Oil and gas production handbook An introduction to oil ... - ABB Group

around 4.5 MPa. During production, the pressure will drop further due to
resistance to flow in the reservoir and well.
The mud enters though the drill pipe, passes through the cone and rises in
the uncompleted well. Mud serves several purposes:
• It brings rock shales (fragments of rock) up to the surface
• It cleans and cools the cone
• It lubricates the drill pipe string and cone
• Fibrous particles attach to the well surface to bind solids
• Mud weight should balance the downhole pressure to avoid leakage
of gas and oil. Often, the well will drill though smaller pockets of
hydrocarbons, which may cause a “blow-out" if the mud weight
cannot balance the pressure. The same might happen when drilling
into the main reservoir.
To prevent an uncontrolled blow-out, a subsurface safety valve is often
installed. This valve has enough closing force to seal off the well and cut the
drill string in an uncontrollable blow-out situation. However, unless casing is
already also in place, hydrocarbons may also leave though other cracks
inside the well and rise to the surface through porous or cracked rock. In
addition to fire and pollution hazards, dissolved gas in seawater rising under
a floating structure significantly reduces buoyancy.
The mud mix is a
special brew designed
to match the desired
flow thickness,
lubrication properties
and specific gravity.
Mud is a common name
used for all kinds of
fluids used in drilling
completion and
workover and can be
oil-based, water-based
or synthetic, and
consists of powdered
clays such as bentonite, oil, water and various additives and chemicals such
as caustic soda, barite (sulfurous mineral), lignite (brown coal), polymers and
emulsifiers. Photo: OSHA.gov
A special high-density mud called “kill fluid” is used to shut down a well for
workover.
28