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USIT 1

Basic USIT Interpretation

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How good is the USIT?

Client: “Do I a good cement job”

Engineer:???? Cement placement - zone isolation Cement coverage - channels Cement compressive strength Cement to casing bond Cement to formation bond Gas filled micro-annulus Liquid filled micro-annulus Top of cement (TOC) Cement composition (gaseous, contaminated)

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USIT 3

How good is the USIT?

Engineer: Cement placement - zone isolation Cement coverage - channels Cement compressive strength Cement to casing bond Cement to formation bond Gas filled micro-annulus Liquid filled micro-annulus Top of cement (TOC) Cement composition (gaseous, contaminated)

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USIT-CBL-VDL

Bond Index Interpretation

CBL & USIT agree

Very low

Very low

Medium

Free gas

Free pipe

Channels in good cement.

USITImpedanceImage

Gas

Liquid

Channels

CBL Bond Index

High

Low / medium

Cement

Cement

Good cement

Liquid microannulus.

Fast formations.

Formation / 2nd casing reflection.

Medium

Medium

High / medium

High / medium / low

Z slightly above ZTCM*

Gas microannulus.

USIT shows more cement

CBL shows more cement

Gas

Z about same as ZTCM*

Gaseous cement.

Light cementPatches or streaks

* ZTCM = USIT Cement Impedance Threshold

Mixed light / heavy cement.

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USIT 5

How good is the USI-CBL/VDL?

Engineer: Cement placement - zone isolation Cement coverage - channels Cement compressive strength Cement to casing bond Cement to formation bond Gas filled micro-annulus Liquid filled micro-annulus Cement thickness Top of cement (TOC) Cement composition (gaseous, contaminated)

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USI Interpretation Examples

Formation reflections Secondary signals

Special casings

Casing rugosity

Micro-annulus

Casing shape

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USIT Job

Formation reflections:

The T3 processing is deliberately short to minimize contamination by reflections. Formation reflections are to be significant when:

1. The 3rd interface is smooth and provides a large

impedance contrast with the cement (i.e., double string or

hard formation).

2. The annulus is thin or the process window is long.

3. The cement is of high density and well set. It will have

high acoustic velocity and low attenuation.

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USIT Job, cont.

Galaxy Patterns:

They are formation/casing reflections that have characteristic pattern of interference fringes on the cement map.

Due to the constructive or destructive signal interference

the apparent impedance is respectively reduced or

increased resulting in fringes oriented parallel to the

casing and centered on the narrowest part of the cement

sheath.

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Galaxy patterns

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USIT Job, cont.

Alloyed steel casing:

In stainless and high alloy the acoustic velocity can vary up to +/- 5% causing

1. A small effect on measured cement impedance (0 - 0.5

Mrayl).

Special Casing Types

2. An error in casing thickness of up to +/- 5%.

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USIT Job, cont.

Fiberglass casing:

This casing cannot be evaluated by the USIT at this time. Why not?

Fiberglass has a much lower acoustic velocity and density than steel, so its acoustic impedance is much lower than steel and similar to neat cement.

The casing cannot resonate and the T3 processing is not suitable for evaluating the thickness or cementation.

Special Casing Types, cont.

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USIT Job, cont.

Protective coatings:

They change the effective impedance in contact with the casing, preventing a cement impedance measurement, but a thickness measurement should be possible.

Special Casing Types, cont.

Special equipment:

Scratchers, centralizers, external casing packers, dump valves and collars affect the measurement.

The change in effective thickness means T3 parameters are out of range.

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USIT Job, cont.

Rugosity:

Internal rugosity reduces the initial reflection, reducing the measured impedance causing gas effect.

External rugosity damps the resonance and increases the measured impedance.

Casing shape, rugosity … etc., cont.

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Highly corroded casing

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USIT Job, cont.

Cleanliness:

Deposits on the inner diameter will change the measured impedance.

Casing shape, rugosity … etc., cont.

Micro-annulus:

A small fluid-filled gap between casing and cement.

This causes a reduction in measured impedance.

Gas micro annulus is seen as Gas...

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Cement inside casing

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USIT Job, cont.

Shape:

Welded casing may cause the appearance of a vertical line on the amplitude and impedance maps, and, in extreme cases, on the radius and thickness maps.

Casing shape, rugosity … etc.

Seamless:

Generally have thickness variations of around 10%, the inner diameter causing the variation in profile.

Typical problems are eccentered bore and bore ovality causing a reduced measured impedance.

Non parallel surface (inner and outer diameter) causes a reduction in resonant energy, and hence, an increased measured impedance.

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Casing shape effects

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Track 5 Internal Radius Image and Track 7 Thickness Image show one joint of casing at 2600 feet in a newly completed well where the image patterns differ indicating that this joint was manufactured by a separate milling process.