Prediction of formation compressibility and secondary gas cap development from seabed and downhole tidal pressure signals in the Lancaster Field

The Lancaster Field is the first, and to date only, fractured basement reservoir development on the United Kingdom Continental Shelf. The field produces under natural depletion and, at the depth of the wells, was initially under-saturated by approximately 300 psi. Reservoir pressure has declined during production, and owing to the lack of a gas export route, regulatory approval for ongoing production below the bubble point has been made conditional on no incremental liberated gas being produced to the surface. Estimation of the potential size of any secondary gas cap that develops is a key part of the surveillance required to meet this condition. In this paper, we show how analysis of the sea bed and downhole tide pressure signals can be used to estimate the pore-volume compressibility for the fractured basement reservoir at initial conditions and how the observed changes in the seabed to downhole attenuation can be related to changes in the fluid compressibility and hence gas saturation. The gas saturations predicted from this analysis are compared with those derived from reservoir simulation modelling and show reasonable agreement.