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Drilling Safely at Well Design Limits - A Critical Well Design Case History
Deep, high pressure exploration wells are expensive and subject to high levels of uncertainty. Extreme conditions often require operations outside recognized design limitations. Strenuous well planning, in conjunction with operational contingencies, can be employed to stretch safety factors beyond conventional limits, while providing mechanisms to limit risk. Risk taking in hole sections with low incident probability allows deeper casing points so that less risk is taken in hole sections with higher incident probability.
These philosophies and practices were employed to reach geological objectives on the Madura CD-1 well located offshore near the eastern end of the Island of Java. The CD-1 was drilled to 16000 ft TVD with 17.3 ppg mud. Six strings of casing were run, the last a 7" liner. A 5-5/8" hole was drilled from 14465 to 16000 ft, and designs put in place to drill deeper if needed.
Factors contributing to risk included abnormal pressure starting at 3500', potentially prolific reservoir productivity coupled with low formation strength in the primary objective at 12400' and pressures greater than 14000 psi in the deeper horizons.
Seismic data was analyzed rigorously to estimate shallow pore pressures where casing point selection critically impacts subsequent casing setting depths. Casing points were picked based on ability to safely control an underground flow rather than just on kick tolerance. Casing above a potentially prolific reservoir was set to contain an infinite influx with minimal risk of an underground flow.
Design parameters of kick tolerance and killable productivity (gas zone thickness x permeability) were monitored continuously by the drilling team.
Contingency plans were developed based on these parameters with the aid of Mobil developed software. Reserve potential for the well was 3 TCF in a shallower primary objective, making this a significant prospect. A secondary objective as deep as 17500 ft contained substantial additional speculative reserves. The benefits of a discovery of this size warranted sound well control risk management to ensure targets could be reached and formations evaluated at total depth.
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