Well Planning for Shallow Water Flows and Overpressures - The Kestrel Well
Authors: Bruce Bob , Bowers Glenn , Borel Robb 
Published: October 2008

Areas of known shallow water flows (SWF) require careful well planning and the latest technology for pressure monitoring while drilling. For the Kestrel well, Mississippi Canyon 632#1, geologists, geophysicists, drilling engineers and pressure prediction specialists worked together to design and drill a well that had to contend with shallow water flow sands and narrow margins between pore pressures and fracture gradients.

The design required understanding the pore pressure and stress environments at offset wells and their relationship to the Kestrel well location. Of particular concern was up-dip transfer of sand pressures. The pore pressure predictions hinged on the seismic data interpretations and the accuracy of the time-depth relation used to predict formation tops. During drilling, pressure while drilling (PWD) and logging while drilling (LWD) technology along with recently developed drilling practices for dynamic pressure control were used to "walk the tightrope" between pore pressure and fracture gradient. The real-time data and leak-off tests indicated that the pore pressures and fracture gradients at Kestrel were close to predicted values and as a result the well was drilled under time and under budget.

Introduction

Drilling in deep waters has forced a more complete understanding of, and appreciation for, pore pressures and fracture gradients. Shallow water flow sands perhaps exemplify the most extreme example of how narrow the window between pore pressures and fracture gradients can be. As stated by Mark Alberty of BP at the 2000 Offshore Technology Conference in OTC11971, shallow water flow sands are overpressured, unconsolidated sands encountered before pressure control is in place on the wellhead assembly. They may flow into the well bore 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 well bore pressures may be less than + 100 PSI for shallow depths below mudline in deep water wells.

The Kestrel well was drilled at a water depth of 2795 ft. in Mississippi Canyon Block 632, an area of welldocumented shallow water flows. Data from nearby wells were incorporated in designing the casing and mud weight program. Essential to the design were predictions of the pore pressures and lithology. Efficient well design required a pore pressure prediction of not only the shallow water flow section but the deeper overpressured sections as well. Accurate predictions required the input of 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. Planning for shallow water flows in this well was very well presented in OTC 7971 in 1996. Figure 1 is a map showing the positions of the Kestrel and MC718#1 wells. Figure 2 is a seismic line extracted from a 3D seismic data volume connecting the well locations.