resistivity logging
TRANSCRIPT
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.