INTERPRETATION FLOW CHART

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6.1 QC Review

If the log is properly documented, it should contain most ofthe information needed for correct interpretation, The majority of the key parts are included in the log header (~3).Check the following four items. Anything missing or incorrectly recorded will at least cause time loss or an invalid log.

Item 1: Theplanning and execution of logging Clear objectives? Effective effort to prevent micro-annulus and eccentricity? Logging actively witnessed— witnessing engineer’s signatures and relevant reports?

Item 2: Thefive data sets are complete and correct (~3.1):

1) General information 2) Well geometry 3) Well fluids

4) Cementing operation

5) Well pressure and temperature

Item 3:The logging engineer’scomments Clear and relevant in addressing any problems. The answers to the questionnaire can also imply his or her competency.

Item 4: The log presentation Header-body-tail-questionnaire, signatures and date complete?

6.2 Quick check ofrangesof the various curves

77 Curves: The normal ranges of U curves depend mainly on the casing ID and mud type/

weight (affecting the sound velocity). Temperature and the type of tool (the size of the transducers) also have some effect. A rough guide for calculating this range is

TT(~is)—(casingIDeg. In mm)/(sound velocity in the mud eg. Inmn’z.~us§10.3)+170(Ms) The last item is the distance of T-R spacing (3ft) times the sound speed in steel (57~ts/ft).

CBL Curves: The normal ranges ofCBL for unbonded pipes depend mainly on the casing sizes (~1O.8)and mud type/weight (~10.7).The ranges for 100% bondedpipes are less reliable because of the difficulty in controlling the test conditions. For foamed cement there have not been sufficient field data or lab results for fully bounded pipes and the interpretation is therefore more difficult.

VDL/Signature log: Check whether the time scale/range are correct (in the above equation replade 170 by 285, which corresponds to Sft), the log has good contrast, and there are any fast formations (~9.4).

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6.3 ExamIne the fl Curves

The purpose of examining the U curves is to explain the curve variation, ifany, and investigate the log validity.

The IT curves variation.The U curves from a properly centralised tool runinuniformly cemented pipes should ideally be straight lines in the expected region (~6.2).Parameters affectingtheU curves include centralisation (~4.2,§9.3), casingID size/weight (ID)changes (e.g.

casing collars) (S4.8), casing fluids change over different depths (~4.6,§9.5), fast formations (~4.5,

§9.4) and temperature (j4.7, §9.6). The U curves will also vary to indicate stretching, cycle skipping (j2.9) and well bonded double-casing string (~4.11,§9.7).

A log validity criterion.A widely quoted log validity criterion in the literature and various manuals is that ifthe U curves vary for more than ±4iis,the log is invalid. Be careful. Variation of the U curves of a real log is rarely within this limit: investigate if the cause was eccentricity!

When the tool is not properly centralised, the U can be shorter than normal. Unfortunately the amplitude will also be reduced. The effects of most other factors mentioned above on the amplitude are likely to be relatively small. Traditionally the log is treated as invalid because of the unknownreduction in the amplitude. This nowhas been better understood (~4.2,§10.5) and the amplitude reduction due to eccentricity can be calculated. However, because the uncertainty of the effects of other factors still exists, it is recommended that similar criterion be used with the U reduction limit being 5.us (~10.5).

Note the transient temperature effect can make the log invalid (~4.7,§9.6) and so can fast formations and micro-annulus— the amplitudes in the interval of interest are not quantitatively reliable.

6.4 ExamIne the CBL Curves

CBL curves: Look for Top of Cement (TOC) if applicable (e.g. a non-liner job), where the CBL curves swing from the low end of the value range to the high (~6.2,§9.1). Is the TOC in the expected

region? Check with the annulus size and pumped _____

cement volume. A low TOC measured is often

associatedwith slurry loss anda high one incomplete mud removal. Ifthe TOC is not found the slurry has been contaminated at least in the top section (~8).

Look for good cement section where the curves are at the low end of the value range. Note ifa lead and tail slurry system was used, a difference should be

seen in the good cement sections found, where the Leadcement tail cement should give a lower reading. These will

be useful in qualitatively evaluating the cement job.

If no good cement section is found, the problem Tail cement could be heavy contamination but refer to the

Special Investigation Chart on page 27.

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Look for “Free Pipe” (uncemented pipe) section if applicable. Do not be misled by high readings of the CBL curves: check for the micro-annulus (~4.1)!The CBL values for free pipe offer references in cement quality evaluation (~6.6).

6.5 ExamIne the VDliSignature log

The VDL log: must have good black and white contrast. It contains much information but now only a small portion is extracted, and this is done by visual examination. The main uses of the

VDL log are: --___________

1) to detect micro-annulus (~9.2) 2) to detect fast formation (~9.4) 3) to confirm free pipe (~9.1)

4) to confirm good bond in double casing strings (~9.7) 5) to confirm good bond to the casing but bad bond to

the formation (where the casing arrivalsare extremely low with little or no formation arrivals and CBL

amplitude indicates a good bond).

The signature lo& whichis often superimposed on the VDL log, produces the wave amplitude information which is not available on the VDL. This information can be useful in confirming changes in bond quality. How-ever, the signature log is not as easy to use as the VDL in detecting, for example, the formation arrivals and that is why it is often combined with the VDL log.

The indications of the U, the amplitude and the VDL logs must be in agreement and their examinations should be in parallel. Ifthey do not agree, there could be a tool problem resulting an invalid log.

L

The rest of the annulus (100%-BPI) is not well bonded, which may be contaminated cement, localised small gaps between the cement and casing, or a channel.

Angle of bonded cement Elf- Elm

BPI (%) - - x 100%

The whole arinulus Elf- Elc

Where the El’s are from the same receiver (e.g., the 3’ receiver), and the subscript m represents the value measured in the zone of interest,

f

cemented pipe.

The BPI applies to any type ofcement system (neat, foam, etc.). Note when a lead and tail slurry system is used, El c should be selected separately for the two slurries. When Elfand El c are available, the corresponding BPI at a given Elm can be found from the above equation, or as shown in Fig.6.l below.

FIg.6.1 Howto findthe BPIgraphically

On the Elm axis, mark El~and Elfvalues. Mark the BPI (%) axis by equal intervals from 0 to 100. Draw a straight line from (Elc,lOO) to (EljçO). Given a Elm value, the corresponding BPI can be found as shown.

To provide a sealwith high confidence, BPI need to be around 95% orhigher for certain lengths (~l0.6).For gas wells this rule should be applied more stringently.

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