Page 232 - Drilling Technology in Nontechnical Language

Chapter 9 – CASING AND CEMENTING 223

If cement was pumped down the casing without a stinger, at the point
that cement starts coming out of the annulus there would still be a huge
volume of cement still inside the casing. This cement has to be removed
from the inside of the casing by pumping down mud behind it. As a result,
a lot of cement is wasted (with surface disposal problems for the large
volume of cement). Furthermore, if there is a problem with the cement, it
might set up inside the casing and have to be drilled out. This would take
a long time and would also be very embarrassing!
Once the cement is displaced into the annulus and the drillpipe stinger
is full of mud, the drillpipe can be pulled out of the hole.

Cement Evaluation behind Casing

It is possible to run wireline logs to evaluate the condition of the
cement behind the casing. This is done by sending out sound waves from
a wireline tool and listening for the returning sound wave a short distance
away. As the sound travels up the cement, the frequency and amplitude are
affected by the quality of the cement and of the cement casing and cement
formation bonds.
A basic cement evaluation log will give an indication of the bonds
but not much else. More sophisticated tools can detect the presence of a
microannulus, channels in the cement, gas bubbles in the cement, and
much more. But of course these tools cost a lot more to use.
Most cement jobs are logged, even if only with the basic tools. In critical
cases, especially where problems are suspected, a better analysis of the
actual problem is required before any remedial work can be considered.

Other Cement Jobs

Secondary cementing

Cement is pretty versatile stuff, thanks to additives, and this gives
many possibilities for using cement to solve a variety of problems. If
in a critical cement job a channel is detected in the cement behind the
casing, it may be possible to perforate the casing and force cement into
the channel under pressure. Repairing a bad primary cement job (primary

_Devereux_Book.indb 223 1/16/12 2:12 PM