Potential Deepwater Geohazards Offshore West Africa: A Review of Data Examples

Abstract

Abstract This paper describes features observed in deepwater areas offshore West Africa that may represent geohazards to exploration and production activities. The range of potential geohazards is discussed with illustrations from different data types. The data examples presented also show the benefits of utilising and combining comprehensive data sets, and the disadvantages of limited data volumes. The recognition of the various geohazards described, combined with their geological setting in new frontier areas, may allow comparisons to be made with similar features from other known areas. Introduction The term 'geohazards' encompasses a range of welldocumented geological features that have the potential to present a risk to safe and efficient offshore drilling and production operations1. Traditionally, in shallow water continental shelf areas, the most serious risks are caused by accumulations of over-pressured shallow gas and varying seabed soil strengths. The former, if undetected, can cause loss of well control and blowouts, whilst the latter can lead to jackup rig instability. Both factors present tangible and proven threats to the safety of the drilling rig and those onboard. The last decade has seen a shift of exploration focus into deeper waters, initially in the Gulf of Mexico and Brazil, and then, with the opening up of acreage internationally, NW Europe, West Africa and the Far East/Australasia. As technology advances, the goalposts for the term 'deepwater' are continually moved to keep up with the latest, deepest, wells drilled. For the purposes of this paper, deepwater means 500 to 2000m, or in other words, the region described as the continental slope and upper part of the continental rise. This increase in water depth has led to a change in emphasis for the term geohazards, and some of the methods by which they are assessed. The continental slope environment, with large thicknesses of unconsolidated sediments, is one influenced by gravity and bottom currents. Thus, slope processes such as slides, slumps and debris flows are important additional factors that may affect the safe and efficient conduct of offshore operations. The universal, industry-wide drive to reduce costs also means that any potential geohazard, which could influence the efficient and cost effective drilling of a well to completion, becomes significant. This is evidenced by the focus on problems encountered with shallow water flows in the Gulf of Mexico. Also, in two separate incidents reported recently, deepwater drillships encountered mechanical problems leading to the loss of risers and blowout preventer stacks2. There has additionally been a subsea blowout reported whilst drilling a well in 605m water depth offshore Italy3. Although not related to geohazards, these episodes serve to remind us that things can go wrong in deepwater frontier areas. The move into deeper water, (with corresponding improvements in 3D data quality over the top hole section), means that 3D is being increasingly relied upon for geohazard assessment. The examples reviewed below attempt to add to the knowledge base of the various deepwater geohazards and allow comparisons to be made with features from other known areas.