Stress characteristics of coal seam underlying the goaf in the Sihe coalfield and its effect on the development of coalbed methane

Extracting the coalbed methane underlying the goaf by surface boreholes through the goaf can reduce the risk level of coal mine gas disaster and realize the co-mining of coal and coalbed methane,which has a good application prospect. Stress distribution in coal seams underlying a goaf is one of the key considerations that affect the production of coalbed methane well through the goaf,but at present,relevant research reports are rare. Taking the eastern Si-he coalfield in the Jincheng mining area Shanxi Province as an example,based on the in-situ stress measurement results and hydraulic fracturing data,the characteristics of stress and hydraulic fracturing parameters of the coal seams underlying the No.3 coal seam in the study area were analyzed and the influence of the coal mining on the gas production of the underlying coal seams was discussed. The results showed that,the stress of No.3 coal seam in the study area fell into a strike-slip faulting stress regime,that is,SHmax>Sv>Shmin,In No.3 coal seam,the stress concentration existed in the syncline axis part where the least principal stress and the fracture propagation pressure were abnormally high,and the difference between the formation breakdown pressure and the fracture propagation pressure was relatively low,the extraction of No.3 coal seam resulted in a drop in overlying load of No. 9 and No. 15 coal seams,and therefore the ratios of horizontal stress to vertical stress of No.9 and No.15 coal seams under No.3 coal seam goaf were higher than those of coal seams that were not affected by mining. For the coalbed methane wells in the unexploited areas,the difference between the formation breakdown pressure and the fracture propagation pressure ranged from -4.9-11.7 MPa,averagely 2.14 MPa,for the coalbed methane wells in the mining-affected areas,the difference between the formation breakdown pressure and the fracture propagation pressure ranged from 0.5-18.0 MPa,averagely 7.1 MPa,and the formation breakdown pressure gradient were obviously higher in the mining-affected coal seams than that in the coal seam that were not affected by mining,and the fracture propagation pressure gradient was lower in the mining-affected coal seam than in the intact coal seam,the coal seam under the goaf and in the anticline axis part had a high pressure difference between the fracture propagation pressure and the formation breakdown pressure and a relatively high permeability,due to the effect of stress concentration,the coal seam under the coal pillar of the protective layer had a low permeability and a low difference between the fracture propagation pressure and the formation breakdown pressure while a high gas production of up to 4 224 m3/d can be achieved from these sections after stimulation by hydraulic fracturing through the surface well.