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Introduction to DownholeLogging
Alberto Malinverno
Well Logging Principles and Applications
G9947 - Seminar in Marine Geophysics
Spring 2008
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in si·tu
in the natural or original position or place
Pronunciation: (")in-'sI-(")tü, -'si-, -(")tyü also-'sE-, -(")chü
Function: adverb or adjective
Etymology: Latin, in position
Date: 1740
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Wireline tool strings
Up to 30m
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Measurements oncore samples
Measurements inthe borehole
Sample
Sensors
“Sample”
Sensors
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6-armcaliper
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• Nuclear Logs –
Natural gamma ray – Density
– Neutron
– Thermal neutron capture
• Sonic
• Electrical / magnetic
• Imaging
• Vertical seismic profile (VSP)• Logging While Drilling
• Examples
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• Gamma ray ! shale indicator
• Gamma ray logGR(API) = 4Th(ppm) + 8U(ppm)
+ 16K(%)
• Compensated gamma rayCGR(API) = 4Th(ppm) +
16K(%)
• CGR ignores U, which is not
always associated with shales
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gamma ray energy
Photoelectricabsorption
Comptonscattering Pair production
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137Cs source of 662 keV gamma rays
Electron density index
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Fast/slow n
Fast n
Fast n Slow n
Fast n sources:- 241Am-9Be (4.35 MeV)- Accelerator (14MeV)Thermal n energy = 0.025eV
Neutronporosity
Capturespectroscopy
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• Neutrons lose mostenergy bouncing off Hnuclei
• Slowing-down lengthfor high-energy
neutrons to reachepithermal energy(0.1-10 eV) depends onwater-filled porosity
• Measurement: flux ofthermal-epithermalscattered neutrons
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Thermal n capturespectroscopy
Detection depends on
• Thermal n absorption cross-section
• Character of gamma ray spectrum
• Elemental abundance
Measured gamma-rayspectrum
Elementalyield
Element gamma-ray spectrum= ! !
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Elemental Capture Spectroscopy (ECS)measures elemental concentrations of
Si, Ca, S, Fe, Ti, and Gd
• = Core plug analysis
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• Nuclear Logs• Sonic – Monopole
– Dipole
• Electrical / magnetic
• Imaging
• Vertical seismic profile (VSP)
• Logging While Drilling• Examples
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Bulk modulus K relatescompressional stress andstrain
Shear modulus ! relatesshear stress and strain
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“Fast” formation “Slow” formation “Slow” formation
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Wyllie’s time-average
Slowness u = 1/V P
V P /V S and lithologyu = (1
−φ)ug + φu f
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• Nuclear Logs
•
Sonic• Electrical / magnetic
– Resistivity• Electrode devices
• Induction
– Magnetic susceptibility
• Imaging
• Vertical seismic profile (VSP)• Logging While Drilling
• Examples
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Electrical properties
Ohm’s law
Resistance R*
Resistivity R
Units of resistivity are "#m (ohm#m)
A
L
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Porosity
Resistivity
Water
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25
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1927
r
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Magnetic susceptibility $
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• Nuclear Logs• Sonic
• Electrical / magnetic
• Imaging – Electrical
– Acoustic
• Vertical seismic profile (VSP)
• Logging While Drilling• Examples
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0.1 in = 2.54 mm
192 electrodes
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Conglomerate in debris flow Rip-up clasts in fluvial channel Cross-beds in tidal sandstone
Burrowing in shelf deposit Overturned sand layers in slump Stylolites in a limestone
1m
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• Nuclear Logs
• Sonic
• Electrical / magnetic
• Imaging
• Vertical seismic profile (VSP)
• Logging While Drilling
• Examples
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After Hardage (1985)
Travel time
Depth
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• Nuclear Logs
• Sonic
• Electrical / magnetic
• Imaging
• Vertical seismic profile (VSP)
• Logging While Drilling
• Examples
Wi li l i L h l d ll
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Wireline logging Logging while drilling(LWD)
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56 samples
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• Nuclear Logs
• Sonic
• Electrical / magnetic
• Imaging
• Vertical seismic profile (VSP)
• Logging While Drilling
• Examples
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Hole-to-hole correlation
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?
H l 798B
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Hole 798B
Opal from diatoms(Glacial)
Terrigenous material(Interglacial)
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Hole 967E
averagecorerecovery
15%