Page 77 - Hydrocarbon

64 Drilling Techniques

hole. The deviation angle is often increased significantly in this interval to reach the
subsurface target, and lateral departures from the surface co-ordinates may reach
several kilometres. Based on pore pressure prediction (from seismic or measured
data from offset wells) the mud weight has to be determined. The pressure exerted
by the mud column has to exceed the formation pressure in order to maintain
overbalance and prevent the hole from collapsing but has to be lower than the
fracture pressure of the formation (Figure 4.14). If the formation strength is
exceeded, fracturing may occur, resulting in mud losses and formation damage.
Borehole/formation stability is the realm of geomechanics. Challenges in well
planning arise when rock strength and thus borehole stability show considerable
variations depending on hole angle and direction, as shown in Figure 4.15. In this
example, the small difference between fracture gradient and collapse gradient at
high deviation may require a revision of the initially planned well trajectory through
the intermediate and/or reservoir section.
An intermediate casing is usually set above the reservoir in order to protect the
water-bearing, hydrostatically pressured zones from influx of possibly overpressured

0

2

fracture
4 gradient

6
Depth x 1000’ 8

10
pore
pressure
gradient
12

14

8.0 10.0 12.0 14.0 16.0 18.0 20.0
pressure gradient - Ib/gal equivalent
pressure profile predictions
Figure 4.14 Mud weight envelope has to be between pore pressure gradient and fracture
gradient.