Offshore drilling costs are affected by the need to drill a borehole for logging purposes, then ream the hole for deeper drilling. The introduction of an entirely new suite of logging tools designed to operate in an 18 inch (or larger) borehole significantly reduces these drilling costs.

The induction and resistivity tools are totally new designs based on mathematical simulations which were verified by experiment. On the other hand, the pad type Big Hole tools are similar to their conventional counterparts except for a redesign of the caliper assemblies.

The induction design measures a signal of 1 mmho from a 24 inch borehole with mud resistivity of 1Ω · m. The depth of investigation of 65 inches is comparable to the ILd but the new design has a reduced shoulder effect. A focused resistivity log specifically designed for large boreholes is also run in combination. For Rm = 0.1Ω · m, Rt ≤ 100Ω · m and borehole diameters between 8 and 20 inches, the error due to borehole effect is less than ± 17%. The nominal bed definition of the focused resistivity log is 2 1/2 feet.

The proposed suite of tools were successfully tested under controlled field conditions. Log examples comparing the conventional and Big Hole tools indicate the improved performance characteristics in big holes.

Keywords:
log analysis, resistivity, big hole tool, borehole, diameter, big hole, correction chart, induction tool, upstream oil & gas, data quality
Subjects:
Formation Evaluation & Management, Open hole/cased hole log analysis
Copyright 1986, Society of Petroleum Engineers
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