A field performance analysis Of turbine-driven bits assumes that the rate of bit penetration is roughly proportional to the mechanical power available from the turbine. The variations in bit performance can then be explained by variations in turbine power output, which is a function of torque and rotational speed and thus is controlled by weight on bit (WOB). The data show that natural diamond bits give their best performance at high WOB, whereas polycrystalline diamond Compact (PDC) bits perform best at low WOB. The data further suggest that at optimum WOB the rate of penetration could be improved by 200–300%. penetration could be improved by 200-300%. A turbine tachometer could enable drilling to be performed at exactly the rpm at which the turbine develops its maximum power and thus gives the highest rate of penetration. Calculations show that the cost per metre drilled may thus be reduced by as much as 50%.
The drilling rate can further be increased by improving the hydraulic capabilities and ef ficiency of the rig, using:
Larger drillpipe, e.g. 3 1/2" instead of 5", to take advantage of the larger bore.
Drill collars and Hevi-Water drillpipe with larger inside diameter.
A minimum amount of drill collars and Hevi-Water drillpipe for themaximum WOB.
Higher standpipe pressures (max. 500 psi).
The reduction of frictional losses in the hydraulic system of the rig thus obtained, together with the higher standpipe pressure allowed, would enable the operator to run turbines with more stages and to Circulate more mud, resulting in up to 50% higher turbine output. Calculations show that in this way a further reduction of perhaps 15% Can be obtained on the cost per metre drilled.
Turbine-driven natural diamond bits and PDC bits have shown considerable variations in drilling performance when run in very similar formations. This inconsistency has been investigated by Comparing a number of runs carried out in landwells in the north-eastern part Of Holland.