Method for joint-mining of coalbed gas and coal

Abstract

Provided is a method for joint-mining of coalbed gas and coal. The method comprises: a well-drilling step; a fracturing and penetration step; a coalbed gas extraction step: an ignition step, and an underground gasification step. The method combines an underground coal gasification technology and a coalbed gas extraction technology, not only allows for utilization of the high temperature of underground gasification to heat a coalbed. thus increasing the permeability of a coal seam. and increasing the recovery rate of the coalbed gas, but also allows for utilization of the coalbed gas to perform the drilling and the fracturing and penetration processes, thus increasing the efficiency of underground coal gasification for mining.

What is claimed is: 
     
         1 . A method for joint-mining of coalbed gas and coal, comprising:
 1) a well-drilling step: drilling from the ground to a coal seam to provide at least one injection borehole and at least one production borehole for communicating the coal seam with the ground;   2) a fracturing and penetration step: injecting a fracturing medium containing CO 2  into the coal seam from the injection borehole, thereby forcing the fracture medium to flow in the coal seam and to discharge from the production borehole so that the injection borehole is brought to communicate with the production borehole in the coal seam;   3) a coalbed gas extraction step: injecting a displacement medium into the coal seam from the injection borehole, displacing coalbed gas in the coal seam, and collecting the coalbed gas from the production borehole;   4) an ignition step: establishing a fire zone in the coal seam at the bottom of the production borehole for igniting the coal seam; and   5) an underground gasification step: injecting a gasification agent containing CO 2  into the coal seam from the injection borehole so that a gasification reaction and a pyrolytic reaction occur between the gasification agent and the coal seam to generate heat energy and a gasified gas, transferring the heat energy towards the inside of the coal seam to generate a pyrolyzed gas, synchronously displacing the coalbed gas to generate free methane, discharging and collecting the free methane, together with the gasified gas and the pyrolyzed gas, from the production borehole.   
     
     
         2 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the fracturing medium is a mixture of CO 2  and O 2 , and the volume concentration of O 2  is below 20%. 
     
     
         3 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, further comprising:
 6) a CO 2  recycling step: recycling CO 2  generated during the underground gasification step, and using a part of the recycled CO 2  as the gasification agent.   
     
     
         4 . The method for joint-mining of coalbed gas and coal according to  claim 3 , characterized in that, further comprising:
 7) a CO 2  storage step: at the end of the underground gasification of the coal seam between the injection borehole and the production borehole, filling a part of the recycled CO 2  into a combustion cavity for burying and storage.   
     
     
         5 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, prior to the ignition step 4), further comprising a safety detection step for ensuring ignition safety. 
     
     
         6 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, in the coalbed gas extraction step 3), the displacing coalbed gas in the coal seam is injecting CO 2  or N 2  for displacement. 
     
     
         7 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the injection borehole or the production borehole is composed of a directional borehole and a vertical borehole. 
     
     
         8 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the injection borehole and the production borehole are composed of vertical boreholes. 
     
     
         9 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the establishing a fire zone is performed by electric ignition or solid fuel ignition. 
     
     
         10 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the underground gasification comprises forward combustion, reverse combustion or retracting injection point combustion. 
     
     
         11 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the well drilling is drilling a horizontal well in a direction vertical to a primary fracture of the coal seam. 
     
     
         12 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the CO 2  used in the fracturing and penetration step is gaseous, liquid or supercritical CO 2 , or a mixed liquid composed of liquid CO 2 , water collagen and chemical additive(s). 
     
     
         13 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, in the fracturing and penetration step, when the pressure in the injection borehole drops rapidly while the gas outflow of the production borehole is above 100 Nm 3 /h, the injection borehole and the production borehole have been communicated with each other in the coal seam. 
     
     
         14 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, in the underground gasification step, the volume concentration of O 2  in the gasification agent containing CO 2  is 20-70% to ensure that ash will not be melted. 
     
     
         15 . The method for joint-mining of coalbed gas and coal according to  claim 14 , characterized in that, in the underground gasification step, the volume concentration of O 2  in the gasification agent containing CO 2  is 20-50% for establishment of a gasification passage. 
     
     
         16 . The method for joint-mining of coalbed gas and coal according to  claim 15 , characterized in that, in the underground gasification step, the volume concentration of O 2  in the gasification agent containing CO 2  is 40-70% for gasification of the coal seam at the end of the establishment of the gasification passage. 
     
     
         17 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, if the content of water in the coal seam is less than the amount of water required for the gasification of the coal seam at the end of extraction of the coalbed gas, the gasification agent containing CO 2  is a mixture of O 2 , CO 2  and water vapor. 
     
     
         18 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, for the conveying of the gasification agent in the underground gasification step, the gasification agent is conveyed from the ground to the fire zone through an annular conveying pipe or a borehole. 
     
     
         19 . The method for joint-mining of coalbed gas and coal according to  claim 3 , characterized in that, the CO 2  recycled in the CO 2  recycling step comes from fume generated during gas power generation or a decarbonization section during gas purification. 
     
     
         20 . The method for joint-mining of coalbed gas and coal according to  claim 1 , characterized in that, the generated gasified gas, pyrolyzed gas and coalbed gas are discharged from the production borehole for synthesis of methane, methane power generation or production of methane-methanol. 
     
     
         21 . The method for joint-mining of coalbed gas and coal according to  claim 3 , characterized in that, the CO 2  recycled in the CO 2  recycling step is used as the fracturing medium for the fracturing and penetration step or the displacement medium for the coalbed gas extraction step.