Method for stepwise construction of preferential gas migration pathway at stope in coal seam

Abstract

A method for stepwise construction of a preferential gas migration pathway at a stope in a coal seam. First, a gas migration pathway is preliminarily formed at a stope depending on a mining effect of mining in a first mined seam. construction and stabilization method of gob-side entry retaining in deep strata, and a method of manual-guided pre-fracturing boreholes are then used to perform active construction respectively in external space and the outside of coal-rock mass to form preferential gas migration pathways. Eventually, under the effect of mining-induced stress, a system of preferential gas migration pathways connected to each other at the stope is further formed.

Claims

1. A method for stepwise construction of a system of gas migration pathways at a stope in a coal seam, comprising: a. performing a conventional mining of a first mined seam, wherein a working face, an auxiliary intake airway, and a primary intake airway form a gas migration pathway outside a coal-rock mass at the stope, at the same time, due to a mining-induced stress and a mining-induced pressure relief effect, mining-induced fractures in the coal seam develop, in-seam mining-induced fractures are formed in the first mined seam, and roof vertical fractures and floor penetrating fractures are respectively formed in a roof stratum and a floor stratum; b. after the working face advances as mining takes place, building an entry-retaining wall rapidly in the first mined seam, thereby rapidly forming a retained-entry preferential gas migration pathway, which is part of the system of the gas migration pathways, behind the working face, that is, forming an efficient guide pathway for gas in a space external to the coal-rock mass at the stope, so that a flowing direction of wind is optimized, gas in the space external to the coal-rock mass flows with an air flow along the guide pathway, the gas in the space external to the coal-rock mass is effectively guided and discharged, and accumulation of gas in local areas in the space external to the coal-rock mass is avoided; c. determining a range of a critical reinforced supporting and stabilizing area of the retained-entry preferential gas migration pathway and the auxiliary intake airway according to the mining-induced stress, and performing sectional reinforced supporting and stabilization on the auxiliary intake airway and the retained-entry preferential gas migration pathway in a mining-induced stress influence area; d. during mining of the first mined seam, for a change condition of a roof, when a hard roof condition occurs, constructing manual-guided pre-fracturing boreholes into a hard roof in advance of the working face in the auxiliary intake airway and the primary intake airway, wherein generated manual-guided fractures induce a formation of a roof-vertical-fracture preferential gas migration pathway in a coal-rock stratum at the stope as the mining-induced stress changes, and promote a formation of an overlying stratum rock fracture area, and an in-seam mining-induced fracture area and a goaf loose rock fracture area are connected to the overlying stratum rock fracture area through the roof-vertical-fracture preferential gas migration pathway, so as to avoid accumulation of gas in a goaf and promote flowing and concentration of gas of the stope; e. after the roof-vertical-fracture preferential gas migration pathway internal to the coal-rock mass and the retained-entry preferential gas migration pathway external to the coal-rock mass have been gradually constructed at the stope, continuing with the advance of the working face, wherein a large amount of gas in the in-seam mining-induced fracture area of the first mined seam is desorbed, diffused, and flows into the working face, the auxiliary intake airway, and the primary intake airway and further flows into the retained-entry preferential gas migration pathway and the goaf along the guide pathway, and a part of gas in the space external to the coal-rock mass in the working face, the auxiliary intake airway, and the primary intake airway and the in-seam mining-induced fracture area migrates upward along the roof-vertical-fracture preferential gas migration pathway and concentrates in the overlying stratum rock fracture area; as the working face advances, due to mining in the first mined seam, the floor penetrating fractures gradually develop into a floor-penetrating-fracture preferential gas migration pathway under the mining-induced pressure relief effect, pressure relief gas in an underlying coal seam migrates upward along the floor-penetrating-fracture preferential gas migration pathway and flows into the working face, the auxiliary intake airway, the primary intake airway, the retained-entry preferential gas migration pathway, and the goaf in the first mined seam, gas concentrates in the goaf loose rock fracture area, and at the same time gas in the goaf migrates upward along the roof-vertical-fracture preferential gas migration pathway and concentrates in the overlying stratum rock fracture area; and f. with further mining in the first mined seam, repeating steps a to e to enable gas to flow in an efficient and orderly manner along the constructed retained-entry preferential gas migration pathway in the space external to the coal-rock mass, where at the same time gas flows and concentrates along the constructed roof-vertical-fracture preferential gas migration pathway and the floor-penetrating-fracture preferential gas migration pathway in the coal-rock stratum, under the effect of mining-induced stress, the system of the gas migration pathways connected to each other at the stope is eventually formed, and gas concentration areas in the in-seam mining-induced fracture area, the goaf loose rock fracture area, the overlying stratum rock fracture area, and an underlying coal-rock stratum rock and coal-seam fracture area are gradually formed, so as to create desirable conditions for centralized diversion and extraction of gas.

