Page 287 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 263

2.7.2
Cementing Program [ ]

Thixotropic cements are useful for ensuring that slurry does not
move after placement (but not for cement plugs, as previously noted) or
when curing lost circulation with smaller slurry volumes where the
cement will gel up in the loss channels after displacing into the loss zone.
Expanding cement. Slurry can be made to expand after setting.
Additives (most commonly, Ettringite) form crystals whose bulk vol-
ume exceeds the total volumes of the reagents. This will help form a
good bond to casing and formation if a microannulus may be formed.
Ettringite forms long needle crystals which impart thixotrophy to the
slurry but if thixotrophy is undesirable, a dispersant will reduce the
thixotropic behavior.
Low temperature cement (permafrost). Cement liberates heat
during setting. If drilling in permafrost, cement heat will lead to thaw-
ing of the permafrost layer and unstable ground close to the surface.
Slurries can be prepared that do not freeze, have a low heat of reaction,
and develop sufficient compressive strength at low temperatures.

2.7.2. Chemical Washes and Spacers

Chemical washes (low viscosity pills with a density between mud
and cement containing dispersants, surfactants, etc.) work by diluting
and thinning the mud. This aids mud removal and the thinning action
helps prevent mud flocculation and gelling. Chemicals in the wash also
help to remove mud film from the casing, which will aid bonding,
especially if oil-based mud is used.
Spacers could be high or low viscosity and have a higher solids
content than washes. The abrasive action of solids may help clean sur-
faces to be cemented.
Correct design of washes and spacers is important and several fac-
tors must be considered.

1. The chemicals used should be compatible with the formation flu-
ids, mud, and cement so they do not cause adverse reactions (for-
mation of viscous fluids) or reservoir damage (precipitates or
emulsions). The chemical wash is meant to disperse the mud and
clean out residual solids. Spacers are used to act as a buffer between

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