1、 中文中文 3250 字，字，2200 英文单词，英文单词，1.1 万英文字符万英文字符 英文原文英文原文 ROTARY DRILLING HOLES IN COALBEDS FOR DEGASIFICATION By Joseph Cervik, H, HI Fields12 and G, N, Aul ABSTRACT Coal is a soft and brittle material. Drilling rates in the P ittsbugh coalbed using a drag bit exceed 3 ft/min at 2,500-pound thrust. How
2、ever,maintaining the bit on a horizontal trajectory or parallel to bedding planes to attain lengths of 1,000 feet is difficult. This Bureau of Mines report presents a drill string configuration that can be guided through the coalbed. The angle of the borehole is measured periodically and by proper c
3、ombinations of bit thrust and rotational speed (r/min), bit trajectory can be lifted or dropped to keep the bit in the coalbed. An example of a hole drilled to 2,126 feet is presented to show levels of thrust and rotational speed used to lift or drop the bit, the seemingly unexplainable events that
4、occur, and the strategy used during drilling. Thrust levels during drilling range from 600 to 2,500 pounds. Hydraulic motors powered by a 30-hp, 440-volt motorprovide ample power for drilling horizontal holes 2,000 feet long. INTRODUCTION Experience has shmn that methane flow rates from horizontal h
5、oles drilled into coalbeds are directly proportional to length. For holes drilled from underground locations in a coal mine in the Pittsburgh coalbed (Fairview, W. Va., area), a characteristic flow rate is 25,000 ft3/d per 100 feet of hole. However, in areas of the coalbed remote from mining, such a
6、s in shafts located 1 mile or more from an underground mine, initial flow rates are much greater and approach 65,000 ft3/d per 100 feet of horizontal hole. The lower flow rate is the result of partial degasification of the coalbed along the periphery of the mine. Maintaining a bit on a horizontal trajectory or inclined to follow the dip of a coalbed is difficult. The natural tendency of the bit during horizontal drilling is to arc downward because of gravity. Changes in hardness of the coalbe
