Page 249 - Hydrocarbon Exploration and Production Second Edition

236 Horizontal Wells

The exact relationship will depend on both fluid and reservoir properties, and
will be investigated during well planning. Poor completion practices may exacerbate
the problem as the lower drawdown on the toe of the well compared to the heel
may prevent proper clean-up of mud, filter cake and completion fluids.
Horizontal wells have a large potential to connect laterally discontinuous features in
heterogeneous or discontinuous reservoirs. If the reservoir quality is locally poor,
the subsequent section of the reservoir may be of better quality, providing a healthy
productivity for the well. If the reservoir is faulted or fractured a horizontal well
may connect a series of fault blocks or natural fractures in a manner which would
require many vertical wells. The ultimate recovery of a horizontal well is likely to be
significantly greater than for a single vertical well (Figure 10.7).
The third main application of horizontal wells is to reduce the effects of coning and
cusping by changing the geometry of drainage close to the well. For example, a horizontal
producing well may be placed along the crest of a tilted fault block to remain as far
away from the advancing oil–water contact as possible during water drive. An
additional advantage is that if the PI for the horizontal well is larger, then the same oil
production can be achieved at much lower drawdown, therefore minimising the effect
of coning or cusping. The result is that oil production is achieved with significantly less
water production, which reduces processing costs and assists in maintaining reservoir
pressure. Horizontal wells have a particularly strong advantage in thin oil columns
(say, less than 40 m thick), which would be prone to coning if developed using
conventional wells. The unwanted fluid in oil rim development may be water or gas,

Well

Naturally fractured Faulted reservoir
reservoir (sealing faults)
Figure 10.7 Increased recovery from a horizontal well.

GOC
Gas cone

Oil column
Horizontal well

Figure 10.8 Gas cresting in oil rim development with horizontal wells.