1、 英文原文 ROTARY DRILLING HOLES IN COALBEDS FOR DEGASIFICATION By Joseph Cervik, H, HI Fields12 and G, N, AulABSTRACT 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. However,maintaining the bit on a horizontal trajecto
2、ry 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 combinations of bit thrust and rotational speed (r
3、/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 occur, and the strategy used during drilling. Thr
4、ust 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 holes drilled into coalbeds are directly proportion
5、al 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 as in shafts located 1 mile or more from an undergr
6、ound 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 coalbe
7、d is difficult. The natural tendency of the bit during horizontal drilling is to arc downward because of gravity. Changes in hardness of the coalbed or hard inclusions such as pyrite balls may deflect the bit in an unpredictable manner. Starting in 1957, the B rit is hconducted experimental drilling
8、 studies to develop tools and equipment for drilling horizontal holes in coal to depths of 450 f eet. Procedures were developed for inducing the bit to rise or fall; no infzrmation was published on bitthrust and rotational speed. A state - of - the - art study on horizontal drilling in 1970 conclude
9、d there are no reliable published data on horsepower, thrust, rotational speed, bit configuration, and the effects of these factors on hole trajectory. In the Pitsburgh coalbed, horizontal degasification holes have been drilled ranging in length from 500 to 850 feet . Hole trajectory was controlled
10、by close - fitting stabilizers (packed hole) that ranged from 2 to10 feetlong. Torque, drilling speed, thrust, and rotational speed were not measured. At a demonstration in a strip pit, a horizontal hole was drilled to a depth of 1,034 feet. Drilling parameTers and a drill - string configuration wer
11、e developed experimentally to control hole trajectory. Holes in excess of 1,000 feet expose alarge are a of the Pittsburgh coalbed to degasification and, therefore, fewer holes are required along the mines periphery to reduce significantly methane flow in to the mine. addition, four orfive horizonta
12、l holes (1,000 feet long) produce about 1 million ft3 / d of methane, which can be piped to the surface through an underground system of pipelines. Reduction in methane flow in to mine openings improves safety, results in a savings in ventilation costs, and improves the potential for increased coal
13、production. The objectives of this Bureau of Mines report are to describe the demonstrated methods of maintaining a bit on a predetermined trajectory and to provide data on drilling parameters and equipment. This report provides guidelines and reviews concepts for (those who are contemplating) a hor
14、i-zontal drilling program. Drilling procedures and parameters have been developed for the Pittsburgh coalbed. Although procedures and parameters may requires light modifications in other coalbeds, the principles of horizontal drilling are unchanged.ACKNOWLEDGMENTS The cooperation of Eastern Associat
15、ed Coal Corp, Pittsburgh, Pa, is greatly appreciated. The authors thank William Laird, vice president - - research and development, for his cooperation in conducting some of the work described in this report and for his continued interest and support. We acknowledge the work of Fenix and Scisson, In
16、c . Tulsa, Okla., on horizontal drilling, which was sponsored by the Bureau of Mines (contract No. H0111355). underlined numbers i n parentheses refer to items in the list of references at the end of this report.DRILLING HARDWARE Coal is a soft and brittle material in comparison to a sandstone and, therefore, presents no special problems in drilling. However, hard inclusions such as pyrite concr
