Page 283 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 259

Cementing Program [ ]
2.7.1

is the main goal, then this type of cement job should meet the objec-
tives. Normally only one slurry is used for very small cement jobs,
such as a liner.
The upper limit on hydrostatic pressure in the well will be dictat-
ed by any weak or loss circulation zones, allowing also for circulating
pressure losses. The lower limit will need to maintain well control.
Casing flotation and collapse should also be calculated.
When lab testing the slurry recipe, ensure that the slurry is still
pumpable at 1 ppg more than the designed weight to allow for mixing
inconsistencies on the rig. On the rig, a pressure balance should be
available to measure the true density of the slurry while mixing.
Fluid loss. Fluid loss properties of the slurry should be measured
and, if necessary, additives should be used to modify this. Fluid loss is
important because when cement slurry is placed across a permeable for-
mation, loss of filtrate into the formation will dehydrate the slurry. This
will affect the setting time (even cause flash setting if extreme), set
strength, and may lead to channeling. In addition, a high fluid loss will
build a thick filter cake that will narrow the annulus, leading to increased
annular pressure drop and possibly induced losses or fracturing.
A general recommendation for maximum API fluid loss is 100
cc/30 min for casings or 50 cc/30 min for liners to prevent channeling.
For horizontal wells, use the lowest fluid loss that can be reasonably
achieved (due to the large exposed permeable formation), <50 cc/30
min. For cementing against gas zones, fluid loss should be as low as
possible; values down to 15 cc/30 min have been used.
Free water. Any water that is used in excess of that needed to com-
pletely hydrate the cement and additives is known as free water. An
excessive free water property will increase settling of solids with water
forming within and on top of the slurry.
In a deviated well this can cause a channel at the high side of the
hole and in a horizontal well this will be even more critical. In gener-
al, the maximum free water should be 0.5%, less in a high-angle well
(>45˚), and zero in a horizontal well (>80˚) or against a gas zone.
Density readings taken at the top and bottom of a vertical 1 ft slurry
1
sample should vary by less than /2 ppg once the sample has stood for
long enough to gel.
Excess free water will promote gas migration and could form a
channel on the high side of the hole if cement settles out with free

259