Study On The Strata Movement Mechanisms And Control In UCG Without Shaft

Underground coal gasification(UCG)can not only solve the environmental problems in conventional mining and really realized the green mining,but also can provide valuable clean oil and gas energy and chemical materials for China's ecomomic development.Meanwhile,this technology could be used to recovery the thin coal seam and the deep coal seam and the coal resources buried under buildings,railways and water-bodies which are very difficult to mining or mining with poor economy.Therefore,environmental protection of our country needs the development of underground coal gasification and this technology is also one of the important technical directions for the future development of coal industry.The development of this thchnology also meets the need of the fluidization development in the coal industry and national low-carbon development.Based on the research methods combining the numerical simulation and theoretical analysis and mechanical experiment and field measurement and similar material simulation,the high-temperature effect of the combustion space area surrounding rock of UCG without shaft,the movement and deformation law of the combustion space area surrounding rock and the surface,the width design methods of the gasifier and isolated coal pillar and the prediction method of the surface subsidence of UCG without shaft were researched systematically,the main results achieved are as follows:(1)The influence scope of the combustion space area temperature field of UCG without shaft on the mechanical properties of the overlying strata,the both sided coal wall and the coal floor was limited.According to the field data analysis of UCG without shaft of Ulan Qab industrial experiment,the influence range of temperature field on the overlying strata of combustion space area is not more than 16.7m,the influence range of temperature field on the both sided coal wall of combustion space area is less than 8.7m,the influence range of temperature field on the floor of combustion space area does not exceed 13.6m.(2)The dynamic and static numerical simulation and similar material simulation methods of UCG without shaft are improved.Based on the research of dynamic sensitivity of the surface movement extreme value to the different geomechanical parameters,a new dynamic geomechanical parameters selection method of numerical model based on weighted inversion method is proposed.In order to ensure the similarity between the strength change characteristics of the combustion space area surrounding rock in similar material model and the actual situation,the time-varying characteristics of the similar material strength were researched and its control measures were proposed.(3)Size effect and high temperature effect of underground combustion space area and different gasification coal types all can influence the overlying strata deformation of combustion space area,vertical stress distribution,plastic zone distribution and surface subsidence.When the width of underground combustion space area is 8m,16 m,24m,and the width of the isolated coal pillar is 8m,16 m,24m and 32 m,the overlying strata wave subsidence curve heights of combustion space area of strip underground gasification are between 13m~43.1m.From the perspective of recovery rate and surface subsidence control,the isolated coal pillar width should be 16 m in the underground gasification project of Ulan Qab and then the combustion space area width of strip underground gasification should be controlled in 16m~24m.(4)The combustion space area form of “strip-regional mining” CRIP is different from the goaf form of strip mining,which lead to the significant difference between the hyperbolic coal pillar and rectangular coal pillar of strip mining.The arch height of hyperbolic coal pillar,the width of coal pillar,the height of coal pillar and the high-temperature effect all could influence the stability of hyperbolic coal pillar.Based on the elastic foundation beam theory and unified strength theory,the limit caving span calculation formula of combustion space area roof under the function together with the high temperature and coal pillar supporting was deduced and the calculation formula of the plastic zone width and elastic zone width of isolated coal pillar in UCG without shaft was also deduced.Based on this,the design method of underground gasifier and isolation pillar width of UCG without shaft under the coupling effect of high temperature and ground stress was proposed,which can provide theoretical and scientific basis for the design and safe operation of UCG without shaft.The stability evaluation method of hyperbolic coal pillar was established through theoretical method.The effectiveness and feasibility of this method was verified by the industrial experiment of UCG without shaft.(5)As the decrease of moisture content of similar meterials,its tensile strength and compressive strength increase exponentially and the relationship curve between the strength and moisture content is different for similar material blocks with different lithology.That is,the similar material strength has time-varying characteristics.The time-varying characteristics of the similar material strength have significant influence on development height of fracture zone and subsidence value of overlying strata.In order to ensure the similarity between the strength change characteristics of the combustion space area surrounding rock in similar material model and the actual situation,the time-varying characteristics of similar materials should be considered and controlled when using the similar material simulation method to research the movement characteristic of combustion space area surrounding rock and the surface.The periodic sampling method can be used to monitor the moisture content of similar materials,and then the strength of similar materials could be monitored which achieves the goal of controlling model strength.There was no collspse of the roof and no instability of the isolated coal pillar in the industrial experiment of UCG without shaft of Ulan Qab and the overlying strata keeps the whole and continuous deformation.Meanwhile,there is no obvious caving zone and fracture zone.The movement pattern of overlying strata of UCG without shaft and underground mining was similar.(6)The prediction methods of surface subsidence of strip underground gasification without shaft were established.The subsidence of the roof-pillar-floor support system was calculated by the continuous media theoty.At the same time,the subsidence of this support system can be taken as the mining space and the random medium theory is applied to calculate the surface subsidence of strip underground gasification based on the triangle subdivision method.Then the surface subsidence prediction method of strip underground gasification was established with the continuous-random theory.The prediction results were close to the measured results when this method is used to industrial experiment of UCG without shaft.The surface subsidence prediction results of this method could be more accurate with the increase of gasification scale.In order to predict surface subsidence of strip underground gasification more conveniently,accurately and rapidly,the surface subsidence prediction method of strip undegerground gaisification is proposed based on the “real mining thickness” according to the characteristic of UCG without shaft.The “real mining thickness” is the coal seam thickness minus the ash thickness.When the isolated coal pillar is stable and the recovery ratio is between 35% and 70%,the surface subsidence prediction parameters of strip mining can be considered as the parameters of the strip underground gasification.The influence of different coal types on the subsidence coefficient and horizontal movement coefficient isn't significant,but they have significant influence on the tangent of the main influence angle.As the gasification coal change from non caking coal to stroung caking coal,the main influence angle tangent increased 0.1~0.6.