Fracture Evolution And Breakage Of Overlying Strata Of Combustion Space Area In Underground Coal Gasification

Aiming at turning the underground coal into a combustible gas by thermal chemicalreaction in situ, underground coal gasification technology is comprehensive utilizationclean coal technology. It is important to guarantee the stability of underground coalgasification and safety production by understating how to accurately find out the burningzone and breaking law of overburden rock fracture evolution in the process of undergroundcoal gasification. Based on the engineering geological condition of a domesticunderground coal gasification field, systematic methods, including physical analogsimulation, numerical simulation, theoretical analysis and measurements in suite wereimployed to investigate deeply and systematically the deformation, damage and fractureextension rules of the overlying rock under the action of temperature and stress. The maininnovative observations are as follows:(1)By conducting a real-time high temperature (room temperature~800℃) thermalphysical properties and thermodynamic properties of lab tests of different lithology sample（Including mudstone, sandy mudstone, siltstone and fine sandstone）, the evolution lawsof the rock parameters, including specific heat, thermal conductivity, compressive strengthand elastic modulus with change in temperature, were obtained. These provide thefoundation data for the research on the combustion zone of overlying rock crack evolutionand breaking rules under the combined action of the temperature field and stress field.(2) Taking the specific engineering geological conditions of a underground coalgasification in domestic field as the prototype, the extension process of the combustioncavity in the process of the underground coal gasification was studied by using the physicalanalog simulation test. The laws of the combustion zone strata breaking, cavingmorphology and height, crack extension evolution under the influence of the temperaturefield and stress field were obtained, which provide an important guidance for practicalengineering design.(3) Accordin to the laboratory rock thermal physical and mechanical properties test,rock heat transfer control equations under the high temperature and the rock damageconstitutive equation wrew established, which were combined with RFPA and COMSOLsoftware to simulate the combustion cavity expansion process under the coupling action ofthe temperature and stress. The results deeply revealed the dynamic evolution laws oftemperature and stress of strata and the expansion laws of overburden rock fracture andfracture in the process of extensing burning zone.(4) Based on the traditional key strata theory in ground control, further by using the theory of thermodynamics and elastic mechanics, combustion cavity overburden rockfracture mechanics model under the action of temperature and stress was established. Theposition of key strata above the combustion cavity and the first weighting interval andperiodic weighting interval were anlysed in theory.Through the method of borehole and observation on the spot to analysis the height ofthe combustion zone overburden rock fracture zone, the measurements data in suite aregood aggrement with our results from similar simulation experiment, numerical simulationand theory analysis, which powerfully proves that the test results and the model isreasonable in this paper. The research results in this paper provid an important significanceguideance in theory and practical engineering of underground coal gasification combustioncavity strata stability control.There are127figures,20tables and195references in this paper.