Mechanical Property And Stability Control Mechanism Of Hot Dry Rock Wellbore Under Thermal Shock

The cost of drilling engineering generally exceeds 50% of the total cost of geothermal energy development of Hot Dry Rock(HDR),and the wellbore is the only access to transport thermal energy.Therefore,the stability control of the borehole is the precondition for the efficient development of HDR geothermal energy.There is a severe thermal shock between the high temperature rock and the low temperature drilling fluid during the drilling process,which leads to the deterioration of the mechanical properties of the rock and the instability of wellbore.Hence,it is great significance that the study of mechanical property and stability control mechanism of HDR wellbore under thermal shock for the efficient extraction of geothermal resources.This article takes the somewhere HDR geothermal energy development demonstration project as the background to study the mechanical property of granite under thermal shock and the stability control mechanism of HDR geothermal drilling engineering.First,the high temperature granite specimens were respectively cooled in 20,60,100 ? constant temperature water and 20 ? air,and then its were carried out by uniaxial compression,Brazilian splitting,and variable angle shear tests to obtain the deterioration laws of mechanical properties of the high temperature granite under thermal shock.Secondly,the cooling curves of granite specimen surface during the thermal shock were measured and numerical simulation means was used to reproduce the internal temperature distribution of the specimen.It is the essence reason that the mechanical properties deterioration of granite after thermal shock explained from the perspective of heat transfer.The concept of ‘thermal shock factor' was proposed,which used to characterize the effect thermal shock on the deterioration degree of mechanical properties of granite.Finally,a multi-field coupled borehole wall stability control mechanical model was established,and the measured mechanical parameters and geological parameters were substituted into the software COMSOL Multiphysics 5.3 for numerical calculations to study the deformation and failure laws of the drilling engineering under the multi-field coupling.The main conclusions are as follows:(1)It is much lower that the compressive strength,tensile strength,and shear strength of high temperature granite within 600 ? under thermal shock in 20 ? water than that of natural cooling in 20 ? air.The reason is that the heat exchange coefficient of water and rocks is much larger than that of air.The surface temperature of the specimen decreases sharply,however,the heat of the specimen interior can not be quickly derived,which lead to a large temperature gradient produced in the surface of the specimen.So,the heat induced cracks develop obviously,and the strength of the rock is reduced.The elastic modulus of high temperature granite under thermal shock cooling in water is much lower than that of natural cooling in air.(2)The high temperature granite within 600 ? is respectively thermal shock in constant temperature water at 20,60,and 100 ?,which compres-sive strength,tensile strength,and shear strength decreases with the increase of water temperature.This result shows that mechanical strength of granite is not only related to the initial temperature,but also the temperature of the cooling medium,and is not determined by the temperature difference.400?600 ? can be regarded as the critical temperature range of granite transition from brittleness to ductility.(3)The concept of ‘thermal shock factor' is proposed,which is defined that the change rates of temperature gradient per unit time during the heat transfer.And it can be used to quantitatively characterize the deterioration degree of mechanical properties of granite under thermal shock.The larger the thermal shock factor,the lower the mechanical strength of the granite.The first 20 s of thermal shock cooling is the most severe stage of thermal shock,which stage has the most severe impact on the mechanical strength of granite.(4)It is researched that the stability controls mechanism of wellbore under thermal-mechanical coupling from the perspectives of burial depth,rock mass temperature and drilling fluid density.The results show that the greater the burial depth and the higher the temperature of the rock mass,the more easily the borehole collapses,and the larger the collapse range.The position where the borehole collapses is parallel to the direction of the minimum horizontal crustal stress.Appropriate drilling fluid density range helps to control the stability of surrounding rock of wellbore.(5)It is analyzed that the deformation and failure laws of the HDR borehole during heat recovery under thermal-hydrologic-mechanical coupling from the perspectives of burial depth and rock mass temperature.The result shows that the injection well is more prone to damage than production well,and the damage is more severe.