Model 520 HT UCA
D01157216
Revision A, October 2015
31
8
Interpreting Test Results
An ultrasonic measurement is a nondestructive test method. It predicts relative
compressive strength development by comparing transit times of known
mechanical compressive strength data to transit times observed in the UCA. Actual
data will have points above and below the best fit curve used to develop the
algorithm. The closer the slurry being tested is to the actual slurry used to develop
the algorithm the closer the data will match.
Algorithm A
= Lightweight slurry (<14 lb/gal)
Algorithm B
= NORMAL SLURRY (standard, used on almost all applications and
should be the default)
Algorithm C
= High density slurry (>17.5 lb/gal)
Algorithm A was developed in the 1970’s for use with lightweight slurries.
However, some modern lightweight slurries may work better on algorithm B. Most
of the modern lightweight slurries react like a standard or normal slurry so
algorithm B is typically a better choice. Algorithm A is intended for slurries where
fly-ash is the major component. The use of algorithm A is recommended in reduced
water slurries. Use algorithm C when there is a weighting agent present to get the
density above 17.5 lb/gal.
We recommend performing a mechanical compressive strength test to see the
actual compressive strength. If the results are within 200 psi then it is considered
the correct algorithm. If not, try a different algorithm. You can easily change
algorithms using the Test Viewer to see which algorithm best fits your data.
The algorithms developed by Halliburton in the 1970's used data from cement
cured at elevated temperatures rather than below ambient temperatures. Algorithms
are being extrapolated to a certain extent at lower temperatures. Modern slurry of a
certain density may be better represented with a different algorithm than a slurry
from 40+ years ago. Having strength values compute a bit low just makes for
conservative results. Fann recommends customers use algorithm B unless they have
reason to think another algorithm might work better. If the crush data seems to
indicate that algorithm A fits a slurry better than algorithm B, then there is no
reason not to use algorithm A.
The strength values obtained using either the API/ISO crush test or the UCA are
indicative of the integrity of the cement under uniaxial loading. In the wellbore, the
cement is subject to complex triaxial loading, and the failure stresses may be
substantially different from those observed in the standard compressive strength
test.
