Page 256 - Hydrocarbon

Well Dynamic Behaviour 243

Pressure (psi)

P
1
3 / 2 " TPC
ΔP DD
5" TPC
P wf

IPR 2

IPR 1

Q Flow Rate (b/d)
Figure 10.15 Reservoir performance and tubing performance.
the minima in the TPR is stable, whilst conditions to the left hand side of the
minima, as well as being inefficient, can suffer flow instability effects such as severe
slugging and are best avoided.
We now have predictions for the pressure drops in the tubing and in the
reservoir. They share a common point or node. The inflow performance relation-
ship (IPR) predicts the wellbore flowing pressure for a given reservoir and reservoir
completion, whilst the TPR predicts the wellbore flowing pressure required to lift
these fluids to surface through the tubing. At the (wellbore) node, the pressure and
the rate must be the same and therefore the point of intersection of the IPR and the
TPR is the predicted well rate and the wellbore flowing pressure. This technique
1
is commonly called NODALt analysis. The same technique can be used for the
intersection of the TPR with the surface facilities pressure drops where the node is
now the surface pressure.
Ignoring the surface facilities pressure drop, the following diagram shows an example
of the equilibrium between the IPR and the TPR for two tubing sizes (Figure 10.15).
For the reservoir with IPR 1 , the larger tubing does not achieve an equilibrium,
1
and the well would not flow if the 5 in. tubing was installed. However, a different
2
reservoir with IPR 2 would benefit from a larger tubing size which would allow
1
greater production, and the correct selection of tubing size would be the 5 in. tubing
2
if one wanted to maximise the early production from the well. An understanding
of the tubing performance and the reservoir performance (which requires reservoir
data gathering) is important for the correct selection of tubing size. Note that most
of the variables (e.g. water cut, reservoir pressure) change substantially with time so
designing for the life of the well will require some compromises.
Returning to the surface pressure drops across the choke and the facilities, these
will also vary over the producing lifetime of the field. The choke is used to isolate the

1
NODAL analysis is a trademark of Flopetrol Johnston, a division of Schlumberger Technology Corporation.