An experiment extracting CBM with surface borehole from mining subsidence area of a 6.25 meters thick coal-bed workface was done in Jincheng city. The test borehole was designed three stages based on the geologic structure of test field. During underground workface passed through borehole, the operational status of well was recorded and the deformation of casing was detected four times. Test has shown that the highest pure flow of surface borehole can reach 8m3/min which proves that the methane extraction technology with surface borehole from mining infection zone has a bright future for application in Jincheng city. But, how to maintain the stability of casing avoiding damage is still a problem, which should be resolved before the drawing technology is used in Jincheng extensively. Besides, the ordinary protecting skills for borehole used in oil exploitation cannot take a satisfactory effect to protect tube against damaging due to overburden rock movement during mining. The casing will be damaged at several positions, especially near the junction of soft and hard strata. It may be right measure to expend the diameter of tube or install special defensive joints at the places where will deform badly under the mining influence.
The paper is based on surface gas extraction technology from boreholes in the goaf in Zhangbei Coal Mine ,Huainan Mining Group .With the method of numerical simulation ,the authors have made analysis on the space-time cracking and moving law about the overburden of mining field during exploiting by contrasting extraction data about boreholes in Huainan Mining Group .Besides ,paper provides conclusion and advices on promoting the stability of borehole in the end .To some extent ,the paper is of instructive significance for the research ,promotion and application of gas extraction technology from the goaf .
Using elastic mechanics and fracture mechanics, analyzing the coal seam hydraulic fracturing breakdown pressure, given its theoretical formula. According to hydraulic fracturing stress status, given the form of two typical hydraulic fracture morphology. Analyzing hydraulic fracturing highly elliptical shape. The displacement field in plane stress state is given, and the theoretical formula of fracturing radius of hydraulic fracturing is deduced. The fracturing technology of underground fracturing is presented, and the fracturing location and fracturing parameters are determined. In Sihe Coal Mine conducted fracturing test, the test results showed that: the average of drainage volume of fracturing hole improved 4.4 times compared with non-pressed-hole. The extraction compliance time is reduced by 38%. Roadway tunneling speed was improved by 15%. It can solve the problem of gas overrun in roadway excavation well, and has a good application and popularization value.
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