

6. How to we focus the current to flow in the zone of interest?

7. On a laterolog tool where do the shallow and deep currents flow to?

8. Why does a laterolog use an insulated bridle?9. What other tool is normally run in combination?10. Which of the following tools has a built in caliper?

(Array Induction, Dual Laterolog, MSFL, Dual Induction)

Resistivity Test 311. Name two factors that can affect the MSFL reading.12. Why does the formation signal for an induction tool not get

swamped by the direct signal from the transmitter coil?13. Induction tools work best in which mud type (Saline/Non

Conductive).14. Laterolog type tools work best in which mud type

(Saline/Non Conductive).15. MSFL type tools work best in which mud type (Saline/Non

Conductive).

Resistivity Test 4

16. SP is an acronym for what borehole property?17. The SP measures a voltage or a current?18. The SP system uses 2 electrodes – where are they

located.19. If the SP deflects in the same direction as the GR is

Rmf greater or less than Rw?20. Name two causes of bad SP logs.