Positive Displacement Motors

In the search for a viable alternative to conventional rotary drilling in which the entire drill string is rotated from surface, various types of downhole motors have been proposed. Drill string rotation can be eliminated by having a motor placed downhole to drive the bit by hydraulic or electrical power. The turbodrill, first patented in 1873, provided a means of driving the bit by harnessing the power of the drilling fluid, but the complexity of the tool and the lack of suitable drilling bits prevented it from being widely used except in the USSR. The electrodrill, dating from 1891, was another tool pioneered largely by the Russians. An electric cable was run down the drill string to power an electric motor mounted above the bit. The motor drove the bit through a gearing mechanism. Although the possibilities of using electrodrills are still being investigated, no commercial tool is presently in use.