This in#ol#es the wellbore pressure bein reater than the
pore-formation pressure, andis the eneral scenario used in the
offshore oil and as industry today when drillinwells.
Equation 2
inshutiswelltheif
Pressure)ic(HydrostatPwitheableinterchangisP
pressureor!ationP
eo"erbalancof #arginP
pressure$ellboreP
Hw
f
%B
w
=
=
=
+=
>
O! f "
f "
P P P P P
The mar in of o#erbalance is often called the safety or trip mar
in, and is the measureof how far abo#e the pore pressure the
wellbore pressure is. If the formation pressurebecomes reater than
the wellbore pressure then formation fluid will flow into
thewellbore, a phenomenon nown as I/ .
In Balance
This scenario in#ol#es the pore pressure for the formation bein
e0actly the same as thewellbore pressure. It is not a common
pressure scenario when drillin in offshore wellsand is shown in
"quation 1.
Equation 3 f " P P =
Underbalanced
&ost con#entional wells are drilled o#erbalance but recent
de#elopments in drillintechnolo y ha#e meant that underbalance
drillin is bein increasin ly used to impro#ewell producti#ity. This
is a specialised drillin technique in which the influ0 of
formationfluid into the wellbore can be deliberately controlled to
minimise or a#oid certainborehole problems such as formation dama
e. Thus the hydrostatic pressure would bedesi ned to be less than
the formation pressure. This is a controlled ic in which the#olume
of fluid flowin and mi0in with the wellbore fluid is nown. Thus, in
this case*
