Lecture 4: Sonic Logs

Instructor: Frzan Ali1

Well Logging I

Tishk International UniversityEngineering FacultyPetroleum and Mining Department

Well Logging I

3rd Grade - Fall Semester 2021-2022

1FRZAN ALI WELL LOGGING I 1

Content

• Reservoir petrophysics

• Porosity

• Porosity Logs

• Sonic tool (BHC)

• Common Industry Sonic Tools

• Wave Types

• Sonic velocity and internal transit time

• Depth of Investigation and Vertical and Bed Resolution

• Log presentation

• The main uses of sonic log

• Deriving the porosity from sonic log measurement

• Correction from shale effects

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Reservoir geology

Petroleum Reservoir is a body of one or more subsurface rock units entraps liquid and/or gases hydrocarbons, commonly sedimentary origin.

Reservoir rock must be :

1-Porous.

2-Permeable.

3-volumeable.

4-Sealed from top by impermeable (seal) rock.

5-Associated with hydrocarbon entrapment.

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Reservoir petrophysics

It is computer assessed detail study of reservoir rocks using suits of well logging to enhanced the essential judgment of the reservoir potentiality.

This approach including the following:

1. log analysis of all porosity data, and data should be corrected for the effect of compaction and hydrocarbon content.

2. Analysis of shaleness and calibrated its influence, shale within reservoir rock occur either as laminar parting or matrix dispersed in the pore spaces , it is responding differently to the compaction influence which would return high water content.

3. Evaluation formation factor and water saturation (Sw).

4. Cross-plotting of pairs of porosity logs (example: N-D logs), to evaluate lithologic characters from both combination.

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Porosity

Porosity is the measurement of the void space within a rock, expressed as a fraction (percentage) of a bulk volume of that rock.

• (0.05) or 5 % > Φ, poor reservoir

• (0.05 –0.10) or 5 –10 % Φ, fair reservoir

• (0.10 –0.15) or 10 –15 % Φ, good reservoir

• ( 0.15 -0.20) or 15 –20 % Φ, very good reservoir

• 0.25or 25 % and more, excellent reservoir.

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Porosity

These class of well log referred as porosity logs, although each produces a Φ value from basic measurements, none measure Φ directly.

The Density and Neutron are nuclear measurement, the Sonic uses acoustic measurements, and the 4th(NMR) senses the magnetic resonance of formation nuclei.

When used individually, each of them has a response to lithology which must be accounted for. But when used in concert, two or three at a time, lithology can be estimated and more accurate Φ derived.

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Sonic tool (BHC)

The sonic tool measure the interval transit time, Δt, or the time in microseconds for an acoustic wave to travel through one foot or meter of formation, along a path parallel to the borehole (reciprocal of velocity). μs/ft or μs/m

The sonic log consists of one or more ultrasonic transmitter ‘T’ (emitter) and two or more receivers ‘R’.

Modern sonic log are borehole compensated (BHC) tools to reduce the effects of borehole size variations and error due to tilt of the tool with respect to the hole axis.

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Common Industry Sonic Tools

Table :The names and mnemonics of common industry sonic tools.

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Wave Types

1. Compressional or longitudinal or pressure wave (P-wave); faster one.

2. Transverse or shear wave (S wave).

3. Rayleigh waves

4. Stoneley waves

5. Mud waves

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The geophysical wave train received by a sonic log.

The Wave Types Used in Sonic Tool

• For the simple sonic log that we are interested in, only the P-waves of interest. When the first P-wave arrival appears, the threshold is exceeded and the timer stops, other waves are masked out of the data

• There are complex tools that make use of both P-waves and S-waves, and some that record the full wave train (full waveform logs).

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Sonic velocity and

internal transit time

Waves

Depth of Investigation

• This is complex and will not be covered in great detail here. In theory, the refracted wave travels along the borehole wall, and hence the depth of penetration is small (2.5 to 25 cm). It is independent of Tx-Rx spacing, but depends upon the wavelength of the elastic wave, with larger wavelengths giving

Vertical and Bed Resolution

• The vertical resolution is equal to the Rx-Tx spacing, and hence is 2 ft. Beds less than this thickness can be observed but will not have the signal fully developed. There are now some special tools which have an even better resolutions (e.g., ACL and DAC).

Logging Speed

• The typical logging speed for the tool is 5000 ft/hr (1500 m/hr), although it is occasionally run at lower speeds to increase the vertical resolution.

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Log tool with Two Receivers (R1, R2) and One Transmitter (T)

• The simplest sonic tool has only one transmitter and two receivers.

• This kind of tool can record the sonic wave that refracts at a critical angle to borehole wall and travels along the side wall .

• The first arrival compressional waves are recorded by two receivers .

• It’s interval transit time (Δt) can be used to estimate formation porosity

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Sonic Tools

Borehole Compensated Device (BHC) with 2 Transmitters (Top; Tx1 & Bottom; Tx2) and 4 Receivers (Rx1 to Rx4), correct for hole size changes and tool tilt.

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Sonic Log Presentation

• The dimension with 40 to 140 μs/ft of Δt

• or μs/m with greater rang of Δt

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75μs/ft

Left Figure represents log tracks, track-1 includes bit size, caliper and GR log, while the track-2 represent the sonic log.

You should have good knowledge of borehole size, hole condition, and shaliness zone from bit size, caliper and GR respectively.

The main uses of sonic log

1. Provision of a record of “seismic” velocity and travel time throughout a borehole.

(calibrate a seismic data).

2. Determination of porosity (together with the FDC and CNL tools).The Sonic log come in

2nd order to calculate Φ after Neutron and Density logs.

3. The sonic used with density log to produce the acoustic impedance (vd).

4. Stratigraphic correlation.

5. Identification of lithologies.

6. Facies recognition.

7. Fracture identification.

8. Identification of compaction.

9. Identification of over-pressures.

10. Identification of source rocks

11. Cement Bond Log (CBL), to determine zones in cased well where the cement may be

imperfect (not adherence the cement to the casing).

12. Pulse-Echo Imaging

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Deriving the porosity from sonic log measurement

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Deriving the porosity from sonic log measurement

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Deriving the porosity from sonic log measurement

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Q/ if the sonic log measurement equal to 63 μs/ft, along the dolomite rock intervals, determine the porosity from chart and equation.

Ans./ by extrapolating a line from Δtaxis (horizontal axis at 63 μs/ft) to intersect dolomite line and then extending the line to intersect the porosity line (vertical axis) at the porosity value equal to 0.17.

Try to determine the porosity with equation and compare the two results.

Typical responses of the sonic log (courtesy of Rider).

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Correction from shale effects:

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