Page 82 - Hydrocarbon
P. 82
Drilling Engineering 69
Figure 4.20 Model of multilateral well to increase reservoir contact.
in reservoirs which are lenticular
where reservoir layers are vertically segregated by permeability barriers.
Whilst drilling and, in particular, completing (see Chapter 10) is more complex,
multilateral wells have the advantage that they only require one borehole from the
surface. If there are no spare conductors on the platform, this is an attractive option.
In mature fields, multilaterals are often best suited to drain remaining pockets of
hydrocarbons. In subsea developments, multilaterals can offer a substantial cost
advantage over conventional wells.
To commence drilling of each branch, either a rotary steerable system or a
whipstock are used. The latter is a curved steel wedge which is inserted into the
borehole, forcing the drilling assembly in the planned azimuth (Figure 4.21).
4.5.6. Extended reach drilling
An extended reach well is loosely defined as having a horizontal displacement of at
least twice the vertical depth. With current technology, a ratio of over 4 (horizontal
displacement/vertical depth) can be achieved.
Extended reach drilling (ERD) wells are technically more difficult to drill and
because of the degree of engineering required for each well, the term ‘designer
well’ is frequently used.
