VSP measurements used as a tool for improving seismic sub-salt imaging in brownfield development
Many of the Paleozoic Rotliegend low-permeability dry gas fields in the Southern Permian Basin of northern Germany are covered by Mesozoic salt dome structures. The resulting complexity in the overburden leads to imaging problems in subsalt reservoirs typically situated at depths between 4000 m to 5000 m TVD. Since structural compartmentalization is often the key challenge in field development, good resolution seismic is essential for well planning. For the study area, only a vintage 3D seismic survey from 1993/94 with limited data quality exists. Acquisition of new 3D seismic data is not possible owing to permitting restrictions and public resistance. Therefore the focus is on the acquisition of 2D VSPs in every new drilled well. The VSP measurements give local insights into the reservoir in the vicinity of the wellbore. With a general structural understanding and production history in mind, those local insights generate immense value for the understanding of the field. The field example presented is located onshore Northern Germany, in one of the country’s most important gas regions, where the operator DEA Deutsche Erdoel AG produces about 59 billion cubic feet of gas per year from its fields. The field has been in production for more than 20 years. Currently two production wells or sidetracks are drilled on average per year. The field itself is a relatively low permeable, deeply buried Rotliegend dry gas field. Despite numerous wells and a comprehensive data set collected over the years, mapping of faults and understanding of dynamic compartmentalisation remains one of the key challenges for further drilling and field development. The main reason for this is that the available surface seismic data set is strongly reduced in quality at reservoir level over the majority of the field. Mostly, owing to intense salt tectonism in the overburden and unfortunate acquisition parameters. The available 3D data set from 1993/94 was acquired using the Northern German Grid survey design, which was the default for each 3D acquisition in the 1990s. This grid is oriented North-South and was not optimized to the local geological setting. This causes problems in illuminating structures underneath the salt bodies. The highest fold for a bin size of 50 m x 50 m is 15 with good-to-moderate imaging, underneath the salt the fold is even less and hence the imaging quality further decreases. Consequently only prominent faults with throws of more than 50 m can be interpreted in parts of the field. Even then the orientation of those faults is often ambiguous. Production experience furthermore suggests that faults that are also well below seismic resolution have a significant baffling influence affecting reservoir communication. Additionally, owing to poor seismic resolution, distinguishing ‘true’ fault structures from seismic processing artefacts is a very difficult task in many places of the field. This is particularly illustrated with attempts to reconcile production data from wells and compartmentalisation seen in seismic data. In various places those data contradict each other significantly. Owing to permitting restrictions and public resistance an acquisition of new 3D data is not possible. DEA is therefore using VSP measurements as a central field development tool owing to the lower impact for the population. In the last seven years DEA has started to record VSPs in deviated outstep wells to image the section below the well path for fault detection in the reservoir. The results have shown to be crucial not only for improving the understanding of the reservoir structure but even more so to de-risk a potential sidetrack location. Thus, early VSP results after drilling represents critical data to reduce the overall project risk of development wells and are therefore now included in the decision-making process of every project.