2. The method for stepwise construction of a system of gas migration pathways at a stope in a coal seam according to claim 1, wherein the critical reinforced supporting and stabilizing area is in a range from a distance a in advance of the working face to a distance b behind from the working face, and both the distance a and the distance b are no less than 200 m.

3. The method for stepwise construction of a system of gas migration pathways at a stope in a coal seam according to claim 1, wherein the entry-retaining wall is built of a high-performance filling material to adapt to a high geostress environment characteristic in the deep first mined seam and achieve a better goaf isolation, thereby implementing stable and efficient guidance of gas by the retained-entry preferential gas migration pathway.

4. The method for stepwise construction of a system of gas migration pathways at a stope in a coal seam according to claim 1, wherein the performing sectional reinforced supporting and stabilization on the auxiliary intake airway and the retained-entry preferential gas migration pathway in a mining-induced stress influence area comprising: combining deep-anchor supporting, a single prop, and U-shaped steel+borehole jet grouting to perform reinforced supporting to ensure that no large deformation occurs in the auxiliary intake airway and the retained-entry preferential gas migration pathway, and determining the density and strength of supporting according to the mining-induced stress to keep the stability of the auxiliary intake airway and the retained-entry preferential gas migration pathway, thereby further implementing stable and efficient guidance of gas in the space external to the coal-rock mass by the retained-entry preferential gas migration pathway.

5. The method for stepwise construction of a system of gas migration pathways at a stope in a coal seam according to claim 1, further comprising optimizing and determining construction angles, orientations, a quantity, and a group interval of the manual-guided pre-fracturing boreholes according to a hardness and a thickness of the hard roof.

6. The method for stepwise construction of a system of gas migration pathways at a stope in a coal seam according to claim 5, wherein the manual-guided pre-fracturing boreholes are constructed by forming the manual-guided fractures inside the hard roof in advance by means of an artificial pre-fracturing technique comprising blasting or hydrofracturing.

7. The method for stepwise construction of a system of gas migration pathways at a stope in a coal seam according to claim 1, wherein the manual-guided pre-fracturing boreholes are constructed by forming the manual-guided fractures inside the hard roof in advance by means of an artificial pre-fracturing technique comprising blasting or hydro fracturing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of overall construction of a preferential gas migration pathway at a stope in a coal seam according to the present invention.

(2) FIG. 2 is a schematic view of construction of a retained-entry preferential gas migration pathway according to the present invention.

(3) FIG. 3 is a schematic view of construction of a preferential gas flow pathway in a roof stratum by using manual-guided pre-fracturing boreholes according to the present invention.

(4) In the drawings: 1first mined seam, 2roof (hard roof), 3roof stratum, 4floor stratum, 5mining-induced fracture, 6roof vertical fracture, 7floor penetrating fracture, 8in-seam mining-induced fracture, 9working face, 10roof-vertical-fracture preferential gas migration pathway, 11floor-penetrating-fracture preferential gas migration pathway, 12overlying stratum rock fracture area, 13underlying coal-rock stratum rock and coal-seam fracture area, 14in-seam mining-induced fracture area, 15goaf loose rock fracture area, 16underlying coal seam, 17auxiliary intake airway, 18primary intake airway, 19retained-entry preferential gas migration pathway, 20entry-retaining wall, 21critical reinforced supporting and stabilizing area, 22air flow, 23manual-guided fracture, 24manual-guided pre-fracturing borehole, and 25goaf.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention is further described below with reference to the embodiments in the accompanying drawings.

