Page 22 - Well Control for Completions and Interventions

16 Well Control for Completions and Interventions

1.1.4.2 Salt beds
Deposition of salt can occur over wide areas. Since salt is impermeable
to fluids, the underlying formations become over-pressured. Abnormal
pressures are frequently found in zones directly below a salt layer.

1.1.4.3 Salt domes or diapirs
Salt domes form when overburden pressure acting on a salt formation
causes it to plastically deform and push up through weaknesses in the
overlying formations. Upwards movement of salt through the sedimentary
strata, and the associated deformation of the formation above, is called
“halokinetics” or salt tectonics. Movement may continue for several 100
million years. Overpressure can occur because of the folding and faulting
of the formation.

1.1.4.4 Tectonic forces
Tectonic movement can give rise to horizontal forces in the formation.
In a normally pressured formation, water is squeezed out of clays as they
are compacted by increasing overburden. However, if the horizontal force
is such that it squeezes the formation laterally, and fluids are prevented
from escaping at a rate equal to the reduction in pore volume, an increase
in pore pressure will result.

1.1.4.5 Faulting
Formation blocks sometimes contain sealed-in pressure that is normal for
the depth of burial. If, however, the formation is uplifted to a shallower
depth because of fault movement, the pressure will be abnormal for the
new depth (Fig. 1.9).

1.1.4.6 Cross-flow
Some completions allow flow between layers in the reservoir. High pres-
sure zones can cross-flow into lower pressured zones. This is sometimes
referred to as an underground blowout. Cross-flow can be very problem-
atic during some intervention activities. It can also complicate and com-
promise well kill operations.

1.1.5 Formation fracture pressure
It is possible to hydraulically fracture a formation by applying pressure
to the wellbore. When a formation fractures, cracks are created within
the rock matrix, and fluid in the wellbore will be lost into the