Drilling Salt Formations Offshore With Sea Water Can Significantly Reduce Well Costs
Stephen M. Willson & Pete Driscoll (BP America Inc.); Arnis Judzis,
Alan Black & Wes Martin (TerraTek, Inc.); Brian Ehgartner & Tom
Hinkebein (Sandia National Laboratories)
This paper was prepared for presentation at the IADC/SPE Drilling Conference held in Dallas, Texas, 2-4 March 2004,
Copyright 2004, IADC/SPE Drilling Conference.
Abstract
Rate of penetration (ROP) increases significantly if salt formations
are drilled using undersaturated fluids. This is especially true when
drilling riserless. Of primary concern is the amount of hole enlargement
that will occur. However, if managed, then a step-change in drilling
performance and costs can be achieved.
This paper presents, in detail, the results of a comprehensive and large-scale
laboratory testing program performed on outcrop salt samples that replicate
drilling salt formations with udersaturated drilling fluids with flow
rates of up to 1000 gpm. The laboratory-testing program includes tests
performed in a CT-scanner to map hole enlargement in real-time, as well
as variously sized borehold leaching tests with borehole diameters of
up to 6". Flow rates are scaled to produce comparable levels of
turbulence occuring in the field.
An analytical hole enlargement prediction model is presented that incorporates
the effects of ROP, pump rate, drill-fluid saturation, disolving salt
drill-cuttings and salt leaching from the borehole wall. This model accurately
predicts, to within 10%, the measured hole enlargements produced in the
scaled laboratory tests, for a wide range of flow rates and fluid saturations.
Predictions of field performance are made, and the implications of the
predicted hole geometry discussed. Provided high rates of penetration
are realized, acceptable hole geometries will result, even when pumping
seawater at flow rates of up to 1600 gmp. A field application is described,
whereby the historical practice of drilling the last 200 ft. of the 20" hole
into salt using a saturated brine drilling fluid was discontinued in
perference to continued use of seawater. The competing influences of
increasing rate of penetration and avoiding the cost of a sacrificial
mud system, offset by the increased cost of cement, resulted in cost
savings of $250,000 per well. Implemented ina multi-well sub-salt development,
cost savings of over $1.5 million will be realized in the drilling program.
Further applications of this technology are now being sought.
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