(6) Specific steps of a method for stepwise construction of a preferential gas migration pathway at a stope in a coal seam of the present invention are as follows.

(7) a. As shown in FIG. 1 and FIG. 2, performing conventional mining of a first mined seam 1, where a working face 9, an auxiliary intake airway 17, and a primary intake airway 18 form a gas migration pathway outside coal-rock mass at a stope, due to a mining-induced stress and a mining-induced pressure relief effect, mining-induced fractures 5 in a coal seam develop, in-seam mining-induced fractures 8 are formed in the first mined seam 1, and roof vertical fractures 6 and floor penetrating fractures 7 are respectively formed in a roof stratum 3 and a floor stratum 4.

(8) b. After the working face 9 advances as mining takes place, building an entry-retaining wall 20 rapidly in the first mined seam 1, forming a Y-type ventilation system, and rapidly forming a retained-entry preferential gas migration pathway 19 behind the working face 9, that is, forming an efficient guide pathway for gas in a space external to the coal-rock mass at the stope, so that a flowing direction of wind is optimized, as shown in FIG. 2, gas in the space external to the coal-rock mass flows with an air flow 22 along the guide pathway, the gas in the space external to the coal-rock mass is effectively guided and discharged, and accumulation of gas in local areas in the space external to the coal-rock mass is avoided; the entry-retaining wall 20 is made of a high-performance filling material to adapt to a high geostress environment characteristic in the deep first mined seam 1 and achieve a better goaf isolation, thereby implementing stable and efficient guidance of gas by the retained-entry preferential gas migration pathway 19; the high-performance filling material has characteristics of high early strength, high bonding property, and high strength, and is formed of cement, pebbles, fly ash, and a specific additive in particular proportions; the mixing amount of the additive is 0.5% to 1.2% of the weight of cement, and the material has high early strength; the eventual consolidation strength may reach 30 MPa, thereby achieving relatively high adaptability to a high geostress environment characteristic in the first mined seam 1; and the particle sizes of the pebbles need to be less than 20 mm to improve the material granularity, thereby ensuring relatively high tightness.

(9) c. Determining a range of a critical reinforced supporting and stabilizing area 21 of the retained-entry preferential gas migration pathway 19 according to variation and distribution characteristics of mining-induced stress, and performing sectional reinforced supporting and stabilization on the auxiliary intake airway 17 and the retained-entry preferential gas migration pathway 19 in a mining-induced stress influence area, where the range of the critical reinforced supporting and stabilizing area 21 of the retained-entry preferential gas migration pathway 19 is determined according to variation and distribution characteristics of mining-induced stress, the range of the critical reinforced supporting and stabilizing area 21 is determined according to a stress distribution characteristic of the retained-entry preferential gas migration pathway, generally the distance in advance of the working face 9 is a, and the distance in rear of the working face 9 is b; the critical reinforced supporting and stabilizing area 21 is in a range from a distance a in advance of the working face 9 to a distance b behind from the working face 9; both the distance a and the distance b are no less than 200 m; and a manner of the performing sectional reinforced supporting and stabilization on the auxiliary intake airway 17 and the retained-entry preferential gas migration pathway 19 in a mining-induced stress influence area is: combining deep-anchor supporting, a single prop, and U-shaped steel+borehole jet grouting to perform reinforced supporting to ensure that no large deformation occurs in the auxiliary intake airway 17 and the retained-entry preferential gas migration pathway 19, and flexibly increasing and decreasing the density and strength of supporting according to a variation characteristic of mining-induced stress to keep the stability of the auxiliary intake airway 17 and the retained-entry preferential gas migration pathway 19, thereby further implementing stable and efficient guidance of gas in the space external to the coal-rock mass by the retained-entry preferential gas migration pathway 19.

