Page 92 - Hydrocarbon Exploration and Production Second Edition

Drilling Engineering 79

When drilling through normally pressured formations, the mud weight in the well
is controlled to maintain a pressure greater than the formation pressure to prevent
the influx of formation fluid. A typical overbalance would be in the order of 200 psi.
A larger overbalance would encourage excessive loss of mud into the formation,
slow down drilling and potentially cause differential sticking. If an influx of
formation fluid into the borehole did occur due to insufficient overbalance, the
lighter formation fluid would reduce the pressure of the mud column, thus
encouraging further influx, and an unstable situation would occur, possibly leading
to a blowout. Hence, it is important to avoid the influx of formation fluid by using
the correct mud weight in the borehole all the time. This is the ‘first line of defence’.
When drilling into an overpressured formation, the mud weight must be increased
to prevent influx. If this increased mud weight could cause large losses in shallower,
normally pressured formations, it is necessary to isolate the normally pressured
formation behind casing before drilling into the overpressured formation. The
prediction of overpressures is therefore important in well design.
Similarly, when drilling into an underpressured formation, the mud weight must be
reduced to avoid excessive losses into the formation. Again, it may be necessary to
set a casing before drilling into underpressures.
Considerable effort will be made to predict the onset of overpressures ahead of
the drill bit. The most reliable indications are gas readings, porosity-depth trends,
ROP and shale density measurements.
If a situation arises whereby formation fluid or gas enters the borehole, the driller
will notice an increase in the total volume of mud. Other indications such as a
sudden increase in penetration rate and a decrease in pump pressure may also indicate
an influx. Much depends on a quick response of the driller to close in the well before
substantial volumes of formation fluid have entered the borehole. Once the BOP is
closed, the new mud gradient required to restore balance to the system can be
calculated. The heavier mud is then circulated in through the drill string and the
lighter mud and influx is circulated out through the choke line. Once overbalance is
restored, the BOP can be opened again and drilling operations continue.

4.8. Costs and Contracts

The actual well costs are divided into

Fixed costs: casing and tubulars, logging, cementing, drill bits, mobilisation
charges, rig move
Daily costs: contractor services, rig time, consumables
Overheads: offices, salaries, pensions, health care, travel.

A fairly significant charge is usually made by the drilling contractor to modify
and prepare the rig for a specific drilling campaign. This is known as a mobilisation
cost. A similar charge will cover ‘once off’ expenses related to terminating the
operations for a particular client, and is called a demobilisation cost. These costs can be
significant, say 5–10 million US$.