Page 422 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 398

[ ] Practical Wellsite Operations
3.5.1

Use of a computer controlled recirculating mixer
Using a “manual” recirculating mixer no faster than the cement can
be accurately mixed

3.5.1. Mud Conditioning for Maximum Displacement

Drilling mud is designed to gel when circulation is stopped. This
property prevents solids such as barite and cuttings from dropping out.
Unfortunately, when mud gels in a washout or in the narrow part of an
eccentric annulus, it is very hard to start it flowing again since the mud
preferentially flows through the easiest path.
Good mud displacement is the single most important factor in hav-
ing a competent cement job. The first step is to condition the mud
prior to pulling out for cementing, to obtain the lowest possible PV, YP,
and gels (without weighting materials dropping out). Normally the
program will include a step for circulating and conditioning
Having properly centralized casing is the second step and mini-
mizing the time that the mud is static is the third. By the time the cas-
ing is in the hole, the first two factors should have been addressed.

3.5.2. Slurry Mixing Options

The best method of mixing cement slurry is by batch. If suitable
tanks are available then mix the cement in them, displacing only when
the slurry properties are exactly according to the program. The thick-
ening time of the slurry is tested at bottom hole temperature and half
an hour on surface should not seriously affect thickening time down-
hole. If there is not enough tank volume for both lead and tail slurries,
consider mixing the tail slurry as a batch (this is the most important
and usually the least volume) before mixing and pumping the lead in
the recirculating mixer. Switch to the batch tank and pump the tail.
Take several samples of each slurry to monitor the setting time.
Use a pressure balance to check the actual slurry weight. The lead slur-
ry should ideally be at least 1 ppg above the mud weight and the tail
slurry also at least 1 ppg above the lead slurry weight. This helps dis-
placement of the mud ahead and discourages mixing of the different
fluids. The spacers should be in between mud and lead slurry density.

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