R-factor recovery via Geertsma’s pressure inversion assisted by engineering concepts

A technique is developed for a North Sea chalk reservoir to estimate the Hatchell-RØste R factors using a reservoir engineering constraint. This provides a way of defining the uncertainty on the R values, given the range of mechanical properties for the field of interest. In our case study the average R for the reservoir and overburden appears to be in the range 7 to 22. R factors are also calculated for regions of pressure relaxation and drawdown identified in the field, and the results confirm the current understanding determined by laboratory experiment and previous studies that the magnitude varies as a function of strain polarity, with the asymmetry being at most a factor of three. These results are validated with full geomechanical modelling followed by time shift modelling, showing that the observed time shifts cannot be created unless this asymmetry is present