Concrete transparency: imaging the outer casing

In a post 10 days ago I announced a new algorithm that could digest any ultrasonic pulse-echo log - acquired by those wireline tools with a rotating head - to measure bonding between cement and formation. There was also a somewhat vague wish that "NAMA can deliver information about [outer casing] cement conditions". But could it possibly?

How can one sleep without trying to answer this tantalizing question? So I plugged the necessary machinery into NAMA and the results are indeed promising. First the full log, then we will zoom into the upper 67 ft, the double string section.

There is no log of the previous casing that we can compare to this image, but we can check a few details to verify if the results are plausible.

First, there are a couple of common patterns between the first and second cement sheaths' impedance maps, but overall they look reasonably different. This suggests we're actually picking up characteristics of the second annulus, and not merely noise from the first one.

Second, the distribution of acoustic impedances obtained from 2 measures (shown below) is realistic: there is a broad peak at 5.9 MRayl, with a standard deviation of 3.5 MRayl. This is consistent with a 15.8 class G cement, a likely choice for the shoe, which has an expected impedance of 6.05 MRayl. What is even more interesting, is that the shape of the distribution doesn't change if we pick another initial guess, unlike the one-measure curve - but then you would expect that to obtain two quantities you need two measures.

The distribution is also broader than the measurement accuracy, for which the calculated standard deviation is 1 MRayl; this implies that the cement properties are not constant. NAMA's performance compares favorably to the existing algorithms: Schlumberger USIT and Halliburton CAST have a posted accuracy of 0.5 MRayl (for the CAST this increases to 1 MRayl when the cement impedance is >6 MRayl).

The same method that delivers the properties of the previous casing can also be used to size the cement-formation microannulus. The graph below shows that the distribution is substantially the same whether we use one or two measures (after all we just want to know one quantity), with a peak around 290-470 μm. This is about double what we measured elsewhere at the casing-cement interface, but it is not surprising: at the inner interface it is very hard to push the steel casing inwards, whereas the rock (and the cement) is much more pliable.

The NAMA algorithm seems to afford a 20/20 vision of what hides behind the first cement sheath, and to do so in a controlled and transparent (pitiful pun intended) way. We were able to generate what are likely to be the first maps ever of microannulus thickness and outer cement impedance.

And it's not over yet: NAMA continues to grow, and it is not hard to incorporate the additional information on the outer cement interface that could be extracted from Schlumberger Isolation Scanner, using an approach already perfected a couple of years back.

Are these maps correct? For this we need to compare the algorithm results to existing logs of the previous casing. Anybody's got good material catching dust in a drawer?

Also, any suggestion on the log presentation are more than welcome: what curves and maps would you like to see?