The auther has stunied on the penetration rates at Tsuchizaki-oki Oil Field and Kumoide Gas Field, especially on the effect of the bit weight and the rotary speed. Resnlts obtained are as follows: 1. At shallower formations the penetration rate per revolution is affected by the bit weight linearly (Figs. 2 and 7). 2. At medium depth the penetration rate per revolution is affected both by the bit weight and the rotary speed. Higher penetration rate per ravolution is obtained at lower rotary speed. At higher rotary speed than a certain limit, the penatration rate dots not incrase with the rncrease of ratary speed (Fig. 8). 3. At deeper formetions the penetration rate is affected only by the bit weight and not by the rotary speed (Figs. 3, 5 and 9).
New drilling mechanism to achieve optimum penetration rates for hard formation has been reduired in the drilling engineering. To get a solution for this, a new-type bit has been studied at the laboratory of Japanese Petroleum Exploitation Company. The bit, based on the new drilling mechanism, consists of cone-cutters and diamond cutting elements. The bit shows higher penetration rate than a conventional rock bit. The result of experiments on this bit is described in this paper.