(10) d. During mining of the first mined seam 1, for a change condition of a roof 2, as shown in FIG. 3, when a hard roof condition occurs, constructing manual-guided pre-fracturing boreholes 24 into a hard roof 2 in advance of the working face 9 in the auxiliary intake airway 17 and the primary intake airway 18, where the height of a group of manual-guided pre-fracturing boreholes 24 needs to exceed the thickness of the hard roof, and construction angles, orientations, a quantity, and a group interval of the manual-guided pre-fracturing boreholes 24 should be optimally set according to the hardness and thickness of the hard roof 2; the manual-guided pre-fracturing boreholes 24 are manual-guided fractures 23 formed inside the hard roof 2 in advance by means of an artificial pre-fracturing technique comprising blasting or hydrofracturing; the generated manual-guided fractures 23 induce the formation of a roof-vertical-fracture preferential gas migration pathway 10 in a coal-rock stratum at the stope as mining-induced stress changes and promotes the formation of an overlying stratum rock fracture area 12, and an in-seam mining-induced fracture area 14 and a goaf loose rock fracture area 15 are connected to the overlying stratum rock fracture area 12 through the roof-vertical-fracture preferential gas migration pathway 10, so as to avoid accumulation of gas in a goaf 25 and promote flowing and concentration of gas of the stope; the hard roof condition is determined according to Roof Classification Scheme for Stopping Working Faces in Gently Inclined and Inclined Coal Seams released in China; the basic concepts of a false roof, an immediate roof, and a main roof are clarified in the scheme first; immediate roofs are classified into four types according to stability; main roofs are classified into four types according to weighting strength; eventually, two categories are combined respectively, and stope roofs are classified into 11 types; and hard roofs are III1, III2, III3, III4, IV4.

(11) e. After the roof-vertical-fracture preferential gas migration pathway internal to the coal-rock mass and the retained-entry preferential gas migration pathway external to the coal-rock mass have been gradually constructed at the stope, as shown in FIG. 3, continuing with the advance of the working face 9, where a large amount of gas in the in-seam mining-induced fracture area 14 of the first mined seam 1 is desorbed, diffused, and flows into the working face 9, the auxiliary intake airway 17, and the primary intake airway 18 and further flows into the retained-entry preferential gas migration pathway 19 and the goaf 25 along the guide pathway, and a part of gas in the space external to the coal-rock mass at the working face 9, the auxiliary intake airway 17, and the primary intake airway 18 and the in-seam mining-induced fracture area 14 migrates upward along the roof-vertical-fracture preferential gas migration pathway 10 and concentrates in the overlying stratum rock fracture area 12.

(12) As the working face 9 advances, due to mining in the first mined seam 1, the floor penetrating fractures 7 gradually develop into a floor-penetrating-fracture preferential gas migration pathway 11 under a mining-induced pressure relief effect, pressure relief gas in an underlying coal seam 16 migrates upward along the floor-penetrating-fracture preferential gas migration pathway 11 and flows into the working face 9, the auxiliary intake airway 17, the primary intake airway 18, the retained-entry preferential gas migration pathway 19, and the goaf 25 in the first mined seam, gas concentrates in the goaf loose rock fracture area 15, and at the same time gas in the goaf 25 migrates upward along the roof-vertical-fracture preferential gas migration pathway 10, and concentrates in the overlying stratum rock fracture area 12.

(13) f. With further mining in the first mined seam 1, continuously repeating steps a to e to enable gas to flow along the constructed retained-entry preferential gas migration pathway 19 in the space external to the coal-rock mass, so that efficient guide is implemented, where at the same time gas flows and concentrates along the constructed roof-vertical-fracture preferential gas migration pathway 10 and floor-penetrating-fracture preferential gas migration pathway 11 in the coal-rock stratum, under the effect of mining-induced stress, a system of preferential gas migration pathways connected to each other at the stope is eventually formed, and gas concentration areas in the in-seam mining-induced fracture area 14, the goaf loose rock fracture area 15, the overlying stratum rock fracture area 12, and an underlying coal-rock stratum rock and coal-seam fracture area 13 are gradually formed, so as to create desirable conditions for centralized diversion and extraction of gas; and scientific and effective stepwise construction of preferential migration pathways for gas at a stope is implemented, and preferential migration and efficient concentration of gas of the stope are promoted.