Energy Geoscience (Jan 2023)
Pre-rig mobilization hazard evaluation in offshore oil and gas prospect drilling: A case study of TM field, offshore Niger Delta
Abstract
TM Field, located offshore Niger Delta in the Gulf of Guinea (Nigeria), has been evaluated for potential drilling hazards prior to the drilling of a proposed oil well. This is in a region where potential hazards that could inhibit successful drilling exercise have not been adequately studied. The study adopted the modern technique of offshore geohazard evaluation that relies mainly on suites of geophysical and shallow geological investigations which include ocean bathymetry using multi-beam echo sounder, side-scan sonar, sub-bottom profiler, magnetic, and 2D-high resolution seismic reflection surveys. The results were integrated to draw inferences about the risk potential of the field. From the bathymetric survey, water depths were found to range from 345 m to 650 m LAT, with seabed relief being 305 m. Water depth at the proposed well location was found to be approximately 450 m and the seabed was found to slope in the northwestern direction. Seafloor gradient was computed as 0.05 across the whole area. Two shallow sub-surface stratigraphic units, labelled A and B, were delineated. Unit A was directly beneath the seafloor and seemed to be composed mainly of clayey/silty sand. Its thickness ranged from 6 m to 70 m across the study area. Unit B appeared to be a layer of sand and its total thickness was not fully delineated. Observable kinks and displacements at the flanks of seabed undulations/depressions on sub-bottom profiler sections suggested that the depressions may be fault-controlled. On the map generated from the marine magnetic data, a magnetic fault was delineated and this appeared to be coincident with the shallower of the two buried faults on the cross-line seismic section cutting through the proposed well-head location. Interpreted seismic reflection data presented this fault as being inactive. Most of the faults mapped, especially the buried ones, appeared to be inactive except a few. Regions with mapped amplitude anomalies are to be avoided during rig-mobilization operations. Though engineering conditions are expected to vary vertically since layers occur as intercalation, significant lateral variation is not expected within uniform layers of bed. Risk of punch-through associated with clayey formation is expected to be low because the geologic layers are not uniformly and entirely clay. The proposed well-head location was found to be free of potential hazard sources.
