Abstract
In preparation for a new phase of development of the deep, >8000 psi depleted, high-pressure, high-temperature (HPHT) Shearwater field in the UK Central Graben of the North Sea, the existing well stock had to be prepared to enable slot availability for sidetracking operations. Here techniques and key learnings gained from the slot preparation activities, in particular with respect to the chalk overburden, are described.
During the slot preparation activities new data on fracture gradients and the extent of porosity in the chalk was acquired. This information was then integrated with the results gained from the use of state of the art logging tools and unique data acquisition (including chalk hydrocarbon molecular and isotope compositions). In addition, an analysis of overburden time shifts from re-processed 4D seismic data was completed, along with geomechanical modelling work.
A new understanding was gained of the effects of reservoir depletion on the chalk, from being a solid uniform tight rock with very low permeability to a more heterogeneous permeable formation with differential reactions to reservoir depletion. The key learnings have been integrated into the ongoing slot recovery operations and future well plans. The overall integrated data allowed a greater understanding of the heterogeneity of the entire chalk section, which has been valuable in considering chalk overburden complexity.
