Page 26 - Well Control for Completions and Interventions

20 Well Control for Completions and Interventions

Note: Always round down to one decimal place when calculating LOT
equivalent mud weight, i.e., 15.69 becomes 15.6 ppg.
When calculating kill fluid weight, always round up to one decimal place,
i.e., 15.12 becomes 15.2 ppg.

1.1.5.2 Formation integrity tests
LOTs are generally restricted to exploration wells, or wells in a develop-
ment area where there is uncertainty about fracture gradient and forma-
tion pressure. Where reliable offset data is available, deliberately fracturing
the formation during drilling is normally avoided. The formation is pres-
sure tested, but at below the anticipated fracture pressure. This type of
test is called a “Formation Integrity Test” (FIT). The advantage of the
FIT is that there is no compromise of the formation fracture pressure.
LOTs and Formation Integrity Tests can only be successfully carried
out with drilling mud in the well. Solids in drilling mud allow a filter
cake to build up on the bore-hole wall, limiting fluid loss into the forma-
tion. With correctly formulated mud in the hole, fluid loss can only be
induced if the formation is fractured. If solid-free fluids are in the well
(i.e., completion brines), fluid will leak off at above pore pressure, but
below fracture pressure. Losses can be controlled using lost circulation
material, such as calcium carbonate (CaCO 3 ) or sized salt. Completion
fluids, their use and properties are described in Chapter 5.

1.1.5.3 Unit systems
To date, no industry-wide standardization of units has been achieved.
Across the world, the oil industry uses a variety of unit systems. Oilfield
units are the most widely used, but metric and SI systems are becoming
more popular. It is not uncommon to find different disciplines at the
same location using different unit systems. For example, drilling teams
often work in oilfield units, whilst the process engineers use SI. It is not
unusual to find a mixture of unit systems in some locations, e.g., some
North Sea operators measure depth and pipe length in meters, but pipe
diameter in inches.
It is not surprising that confusion over unit systems and conversion
factors leads to mistakes. The safest approach is for an individual to use
the unit system they are most comfortable with. Simple arithmetical
errors are more likely to be recognized, since the magnitude of the
answer will be anticipated.