Experimental Investigation Of The High-temperature Permeability And Microscopic Characteristics Of Lignite And Numerical Simulation Of The In-situ Oil-gas Exploitation Of Lignite By Steam Injection

Utilizing coal resource clean and efficiency is attracting more and more attention since our human noticed the disadvantages of traditional extensive utilization ways.Exporting and utilizing lignite is usually low efficiencyand polluted to the environment due to its characters like high water content,high volatile component and low heating value.Therefore,based on the low thermal stability and pyrolysis easily properties of lignite,lignite in-situ steam injection exploitation technique has been proposed by Zhao yangsheng of Taiyuan University of Technology.Based on the above background,in this thesis,we have systematically investigated the permeability and microscopic structure of lignite under pressure gas pyrolysis condition at high temperature both from the macroscopic and microscopic view of experiments.By considering the multi-fields(solid,fluid and heat filed)coupling effect theory,we have simulated the temperature field,stress field and displacement of stratum under in-situ heating exploration process.The main research contents and conclusions are as follows:(1)By adopting a triaxial rock permeability testing system,the permeability of lignite from room temperature to 650?has been tested,and we have found that the permeability of lignite in general increases with the increasing of temperature at different pore pressures.Furthermore,the permeability of lignite above 200? is much higher than that of room temperature.Three dramatic decreasing of the permeability can be observed in the middle-high temperature range(200~650?),but the permeability of lignite is already remaining above 110 md,which indicating good permeability property of lignite at high temperature.By analyzing the permeability curve of lignite versus temperature,we have found that the dehydration and pyrolysisare the two dominant factors influencing the permeability of lignite.By analyzing the correlation of the deformation of lignite with temperature,we have observed dramatic deformation happened to lignite in the temperature range of 25~200? and 300~650?.(2)By using our newly designed high-temperature gas heating system,we have studied the structural features,pore and crack structures,of lignite both by NMR and X-CT methods.Based on NMR testing results,we have found that the pore size increases dramatically with the increasing of temperature.And at 450? the pores and cracks with size over 1?m size become the dominant pores and cracks in lignite,which holds 55% of all pores and cracks.By utilizing the relative physical model,the permeability feature of lignite calculated based on the NMR results is in good agreement with our macroscopic testing results.From the X-Cr results,we have found that the porosity of lignite increases from room temperature to 250?,following by decreasing at 350?,and then increase again to 16.02% at 450?,which is 12.7 times higher than that of room temperature,indicating well developed of the seepage channel.The porosities of lignite at the same research scale obtained by the CT and NMR methods show exactly the same variant tendency.The resolution of NMR can reach nanoscale,while CT can provide a more general microstructure of lignite.By adopting both methods,a more accurate and clear picture of the interior structure of lignite,e.g.pore and crack type,effective porosity,distribution and quantitative characterization of pore and crack in three-dimensional space.(3)By systematically investigating the variant tendency of the pore structure of lignite heated by high-temperature gas with the pressure of 0.1 MPa,0.5 MPa and 1 MPa,we have found that the porosity of lignite increases with the increasing of temperature.At 500?,the porosities of lignite with 0.1 MPa,0.5 MPa and 1 MPa gas pressure are 23.01%,27.11% and 28.63%,respectively.By analyzing the quantity and volume of pore clusters with different sizes under different pyrolysis conditions,we have found that gas pressure can benefit the developing of the small cluster and higher gas pressure can promote the connecting of different pore clusters,which resulting in the formation of seepage channel.By comparing lignite under heat radiation heating and pressure gas heating conditions,we have found that the pyrolysis of lignite can be influenced by the interaction effect of gas with pressure and the inner gas of particles in lignite,thereafter the pore structure of lignite is changed.(4)Based on the multi-filed coupling theory of porous matter,we have applied the solid-liquid-heat filed coupling model of the in-situ lignite exploration technique,and using relative fundamental parameters,to simulate the temperature field,stress field and displacement of the stratum of lignite during exploration.We have found that the temperature along the direction of stratum extends away from the heat injection well with shape of a disc.The temperature along the vertical stratum spreads around the fracturing layer.It only takes 5 hours for the temperature extension from the fracturing layer to the producing well.All the regular lignite layers from the heat injection well to the producing well is heated over 500? after 204.7h heat injection.Along with the temperature increasing of the stratum,?z changes gradually from compression stress to tensile stress,and the tensile stress zone spreads around the heat injection well and fracturing layer.Along with the heat injection process,dramatic expansion deformation can be observed between the heat injection well and producing well,and the bump height increases as time goes on.After 1311 h heat injection,the bump heights of the heat injection well and producing well are 8.5 mm and 4 mm,respectively.