Abstract

The Cygnus gas field is being developed by ENGIE E&P UK Limited in the UK Southern North Sea and is one of the largest discoveries in the Southern North Sea in the last 30 years. The field has two gas bearing reservoirs in the Carboniferous Lower Ketch Member of the Ketch Formation and the Permian Lower Leman sandstone. The Lower Leman Sandstone is the main reservoir target for 7 of the 10 wells in the development. The Lower Leman Sandstone is highly layered comprising fluvial influenced playa shoreline facies. The better reservoir quality intervals are restricted to thinly bedded laterally extensive sand rich intervals related to drier climatic events. Through utilization of reservoir navigation LWD tools the production wells have preferentially targeted these better quality thin intervals in order to maximize well productivity. To geosteer in these sands provided several challenges such as uncertainty in dip and the presence of sub-seismic faults. Furthermore the reservoir shows only subtle variations in log response from Gamma Ray and resistivity tools. This made correlations with offset well data difficult. As a result, the integration of information from multiple LWD tools and types of analysis was required to delineate the geological structure and to identify the stratigraphic position of the trajectory in order to place the well in the target interval. The key data sources for the leman productions wells have been the correlation to offset data, real-time Density images and utilization of Deep Directional Azimuthal Resistivity's. The limit of the Deep Directional Resistivity tool was tested due to the very low resistivity contrast reservoir (commonly 1-2 Ohm.m) but the data has still been utilized during geosteering operations. The quality of the density image data has also allowed for a real-time True Stratigraphic Thickness (TST) calculation service to be provided where extra stratigraphic control was required.

Five of the Leman production wells have been successfully drilled and completed with results that were at the high end of expectations. This will contribute to maximizing the productivity for the field.

In this paper the case studies showcased will demonstrate the Geosteering methodology utilized in this complex reservoir. For ENGIE, these examples demonstrate the value of investing in technologies for acquiring multiple data sets for successfully Geosteering in challenging geological settings.

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