dsi 1 dsi* dipole shear sonic imager. dsi 2 lecture plan ä wave propagation - monopole & dipole...

DSI 1

DSI* DipoleShear SonicImager

DSI 2

Lecture Plan

Wave Propagation - Monopole & Dipole

Hardware Waveform Processing Operations

Applications

DSI 3


Wave Propagation

DSI 4

• Compressional:– Particle vibration parallel to direction of wave propagation

• Shear:– Particle vibration perpendicular to direction of wave propagation

Wave Propagation Modes

DSI 5


• Slowness depends on rock mechanical properties:

– rock density– elastic dynamic constants

• Shear slowness:– stiffness of the rock

• Compressional slowness:– stiffness of the rock– compressibility

DSI 6


• Fluid-saturated rocks, slowness depends on:

– amount and type of fluid– the makeup of the rock grains– degree of intergrain cementation

• Soft, loosely consolidated rocks:– generally less stiff and more compressible than hard rocks– sound waves travel slower in soft

rocks than in hard ones

• Fluids (completely unconsolidated rocks):

– have no stiffness at all– will not support shear wave propagation

DSI 7

• Stoneley:– Surface wave guided by the borehole– Travels slower than mud– Does not penetrate the formation– It’s energy concentrates on the bore-

hole surface– It’s amplitude exceeds that of other

waveforms– At low frequency there is little energy

decay by comparison to high frequency


DSI 8

• Non-directional pressure source

• Pulse created in bore-hole & propagates into the formation

• Excites both P & S waveforms in the formation

• Head-waves are created in the mud & detected by the receivers

• Operates in the 10 Khz to 20 Khz range (Not suitable for Stoneley)

Monopole Source

DSI 9

Hard (Fast) Formation — Monopole

• Formation shear slowness less than mud compressional slowness

Vmud < Vshear

• Both the compressional and shear formation waves propagate along the borehole

• Energy leaks back into the borehole as headwaves, which are detected

Head waves

Fluid waveOmnidirectional source

Formation

Compressionalwave

Shearwave

Compressionalwave

Shearwave

Stoneleywave

Compressionalwave

Wellbore

DSI 10

Soft (Slow) Formation — Monopole

• Formation shear slowness greater than mud compressive slowness

Vmud > Vshear

• Snell’s Law predicts in slow formations the shear wave transmitted into the formation travels away from the borehole

•The shear headwave in the borehole is only marginally detectable or absent

• Shear curve is discontinuous when ‘slow’ zones are present, log is of limited value

Head wave

Fluid waveOmnidirectional source

Compressionalwave

Shearwave

Compressionalwave

Stoneleywave

Wellbore Formation

DSI 11

• Directional pressure source

• Pulse created on one side of the bore-hole causing a small amount of flexing. (Flexural Wave)

• Excites both P & S waveforms in the formation

• Flexural waves travel up the bore-hole & are detected by the directional receivers.

• Operates at low frequencies (~2.2 Khz)

Dipole Source

DSI 12

DSI Transducer - Dipole

• A dipole tool utilises a directional source and receivers

• The dipole source creates a pressure increase on one side of the hole and a decrease on the other

• This causes a small flexing of the borehole wall which directly excites compressional and shear waves in the formation

DSI 13

DSI Transducer - Dipole

• Propagation of this flexural wave is coaxial with the borehole

• Displacement is at right angles to the borehole axis and in line with the transducer

• Dipole has low operating frequencies, below 4 kHz where excitation of these waves is optimum

DSI 14

Soft (Slow) Formation - Dipole

• Compressional and shear waves radiate straight out into the formation

• An additional shear/flexural wave propagating up the borehole. It creates a "dipole-type” pressure disturbance in the borehole fluid

It is this pressure disturbance that the directional receivers detect

Wellbore

Flexural waveDirectional source

Formation Compressional

wave

Shearwave

Compressionalwave

Flexuralwave

Shearwave

DSI 15

Soft (Slow) Formation - Dipole

• The shear/flexural wave, initiated by the flexing action of the borehole, is dispersive

• At low frequencies it travels at the same speed as the shear wave; at higher frequencies it travels at a slower speed

• Unlike monopole-only tools, the dipole tool can record a shear/flexural wave even in slow formations

Wellbore

Flexural waveDirectional source

Formation Compressional

wave

Shearwave

Compressionalwave

Flexuralwave

Shearwave

DSI 16

• Shear/flexural wave is:– short in duration– concentrated at lower frequencies

• Additional higher-frequency compressional arrival

• In this typical slow formation example, there is a clear flexural wave from which the shear slowness is inferred

Compressionalwave

Flexuralwave

Shearwave

Dipole Waveforms - Slow Formation

DSI 17

Dipole Waveforms - Fast Formation

• Shear/flexural wave is:– long in duration– very dispersive

• Low frequency components, traveling near the shear slowness, become fairly well separated from the slower, higher frequency components

• Shear can often be detected and the formation shear slowness estimated directly from the waveforms

Shear Flexural Mode

DSI 18


Hardware

DSI 19

Monopole Compressional and

Dipole Shear measurements

provide Sonic data in hard and

soft formations

DSI-Dipole Shear Sonic Imager

DSI 20

• The borehole physics limitation of a

Monopole Sonic to acquire DT shear in

formations where DT shear > DT mud.

