Page 81 - Hydrocarbon

68 Drilling Techniques

Long Radius Medium Radius Short Radius

1 1 /2° to 3° /ft
kickoff 40 to 20 foot radius
(12 to 6 m)
8° to 50° / 100 ft
700 to 125 ft radius
(200 to 40 m)
800 ft (240 m)
2° to 6° / 100 ft
3000 to 1000 ft radius
(900 to 300 m)
Average 3000 ft (900 m)

Over 3500 ft (1070 m)
Figure 4.19 Horizontal well radii.

The success of horizontal wells was largely dependent on the development of
tools which relay the subsurface position of the drill bit in real time to the drill floor.
Improvements in this technology have greatly improved the accuracy with which
well trajectories can be targeted. MWD is achieved by the insertion of a sonde into
the drill string close to the bit. Initially providing only directional data, the tools
have been improved to the point where petrophysical data gathering (gamma ray
[GR], resistivity, density and porosity) can be carried out whilst drilling.
Most reservoirs are characterised by marked lateral changes in reservoir quality
corresponding to variations in lithology. Computing tools now commercially
available allow the modelling of expected formation responses ‘ahead of the bit’.
This is possible in areas where a data set of the formations to be drilled has been
acquired in previous wells. The expected GR and density response is then simulated
and compared to the corresponding signature picked up by the tool. Thus, in
theory, it is possible to direct the bit towards the high-quality parts of the reservoir.
Resistivity measurements enable the driller to steer the bit above a hydrocarbon
water contact (HCWC), a technique used, for example, to produce thin oil rims.
These techniques, known as geosteering, are increasingly being applied to field
development optimisation. Geosteering also relies on the availability of high-quality
seismic and possibly detailed palaeontological sampling.

4.5.5. Multilateral wells
Drilling a number of holes, branching from a central borehole, is an attractive
option in the following cases:
where reservoir productivity is low but can be significantly improved by
increasing the reservoir surface area exposed to the well (Figure 4.20)