To drill an oil well at a minimum cost, it will be essential to use the most suitable bit type for the ground and to control both the bit load and the rotary speed at the optimum values respectively every moment. The authors have first discussed on the equations for the rate of drilling and the rate of wear of the bit teeth and bearings. Secondly they have investigated the values of several constants involved in the equations by referring to the drilling bit records taken by the Japan Petroleum Exploration Co., Ltd. It results in that four of the seven constants are found, while for the remaining three a method of determining them from a drill test which is to be carried out at the begining of each drilling operation is proposed. Lastly they have shown how to determine the optimum bit load and the rotary speed every moment, and have proved theoretically the profit which may be obtained by the minimum cost drilling by comparing its total cost with those of conventional drilling under various conditions.
