Page 188 - Well Control for Completions and Interventions

180 Well Control for Completions and Interventions

separator vessel is equal to the hydrostatic pressure of the fluid in the mud
leg. If this pressure is exceeded, the blow through will occur. An accurate,
easy-to-read, and properly calibrated low pressure gauge must be fitted to
the separator vessel. Gauges are typically in the 15 20 psi range, and ide-
ally should be visible from the choke console. Many gas separators are fit-
ted with audible alarms to warn of overpressure.

4.12 FLUID STORAGE

During drilling and completion operations, a large volume of fluid
needs to be held at the wellsite. On land rigs, most of the fluid is stored
in partitioned rectangular steel tanks, with each compartment holding
approximately 200 bbls. In addition to the bulk storage, several smaller
tanks are used for specific purposes, for example the storage of a kill pill.
At some land locations, large volume pits are made by forming earth into
raised banks then lining the construction with impermeable sheeting
(normally plastic). In recent years, earth pits have become less common,
principally because of environmental concerns. Where they are used, it is
usually for the storage of waste mud and cuttings prior to disposal.
On offshore drilling rigs and platforms, fluid is stored in several large
tanks each holding as much as 1000 bbls. Although individual tanks are
generally larger, the cumulative volume stored is often less than at a land
location, since space is limited. Table 4.4 is representative of the pit
capacity of a medium sized jack-up drilling rig (Fig. 4.30).
Insufficient fluid storage can create logistical problems, especially
when transitioning from the drilling to the completion phase of the well
construction process. For some completion operations, for example
open-hole gravel packing, a large volume of completion brine is required.
Moreover, the entire fluid system needs to be thoroughly cleaned before
the fluid can be stored. In practical terms, this normally means that when
the drilling mud is displaced from the well and replaced by completion
brine, it must be sent to a support vessel rather than returned to the pits.
The need to transfer the drilling mud to a vessel rather than back to the
pits has obvious implications for well control. It becomes more difficult
to monitor fluid returns, and an influx can go unnoticed for longer. In
some circumstances, the overbalanced drill fluids are replaced with under-
balanced completion fluids, adding to the risk.