A survey of 25 novel drills1 has shown that spark drills have potential for drilling oil wells and blastholes at higher rates than rotary drills. Available laboratory and field data were extrapolated to drilling rates of 70 to 280 fph for full-scale spark drills. Most of the data used in these extrapolations were from small-scale laboratory tests at atmospheric pressure. Accurate extrapolation of the small-scale spark drill tests to full-scale field drills was complicated by the fact that the effect of fluid pressure in the well bore and the effect of spark size were not known. In order to answer these questions and to more accurately evaluate the potential of spark drills for drilling blastholes and oil wells economically, a high-voltage spark discharge system was assembled. This equipment produced powerful sparks (10,000 joules) equivalent to those that would be used on a 100-kw full-scale field drill. These sparks are 100 times more powerful than those previously used on spark drills, thus eliminating extrapolation of the spark size. In addition, a pressure chamber was modified to enable using these sparks to produce craters in rocks at fluid pressures up to 15,000 psi, thus eliminating extrapolation from atmospheric pressure to the high pressure existing in deep oil wells. These tests have provided considerable information for more accurately evaluating spark drills for oil-field drilling.
When a high-voltage capacitor is discharged across two electrodes, the dissipated energy produces a high-temperature, high-pressure plasma for a fewµsec. When these sparks are discharged in air, they undergo considerable expansion ( Fig. 1) and produce low-pressure pulses (103 [Fig. 1--Air (left) and underwater (right) sparks. (Available in full paper)], 104 psi). Sparks fired in water are confined by the water surrounding the spark, thus forcing the electrical energy to flow through a narrower channel and forming much higher pressures (105-106 psi). Because of their higher pressure, underwater sparks are much more effective for drilling rock than air sparks. For this reason, spark drills have to be used in wells filled with water or mud.
The spark pressure pulses remove rock by forming craters similar in shape to the craters formed under impacting bit teeth (Fig. 2). The spark temperatures are sufficiently high to fuse the rock. but since these sparks last only 1 to 50 µsec, fusion is of little importance.
The drilling rate, R, for any drill is R = P /AE (ipm) where P is power output (ft-lb per rain), A is hole cross-sectional area (sq in.), and E is specific energy required to remove rock (ft-lb per cu in.). Fig. 2--Spark rock-cratering mechanism (Available in full paper)
For most drills, the specific energy required to remove rock remains nearly constant as the power output and the hole diameter are varied, provided threshold values of force or energy are exceeded. Eq. 1 is, therefore, useful for extrapolating small-scale laboratory tests to fullscale field drilling tests. To do this, it is necessary to estimate both the amount of power that can be delivered to the bottom of a well and the power output of a full-scale field drill
