Well Integrity Technical Section

Dear Community, Greetings. Need your professional opinion. 

In my company Drilling Team spread the rumor that acoustic cement evaluation logs can't provide reliable data across double casing area, for example:

• for 9-5/8" casing interval located inside 13-3/8" casing or
• for 7" casing located in 9-5/8" casing.

Drilling team claims that service providers (SLB, Hall, BH, WFD, etc..) are concerned parties interested in more job volume, therefore their opinion shouldn't be considered.

Therefore I am looking for the support from operating companies in these regards. 

Kindly could you please share you opinion for this subject? (I know that conventional acoustic tools might be affected which shouldn't be the case for ultrasonic tools measuring the impedance, but I might be wrong). 

If you can share contacts of cased hole Petrophysicists from major operating companies - that would also be appreciated. 

thanks

rgds

Hi Andrey,

This is an old canard, probably born with the Jutten & Corrigall (1989) SPE paper. There are actually two separate problems in narrow double string annuli with good, fast cement:

• The CBL peak may be polluted by arrivals from the outer casing, or even from the formation. Fast formation signature helps identify this scenario: erratic transit time correlated with noisy amplitude, and the wavy interfering arrival threading in and out of the casing arrival on the VDL. Is the "fast formation" hiding an intermittent mud pocket? The CBL may not be able to rule it out completely (after all, interference is strongest across narrow annuli - where gelled mud lurks), but if you line up all evidence you may conclude that cement is good enough.
• Pulse-echo ultrasonic tools (SLB USIT, Halliburton CAST...) are affected by galaxy patterns. These resemble target signs, or woodwork patterns: concentric, elongated ellipses with alternating high and low impedance - very often as low as gas. The question again is: are these noisy features hiding a mud channel? Here you have a qualitative and a quantitative method to rule this out: qualitatively, mud pockets will appear as stains that interrupt the alternating patterns. Very visible. Quantitatively, if you take the average of acoustic impedance over a peak-to-peak cycle (either along the length of the casing or around it), you will find the annular material impedance: if it is around 2 MRayl it's mud, if around 6 MRayl it's tail cement.

Mind that the usual annuli (7"-9⅝" and 9⅝"-13⅜") are mostly immune from this noise source. Keep running logs and, if you can, always pair sonic and ultrasonic together.

Best regards,

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Matteo Loizzo
Well integrity consultant
matteo.loizzo@mac.com
Berlin, Germany
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