The accurate quantification and management of seismic positioning uncertainties have been paramount in securing the drilling operations and maximizing well life in two recent High Pressure/High Temperature (HPHT) developments in the UK Central North Sea.
In these HPHT fields, all development wells need to be drilled before onset of production, as drilling into depleted reservoirs is considered as high risk. Identifying the optimal development well locations that balance the needs for a safe drilling margin (downdip) vs maximizing well life (updip) is complicated by the presence of large seismic positioning uncertainties.
Very close cooperation and mutual understanding between the well engineer and the seismologist/geologist is required to develop the optimal drilling strategy. The first well was placed in an intermediate location from which recovery would have been feasible had the worst possible raybending effects materialized. While drilling, a predictive Vertical Seismic Profile (VSP) was acquired above the reservoir to check the relative position of the well and the presence/thickness of the Cromer Knoll, the seat for the production casing. Based on the first wells, a 3D Pre Stack Depth Migration finally resolved the positioning uncertainties. Risky and costly contingency operations have been avoided.