Summary
To obtain improved oil recovery (IOR), it is crucial to have a best-possible
description of the reservoir and the reservoir dynamics. In addition to
production data, information can be obtained from 4D seismic and from tracer
monitoring. Interwell tracer testing (IWTT) has been established as a proven
and efficient technology to obtain information on well-to-well communication,
heterogeneities, and fluid dynamics. During such tests, chemical or radioactive
tracers are used to label water or gas from specific wells. The tracers then
are used to trace the fluids as they move through the reservoir together with
the injection phase.
At first tracer breakthrough, IWTT yields immediate and unambiguous
information on injector/producer communication. Nevertheless, IWTT is still
underused in the petroleum industry, and data may not be used to their full
capacity--most tracer data are used in a qualitative manner (Du and Guan 2005).
To improve this situation, we combine tracer-data evaluation, 4D seismic, and
available production data in an integrated process. The integration is
demonstrated using data from the Snorre field in the North Sea. In addition to
production data, extensive tracer data (dating back to 1993) and results from
three seismic surveys acquired in 1983, 1997, and 2001 were considered.
Briefly this study shows that
- Seismic and tracer data applied in combination can reduce the uncertainties
in interpretations of the drainage patterns.
- Waterfronts interpreted independently by tracer response and seismic
dimming compare well.
- Seismic brightening interpreted as gas accumulation is supported by the
gas-tracer responses.
Introduction
The Snorre field is located in the Tampen Spur area on the Norwegian
continental shelf and is a system of rotated fault blocks with beds dipping 4
to 10° toward the northwest. The reservoir sections are truncated by the Base
Cretaceous unconformity. The reservoir sections consist of fluvial deposits of
the Statfjord and Lunde formations. The reservoir units contain thin sand
layers with alternating shale in a complex fault pattern. A challenge regarding
optimization of the reservoir drainage, as well as oil production, is to
understand how the different sand layers communicate and to what degree the
faults act as barriers or not.
The present work concentrates on the integration of 4D-seismic and tracer
methods to obtain information on fluid flow in the Upper Statfjord (US) and
Lower Statfjord (LS) formations in the Central Fault Block (CFB). The outline
of this fault block is indicated in Fig. 1. The net/gross ratio is higher and
the reservoir quality is generally better in the US than the LS formation. The
CFB is produced by water-alternating-gas (WAG) injection as the drive
mechanism, where the injectors are placed downdip and the producers updip. The
average reservoir pressure in the CFB is 300 bar, and the reservoir temperature
is 90°C.
Tracer data are used to understand fluid flow in the reservoir. The data
give valuable information about the dynamic behavior and well communication,
but in some cases the interpretation may be complicated by reinjection of
produced gas and water. Tracer studies in the Snorre field have been presented
previously in several papers (Dugstad et al. 1999; Ali et al. 2000; Aurdal et
al. 2001). To use the data fully, however, integration with other types of
reservoir data is important.
The main objectives of the seismic monitoring of Snorre are to contribute to
increased oil recovery and to optimize placement of new wells. 4D analysis,
together with tracers, should potentially increase the multidisciplinary
understanding of the drainage pattern in the reservoirs. The results should, in
addition to all the reservoir and production data, be used actively in
target-remaining-oil processes and in well planning. In addition, the 4D data
can give input to update the geological model and simulation model (history
matching) and to identify possible well interventions. There is also a
potential to include the data in workflows to identify lithology changes.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
9 January 2007
- Meeting paper published:
11 March 2007
- Revised manuscript received:
23 January 2008
- Manuscript approved:
20 March 2008
- Version of record:
20 August 2008
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