Page 420 - Hydrocarbon Exploration and Production Second Edition

Managing Decline 407

Pressure P 0 P

P 1 2

well 1 (well 3 ?) well 2

Distance
original reservior pressure profile (P )
0
average abandonment pressure including well 3 (P )
1
average abandonment pressure including well 3 (P )
2
Figure 17.3 In£uence of an in¢ll drainage point.

time value of money. Compared to the base case development plan, additional wells
may access reserves which would not necessarily be produced within the field
economic lifetime, simply because the original wells were too far apart. This is
illustrated in Figure 17.3 by considering the pressure distribution in a reservoir
under depletion drive. A third well in this situation could recover additional reserves
before the wells reach their abandonment pressure. The additional well would have
to be justified economically; the incremental recovery alone does not imply that the
third well is attractive.

17.2. Workover Activity

Wells are ‘worked over’ to increase production, reduce operating cost or
reinstate their technical integrity. In terms of economics alone (neglecting safety
aspects), a workover can be justified if the NPV of the workover activity is positive
(and assuming no other constraints exist). The appropriate discount rate is the
company’s cost of capital (Figure 17.4).
Well production potential is the rate at which a well can produce with no external
constraints and no well damage restricting flow. Actual well production may fall
below the well potential for a number of reasons, which include

mechanical damage such as corroded tubing or stuck equipment
formation productivity impairment around the wellbore
flow restriction due to sand production or wax and scale deposition
water or gas breakthrough in high-permeability layers
cross flow in the well or behind casing.

If mechanical damage is severe enough to warrant a workover, the production
tubing will normally have to be removed, either to replace the damaged section or
gain access for a casing repair. Such an operation will require a rig or workover
hoist, and on an offshore platform may involve closing in neighbouring wells for