BPICalculationExample
Below is a section around the shoe ofa 7’, 29 lb/ft linerjob run atthe Gulf ofMexico. Assuming that the log has passed the QC and quick examinations, let us see how the BPI is calculated.
Suppose the cementing operation and other well conditions allow us to believe that at least a section of
perfect cement job can be achieved, we can then select the lowest reading, 5mV, as 100% bonded value Elc. The “free pipe” value Elf is nonexistent(hopefully!) in a linerjob, we therefore look itup from §10.8 anduse 62mV (or we can use an available value from a log with close conditions as the job in hand).
Between points Aand B the average reading of7mV gives the BPI-(62-7)/(62-5)-96.5%; BetweenB and C the average reading is about9mV. The effect of the slight U reduction (3—4~.ts)caused by eccentricity can be compensated for (~10.5),in this casebyincreasethe amplitude by about 10% to lOmV. The
corresponding BPI here is therefore (62-10)/(62-5)91%.
If El~cannot be clearly def’med on the log, we can also use a reasonable value elsewhere, e.g. 2.4mV from §10.8. The BPI for the two intervals will be (62-7)/(62-2.4)92% and (62-l0)/(62-2.4)—87%
respectively. As there exist about90% bonded intervals for30ft, the probability of zonal isolation is high.
Notemore than 50%reduction in Elc(from5mVto 2.4mV) haschangedthe BPI valuefor <5%. Variations in Elf does not affect the BPI much either. This means that for conventional jobs the selection of the reference valuesis important but not critical for a reliable interpretation. Preferablyboth 0and Elf are decided from the log whenever possible. This method applies to all types of slurry design.
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* Either go to the next action or resume the main interpretation flow chart on previous page
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6.7 Finding Information
If information in the log is incomplete, or other information is required such as open hole logs or some detail of cementing operation, contact the right person to obtain it. Keep an updated list of contacts for necessary help: their name, specialities, company, base, telephone and fax numbers etc..
6.8 AnalysIng abnormal log behaviour
1) Review all factors which are likely to affect the log (~4).
2) Eliminate one by one around the key problem in hand until only such factors whose effects may be significant.
3) Judge whether these factors have invalidated the log— a good understanding of all the factors (study §4) may be very useful.
6.9 CalculatIng the probability ofzonal Isolation
Cementing operation can offer valuable common sense evaluation which should not be discounted (~l.4).If any steps in the operation were altered from planned, itis suggested that the post-job CPS be run (~7).A starting point is to judge the likelihood of at least some sections which are channel free. Ifno such sections can be assured and the log shows a very bad cement job, a remedy job may have to be considered (~8).
For the BPI calculation, the El value for 100% bond (Sl0.8) may have to be found elsewhere if that value for the given conditions is not available in the literature. The log of a well of close location with similar conditions can prove useful to provide this reference. The value can be chosen by finding the lowest reading at intervals where isolation was required and achieved.
Make sure there were no fast formations or micro-annulus (~4.5,~4.1).
6.10 Keep a record of Interpretation
It is recommended that the interpretation of a log is recorded, eg, on the back of a log, for ease of later reference. The rubber stamp issued together with this manual is for this purpose. The table from the stamp is designed to summarise the log with minimal amount of information.
Stamp the back of the log and fill in the form as you complete the interpretation.
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~7. Cementing Operation
1. What to check~
Do not come to conclusions on the quality of the cement job without reviewing the cementing operation. Every step of the operation is carefully designedto ensure a good job. Problems may occur if any recommended step was not followed correctly. Common sense can often tell you the nature of the problem.
Cement placement itself is a large and complex subject. But simplistically speaking, there are somerules ofthumb. For example, the more centralized the casing string, the better the cement job; Equally important are the displacement rates of mud, spacer and cement slurry. Usually the higher the flow rates, the better the displacement. The flow rates are limited by the formation strength and the displacement facilities. The rheology and density of the fluids are similarly important. The idea is to remove the viscous mud by spacer as completely as possible, then to displace the spacer by the cement slurry. The spacer here is designedto make easy the removal of mud and the placement of cement. Usually a heavier fluid following a lighter one can help the displacement. Remember, non-vertical wells are not only difficult to log, but also difficult to cement because ofthe casing centralisation problem and the complicated flow regimes in an inclined annulus.
Without being deeply involved in cementing technology, you can always get some idea about the quality of the cement job by checking the following:
- Was there any lost circulation?
- Was the casing string centralised?
- Were the pumping rates and displacement timing as planned?
- Were the well-site-measuredfluids properties (density, viscosity, additives quantities etc.) as designed?
Answer “No” to any of these questions could mean a less than perfect cement job. Ifthere were losses to the formation or other operational problem encountered during cement placement, it could be a bad job and log interpretation must take this into account.
2. The Cement Placement Simulator (CPS)
This simulator, developed by the Fluid Mechanics Team at RCS, can tell you how to carry out the cement job and what the cement job should look like. The CPS is located at Dyce, Houston and Sunbury and wifi also be incorporated in DEAP. If a bad cement job is suspected it is suggested that the CPS be run. The result is another reference for interpreting the log.
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