Page 72 - Hydrocarbon

Drilling Engineering 59

performance of OBM but are environmentally benign (e.g. synthetic oil-based mud
[SOBM]).
The choice of drilling fluid has a major impact on the evaluation and production
of a well. Later in this section, we will investigate the interaction between drilling
fluids, logging operations and the potential damage to well productivity caused by
mud invasion into the formation.
An important safety feature on every modern rig is the BOP. As discussed earlier
on, one of the purposes of the drilling mud is to provide a hydrostatic head of fluid
to counterbalance the pore pressure of fluids in permeable formations. However, for
a variety of reasons (see Section 4.7) the well may ‘kick’, that is formation fluids may
enter the wellbore, upsetting the balance of the system, pushing mud out of the
hole, and exposing the upper part of the hole and equipment to the higher pressures
of the deep subsurface. If left uncontrolled, this can lead to a blowout, a situation
where formation fluids flow to the surface in an uncontrolled manner.
The BOPs are a series of powerful sealing elements designed to close off the
annular space between the pipe and the hole through which the mud normally
returns to the surface. By closing off this route, the well can be ‘shut in’ and the
mud and/or formation fluids forced to flow through a controllable choke, or
adjustable valve. This choke allows the drilling crew to control the pressure that
reaches the surface and to follow the necessary steps for ‘killing’ the well, that is
restoring a balanced system. Figure 4.12 shows a typical set of BOPs. The annular

bell nipple

flow line
fill line

annular preventer

pipe ram
shear/blind ram

kill line drilling choke
spool flow

pipe ram

BOP riser
emergency casing emergency
head
kill choke

Figure 4.12 Blowout preventer.