In-Situ reservoir compaction measurements have been performed in the Groningen field since 1974. The technique is based on regularly monitoring the distance between radio-active markers shot at regular distances across the reservoir. The application of this technique in the Groningen field is unique in the sense that it is normally used to monitor compaction in unconsolidated reservoirs, that compact much more than the Groningen reservoir. In practice, it has been proven difficult to obtain the required accuracy, 1 mm per 10 m. Until recently, the vertical rock compressibility could only be measured in three of the eight wells. The reservoir rock is so competent, that the target resolution is very close to the instrument resolution. Over the years, much effort has been put into improving the accuracy and reliability of the measurements, improvement on tools as well as on interpretation techniques. In 1993/1994, part of the surveys has been re-interpreted, using a new marker position algorithm, and a selection mechanism has been developed to identify statistically significantly deviating data. As a result, in all but one well could the vertical compressibility be obtained. The in-situ measured compressibilities match with the compressibility as inferred from the surface subsidence over the field.

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