VSP While Drilling: A Deepwater Case Study Offshore West Africa

Rank wildcat wells in deepwater represent some of the greatest opportunities for operators today but also present major challenges. 3D surface seismic velocity data without nearby well calibration has significant uncertainty from the conversion of two-way time to horizon depths, which further induces risk in the predrill pore pressure model created from limited available data. This case study is about a deepwater exploration well drilled offshore West Africa where seismic while drilling technology was used extensively to reduce depth uncertainties and drilling risks from spudding to total depth of the well. This allowed for the target depth uncertainty to be reduced by more than 90% along the three consecutive drilled sections, from greater than +/-100 meters to less than +/-5 meters. Furthermore it allowed this 2D well trajectory to avoid accidental penetration of the first target, avoiding a kick and placing the 13 3/8” casing shoe safely above the first target, 80 meters shallower than predicted, and just 20 meters from the top formation. Placing the 13 3/8” casing shoe as deep as possible increased the mud weight window for the 12 1/4” hole section, and contributed towards saving a casing string close to the bottom of the well.