| The geothermal mining technology is of prime significance for the collaborative exploitation of coal resources and hydrothermal resources.The key to the successful implementation of this technology lies in determining superior geothermal well locations in storage areas of coal-bearing geothermal resources,so as to ensure the stability of geothermal well wall and the long-term exploitation of geothermal resources under mining influence.To this end,first,the mechanism and influencing factors of geothermal well wall instability under mining influence need to be explored;subsequently,the instability mechanism and geothermal productivity should be comprehensively considered;on this basis,long-term stable superior geothermal well locations in geothermal mining areas can be selected.In this study,with geothermal mining in Pingdingshan No.10 Mine taken as the engineering background,the mechanism of geothermal well wall instability under mining influence was revealed,and the layout and optimization of well locations were investigated in depth.The evolution law of stress distribution in the floor during geothermal mining was analyzed,and a physical model specimen was prepared to study well wall stability.Besides,uniaxial compression tests,triaxial compression tests,and triaxial confining pressure unloading tests were designed and conducted on hollow cylindrical rock body based on the stress distribution around wells under mining influence.Accordingly,the damage criterion of hollow cylindrical rock body under unloading conditions was determined.Moreover,a model for stress distribution around wells under mining influence was established,and the mechanism of geothermal well wall instability was revealed.Finally,the weight values of factors affecting the exploitation of geothermal resources were calculated,and a decision-making method for optimizing the layout of geothermal wells in mining areas was proposed based on the mechanism of geothermal well wall instability and the influencing factors of geothermal mining.The main achievements are as follows.(1)A physical model was prepared to investigate the stability of well wall,and the mechanical properties of hollow cylindrical rock body were analyzed.Specifically: 1)Under uniaxial compression,the peak stress of hollow cylindrical rock body decreases with the increase of pore size while it increases with the rise of loading rate.2)Under triaxial compression,the peak stress of hollow cylindrical rock body rises with the increase of confining pressure,and its elastic modulus tends to increase first and then stabilize.(2)Unloading is an objective stress environment for the geothermal mining process in mines.For this reason,confining pressure unloading tests that conformed to the mining-induced unloading paths were designed,and the mechanical response characteristics and crack propagation modes of hollow cylindrical rock mass specimens under different unloading conditions were obtained.Specifically: 1)The peak stress increases with the rise of confining pressure and unloading level,while it decreases with the increase in the confining pressure unloading rate.hollow cylindrical rock mass specimens exhibit the best stability under 70% unloading level.2)Under different confining pressures,the axial and circumferential strain increments rise linearly first,and then grow exponentially.The circumferential strain increment starts to grow exponentially earlier than the axial strain increment.Under different unloading levels,as the unloading level goes up,the axial and circumferential strain increments both gradually shift to single linear growth.Under different confining pressure unloading rates,the axial and circumferential strain increments both decrease with the increase in the unloading rate.3)The confining pressure unloading and expansion characteristics fall into three stages,among which the expansion stage is the most sensitive.4)Unloading conditions significantly influence the deformation modulus,and the generalized Poisson’s ratio increases with the unloading of confining pressure.(3)The damage criterion of hollow cylindrical rock body was determined,and a model for stress distribution around wells under the coupling effect of “mining disturbance-extraction unloading” was established,and the mechanism of geothermal well wall instability under mining influence was revealed.Specifically: 1)A numerical model was constructed and verified based on the Mogi-Coulomb strength criterion which more accurately describes the critical state of hollow cylindrical rock body failure.2)Under the coupling effect of “mining disturbance-extraction unloading”,the vertical stress of the floor is asymmetrically distributed,and the distribution range and attenuation degree of horizontal stress both expand with the increase of unloading.Mining disturbance is the main factor affecting stress distribution.3)A model for stress distribution around wells under the coupling effect of “mining disturbance-extraction unloading” was established,and the criterion for critical instability of geothermal well wall was proposed based on the Mogi-Coulomb strength criterion.4)The well wall undergoes high-angle spiral shear failure when the horizontal stress difference is smaller than 20%,while annular shear failure occurs when the difference is greater than20%.Additionally,the instability mode is high-angle spiral shear failure when the lateral pressure coefficient is smaller than 1.0,while annular shear failure occurs when the coefficient is larger than 1.0.(4)The stability of geothermal well wall in the geothermal mining area of Pingdingshan No.10 Mine was researched,and the evolution law of stress distribution around geothermal wells and the failure mode of well wall instability were analyzed.Furthermore,and the reasonable spacing between the working face and geothermal wells was determined with consideration of both well wall stability and geothermal productivity.Specifically: 1)Compared with the axial stress,the circumferential stress and radial stress are more sensitive to changes in mining intensity.The displacement field is enhanced with the increase of mining intensity,and the well wall fails in the form of annular shear failure.2)Compared with the axial stress,the circumferential stress and radial stress are more sensitive to changes in lateral pressure coefficient and are positively correlated with it.The maximum displacement of well wall increases with the growth of lateral pressure coefficient.3)The permeability of the reservoir under solid coal on both sides of the working face is positively correlated with the mining step distance,while the permeability of the reservoir under the goaf is negatively correlated with the mining step distance.4)The production of geothermal wells shows a tendency to increase and then decrease with the increase of mining step,and the reasonable spacing between geothermal wells and the working face is 30 m.The production of geothermal wells is also shown to increase with the increase of mining step.5)The geothermal well wall stability zone is mainly concentrated in the geothermal well head and bottom,and the shear damage zone is mainly concentrated in the geothermal well body section from 10 m to 55 m.The shear damage zone is mainly concentrated in the geothermal well body section.(5)An SOA-RFR-GA intelligent prediction model for geothermal productivity was established,and the weight values of heat-storage characteristic parameters in geothermal mining were determined.In addition,the interaction mechanism between coal mining system and geothermal system was revealed,and the geological conditions a of the eastern mining area of Pingdingshan coalfield were clarified.Eventually,an optimized decision-making method for geothermal well location layout based on the fuzzy analytic hierarchy process was proposed.Specifically: 1)How changes in parameters(specific heat capacity,geothermal gradient,porosity,and fracture geometry)influence geothermal productivity was explored by using the established model.An SOA-RFR-GA hybrid intelligent prediction model that could well predict geothermal productivity was established,with a maximum error of not more than 5.39%.The importance of characteristic parameters shows that geothermal productivity is mainly affected by geothermal gradient and specific heat capacity of bedrock.2)Affected by tectonics of Likou syncline,Pingdingshan Mining Area is characterized by fault block uplift and surrounding depression,heat accumulates.3)Water-thermal coupling simulation demonstrates that in the recharge area,as cold water continuously infiltrates along the karst aquifer exposed on the surface,the formation temperature becomes increasingly lower.In the discharge area,cold water from the recharge area is heated by deep circulation and then discharged upwards,heating the surrounding rock strata.4)The results of the fuzzy analytic hierarchy process demonstrate that the importance of factors affecting the layout of geothermal wells follows the order: mining intensity>location of mining area>geothermal gradient>supporting facilities for mines>specific heat capacity.5)An optimized decision-making method for geothermal well location layout based on the fuzzy analytic hierarchy process was proposed and the application of geothermal well layout in Pingdingshan Coalfield.. |
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