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Well planning for SWF and overpressures at the Kestrel well

BHP Petroleum, Houston, Texas, U.S.

Glenn Bowers

Applied Mechanics Technologies, Houston, Texas, U.S.

Corresponding author: Bob.Bruce@BHPBilliton.com

Editor's note: This article is excerpted from a presentation at the 2001 OTC.

The first 20% of the full text of this article appears below.

Drilling in deepwater has forced a more complete understanding of and appreciation for pore pressures and fracture gradients. Shallow-water flow (SWF) sands perhaps exemplify the most extreme example of how narrow the window between pore pressures and fracture gradients can be. As stated by BP's Mark Alberty at the 2000 OTC (paper OTC11971, SWF sands are overpressured, unconsolidated sands encountered before pressure control is in place on the wellhead assembly. They may flow into the well if their pore pressures are not balanced by the mud weights. However, if the mud weights are too highly overbalanced, the sands may fracture—resulting in lost returns and loss of well control. The margins for wellbore pressures may be less than ±100 psi for shallow depths below mudline in deepwater wells.

The Kestrel well was drilled in a water depth of 2795 ft in Mississippi Canyon Block 632, an area of well-documented SWF. Data from nearby wells were incorporated in designing the casing and mud weight program. Essential to the design were predictions of pore pressures and lithology. Efficient well design required a pore-pressure prediction for the SWF section and the deeper overpressured sections. Accurate predictions required input from a pore-pressure analyst, geophysicists, seismic interpreters, operations geologists, and drilling engineers.

	Planning of the well

 
The Kestrel well was designed with information from a number of wells, but a great deal of the most important information came from the Mississippi Canyon 718 1 well, approximately 7 miles WSW of the Kestrel well. Planning for SWF in this well was very well presented at the OTC in 1996 ("Identification of ‘flowing water sand’ drilling hazards in the deepwater Gulf of Mexico" by Byrd et al.). Figure 1 is a seismic line extracted from a 3D seismic data volume connecting the wells. The interpretation of the significant sequences is included. . . . [Full Text of this Article]