Abstract
In the 1st report, an experimental reappearance of the crooked-hole phenomenon of a deep boring was carried out, and the mechanism of the crook and the influence of the circulating rate of water to the crook were described.In this paper, the influences of the clearance angle & the mean width of wearlands of tips of the bit and the thrust on the bit affected to the crook of the borehole by using the same equipment mentioned at the 1st report were investigated, by means of the drilling of an artificial rock sample which has the inclined angle 30° of the boundary of layers of different strengths under the condition of the circulating rate of water 2e/min. For this investigation the ratio of the drilling rates on both rocks and the torque increase through inclined boundary were considered.Consequently, the followings were obtained within this experiment.(1) The smaller the clearance angle of tips of the bit was, the larger the drift angle mentioned as the inclination in Fig. 7 in the 1st report of the borehole from vertical was.(2) The tendency of the increase of the drift angle of crook according as the wearlands of tips becomes larger was recognized. But this drift angle was decreased after reaching the maximum value with the excess wearlands.(3) The appearance of the maximum drift angle was occured at the smaller wearlands, in the case of the light thrust compared with the heavy one.(4) The relation of wearlands to the drift angle was explained by the ratio of drilling rates on both rocks.(5) The existence of the relation of the rate of the torque increase at the inclined boundary to the drift angle of crook was found.(6) When the bit drilled through the inclined boundary, the appearance of the horizontal shift of borehole to the direction of down-dip side along the soft rock previous to the crook to that of up-dip side into the hard rock was recognized.(7) Besides the appearance of the drift angle, the considerable clock-wise deflection of the orientation shown as the twisted angle was found.
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