Page 248 - Hydrocarbon Exploration and Production Second Edition

Well Dynamic Behaviour 235

The geometry and reservoir quality have a very important influence on whether
horizontal wells will realise a benefit compared to a vertical well, as demonstrated by
the following example (Figure 10.5).
In the case of the very low vertical permeability, the horizontal well actually
produces at a lower rate than the vertical well. Each of these examples assumes that the
reservoir is a block, with uniform properties. The ultimate recovery from the horizontal
well in the above examples is unlikely to be different to that of the vertical well, and the
major benefit is in the accelerated production achieved by the horizontal well.
The PIF estimate is only a qualitative check on the potential initial benefit of
a horizontal well. The stabilised flowrate benefits of horizontal wells compared
to vertical wells are more rigorously handled by relationships derived by Joshi
(ref. Horizontal Well Technology, Pennwell, 1991). Also, in high permeability
reservoirs there is actually a diminishing return of production rate on the length of
well drilled, due to increasing friction pressure drops with increasing well length,
shown schematically in Figure 10.6.

200 m
100 m
200 m
400 m 400 m
200 m
k = k h k v = k h k = 0.01 kh
v
v
PIF = 1.0 PIF = 4.0 PIF = 0.4

L k v
PIF
h
k
h
Figure 10.5 Productivity improvement factor (PIF) for horizontal wells.

Production
rate

0 1000 2000 3000 4000
Well length (m)
Figure 10.6 Production rate vs. horizontal well length.