• Dipole Sonic acquisition overcomes this

limitation & DT formation >> DT mud

are acquired.

DSI vs DSLT

DSI 21

DSI Hardware

SPAC (Sonic Parallel Acquisition Cartridge)

Microprocessor controls:

• Digitizing• Stacking• Transmitting signals up-hole• Sending commands to the other tool

components via a dedicated serial link

DSI 22

SMDR• 8x receiver stations

• Each station has 2x hydrophone pairs:

– 1x oriented in line with the upper dipole transmitter (odd pair)– 1x oriented in line with the lower dipole transmitter (even pair)

• The outputs from each pair are:– Differenced for dipole reception– Summed for monopole reception

• Receivers are carefully matched during manufacture . Selectable filters and programmable amplifiers are also in the SMDR sonde.

DSI Hardware

DSI 23

DSI Hardware

SSIJ (Sonic Sonde Isolation Joint)

• Mechanical shock absorber to prevent:

– Direct acoustical tool arrivals from the transmitters– Reduces noises coming from below the receiver section

• Do not log with more than 900-

lbs.of weight below the SSIJ

• Do not log without the SSIJ

DSI 24

DSI HardwareSMDX• 3x transmitter elements:

– 1x omni-directional monopole, ceramic transducer– 2x unidirectional dipole transducers oriented perpendicular to each other

• Monopole transducer:– Low frequency pulse for Stoneley– High or low frequency pulse for compressional and shear

• Dipole transducers:– Standard frequency– Low frequency (for large borehole and very slow formations)

• All transducers can be fired at a rate of up to 15 Hz.

DSI 25

DSI Specifications

Temperature rating 350°F [175°C]

Pressure rating 20,000 psi [138 MPa]

Tool diameter 35/8 in. [92 cm]

Minimum hole size 51/2 in. [13.9 cm]

Maximum hole size 21 in. [53.3 cm]

Tool length 51 ft [15.5 m]

Maximum logging speed

One eight-waveform set 3600 ft/hr(single mode)

All six modes simultaneously 900 ft/hr(without BCR)

Digitizer precision 12 bits

Digitizer sampling interval limits Variable from 10 to 32,700 µsec per sample

Digitized waveform duration limits Up to 15,000 samples/ all waveforms

Acoustic bandwidth

Dipole and Stonely 80 Hz to 5 kHz

High-frequency monopole 8 to 30 kHz

Combinability All MAXIS tools, any resistivity tool

DSI 26

• Depths of investigation for sonic devices is a function of:

– formation type– shear and compressional slowness– transmitter-to-receiver – source frequency (wavelength)– etc

Depth of Investigation

DSI 27

DSI Hardware Versions: DSI-A

• CTS telemetry , TCC ( DTC/DTA ) +SPAC-A+SMDR-

A+SSIJ-B+SMDX-A

• DSST-A is obsolete; its production started in August 1990 and

stopped in July 1995. There will be no support for DSST-A in OP

9.2 and later OP versions. SKK recommends that you upgrade all

your current SPAC-A tools to the SPAC-B version

DSI 28

DSI Hardware Versions: DSI-B

• DTS telemetry , DTC-A/H +SPAC-B+SMDR-A+SSIJ-

B+SMDX-A

• DSST-B production started in July 1995. The SMDR-AA

receiver sonde is also going to become obsolete soon. SKK

recommends that you upgrade all your current SMDR-A sondes

to the SMDR-BD/BE version.

DSI 29

DSI Hardware Versions: DSI-II (DSI-Plus)

• Enhances the measurement quality & improves

tool reliability. • Enhances shear slowness measurement by

improving waveform amplitude• The SMDR receiver array has been redesigned

to achieve these improvements • To implement the upgrade of DSI to DSI-II,

SMDR-AA MR-2 & SSIJ-BA MR-2 is needed.• These are mandatory modifications that must

be implemented only by trained and qualified

personnel• The SMDR-AA becomes SMDR-BD

DSI 30

DSI Hardware Versions: S-DSI

• DSI logs run in slow formations with the standard DSI

sleeve result in a strong sleeve arrival in the data.

• Do not run the standard SMDR slotted sleeve in

formations slower than 500 us/ft.

• When in doubt about DTs & when logging surface

formations, use S-DSI and not DSI-II (DSI Plus).

