Page 59 - Primer on Enhanced Oil Recovery

50 Primer on Enhanced Oil Recovery

According to the results of production testing, exploration and production wells
are divided into successful and unsuccessful (dry).
The spatial position allows to distinguish between vertical, inclined, and horizon-
tal wells.
A vertical is a well whose axis deviation from the vertical passing through its
mouth is within some prescribed limits. Historically, this is the oldest type of wells.
Inclined well is deliberately drilled along a predetermined inclined to vertical tra-
jectory. A horizontal well is an inclined well with the final interval deviating from
vertical by more than 80 degrees. In many cases inclined and horizontal wells are
referred to as deviated wells
Deviated wells are more expensive to drill as compared to vertical wells but if
planned and implemented well they allow much better oil recovery from an oil
field.
Horizontal wells are effectively used in the following cases:

in fractured reservoirs, the location of a horizontal wellbore allows crossing fractures in
the reservoir for effective drainage of the reservoir
in reservoirs with the risk of water and gas breakthroughs, horizontal wells were used to
minimize water content on the pumped to the surface liquids
to increase oil recovery, especially in combination with the use of secondary and tertiary
methods of stimulation.

6.4 Multilateral wells

It was justified long time ago that oil production from an oil horizon can be
increased by the bigger diameter borehole well. It is reported that A. Grigoryan in
1949 in the former USSR had developed this idea further and drilled first well with
the horizontal branching. While the horizon penetration increased only 5 times the
oil production increased almost 20 fold.
Multilateral wells are wells that have branching in the part of the main bore in
the form of two or more long horizontal, inclined or wave-shaped trunks. Oil pro-
duction is carried out in the main trunk, and the branching serve as additional drain-
age channels through which oil comes from distant parts of the reservoir. The
branches can access oil in highly productive cracks or lenses. At the same time
branches can reach zones not affected by the previous reservoir development and
stagnant zones of the reservoir.
The choice of the form of branching, the radius of curvature of boreholes
depends on the geological, geophysical and lithological characteristics of productive
reservoirs, reservoir pressure, natural reservoir conditions, methods of maintaining
reservoir pressure, etc.
Currently, many different forms of branching and stem profiles of multilateral
wells have been developed, differing from each other in the number of branches,
their shape and length.