Page 19 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 13

Table 1.4 Regional variations for pore pressure gradient
Region Pore pressure gradient

psi/ft KPa/m
Anadarko basin 0.433 9.64
California 0.439 9.77
Gulf of Mexico 0.465 10.35
Malaysia 0.442 9.84
North Sea 0.452 10.06
Rocky Mountains 0.436 9.71
West Africa 0.452 9.84
West Texas 0.433 9.64

Where h 5 the vertical depth (ft or m); G w 5 the pressure gradient
(psi/ft or kPa/m).
Formation water is normally saline and more dense than fresh water.
However, increasing temperature with depth reduces fluid density, so a
common “normal” value used is the fresh water gradient (0.433 psi/ft or
9.80 kPa/m). Gradients within the range 0.433 0.5 psi/ft are considered
normal (Table 1.4).
Pressure at the top of a hydrocarbon-bearing structure can be
expected to be higher than the hydrostatic gradient extrapolated from the
hydrocarbon/water contact caused by the reduced pressure gradient in
the oil and gas column (Fig. 1.6).
Where pressure in the formation is greater than that caused by a column
of formation brine, the pressure is considered “abnormal.” Although the
term “abnormal” is used, the condition is in fact quite common, and a char-
acteristic of some of the best oil and gas reservoirs. There are several causes.
Where the vertical depth of the water column is more than well
depth, the well will be abnormally pressured. Perhaps the best example of
this is the artesian well Fig. 1.7.
When drilling a low-lying area of a mountainous region, a relatively
short borehole can penetrate a formation that is pressurized by a fluid col-
umn that has a higher elevation than wellbore ground level. Balancing
fluid needs to be very dense to prevent uncontrolled flow. Similarly, in
dipping and folded permeable reservoirs, pressure from the deepest part of
the formation can be transmitted to the shallowest part. Whilst pressure at
the deepest point may be normal for the depth, pressure at the crest can
be significantly higher than “normal”. For example, the pore pressure gra-
dient in the North Sea is generally given as 0.452 psi/ft (10.06 KPa/m).