Beginners guide to the soundtrack of the borehole. · Beginners guide to the soundtrack of the borehole. Peter Fitch Department of Earth Science and Engineering. [email protected]

Beginners guide to the soundtrack

of the borehole.

Peter Fitch

Department of Earth Science and Engineering.

[email protected] (@peterophysics) LPS Seminar 23rd June 2016

mailto:[email protected]

Basic petrophysics concepts

Petrophysics Caution Interpretation Fundamentals

Matrix (grains) Clay (“Shale”)

Clay bound water

Capillary bound water

Free water

Hydrocarbon

Effective porosity

Total porosity

Clay or shale volume

Bulk volume

Basic petrophysics concepts


100 µm 100 µm

Basic sound wave concepts


https://www.youtube.com/watch?v=gl4FvHKzAlU

Basic sound wave – rock interaction


100 µm

♫ More than a feeling (Boston)

Measurement of the time taken for a sound wave to

move through the formation

• Traditionally, through one foot

• μsec/ft (micro-seconds per foot)

• μsec/m (micro-seconds per metre)

Travel or transit time (Δt or DT)

• Inverse of velocity (e.g. ft/μsec)

Influenced by lithology, porosity and fluids


♫ With or without you (U2)


[Rider & Kennedy, 2011]

♫ With or without you (U2)


[Schlumberger, 1991]

Sound waves detected:

• P-wave

• compressional

• S-wave

• shear

• Stoneley wave

Presented in terms

of transit time, or

slowness

♫ Pinball Wizard (The Who)

In a dense, well cemented / consolidated rock with no

porosity P-waves will move quickly

• Low transit times


Rock type

(no porosity)

Velocity

(ft / sec)

Transit time

(μsec / ft)

Sandstone 18,000-19,500 56 – 51

Limestone 21,000-23,000 48 – 44

Dolomite 23,000 44

Anhydrite 15,000 50

Salt 17,500 66

Shale 5,880-16,660 170 – 60

♫ Handbags & Gladrags (Rod Stewart)

Piezoelectric transducers, transmitter and receivers


Receiv

er

Tra

nsducer

Tra

nsm

itte

r

Tra

nsducer

Ele

ctr

onic

pro

cessor

Electronic signal

Electronic signal

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjYnoSq_a7NAhVaOMAKHRJBC0gQjRwIBw&url=http://animatedviews.com/2007/gromit-hears-his-masters-voice/&psig=AFQjCNEUTfV5pVx-wf9cogWttiqVDctf5A&ust=1466250033397820

♫ Drove all night (Cindy Lauper)

Mud wave

Refracted wave

Critically refracted wave


♫ Are you gonna go my way (Lenny Kravitz)


Borehole Compensated

(BHC)

Two sets of a transmitted to 2

receiver pairs

• Near = 2 ft

• Far = 3 ft

Reduced borehole effects

Signal degrades if tool not

centred in borehole

Large borehole = mud wave

arrives first

Long-Spaced Sonic (LSS)

Two sets of a transmitted to 2

receiver pairs

• Near = 10 ft

• Far = 12 ft

Deeper penetration into the

formation

• Much reduced borehole

effect

Less impacted by borehole

size


♫ Are you gonna go my way (Lenny Kravitz)



♫ Got the time (Anthrax)


Array Sonic Tools (AST)

• Eight additional

piezoelectric receivers

• Full waveform measured

• Slowness-time coherence

• P-wave, S-wave & Stoneley

wave

• Continuous mud wave


♫ Got the time (Anthrax)

Typical acoustic tools: •

• Shallow depth of investigation

• 2.5 – 25 cm ( 1 – 10 inch)

• Runs central to borehole

• Less sensitive to borehole size

• but very large boreholes cause erratic tool motion, noise spikes and

cycle skipping

• Array sonic tools

• Monopole (multidirectional) or dipole (polarised/directional)

• Allow for compensation of sonic signal for borehole conditions

• Allow signal stacking – for noise elimination


Typical acoustic log presentation


[Baker Hughes, 2011]

♫ Crazy (Aerosmith)



Log suite example – LPS dataset


♫ Don’t stop believing’ (Journey)

Estimating porosity from the acoustic log…

• P-wave

• More in following talks…

• Two common relationships used

» Wyllie Time-Average

» Raymer-Hunt-Gardner


Wyllie Time-Average Equation

Relationship between porosity and P-wave transit time

• Clean unconsolidated formation

• Uniformly distributed small pores

• Fluid transit time 189 μsec/ft for freshwater mud

∆𝑡𝑙𝑜𝑔 = ∅𝑒 × ∆𝑡𝑓 + 1 − ∅𝑒 × ∆𝑡𝑚𝑎

∅𝑒 = ∆𝑡𝑙𝑜𝑔 − ∆𝑡𝑚𝑎∆𝑡𝑓 − ∆𝑡𝑚𝑎


Wyllie Time-Average - caution

High porosity, gas-bearing formation

• over estimate porosity

Shaly sandstone

• over estimate porosity

Carbonates

• Ok for intergranular porosity

• Secondary porosity = under estimate porosity

Unconsolidated sandstone

• Over estimate porosity

• Compaction correction available



Wyllie Time-Average Chart



Raymer-Hunt-Gardner


Raymer-Hunt-Gardner Equation

Popular alternative to Wyllie Time-Average

Completely empirical relationship, based on their

measurements (previous graph)

Constant (C) of 0.63 can vary as fitting parameter

∅𝑒 = 0.63 × 1 −∆𝑡𝑚𝑎∆𝑡𝑙𝑜𝑔


Wyllie vs. RHG approach


Wyllie Equation

Ø = ((Δtlog- Δtmat )/ (Δtflu -

Δtmat ))*(1/Cp)

Δtmat = 55 Sandstone

Δtmat = 47 Limestone

Δtmat = 43 Dolomite

Δtflu = 189 Fresh Water

Cp = Compaction Correction ~

Δtshale/100

Raymer et al, 1980*

log

163.0t

tmat

*Note this equation is a best fit, or empirical relationship, through experimental data points – while it often works it has no real physical basis

0

0.1

0.2

0.3

0.4

0.5

0.6

50 70 90 110 130

DT (us/ft)

po

ros

ity

RGH

Wyllie

♫ Cum on feel the noise (Slade)

Cycle skipping


[Rider, 1996]


Borehole damage


[Rider, 1996]


As pore pressure increases

transit time will increase

• Velocity decreases

Shale overlying a formation

with abnormally high

pressure will typically have

excess water in pore space

• Overpressured

• Departure from the normal

compaction trend



Summary – acoustic log

Measurement of the time taken for a sound wave to

move through the formation

• Traditionally, μsec/ft (micro-seconds per foot)

Travel or transit time (Δt or DT)

• Inverse of velocity (e.g. ft/μsec)

Influenced by lithology, porosity and fluids

Porosity can be estimated using relationships such as

Wyllie Time-Average and Raymer-Hunt-Gardner


BONUS TRACK

[email protected]

@peterophysics

https://www.youtube.com/watch?v=ClJ5lwl_wM0



Beginners guide to the soundtrack of the borehole. · Beginners guide to the soundtrack of the borehole. Peter Fitch Department of Earth Science and Engineering. [email protected]

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