Page 285 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 261

2.7.1
Cementing Program [ ]

Cement is placed in a 2 in cube mold and set. After removing it
from the mold, the cube is subjected to a uniaxial force using a
hydraulic press until it fails. The reason why it does not give actual fail-
ure pressure downhole is because it is unconfined at the sides. In the
well, the cement is confined sideways by the formation and casing, and
downhole conditions of temperature and pressure will alter the true
compressive strength.
A more recent innovation is the use of ultrasound for measuring
compressive strength. Since this is nondestructive and correlates well
with physical measurements, compressive strength development can
be continuously monitored during curing.
For supporting casings, a minimum measured compressive
strength of 500 psi is recommended. For perforating, 2000 psi is
recommended.
Temperature rating. The thickening time and compressive
strength buildup are dependent on well temperature. The slurry must
have sufficient pumpable time to complete the job, with a safety mar-
gin in case of problems. In addition, the thickening time should not be
so long that rig operations are unnecessarily delayed while waiting on
cement. The thickening time is determined in the laboratory using
samples of cement and mixwater sent in from the rig.
Accelerators or retarders can be used to lessen or lengthen the
pumpable time and will similarly affect the rate of compressive
strength buildup.
Under static (nonpumping) conditions, the well will have a tem-
perature gradient as the formations get hotter with depth. Circulating
will decrease the local temperature around the wellbore. Thus at any
particular depth, two working temperatures will be relevant to cement-
ing operations: circulating and static. A temperature log run some
hours after finishing circulating will give the bottom hole static tem-
perature (BHST) at the bottom of the well. The bottom hole circulating
temperature (BHCT) at depth can be calculated by reference to API
specification 10, which contains temperature schedules. It is also pos-
sible to measure this temperature directly during circulating with small
thermosensitive probes.
Of these two temperatures, BHST is relevant to investigating
cement stability and compressive strength development with time.
BHCT is used when calculating pumpable time. As a rule of thumb, the
static temperature at the depth of the top of cement should not be less
than BHCT used in slurry design. If it is significantly less, it may take

261