Page 75 - Hydrocarbon

62 Drilling Techniques

A typical survey area is some 4 by 4 km, centred on the planned location. Surveys
may include

Seabed survey: employing high-resolution echo-sounding and side scan sonar
imaging, an accurate picture of the sea bottom is created. The technique allows
the interpreter to recognise features such as pipelines, reefs and wreckage.
Particularly if a jack-up rig is considered, an accurate map of these obstructions is
required to position the jack-up legs safely. Such a survey will sometimes reveal
crater-like structures (pockmarks), which are quite common in many areas. These
are the result of gas escape from deeper strata to the surface and could indicate
danger from shallow gas accumulations.
Shallow seismic: unlike ‘deep’ seismic surveys aimed at the reservoir section, the
acquisition parameters of shallow surveys are selected to provide maximum
resolution within the near-surface sedimentary layers (i.e. the top 800 m). The
objective is to detect indications of shallow gas pockets or water zones. The gas
may be trapped within sand lenses close to the surface and may enter the borehole
if penetrated by the drill bit, resulting in a potential blowout situation. Gas
chimneys are large-scale escape structures where leakage from a reservoir has
created a gas-charged zone in the overburden. If shallow water zones are
penetrated, they may flow to the surface of the seabed and reduce the load-
bearing capacity of the conductor pile.
Soil boring: where planned structures require soil support, for example drilling
jackets or jack-up rigs, the load-bearing capacity has to be evaluated ( just like on
a land location). Usually a series of shallow cores are taken to obtain a sample of
the sediment layers for investigation in a laboratory.

Particularly for jack-up rigs, site surveys may have to be carried out prior to each
re-employment to ensure that the rig is positioned away from the previously formed
‘footprints’ (depressions on the seabed left by the jack-up legs on a previous job).

4.5. Drilling Techniques

If we consider a well trajectory from surface to total depth (TD), it is helpful to
look at the shallow section and the intermediate and reservoir intervals separately.
The shallow section, usually referred to as top hole, consists of rather unconsolidated
sediments, hence the formation strength is low and drilling parameters and
equipment have to be selected accordingly.
The reservoir section is more consolidated and is the main objective to which
the well is being drilled, hence the drilling process has to ensure that any productive
interval is not damaged.

4.5.1. Top hole drilling

For the very first section of the borehole, a base from which to commence drilling is
required. In a land location, this will be a cemented ‘cellar’ in which a conductor or