Identification of "Flowing Water Sand" Drilling Hazards in the Deepwater Gulf of Mexico

Abstract

Abstract Shallow flowing water sands impose costly well designs and delays for deepwater Gulf of Mexico wells. Typically detailed site surveys overlook the regional distribution of shallow acquifer sands and seals which combine to induce flow. A new approach to site clearance and well design emphasizes integration of specially processed, high resolution airgun data with 3D seismic datasets, correlation wells, seismic stratigraphic analysis and mapping. The techniques were applied for a proposed wellsite in the Missisippi Canyon area. The 3D data density and regional coverage permitted seismic correlation and facies calibration to nearby wells land drilling histories. Interpretation and mapping on the 3D data provided a sub-regional stratigraphic and structural framework. To enhance stratigraphic detail, conventional 2D hazard airgun data were specially processed and both datasets were interpreted on a workstation. Mapping of sands, seals and overburden distribution identified potential flowing water sand zones. The resulting overpressured sands prognosis was used for casing and cement design and as a guideline for well operations in the first 5000 ft below mudline (bml). The analysis identified a regional seal with two underlying zones of overpressured sands. Initial interpretation of the 3D data at the well location suggested a continuous sand zone cut by a large channel. Instead, the high frequency 2D data revealed rotated fault blocks with local seals. The first overpressured sand zone was avoided by locating the well in a rotated fault block and setting the 20" casing point there. The BOP stack was set prior to drilling deeper overpressured sands. Understanding the source of some deep water drilling problems such as "flowing water sands" indicates a need for better regional stratigraphic mapping of the shallow section. Access to nearby drilling histories can focus on particular problem sections. Integration of regional data, high resolution seismic data and stratigraphic interpretation is key to improved identification of shallow flowing water sands. Introduction Planning the location of exploratory wells typically focuses on optimizing penetrations of prospective hydrocarbon targets and engineering the well to get there. Deepwater drilling costs and extended targets have challenged geoscientists and engineers to optimize well locations and casing designs to avoid or mitigate hazards. Traditional site clearance and well planning have relied on pringer profiles and shallow airgun 2D hazard surveys to detect shallow gas zones faults and unstable substrates. However, 3D seismic is becoming a more commonly used exploration toll in the Gulf of Mexico and can also be applied to timely and cost-effecitve hazard prediction. Often unavoidable, overpressured sand zones or "flowing water sands" can pose a costly drilling problem once a well is committed to the location. The problem is acute in the shallow section above the 20-inch casing point wher ethe well is drilled without the benefit of a BOP and riser system. Prediction on the site zones must be coordinated with casing and cement designs. This paper outlines the application of a technique and the results of predicting shallow flowing water sands for an exploratory well in Mississippi Canyon 718.