7/25/2019 Chapter 04 Drilling
1/2
PUC Minas Virtual 1
The drilling process is probably seen from the outside as the
most intriguing and spectacular
aspect of the petroleum industry. Huge land drilling rigs can
reach depths of over 10000 m
and over the years oil and gas well drilling has expanded from
land to offshore where over the
years greater and greater water depths have been overcome.
Drilling is part of all stages of the
oil and gas field development life cycle. Wells are drilled in
the exploration phase. Thesewells are the most difficult and costly
as many uncertainties exist in new areas and if
successful many measurements are taken to gain better
understanding of the discovery.
During the next appraisal phase the field boundaries are
delineated and again an extensive
data acquisition program is embarked on to give the basis of the
development plan.
Geoscientists and Engineers and work together to design a plan
(the field development plan)which ultimately leads to the
production phase. During the development phase the most
number of wells are drilled and significant part of the total
investment (30-50%) is consumed
by the drilling and completion operations.
Offshore drilling technology has evolved over the years to
overcome ever greater water
depths. Initially land rigs were simply mounted on stilts or
jackets and later greater mobility
was achieved with so called Jack-Ups. These are essentially
floating platforms that when
brought to location drop legs into the ground and self-rise
above the sea level to be safe from
wave action and swell.
At greater depths (300-400ft), floating devices are required.
These are called semi-
submersibles because a great part of their structures and
chambers remain under water to
provide the required buoyancy. These drilling devices have to be
held on location either by
anchors or by dynamic positioning them with GPS and computer
controlled thrusters. Greater
water depths can be overcome with TLP (Tension Leg Platforms)
and Drillships.
The drilling process normally used in the petroleum industry is
rotary drilling, whereby a
drill-bit is mechanically rotated to cut the rock in a grinding
action. The drill bit is connected
to an ever increasing number of drill pipe, which at the bottom
is extra heavy (Heavy weights
and Drill collars). This ensures that sufficient weight can be
transferred to the bit while the
majority of the drill string is held in tension and not in
compression. The drill pipe is hollow
and the bit, which is of a larger diameter, allows circulation
of drilling fluid from the surface
through the drill pipe and back in the annular space between the
freshly cut well bore wall and
the outside of the drill pipe. The drilling mud exits the bit
through nozzles and cools the bit,while at the same time
transporting the cuttings (rock cut by the bit while making hole)
to
surface. The hydraulic pressure and chemical properties of the
mud give hydraulic
overpressure against formation fluids and give stability to the
hole. Nevertheless section of
hole have to be secured with pipe (casing) and cemented before a
new section, smaller in
diameter, can be started on. On the surface the casing strings
are connected with a safety
critical device called the BOP (Blow Out Preventer), which in
emergency cases can be closed
around the pipe or even shear the pipe off.
When the well finally reaches the reservoir, this last section
is also usually secured with
production casing. To allow the reservoir fluids to enter the
well, this casing has to be
perforated. This is done by lowering guns into the hole which
punch small holes with
penetrations of up to 1-2 m into the rock.The well is now ready
for production. Sometimes the reservoir fluids flow to surface
by
themselves. We call these wells free-flowing. If reservoir
pressure is insufficient pumping
Ttulo: Chapter 4: Drilling
Autor: Oliver Maximilian Seybold
