Page 78 - Hydrocarbon Exploration and Production Second Edition

Drilling Engineering 65

2.4
fracture
gradient
2.0
stable
mud weight (SG) 1.2 collapse
1.6

gradient

0.8

0.0

hole angle
0.0
0 20 40 60 80
Figure 4.15 Example of relationship between mud weight and borehole stability.

hydrocarbons and to guarantee the integrity of the wellbore above the objective
zone. In mature fields where production has been ongoing for many years, the
reservoir may show depletion pressures considerably lower than the hydrostatically
pressured zones above. Casing and cementing operations are covered in Section 4.6.
Before continuing to examine the aspects of drilling through the reservoir,
remember that the reservoir is the prime objective of the well and a very significant
future asset to the company. If the drilling process has impaired the formation,
production may be deferred or totally lost. In exploration wells, the information
from logging and testing may not be sufficient to fully evaluate the prospect
if the hole is not on gauge, necessitating sidetracking or even an additional well.
On the other hand, there is considerable scope to improve productivity and
information value of the well by carefully selecting the appropriate technology and
practices.
In some areas such as the Central North Sea, offshore Canada, and onshore
California, high pressure high temperature (HPHT) accumulations are present. Wells
may encounter reservoir temperatures in excess of 3701F (1901C) and pressures
above 15,000 psi. These are challenging conditions for drilling fluids, mud motors,
gauges and logging tools. In particular, components such as batteries, sensors,
electronics and seal elastomers had to be specifically developed for these extreme
conditions.

4.5.3. Directional drilling

Directional drilling is usually done with a rotary steerable system (Figure 4.16).
A downhole steering and control unit is located in the near-bit assembly. A set of small
electronically controlled rotating stabiliser pads (actuators) exert a continuous
directional force onto a drive shaft which orients the drill bit into the desired