Page 88 - Hydrocarbon Exploration and Production Second Edition

Drilling Engineering 75

contamination of the cement with drilling fluid which would cause a bad cement
bond between borehole wall and casing. Once the bottom plug bumps into the float
collar, it ruptures and the cement slurry is pushed down through the guide shoe and
upwards outside the casing. Thus, the annulus between casing and borehole wall is
filled with cement.
The success of a cement job depends partly on the velocities of the cement
slurry in the annulus. A high pump rate will result in turbulent flow which results in
a better bond than the slower, laminar flow. The cement has to be placed evenly
around each casing joint. This becomes more difficult with increasing deviation
angle since the casing joints will tend to lie on the lower side of the borehole
preventing cement slurry entering between casing and borehole wall. To avoid this
happening, steel springs or centralisers are placed at intervals outside the string to
centralise the casing in the borehole.
Once the cementation has been completed, the rig will ‘wait on cement’
(WOC), that is wait until the cement hardens prior to running in with a new
assembly to drill out the plugs, float collar and shoe, all of which are made of easily
drillable materials.
The process described so far is called primary cementation, the main purpose of
which is to

bond the casing to the formation and thereby support the borehole wall
prevent the casing from buckling in critical sections
separate the different zones behind the casing and thereby prevent fluid
movement between permeable formations
seal off troublesome horizons such as lost circulation zones.

Sometimes primary cementations are not successful, for instance if the cement
volume has been wrongly calculated, if cement is lost into the formation or if the
cement has been contaminated with drilling fluids. In this case, a remedial or
secondary cementation is required. This may necessitate perforating the casing at a
given depth and then pumping cement through the perforations.
A similar technique may also be applied later in the well’s life to seal off
perforations through which communication with the formation has become
undesirable, for instance if water breakthrough has occurred (squeeze cementation).
Plug back cementations, that is cement placement inside the casing and across the
perforations may be required prior to sidetracking a well or in the course of
decommissioning.
The chemistry of cement slurries is complex. Additives will be used to ensure
the slurry remains pumpable long enough at the prevailing downhole pressures and
temperatures but sets (hardens) quickly enough to avoid unnecessary delays in the
drilling of the next hole section. The cement also has to attain sufficient
compressive strength to withstand the forces exerted by the formation over time.
A spacer fluid is often pumped ahead of the slurry to clean the borehole of mudcake
and thereby achieve a better cement bond between formation and cement.