Page 71 - Hydrocarbon Exploration and Production Second Edition

58 Drilling Systems and Equipment

desilter desander

mud
tanks
shale shaker

swivel
stand
pipe rotary
hose
kelly
drilling floor
mud pumps

BOP
stack
clean mud
casing
mud returns
drillpipe

annulus
bit
Figure 4.11 Mud circulation system.

In order to effectively lift the cuttings out of the hole a certain fluid viscosity
needs to be achieved; yet the mud must remain pumpable. If the mud circulation
stops, for instance to change the bit, the mud must gel and any material suspended in
it must remain in suspension to avoid settling out at the bottom of the hole. It has to
be stable under high temperatures and pressures as well as at surface conditions. Mud
chemicals should not be removed by the mud-cleaning process. Drilling fluids have
to be capable of carrying weighing material such as barites in order to control the
formation pressures. They have to be compatible with the formations being drilled,
for example they should prevent the swelling of formation clay and not permanently
damage the reservoir zone. Last but not least, since these fluids are pumped,
transported and disposed in large quantities, they should be environmentally
friendly and cheap!
Often drilling fluids are made up using water and are called water-based muds
(WBM). Another frequently employed system is based on oil, oil-based mud (OBM).
The advantage of OBM is better lubrication of the drill string, compatibility with
clay or salt formations and they give a much higher ROP. Diesel fuel was usually
used for the preparation of OBM. During operations, large quantities of
contaminated cuttings were formerly disposed of onto the seabed. This practice is
no longer considered environmentally acceptable. If OBM or any other hazardous
fluid is contained in the cuttings, a closed-loop mud system will be required. The
cuttings will be either decontaminated in a dedicated onshore plant or re-injected as
a slurry into a suitable formation. New mud compositions and systems are
continuously being developed, for instance synthetic drilling fluids which rival the