Page 252 - Well Control for Completions and Interventions

246 Well Control for Completions and Interventions

• Rig pumps/mud pumps. If a rig is on location for the well kill, the
mud pumps will probably be used. These are high volume pumps, but
are sometimes tricky to operate consistently if very small volumes are
required. If small volume and consistency of output is required, a
smaller capacity pump might be preferred, for example a cement unit.
• Surface equipment limitations. Be aware of any limitations on surface
equipment downstream of the choke, i.e., the choke line, mud/gas
separator. The maximum pump rate must be lower than the handling
capacity of this equipment.
• Fluid filter throughput limits. Whilst not strictly a well kill concern,
some circulating operations, for example gravel packing or wellbore
cleaning, work in a closed loop where brine returns are filtered before
they are returned to the pits for re-use. The pump rate will be limited
to the capacity of the filtration system.

7.3.4 U tube pressure

When circulating, the tubing and annulus are in hydraulic communica-
tion. If the tubing and annulus contain fluids of differing density, any
resulting difference in HP will cause the fluid level to change until equi-
librium is reached. If the well is shut-in, higher pressure will be observed
at the top of the fluid column of lower density (tubing or annulus).
Example:
The tubing is filled with base oil with a density of 6.5 ppg (0.78 sg)
and the annulus is filled with a 10 ppg (1.2 sg) brine. The tubing end is
at 5000 ft. (1524 m) and the resulting pressure at the surface is as follows.
Oilfield units:
HP of the brine column 5 10 3 0.052 3 5000 5 2600 psi.
HP of the base oil column 5 6.5 3 0.052 3 5000 5 1690 psi.
Tubing pressure at the surface 5 2600-1690 5 910 psi.
SI units:
Brine hydrostatic 5 (1.2 4 10.2) 4 1524 5 179.294 bar (17,929 kPa).
Base oil hydrostatic5 (0.78 4 10.2)/15245 116.541 bar (11,654 kPa).
Tubing pressure at the surface 5 179 2 117 5 62 bar (6200 kPa).
There are several occasions where U tube pressure will be present
during workover, completion, and intervention operations:
• Circulation of a weighted-up fluid to kill a well following a kick.
• Circulation of a kill weight fluid into the well prior to a workover or
intervention.