Sweet Spots Selection For Coalbed Methane Development And Negative Pressure Extraction Technology From Closed Gob In Jixi Mining Area

The Jixi mining area is taken as the study area in this thesis which adopts the research methods of theoretical analysis,numerical simulation and engineering test.The development characteristics of coalbed methane storage space in closed gob are clarified.The method of sweet spots selection for coalbed methane development in closed gob is proposed.The negative pressure extraction technology of coalbed methane well in closed gob is discussed.The main conclusions are as follows:The overlying rock fractures of the coalbed methane storage space in the closed gob are developed as "trapezoid" and "O"-shaped rings on the vertical and horizontal profile,respectively.The stress distribution of the gob can be divided into stress concentration area,full pressure relief area and stress recovery area from the coal pillar area to the middle of the gob.The roof height of coalbed methane storage space can be estimated by a formula derived from the perspective of hard rock ratio coefficient,and the floor depth and lateral boundary can be estimated by statistical formulas and pressure relief angles,respectively.Seven "one-vote veto" conditions has been formulated as the principle to select the closed gob for coalbed methane extraction from the two aspects of coalbed methane resource potential and engineering development.The estimation model for the geological reserves and recoverable reserves of coalbed methane in closed gob is established from the perspective of resource composition.An evaluation indicator system for the development potential of coalbed methane in closed gob is established,including three primary indicators of resource conditions,preservation conditions,and engineering conditions,as well as 9 secondary indicators.A method for selecting sweet spots for coalbed methane development in closed gob is proposed based on AHPentropy weight model and optimal segmentation method.The results of the sweet spot selection in Jixi mining area have guided the engineering of the coalbed methane wells in the closed gob of this area.The production process of the surface well for coalbed methane extraction in closed gob will undergo near-well extraction stage,extraction radius expansion stage,and methane concentration decay stage in each extraction period.The changes in surface atmospheric pressure and the value of the negative pressure are the main reasons of periodic changes in methane concentration.There is an optimal extraction negative pressure lower than 60 k Pa and higher than 31 k Pa,which can maximize the gas production flow of surface well in the closed gob.When the negative pressure is lower or higher than the negative pressure,the gas production flow will decrease.The stop extracting indicator of the intermittent extraction technology is that the methane concentration drops to the low utilization limit or the produced gas temperature and CO concentration are abnormal,and other signs of spontaneous combustion.The restart extracting indicator is that the methane concentration is stable and significantly higher than before stopping extracting,and the gas temperature returns to normal.This article is about 54,000 words,including 41 illustrations,26 tables,and 128 references.