Time dependent signal of a chalk field: The South Arne Field, Danish North Sea
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Time-lapse seismic analysis is applied to a producing chalk field, with the aim to understand the field time-varying behaviour with respect to reservoir structure and fluid migration. The study area is the South Arne field in the Danish Central Graben (North Sea). The field lies in an elongated anticline, and the reservoir consists of fractured chalk of the Tor and Ekofisk formations. The reservoir is highly heterogeneous, with varying reservoir quality (i.e., porosity and water saturation). The time-lapse study includes the interpretation of rock physics properties in order to explain how and why the seismic response changes with production. The analysis of the reservoir structure is largely based on seismic time-shift data to detect reservoir compaction, and difference in amplitude response of faults in order to analyse possible fault reactivation. Seismic amplitudes, AVO (amplitude variation with offset) response, and coloured inversion results are analysed to detect fluid movements. A significant amount of production-induced compaction is observed for a limited part of the reservoir, which correlates strongly with high porosity and low water saturation, and the location of several production and injection wells. Based on the rock physics analysis, this part of the reservoir is less stiff than the surrounding reservoir formations, which explains why it is subject to compaction. This may also explain the observation that faults in this part of the reservoir are highly reactivated. The observed changes in seismic response with production time include reduction in amplitudes, decrease in AVO response, and hardening effect from the coloured inversion. These effects are caused by changes in fluid saturation and by compaction. The changes in seismic response are strongly correlated with the compaction, which indicates that the time-lapse seismic response is more sensitive to the reduction in porosity caused by compaction than to fluid changes. This is supported by the well data, which show that the acoustic impedance of the reservoirs is more influenced by porosity than water saturation. Nevertheless, based on the hardening effects, the oil accumulations seem to be separated by partially sealing faults, causing the fluids to move differentially for the different fault blocks.
Master's thesis in Petroleum Geosciences Engineering