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 4-10o towards North-West. The reservoir sections consist of fluvial deposits and reservoir units contain thin sand layers with alternating shale in a complex fault pattern. The average reservoir pressure in the CFB is 300 bar and the reservoir temperature is 90oC.
To understand sand layer communication and to what degree the faults act as barriers or not, a significant tracer program (with more than 50 individual injections) has been executed. The tracer program started early in 1993, about 1/2 year after production start, and still continues today. During execution of a water-alternating-gas (WAG) program, tracers have been injected both in the water phase and in the gas phase. Here we focus on two water tracer injections in the Central Fault Block (CFB).
Water fronts from seismic data
In the Snorre field, 4-D seismic surveys and corresponding interpretation show clear dimming (seismic amplitude decrease) as oil is substituted by water (cf. figures below). These surveys were performed after five years of production (first figure) and after an additional four years of production (second figure). The dark color in the figures corresponds to reservoir areas where oil has been substituted by water. In the first figure it can be noted that the water front has reached about halfway between the producers P-40 and P-38.
Tracer information confirms 4D seismic data
As part of the extensive tracer program in the Snorre field, three distinct tracers were injected in P-25, P-28 and P-34 in the same month at an early stage of the water injection in the CFB. By monitoring these tracers in the producers it is possible to track the water front moving North and East in the CFB in Snorre.
The results from this tracer survey are summarized in the figures above (see also pdf-download for more details). Comparing with the 4-D seismic data the tracer data are in excellent agreement with the water front inferred from the seismic study. In addition, the tracers add significant information, revealing that the water in the P-38/P-40 area originates from two sources, namely P-34 and P-28.
In conclusion we note that tracer data can be used to confirm 4-D seismic interpretations. Moreover, the 4-D seismic data confirms that tracer data provides an excellent tool to track water fronts in petroleum reservoirs.
In the Statoil operated North Sea Snorre field, production data, 4D seismic and tracer monitoring were used in an integrated process to map reservoir flow (SPE 105288). Tracer data confirmed interpretations of water fronts indicated by 4D seismic dimming, thus giving unambiguous interpretation of drainage patterns.
For more information regarding this case study, please download the whitepaper below.
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