This work was carried out to obtain a more realistic method for prediction of bottom-hole pressures and air or gas volumes required in air and gas drilling. The approach presented allows a prediction of the volume of air or gas required to lift particles from the point in the annulus of critical lift: the region just above the drill collars. Besides particle-lift considerations, this prediction method considers particle-wall and gas-wall frictional effects, real gas behavior and other pertinent variables such as changes in the temperature due to depth, effect of co-current vertical flow and penetration rate. Calculations are presented for a number of examples which show the volume of air required as a function of drilling rate and depth. Bottom-hole pressures at the design point are also determined. Calculated results are shown to agree more closely with field observations than do volumes predicted by the Martin, Angel, or McCray and Cole methods. The proposed method may eliminate the need for experience factors in determining appropriate air or gas volumes.
Development of the technique in which air or gas is used as the circulating fluid represents one of the most significant breakthroughs in cost reduction that the drilling industry has experienced in the last 15 years. Credit for this achievement is due the many drilling people whose initiative and accumulated experiences have refined the method and determined situations where the technique is most applicable. From this accumulated knowledge, much is now known of geographical areas and depths where the method has the advantage over more conventional circulating fluids including the use of air or gas as a completion fluid to overcome formation damage, in drilling water wells and in drilling big holes for mine shafts and underground nuclear testing. The method as now practiced is limited in more universal applications by only its inability to cope with high formation pressures, unstable hole conditions, or large rates of water influx.
Present standards for the selection of air volumes are inadequate and require an experience favor to determine requirements under any given condition. The presence of water makes the present criteria even less effective. A need is seen for a prediction method which eliminates the need for such factors.
From drilling of limestone rocks in quarry operations, it was found that air volumes sufficient to give an annular velocity of 3,000 ft/min. evaluated using standard density air provided both adequate cleaning ahead of the bit as well as efficient removal of the drilled solids. However, in deeper drilling or with different rock formations, a general uncertainty exists as to the air or gas volume required for effective cuttings removal. It is in such instances that an experience factor is used in determining air or gas volume requirements.
The first method proposed for predicting volume requirements in air and gas drilling was presented by Martin of Hughes Tool Co. His predictions were based on application of the Weymouth formula for horizontal gas flow and did not attempt to include such effects as the drilled solids or variations in temperature with depth. As has been generally observed in practice, Martin's method gives a much smaller volume than is actually required. Angel's method of predicting volumes required included the effect of drilled solids by considering drilling rate as a parameter and includes depth corrections for pressure and temperature. His approach assumes that the mixture of gas and solid particles behaves hydrostatically as a perfect gas.
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