Abstract

Once the drilling decision is made, following a full prospect evaluation, the well planning and design phase starts. A key component of planning is to assess and mitigate any hazards associated with drilling, for which the operator is responsible. The traditional approach is to acquire a dedicated site survey, which can be time-consuming and expensive. The objective of a site survey is to identify hazards, which may affect the operational or environmental integrity of a proposed drilling operation. These hazards maybe geological or man-made. This is usually achieved by acquiring dedicated high-resolution (HR) 2D or 3D seismic site surveys. For cost and time reasons, the site surveys are often acquired as a grid of 2D lines around the planned surface locations of the well, and typically cover a small area. This implies that the approximate location of the well is decided before conducting the site survey, which in turn will help to further refine the location based on a hazard analysis of the data. A new site survey might be necessary if the hazard assessment indicates the planned well has to be moved. Although rare, such risks do exist and may have cost and time implications for a project. Site surveys are designed to image the shallowest overburden, and are normally acquired with shallow towed sources and receivers, making the weather conditions a key factor for their success. To control the cost, typical site surveys are acquired with short offsets, which means they may rely on earlier exploration seismic surveys to obtain an accurate near-surface velocity model.

The International Association of Oil and Gas Producers (IOGP) produces guidelines for the conduct of offshore drilling hazard site surveys (IOGP report number 373-18-1, 2017). The report suggests as long as the data fulfills these guidelines, it should have sufficient quality and resolution to enable an effective geohazard analysis. In deep water, the use of large-scale 3D seismic volumes will provide data fit for purpose assuming the acquisition geometry provides the appropriate spatial sampling. In shallow water (<250 m), the acquisition geometry may not be suitable for near-surface imaging with conventional algorithms. The interplay between shallow water data and conventional 3D seismic acquisition near offsets, can mean only high reflection angles are recorded. This can cause imaging gaps in seismic data that are visible as an illumination footprint. In shallow water a separate site survey data set might be needed.

What if we could produce a product needed for hazard analysis without acquiring new data? What if we could cover an area 10 to 20 times larger than the one of 2D HR seismic surveys, and with a full 3D perspective? What if we can perform this at any time we needed it? Having such a solution can have several advantages: there will be more time available for well trajectory design; the same geohazard products can be used for more than one well over the licence area; no dependency on weather condition; a more environment friendly solution as no new data set needs to be acquired. In this paper one such solution is proposed, explained in detail and illustrated with case studies.

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