Page 231 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

200 Reservoir Engineering

TOTAL TRAVEL TIYE
Wh

A

Figure 5-123. CBL-VDL log run in casing with a good cement-casing bond
and a poor cement-formation bond [220].

Microannulus or Channeling in Cement. Microannulus occurs when the cement
is emplaced and the casing is pressurized. When pressure is released after the
cement has set, the casing “pops” away from the cement sheath. This generates
a gap or microannulus between the casing and the cement. This can also occur
if excessive pipe dope or varnish is present on the pipe. Microannulus due to
pressurization primarily occurs opposite washed out portions of the borehole.
Channeling occurs when cement is in the annulus but does not completely
surround, or is not bonded to, the pipe. This condition will not have proper
fluid seal which allows oil, gas or water to pass up the hole outside the casing.
Microannulus, on the other hand, may have a proper seal even though a small
gap exists. It is very important to be able to distinguish between microannulus
and channeling; squeezing may eliminate the channel altogether.
Figure 5-124 shows a case of microannulus. Figure 5-124a shows strong to weak
casing-bond on the CBL amplitude and weak formation-arrivals on the VDL.
This indicates poor acoustic coupling between casing, cement, and formation.
The weaker the formation signal, the more pronounced the microannulus.
Microannulus can be easily differentiated from channeling by:

1. Pressurizing the pipe and rerunning the CBI-VDL. Microannulus conditions
are confirmed by strengthened formation signals on the VDL and decreased
CBL amplitude indicating better casingcement acoustic coupling. Typically,
channeling will produce little or no improvement in the signal when the
casing is pressurized. Figure 5- 124b shows a case of stronger formation
arrivals indicating the presence of microannulus rather than channeling.