Outburst prevention mode of ground and underground technology joint and its dynamic evaluation in contiguous coal seams

In view of the lack of systematic and effective outburst prevention mode and dynamic evaluation method of combined surface and underground gas extraction in close distance coal seams,through statistical analysis of outburst information in coal-fields similar to Shaqu No. 1 coal mine,China,the characteristics of large in-situ stress gas poten- tial,scattered outburst sources and low coal strength and permeability are obtained. Using theoretical analysis, the stress-energy transfer and outburst start mechanism of coal seam mining are revealed from the aspects of stress super- position and relieving high-energy gas migration. Then,the outburst prevention ideas of multiple coal seam collabora- tive release potential and “ multiple coal seam-regional-local” step-by-step enhanced extraction are inspired. Surface and underground joint outburst prevention technological system is optimized and integrated by systematic analysis and inductive deduction. Combined surface and underground gas extraction,the conventional wells,outburst prevention and fracturing wells and multiple branch horizontal wells are combined to pump and reduce outburst in the far area. In the development area,the protective layer mining + directional long hole group or multi branch horizontal well hole doc- king extraction+directional long hole group three-dimensional regional extraction and outburst prevention are applied. In the production area,the directional long borehole group strip pre extraction+progressive large aperture borehole ex- traction are adopted to eliminate outburst. In soft and low permeable coal seam,the controllable shock wave,low-pres- sure pulsating hydraulic fracturing and other fatigue damage enhancement methods are needed to strengthen the effect of outburst elimination,and their key technical parameters are determined in field test. Meanwhile,the progressive dy- namic evaluation function of the application process stage of the model is established,the expressions of the four pa- rameters of the two-level indicators in the three stages with time are given,and the corresponding critical values are de- termined. Engineering practice results show that the surface and underground combined gas extraction and outburst elimination mode basically realizes the coordinated outburst elimination of the upper four coal seams in the planning time. The conversion rate of coal in the three stages of mining is 95% ,85% ,90% respectively,and the overall dynam- ic conversion rate is 70. 5% ,which shows that the connection between extraction and mining is relatively tight,mainly because the extraction and outburst prevention efficiency in the development area is low. After using this model,the output of high-quality CBM increased by 0. 8 million tons and 4. 2 million cubic meters.