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Sonic Measurement

Sonic Measurement

© Schlumberger 1999

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Sonic Measurement

Sonic ToolThe sonic tools create an acoustic signal and measure how long it takes to pass through a rock.

By simply measuring this time we get an indication of the formation properties.

The amplitude of the signal will also give information about the formation.

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Sonic Measurement

sonic borehole waves

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Sonic Measurement

waves 2

In a fast formation both compressional and shear waves are created.

The head waves in the borehole are the signals seen by the receivers.

The array of receivers see the signal at different times as they are at different distances from the transmitter.

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Sonic Measurement

Sonic -BHC

A simple tool that uses a pair of transmitters and four receivers to compensate for caves and sonde tilt.

The normal spacing between the transmitters and receivers is 3' - 5'.

It produces a compressional slowness by measuring the first arrival transit times.

Used for:Correlation.Porosity.Lithology.Seismic tie in /time-to-depth conversion.

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Sonic Measurement

Long Spacing Sonic

The BHC tool is affected by near borehole altered zones hence a longer spacing is needed with a larger depth of investigation.

The tool spacings are 8' - 10', 10' - 12'.

The tool cannot be built with transmitters at each end like a BHC sonde, hence there are two transmitters at the bottom.

A system called DDBHC - depth derived borehole compensation, is used to compute the transmit time.

The uses of this tool are the same as the BHC tool.

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Sonic Measurement

Array SonicMulti-spacing digital tool.

First to use STC processing.

Able to measure shear waves and Stoneley waves in hard formations.

Used for:Porosity.Lithology.Seismic tie in / time-to-depth conversion.Mechanical properties (from shear and compressional).Fracture identification (from shear and Stoneley).Permeability (from Stoneley).

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Sonic Measurement

DSI General

In a slow formation the shear wave from a monopole source never creates a head wave. The fluid wave is the first arrival after the compressional.

A dipole source is directional.

It creates a flexural wave on the borehole wall and shear and compressional in the formation.

The shear wave is recorded whether the formation is soft or hard.

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Sonic Measurement

DSI tool

Generates both monopole and dipole signals.Generates different frequencies for measuring a range of waves.

Measures:Compressional and shear Two orthogonal shear - dipole signalsStoneley

Application:Seismic.Mechanical properties (from shear and

compressional).Fracture identification (shear and

Stoneley).Permeability computation (Stoneley).Porosity / Lithology.Gas shows.

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Sonic Measurement

STC Processing

This type of processing is necessary to extract the shear and Stoneley information from the waveform.The processing applies a "semblance algorithm" to the recorded set of traces.

This means looking for the same part of the wave (e.g. shear) on each wavetrain.

Once this has been done the transit time can be computed.

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Sonic Measurement

STC Map

At a given depth, the slowness can be plotted against time.Regions of large coherence appear as contours.

These correspond to the compressional (fastest), shear (close to the compressional) and Stoneley (furthest away).

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Sonic Measurement

STC Output

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Sonic Measurement

Tools Summary

Measurements: BHC LSS Array DipoleSonic Sonic

Compressional x x x x

Shear/Stoneley:Hard rock - - x xSoft rock - - - x

Computations:Porosity x x x xLithology x x x xSeismic tie in x x x x

Mechanical properties:Hard rock - - x xSoft rock - - - x

Fracture detection- - x xPermeability - - - x

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Sonic Measurement

Borehole Effects

As the sonic tool is measuring the time for the signal to go from the transmitter to the receiver there are two types of erroneous responses.Cycle skipping

Road noise

This is noise at the receivers that is due to the borehole environment and has nothing to do with the signal being measured.

If the signal strength is too low the detection goes to the

next peak.This means that the final transmit time will be wrong.

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Sonic Measurement

Borehole Effects 2

There are a number of borehole phenomena which cause these effects:

Borehole rugosity - causes the tool motion to be erratic, the signal may be distorted and give road noise or cycle skipping.

Large holes - if the borehole diameter is very large the mud signal may arrive at a receiver before the formation signal. The proper tool set-up for each condition has to be picked before the job.

This means choosing whether to centralise or excentralise the tool and the equipment to

be used.

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Sonic Measurement

Borehole Effects 3

Gas in the wellThe acoustic impedance of gas is very low, hence the signal will be strongly attenuated. There may be skipping.

Altered zoneThis is largely overcome by using a long spacing tool to read deeper into the formation.

CavesCan create problems in spite of compensation as they will also reduce signal amplitude.

FracturesReduce the signal amplitude especially the shear and Stoneley waves.

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Sonic Measurement

Porosity - 1

The porosity from the sonic slowness is different than that from the density or neutron tools.

It reacts to primary porosity only, i.e. it does not "see" the fractures or vugs.

The basic equation for sonic porosity is the Wyllie Time Average:

( ) maf ttt ∆−+∆=∆ φφ 1log

maf

ma

tttt

∆−∆∆−∆

= logφ

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Sonic Measurement

porosity 2

There is another possibility for transforming slowness to porosity, called Raymer Gardner Hunt.This formula tries to take into account some

irregularities seen in the field.

The basic equation is:

A simplified version used on the Maxis is:

C is a constant, usually taken as 0.67.

( )fmac ttt ∆

+∆−

=∆

φφ 211

log

log

ttt

C ma

∆∆−∆

=φ

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Sonic Measurement

Porosity 3

This chart shows the relationship between the sonic compressional slowness and the porosity. Both the lithology and the equation must be known prior to using this chart.

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Sonic Measurement

Crossplots

The sonic measurements can be cross-plotted with the density or the neutron readings to give porosity and lithology information as with the density-neutron crossplot, however:

The neutron - sonic (TNPH-Dt) Has problems because there are two possible equations.

The density - sonic (Dt-RHOB) Has problems with the transforms as there is no separation between the lithology lines.

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Sonic Measurement

mechanical properties

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Sonic Measurement

Mechanical Properties and Sonics

A combination of compressional, shear and density measurements gives the rocks' dynamic elastic moduli.

These are used to obtain the formation's mechanical properties.

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Mechanical Properties uses 1

Main uses of mechanical properties in soft formations are:

Sand stability evaluation, i.e.The prediction of the formation collapse under

producing conditions.

Using theoretical failure criteria it is possible to predict if the perforation will produce sand.

Well bore stability, i.e.The prediction of formation failure / collapse while drilling.This is especially relevant in deviated wells when drilling at high angles through soft rock can be problematic.The physical mechanism is similar to that of sand stability evaluation.

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Sonic Measurement

Mechanical Properties uses 2

The major use of mechanical properties in hard rocks is to predict how they will behave under "excess" pressure:Drilling:Will the formation fracture and the drilling mud

disappear?

Hydraulic fracturing: How much pressure will fracture the formation and how far will the fracture extend?

Experimental models are used to compute parameters such as tensile strength.

Simulations are used to predict the pressures that will "crack" the rock and lengths of fractures.

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Sonic Measurement

Sonic Parameters

Vertical resolution:

Standard (BHC, LSS, MSTC) 24"STC 36"6"DT 6"

Depth of investigation:

BHC 5"LSS-SDT 12" (12 ft spacing)

Readings in(ms/ft)

Limestone (0pu) 47.5Sandstone (0pu) 51-55Dolomite (0pu) 43.5Anhydrite 50Salt 67Shale >90Coal >120Steel (casing) 57