Page 286 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 262

[ ] Well Programming
2.7.1

an unacceptable length of time to cure; in this case, extra testing
should be done at the actual TOC static temperature to see if the
cement characteristics are still acceptable. Consider a multistage
cement job if the slurry at required TOC will not cure as needed. Note
that for deep, hot wells (BHST > 110°C [230°F]), the long-term stabil-
ity of Portland cement requires the addition of silica flour.
Thixotrophic cement. A fluid is said to be thixotropic when it
forms a structure exhibiting high gel strength when shear stress is
removed but will thin again once shear stress is reapplied. The viscos-
ity of the fluid will vary for a particular shear rate depending on
whether the shear rate is increasing or decreasing; therefore, viscosity
is also dependent on history as well as shear rate.
If the fluid is at rest and has gelled, then a high stress has to be
applied as shown in Figure 2-8. However, if the fluid is moving and
measurements are taken while reducing the shear rate, the fluid looks
Bingham in nature.

Shear Stress

Viscosity for decreasing shear rate
Gel Viscosity for increasing shear rate
strength

Yield
point

Shear Rate

Fig. 2-8 Viscosity of a Fluid that Varies with Increasing or Decreasing Shear Rate

Thixotropic cements also tend to show increasing yield points and
gel strengths after each rest-pump cycle. This makes it particularly
important not to stop pumping unnecessarily during the job, otherwise
high pressures can be imposed downhole when starting to pump.

262