Cement Bond Logging - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 210

Quick-Look for Conventional CBL Interpretation

Table 12.3 provides a quick-look summary of the diagnostics for the commonly
found cement bond cases.

Table 12.3 Quick-look CBL diagnostics

Cement Bonding

Log

None Some Complete 100%

Track Parameter Free Pipe Partial Bond Bond Fm. & Csng

1 Δt (μsec/ft) Steady Steady Ratty

2 Amplitude (mV) High Intermediate Low

Strong Csng Strong Fm.

3 VDL Signature Intermediate

Weak Fm. Weak Csng.

Microannulus/Channeling

It is not uncommon to fi nd cases where it is diffi cult to assess whether partial bond-
ing is due to channels or due to what is known as microannulus. Both cases may
exhibit:

￿ Medium amplitude

￿ Moderate casing arrivals

￿ Moderate formation arrivals

One way to determine the cause of partial bonding is to rerun the log with pres-
sure on the casing. The microannulus is a microscopic gap between the cement and
the casing that forms very poor acoustic coupling. Thus, although cement is present,
the log suggests otherwise. The root cause of the microannulus may be due to one
or the other of the following:

(a) Pressure was held on the casing while the cement cured. Subsequent release of
the casing pressure allowed the microannulus to form.
(b) The casing expanded due to the heat generated by the curing process and sub-
sequently cooled and shrank back to normal size.
(c) Cold completion fl uid was circulated just prior to running the CBL, causing
thermal contraction of the casing.
(d) A completion fl uid of lesser density was used to circulate out a heavy mud used
to displace cement. The resulting reduction in pressure in the casing allowed the
microannulus to form.