Summary
Relative permeabilities are fundamental to any assessment of reserves and
reservoir management. When measurements on core samples are available, however,
they often predict initial water production that is not experienced by
individual wells. For example, dry oil production occurs from portions of
reservoirs where the local water saturation is relatively high, even though the
relative permeability data would predict a water cut in the range of 30 to 60%.
This lack of agreement means that effective reservoir management is hampered
because it is difficult for simulation models to mimic the observed reservoir
production without use of data that may bear little resemblance to
measurements.
After a brief discussion of relative permeability, the focus of this paper
is first to examine the uncertainties in the data that are used for the
predictions. This then provides a numerically structured approach to
adjustments that need to be made to data so that history matching of simulation
models can be achieved. The relative permeabilities, rather than saturations
and fluid properties, are shown to be the least certain of the relevant
data.
The second focus in the paper is to explore the reasons why the relative
permeability data are so uncertain. The evidence points to the fact that oil
emplacement and the subsequent geological history of the reservoirs have not
been considered sufficiently in preparing core samples before making
measurements. Greater reliance on drillstem and early production tests is,
therefore, crucial for deriving reservoir relative permeabilities until
laboratories are able to mimic oil emplacement within rock samples as
experienced in the reservoir.
The main source of data is the abandoned UK North Sea reservoir Maureen
(Block 16/29a). Inevitably, during the 36 years since discovery, some data have
been misplaced. Nevertheless, sufficient data exist to highlight the potential
need for a paradigm shift in understanding how relative permeabilities should
be obtained for reservoir simulation.
Introduction
There are many examples of dry oil production from portions of reservoirs
where the local water saturation is relatively high (Matthews 2004). On the
occasions when relative permeability data are available, predictions of the
expected water cut are not zero but typically in the range of 30 to 60%. A
particular example is that of the abandoned UK North Sea reservoir Maureen
(Cutts 1991). This lack of agreement means that effective reservoir management
is hampered because it is difficult for simulation models to mimic the observed
early reservoir production without use of data that may bear little resemblance
to measurements.
The focus of this paper is first to examine the uncertainties in the data
that are used for the predictions. The Maureen reservoir--its data were placed
in the public domain for research and training purposes by Phillips after it
was abandoned (Gringarten et al. 2000)--provides the main source of
information. The data examined are viscosity, saturation, and relative
permeability.
Having established which data are the most uncertain, the paper then
includes a brief discussion of the transition zone and oil emplacement to
understand the nature of the uncertainties in relative permeability
measurements and, in particular, measurements of the irreducible water
saturation. From this, a new avenue of research related to oil emplacement can
be identified that, if pursued, may lead ultimately to better
reservoir-management models.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
2 June 2007
- Meeting paper published:
4 September 2007
- Revised manuscript received:
15 April 2008
- Manuscript approved:
16 April 2008
- Version of record:
29 December 2008
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