• S-DSI uses a slow-formation sleeve that replaces the

standard SMDR sleeve to extend the range of DSI-II dipole

slowness measurement from 500 usec/ft to 1200 usec/ft.

• To implement the upgrade of DSI-II to S-DSI a Slotted

Sleeve is required, which is SMDR-BD MR-3 (optional).

• The slotted sleeve is intended for use only with DSI-II

tools.

DSI 31

DSI Operating Limits

DSI 32

DSI Hardware Versions: BARS

• DSST-C ( BARS )

• DTS telemetry , DTC-A/H +SPAC-B+SMDR-C+SSIJ-

B+SMDX-A

• DSST-C is available in OP 9.1 and later version as an experimental

tool for BARS (Borehole Acoustic Reflection Survey) operations.

SMDR-C is identical in every aspect to the SMDR-B with the exception

of its PGA board;

DSI 33


Waveform Processing

DSI 34

DSI Acquisition & Processing

• Acquisition: Waveforms of the acquired modes, one for the Rx & one (optionally) for the Tx are built into sets.

• STC (Slowness Time Coherence): Rx & Tx waveform sets are processed to identify coherent arrivals.

• Labelling: Detects the desired arrival from among the peaks identified by STC.

DSI 35

STC Computation - 1 Array Waveforms

STC - Slowness-time-coherence processing

DSI 36

STC Computation - 2 Contour Plot

DSI 37

Poisson's Ratio

.25 .50Gamma Ray

Caliper

6 16

0 100

Delta-T Comp.

100 200Delta-T Shear

0 1. 1. 0

100 500Dtc Coherence Dts

10200

10250

10300

10350

Slowness Time PlaneProjection

Labeling

DSI 38

Dipole Waveforms - Bias Correction

• Bias correction is small for fast formations and averages about 5 percent in slow formations

Shear Flexural Mode

Flexural ModeShear

Flexureslowness

Shearslowness

BiasCorrection

DSI 39

10250


• One of the coherence peaks will correspond to the dispersive flexural mode

• The slowness of this peak is always greater (slower) than the true shear slowness

• In fast formations a low-frequency band pass filter usually produces a coherence peak very close to the true shear slowness

• In slow formations the formation shear must be estimated from the flexural data

DSI 40

10250

Dipole Waveforms - Bias Correction• Low-frequency source tends to minimize the dispersion

• Some correction is still needed to obtain the true formation shear

• A precomputed correction, derived using data generated from numerical modeling, is included in the processing to correct for the bias caused by flexural wave dispersion

• Amount of correction depends on:– the acoustic response signature of the source– the STC filter characteristics– the borehole size– shear slowness

DSI 41

10250


• Bias correction is small for fast formations and averages about 5 percent in slow formations

DSI 42

DFMD - Digital First Motion Detection

• Amplitude threshold-crossing

times derived in the cartridge for

each receiver waveform

• Input into Identification and

tracking algorithm

• Algorithm selects crossing time

the one on each waveform that

corresponds to first motion, and

tracks it over depth

DSI 44


Environment

DSI 45

DDBHC = average (RA & TA)

RA - Receiver array– derived from one tool position

TA - pseudo-transmitter array– derived from several tool positions

DSI Borehole Compensation

DSI 46

SAM 1 & 2 (Dipole) - RA only

SAM 3 (Stoneley) - RA only

SAM 4 (P&S) - DDBHC

SAM 5 (DFMD) - N/A

SAM X (Expert) - N/A

Note:There is no theoretical basis for borehole compensation for the Stoneley mode or for the dipole modes

DSI Borehole Compensation

DSI 47

Road Noise

• Cause: contact between centralisers / standoffs and borehole wall

• Use correct method and placement of centralisation

• Reduce OD of CME-Z close to BS to reduce road noise

• New LCME-A (CME-Y) shows similar order of noise as CME-Z’s

MST - Monopole Stoneley

LDP - Lower Dipole

CME-Y new centralizer

LCME-A

DSI 48


Tool Maintenance

DSI 49

Air Volume Check Procedure

DSI 50


Applications

DSI 51

• Mechanical property analysis

— sanding analysis

— fracture height

— wellbore stability

• Formation evaluation

— gas detection

— fractures

— permeability

• Geophysical interpretation

— synthetic seismograms

— VSP

— AVO

• Formation Shear Anisotropy

Applications

DSI 67

Summary

• The DSST tool has advantages over the previous sonic tools like SDT

• It utilises both Dipole and Monopole transducers

• The dipole capability allows the tool to measure the shear slowness in

typical slow or unconsolidated formations to overcome the limitation of

monopole

Remember:– Always use DSI job planner and understand why the job is being run– Always run the tool well-centred and under the recommended logging speed limit– Never run the tool without SSIJ– Run DSST with GPIT for BCR mode

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