In order to clarify the jet stream behavior of rock bit, pressures at a bottom plane are measured using the 33/4-in. soft formation type jet bit and regular bit. Water is used as the circulation fluid. About the jet bit, the velocity of impingement at bottom plane and the velocity of crossflow along the bottom surface are the function of impact force, ie. product of flow rate and jet velocity. They are proportional to the square root of the impact force. If nozzle velocity is same at different size nozzle, the jet impingement velocity and the crossflow velocity are greater at larger size nozzle, and if flow rate is same, the velocities are greater at smaller size nozzle. About the regular bit, pressure distribution under a cutter differs at each cutter. The smaller size nozzle makes greater crossflow velocity. The greater size nozzle has greater ability of washing cutters. The bottom cleaning ability of jet bit is proportional to the product of the impact force and is inversely proportional to the square of the hit diameter. The larger size bit requires greater impact force than the smaller one in order to obtain a perfect cleaning action.
Using the synthetic muds with additives, the mud resistivity, mud filtrate resistivity and the mud cake resistivity were measured, and these relationships were investigated from the standpoint of electric log interpretation.