Page 76 - Hydrocarbon Exploration and Production Second Edition

Drilling Engineering 63

Figure 4.13 Cellar with Christmas tree on a land location.

stove pipe will be piled prior to the rig moving in. The cellar will later accommodate
the ‘Christmas tree’ (an arrangement of seals and valves to control production), once
the well has been completed and the rig has moved off location (Figure 4.13).
As in the construction industry, piling of the conductor is done by dropping
weights onto the pipe or using a hydraulic hammer until no further penetration
occurs. In an offshore environment, the conductor is either piled (e.g. on a
platform) or a large-diameter hole is actually drilled, into which the conductor is
lowered and cemented. Once the drill bit has drilled below the conductor the well
is said to have been spudded.
The top hole will usually be drilled with a large-diameter bit (between 22 and
27 in. diameter). The drill bit (roller cone type) will be designed to drill
predominantly soft formations. As a result of the hole diameter and the rapid
penetration rate, vast quantities of drilled formation will have to be treated and
removed from the mud circulation system. Often the ROP will be reduced to allow
adequate removal of cuttings and conditioning of mud. In some cases, the problem
1
is alleviated by first drilling a pilot hole with a smaller diameter bit (12 in.) and later
4
redrilling the section to the required size using a hole opener. This is essentially a
larger diameter drill bit above the smaller diameter bit. Hole openers are also run if
the hole has to be logged (most logging tools are not designed for diameters above
1
17 in.) and if accurate directional drilling is required.
2
A surface casing is finally cemented to prevent hole collapse and protect shallow
aquifers.

4.5.2. Intermediate and reservoir section

Between the top hole and the reservoir section, in most cases, an intermediate section
will need to be drilled. This section consists of more consolidated rocks than the top