Page 401 - Hydrocarbon Exploration and Production Second Edition

388 Managing the Subsurface

Sand Pressure
A

B

C

D

E

Depth Original
pressure
regime
Figure 16.3 RFT pressure plot in a development well.

is at least partly sealing. The shale layers (or faults) between sands C and D and
between D and E must be fully sealing, since sand D is still at the original pressure.
The vertical pressure communication of the reservoir is therefore limited by these fea-
tures. Assuming that the reservoir in this example is being produced by natural
depletion, then it can be seen that production from layers B and C (which are in
vertical pressure communication) is faster than from the other sands, indicating either
that the sands have better permeability, or are limited in extent. Meanwhile, no
production is occurring from the D sand in this area, since the pressure remains un-
depleted. The RFT data can therefore be used to derive more than simply a pressure.
The modern equivalent of the RFT is the MDT which is a Schlumberger tool.
Monitoring the reservoir pressure will also indicate whether the desired reservoir
depletion policy is being achieved. For example, if the development plan was
intended to maintain reservoir pressure at a chosen level by water injection, measure-
ments of the pressure in key wells would show whether all areas are receiving the
required pressure support, and may lead to the redistribution of water injection or
highlight the need for additional water injectors. If the chosen reservoir drive
mechanism was depletion drive, then reservoir pressure in key wells will indicate if
the depletion is evenly distributed around the field. A relatively undepleted pressure
would indicate that the area around that well is not in pressure communication with
the rest of the field, and may lead to the conclusion that more wells are required to
drain this area to the same degree as the rest of the field. The presence of an active
natural aquifer can also be detected by measuring the reservoir pressure and the
produced volumes; the contribution of the aquifer support for the reservoir pressure
would be calculated by the reservoir engineer using the technique of material balance
(Section 9.1, Chapter 9).
In a reservoir consisting of layers of sands, the sweep of the reservoir may be
estimated by measuring the production rate of each layer using the PLT. This is a tool