Page 28 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

meter. For a 3000 meter deep well with 30 MPa downhole pressure and
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normal crude oil at 850 kg/m , the wellhead static pressure will only be
around 4.5 MPa. During production, the pressure will drop further due
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 specialist 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

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