The discussions were focused on the detailed design criteria and application on specialist operations. The first question was initiated by H. F. SPOERKER (Austria) who wanted to know the experience with oil based drilling fluids and heavyweight, high solid muds. He also asked what were the temperature limitations on the motors. Mr. FORREST replied that there have been no problems experienced so far using oil based drilling fluids; on the contrary, it enhances the bearing life. The heaviest mud weight that has been used was 19.7 Ib/gal. The combination of very high mud weight and high flow rates tends to decrease the life of the rotor. The highest formation temperature in which the motors have been run is 300°F which is very close to an equivalent motor elastomer temperature of 220–240 "F. The life of the elastomer is dependent on the temperature of the fluid that flows through the motor and not the static bottom hole temperature.

The next question was asked by R. C. GARG (India) as to whether there are any established curves for determining torque/RPM characteristics of these motors under various axial loading conditions. The speaker said that test runs were made by loading the bit from 0–60 O00 lb (axial load) using water as the drilling fluid. The torque output of the motor was virtually independent of the axial load, and may be assumed to be so for all practical applications.

M. GEREGA (UK) then asked if there is a positive indication method available at the surface for stall characteristics and if not, how is the actual RPM of the motor determined? The speaker replied in the affirmative that there will be definite indication at the surface of stall. As the motor approaches the stall regime, there will be a rise in the standpipe pressure which will follow a linear relationship until the stall occurs. Beyond this, there will be a further rise in the standpipe pressure up to 40%. Typically, the operating pressure drop is in the order of 500 psi and if stalling occurs, the standpipe pressure will rapidly rise to 200–300 psi beyond the operating pressure.

Figure 4 in the paper gives detailed performance characteristics.

Replying to a question from O. J. HEBERT (USA) on whether the motor has a tendency to cause left-hand walk as does turbines, the speaker stated that on the contrary, the motor tends to cause righthand walk in the same manner as rotary drilling and thus can be used in conjunction with turbodrilling to control azimuth.

J. R. C. HODGE (UK) then asked if the rotor and stator could be locked in order to back-off, if the bit becomes stuck. The speaker stated that there is no special locking device. However, torque in the range of 7000 ft/lbs can be developed at the bit under stall conditions which with pulling on the pipe woul

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