Well-slot method for degassing coal beds in which the rock pressure has not been discharged

Abstract

A method developed at the Mining Institute (Siberian Branch of the Russian Academy of Sciences) for directional fluid fracturing (DFF) of rock masses can be used to create an extended slot in the rock mass in a single operation. This is done by introducing a specially designed mechanism into the well at the required depth; this mechanism cuts an initiating slot of the required size and shape into the rock at the wall of the well. Through a sealing device, a fluid is fed under pressure to the initiating slot; the fluid may be any substance that will flow, including water. When a critical pressure of the fluid is reached, the frontal part of the initiating slot is moved from place; and, as it grows in the assigned direction, it is converted to an extended, large-area slot. Information on the essence of the DFF method and results of tests in coal and hardrock mines have been presented in scientific reports and a number of articles published in the period from 1982 to 1996, mainly in the present journal (Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh). A slot that is being created in a rock mass can be given various properties. In the particular case with which we are concerned, it must have the property of a highly porous collector that is capable, in the ftrst place, of draining the surrounding mass and, in the second place, of transporting the gas through the slot/collector to the well and then to the gas line. If the creation of an extended slot in various rock masses is performed repeatedly, then the subsequent work must resolve the problem of efficient filling of the slot with solid granular material. Apart from the concepts themselves, in our case we can use practical experience in hydraulic fracturing operations in coal beds and oil-bearing formations. Here it is appropriate to note the main difference between the method of conventional hydraulic fracturing [1] and the DFF method [2]. In the first case, the liquid is propagated only along natural exogenic and endogenic cracks, which are for the most part large and are randomly oriented in space; therefore, we can hardly consider that area degassing of a coal bed through wells at a distance from each other will be accomplished uniformly. This is illustrated graphically by circle charts that have been plotted for the natural fracturing with the imposition of cracks formed by hydraulic fracturing, and also by sketches made repeatedly of the actual zones of hydraulic fracturing of beds after they have been exposed by underground mining operations [1]. In the second case, when it is proposed to use the method of directional fluid fracturing [2], there is created in a single operation just one extended slot that is located in space in such a place and such an orientation that the process of degassing a gasbearing coal bed will be accomplished uniformly. This requirement is satisfied to the greatest degree by the location of a large-area slot within the seam and along its bedding. A qualitative evaluation of the degassing capacity of a slot can be obtained from an analysis of the simplest Darcy formula