By the use of drilling records taken in oil fields in Japan, the factors affecting penetration rate of a rotary drilling were analysed. Some of the results obtained are as follows: (1) In some fields there is a relationship between load on a bit and penetration rate both for vertical wells and directional wells drilled with a vertical deviation angle of less than 40 degree. (2) In other fields there is no special relationship between bit load and penetration rate. In these fields hydraulic

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... rs must be considered to analyse the penetration rate. (3) When soft mudstone (its compressive strength: 135kg/cm2) at a depth of 3700 meter was drilled with heavy bit load, it gave a very low penetration rate because the pressure caused by a long column of mud fluid was very high.