Study On The Overlying Strata Movement By UCG Mining In Matigou Coal Mine

Underground Coal Gasification (UCG) is a process that combustible gas can beproduced by making the coal burning and gasifying in situ with chemical miningmethod. As an autothermal combustion gasification, the temperature of UCG stopeand its surrounding combustion space area can reach to above1000℃. The directionalmovement of flame working face makes an unsteady temperature field existing insurrounding rocks. Dynamic changing of temperature field impacts the dynamicdistribution of stress field and displacement field in surrounding rocks, and willimpact the rock pressure distribution and overlying strata movement. It has significantvalue to theoretical study and engineering practice for UCG, by studying dynamicdistribution of temperature field, stress field and displacement field and the surfacemovement and deformation law. In this paper, the mine pressure distribution andoverlying strata movement of shallow overburden and small mining width UCGmining in Matigou coal mine is studied by methods of theoretical analysis, numericalsimulation and field measurement. The main innovative achievements are asfollowings:(1) The mathematical model for unsteady temperature distribution in surroundingrock of combustion space area is built, and its analytical solution is obtained by usingthe Laplace transform. The temperature field equations of roof and floor in verticaldirection and the temperature field equations of coal seam in horizontal direction areobtained. It shows that with UCG working face directional moving during the wholegasification time quantum, the temperature in surrounding rock increases first andthen decreases, and the peak values of temperature curves decrease gradually and thepositions of peak values move to the interior of surrounding rocks. There are envelopecurves of temperature curve clusters for each moment, and the envelope curves areput forward as a new method of calculating basis for burnt surrounding rocks range,coking cycle range and heat influence range. The judging standards of ranges forburnt surrounding rocks, coal-wall coking cycle and heat influence are given, and thevalues of those ranges are worked out.(2) Based on the elastic foundation beam theory, a mechanical model ofmulti-layers thermoelastic foundation beam with span and containing temperaturefield and damaged coal seam elastic foundation is built. The direct and indirectinfluences of high temperature on foundation beam deflection are analyzed, and the deflection analytical solutions of one to two layers are obtained. It is found, becauseof the existence of temperature field, the deflection and bending moment values ofthermal elastic foundation beam are higher than they are at normal temperature, andthe values increase first and then decreases with time. The mining width deceaseswhen the beam reaches to the ultimate bending moment value. It is calculated that themining width15m of Matigou coal mine gasifier is smaller than the limited span20.34m of immediate roof, which meeting the design requirement.(3) Numerical simulation of two-dimensional unsteady heat conduction andthermal-mechanical coupling in surrounding rocks of combustion space area isprocessed by COMSOL multi-field coupling numerical simulation software. Thisnumerical simulation has contained the changing rule of thermophysical parameters ofimmediate roof and coal seam increased with temperature. Dynamical distribution lawof temperature field, stress field and displacement field in surrounding rocks areobtained. It is found that the manifestation of stress field and displacement field isobvious due to the existence of temperature, and the maximums of stress anddisplacement increase first and then decreases with time. The temperature values bythermal-mechanical coupling numerical simulation are higher than by separatetemperature field numerical simulation and theoretic calculation, and envelope curverange and value and the ranges of burnt surrounding rock, coal-wall coking cycle andheat influence are increased also.(4) Matigou coal mine observation station for overlying strata movement andsurface subsidence was established for the first time in China, it is for shallowoverburden, small mining width and super-subcritical UCG mining. The form andlocation of combustion space area was detected by geophysical prospecting methods.The observation results show that subsidence curves appear as the fluctuant andstepwise subsidence pattern as five stages (sinkingâ†'risingâ†'sinkingâ†'risingâ†'sinking). It shows that the projection plane of combustion space area seems like anoval, wide in the middle and narrow at both ends with the short axis about27m andthe long axis about74m. The upper part, the lower part and the left of gasifier exceedthe designed boundary of gasifier. Compared with the theoretical calculation resultsand numerical simulation results, the field measured curves show obvious nonlinear,noncontinuity and volatility in term of the overall form; the field measured resultsshow obvious hysteretic nature in term of the time behavior; the field measuredresults have a bigger subsidence value in term of final settlement because of virtual gasification mining width increase. The boundary angle of incline main section, thegreatest subsidence angle and the maximum surface movement and deformationparameters of subsidence basin are calculated as follows: β0=61.0°, γ0=72.5°, θ=69.0°, imax=1.385mm/m, εmax=0.516mm/m, Kmax=0.275mm/m2, These datashows that no apparent subsidence basin forms, and it does not reach the criticalmoving boundary and does not destroy the surface building in the observation period.