Page 247 - Hydrocarbon Exploration and Production Second Edition

234 Horizontal Wells

The tendency for coning and cusping increases if
the flowrate in the well increases
the distance between the stabilised OWC and the perforations reduces
the vertical permeability increases
the density difference between the oil and water reduces.
To reduce this tendency the well should be produced at low rate, and the
perforations should be as far away from the OWC as possible. Once the unwanted
fluid breaks through to a well, it may be recompleted by changing the position of
the perforations during a workover, or the production rate may be reduced.
The above examples are shown for water coning and cusping. The same
phenomena may be observed with overlying gas being pulled down into the
producing oil well. This would be called gas coning or cusping.
The height and width of the cones or cusps depend on the fluid and reservoir
properties, and on the rates at which the wells are being produced. In a good quality
reservoir with high production rates (say 20 Mb/d), a cone may reach more than
200 ft high, and extend out into the reservoir by hundreds of feet. Clearly this
would be a major disadvantage in thin oil columns, where coning would give rise to
high water cuts at relatively low production rates. In this instance, horizontal wells
offer a distinct advantage over conventional vertical or deviated wells.

10.3. Horizontal Wells

Horizontal wells were drilled as far back as the 1950s, but gained great
popularity from the 1980s onwards as directional drilling technology progressed and
cost pressure mounted. Horizontal wells have potential advantages over vertical or
deviated wells for three main reasons

increased exposure to the reservoir giving higher productivity indices (PIs)
ability to connect laterally discontinuous features, for example fractures, fault blocks
changing the geometry of drainage, for example being parallel to fluid contacts.
The increased exposure to the reservoir results from the long horizontal sections
which can be attained (sections many kilometres in length are now routine in many
fields). Because the PI is a function of the length of reservoir drained by a well,
horizontal wells can give higher productivities in laterally extensive reservoirs. As an
estimate of the initial potential benefit of horizontal wells, one can use a rough rule
of thumb, the productivity improvement factor (PIF) which compares the initial
productivity of a horizontal well to that of a vertical well in the same reservoir,
during early time radial flow
r ffiffiffiffiffi
L k v
PIF ¼
h k h

where L is the length of the reservoir; h the height of the reservoir; k h the
horizontal permeability of the reservoir; k v the vertical permeability of the reservoir.