Downhole Logging

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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


• 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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1927

r


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Magnetic susceptibility $


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• Nuclear Logs• Sonic


• Imaging – Electrical

– Acoustic


• 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


• Imaging


• Logging While Drilling

• Examples


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After Hardage (1985)

Travel time

Depth


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• Nuclear Logs

• Sonic


• Imaging



• 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


• Imaging



• 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%

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