Summary
Accurately modeling water-saturation variation in transition zones is
important to reservoir simulation for predicting recoverable oil and guiding
field-development plans. The large transition zone of a heterogeneous Middle
East reservoir was challenging to model. Core-calibrated, log-derived water
saturations were used to generate saturation-height-function groups for nine
reservoir-rock types. To match the large span of log water saturation
(Sw ) in the transition zone from the free-water level (FWL)
to minimum Sw high in the oil column, three saturation-height
functions per rock type (RT) were developed, one each for the low-, medium-,
and high-porosity range.
Though developed on a different scale from the simulation-model cells, the
saturation profiles generated are a good statistical match to the
wireline-log-interpreted Sw, and bulk volume of water (BVW)
and fluid volumetrics agree with the geological model. RT-guided
saturation-height functions proved a good method for modeling water saturation
in the simulation model.
The technique emphasizes the importance of oil/brine capillary pressures
measured under reservoir conditions and of collecting an adequate number of
Archie saturation and cementation exponents to reduce uncertainties in well-log
interpretation.
Introduction
The heterogeneous carbonate reservoir in this study is composed of both
limestone and dolomite layers frequently separated by non-reservoir anhydrite
layers (Ghedan et al. 2002). Because of its heterogeneity, this reservoir, like
other carbonate reservoirs, contains long saturation-transition zones of
significant sizes. Transition zones are conventionally defined as that part of
the reservoir between the FWL and the level at which water saturation reaches a
minimum near-constant (irreducible water saturation, Swirr )
high in the reservoir (Masalmeh 2000). For the purpose of this paper, however,
we define transition zones as those parts of the reservoir between the FWL and
the dry-oil limit (DOL), where both water and oil are mobile irrespective of
the saturation level. Both water and oil are mobile in the transition zone,
while only oil is mobile above the transition zone. By either definition, the
oil/water transition zone contains a sizable part of this field’s oil in
place.
Predicting the amount of recoverable oil in a transition zone through
simulation depends on (among other things) the distribution of initial oil
saturation as a function of depth as well as the mobility of the oil in these
zones (Masalmeh 2000). Therefore, the characterization of transition zones in
terms of original water and oil distribution has a potentially large effect on
reservoir recoverable reserves and, in turn, reservoir economics.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
8 December 2004
- Revised manuscript received:
4 September 2006
- Manuscript approved:
10 October 2006
- Version of record:
20 December 2006
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