| High-temperature geothermal resource is a kind of highly competitive clean and renewable energy which can be developed and utilized through the construction of enhanced geothermal system.One of the key technologies for building the system is hydro-shearing permeability enhancement,i.e.to induce the shear sliding of fractures in the geothermal reservoir through water injection,thus opening the fractures and improve the permeability of geothermal reservoir.Therefore,it is necessary to study the hydraulic shear slip mechanism,shear-seepage-heat transfer characteristics of fractured rock mass.In this dissertation,the shear-slip mechanism and judgment criteria of fractured rock mass under the influence of multi-factors were put forward by means of theoretical analyses,experiments,mathematical and mechanical models and numerical tests.The uniaxial compression test of granite and the direct shear test of granite fracture were carried out at high temperature in real time.Through the tests,the influence of temperature on the physical and mechanical properties of rock was grasped,and the mechanics and deformation characteristics of shear slip of fractured rock mass at high temperature were revealed.Besides,in order to characterize the shear behavior of the rock mass,the principle of surface roughness degradation in the shear process was put forward,and the shear-constitutive model and the shear-seepage model of the fractured rock mass were established.Furthermore,shear-seepage-heat transfer characteristics of rock masses with a single fracture and a fracture network were simulated by using the established model,respectively,and the evolution laws of seepage field and temperature field of fractured rock mass in the shear process were revealed.Finally,based on the geological conditions of the geothermal field in Yangbajing geothermal power station,the model of the fault geothermal development system was established,and meanwhile the industrial analysis was made.The study provides a new mode for the development and utilization of the high-temperature geothermal resources.The main results are as follows:(1)Based on the Mohr-Coulomb failure criterion,the shear failure criterion of fractured rock mass under the influence of fracture surface roughness,dip angle,water injection pressure,in-situ stress and other factors was proposed.Next,the characteristics of hydraulic shear slip in fracture network were analyzed by using the criterion,and the mechanism of hydraulic shear slip in fractured rock mass under the influences of multiple factors was revealed.The main results are as follows:1)The occurrence of hydraulic shear is periodic and sudden.In the fracture network,with the increase of water injection pressure,the number of hydraulic shear fractures remains basically constant first and then increases sharply.2)The hydraulic shear of fractures depends on the side-pressure coefficient.The deeper the thermal reservoir is,the better the hydraulic shearing technology can be applied.3)Surface roughness and fracture dip angle have an important effect on the hydraulic shear of fractures.Increasing water injection pressure cannot cause the shear failure of fractures with relatively small dip angles.(2)The mechanical and deformation failure experiments of granite under uniaxial compression were carried out at high temperature in real time,so as to reveal the effects of temperature on the strength,elastic modulus and failure mode of granite.In addition,the experiment was performed on particle size classification of granite to study the dependence of particle size classification of granite on temperature.The main results are as follows:1)The rise of temperature weakens the compressive strength and elastic modulus of rock.2)High temperature aggravates the degree of rock fragmentation.With the rise of temperature,the middle-size detritus shows a decreasing trend on the whole,while fine and small detritus keeps increasing.3)The high-temperature environment of rock mass can improve the effect of hydraulic shear stimulation and enhance the permeability of heat reservoir.To be specific,the increase of temperature not only weakens the physical and mechanical properties of rock and reduces the water injection pressure stimulated by hydraulic shear,but also raises the area of contact with geothermal fluid by promoting rock fragmentation and increasing internal fractures(3)A direct shear test system of fractured rock mass at high temperature was developed.Based on the system,the mechanical and deformation characteristics of fractured rock mass under the joint action of crack surface roughness,normal stress,temperature and other factors were studied.In this way,the mechanisms of influences of temperature and roughness on shear stress and normal displacement of fractured rock mass were revealed.The main results are as follows:1)In the shear process of fractured granite at high temperature,the shear stress-shear displacement curve can be divided into four stages:the stress adjustment stage,the stress rise stage,the peak stage and the post-peak stage.With the increase of shear displacement,the normal displacement can be divided into three stages:the displacement adjustment stage,the displacement increase stage and the stable stage.2)With the increase of roughness,the shear strength increases and the maximum normal displacement increases gradually.With the increase of temperature,the shear strength decreases slightly and the maximum normal displacement decreases gradually.3)The shear failure mechanism of fractured rock mass at high temperature is shown in three aspects:the bite fracture,the friction rolling and the mutual filling of the concave body and the convex body.4)In the process of hydro-shearing,the stress concentration caused by occlusion of concave and convex bodies will produce tensile cracks around the original cracks,thus increasing the effective heat exchange area.(4)In order to characterize the shear behavior of rock mass,the basic principle of crack surface roughness degradation was put forward,and the mathematical models of roughness degradation and post-peak accelerated degradation were established.Furthermore,the shear constitutive model and shear seepage model of fractured rock mass were established.Then,taking the change in fracture opening caused by shear process as a bridge,the shear-seepage-heat transfer model of fractured rock mass was established under the framework of heat-fluid-solid coupling equation,and meanwhile the corresponding solution method was put forward.(5)Based on the shear-seepage-heat transfer model of fractured rock mass under heat-fluid-solid coupling,numerical experiments of seepage-heat transfer in a single fracture and a fracture network were carried out.The seepage characteristics and temperature field evolution of fractured rock mass in shear process were revealed.The main results are as follows:1)The distribution pattern of the temperature-falling zone in bedrock is consistent with that of fractures.2)Hydraulic shear enhances the permeability of fracture,but it decreases the temperature around the fracture rapidly.The effect of hydraulic shear fracturing on rough fractures is better in the process of actual heat extraction.3)After the completion of hydraulic shear,the main controlling factor affecting the heat extraction transfer from the discharge rate to the temperature of the outlet.4)In geothermal production,the“necking”phenomenon restricts the efficiency of heat extraction,so it is necessary to minimize the generation of“necking”path and try to form a more uniformly distributed fracture network.(6)According to the geological conditions of Yangbajing geothermal power station,the development and utilization model of fault geothermal energy was established,and the single-well and multi-well production were analyzed.The main results are as follows:1)The fault geothermal system boasts a good the thermal reservoir to which the model is applicable.The temperature of the fault near horizontal shear zone(500-1000 m from the magmatic capsule)is between 350?and 450?.This zone can be regarded as a connected fracture network which contains multiple U-shaped loops for continuous production.2)The outlet temperature is related to the injection rate and the production mode.Because of the long underground seepage path for long-distance production,short-distance production has some advantages in heat extraction.3)In multi-well production,the influence of back pressure difference on long-distance production well is greater than that on short-range production well.The back pressure of long-distance production well should not excess that of short-distance one,mainly because the high back pressure will lead to a low flow rate which will further aggravate the imbalance between production wells.4)It is conservatively estimated that a 30 MW fault geothermal demonstration project,which requires an investment of about 300million RMB,can be built in Yangbajing geothermal field with an annual output of2.6×10~8 kW·h.5)Despite the huge scale of deep geothermal energy,the amount of heat produced is extremely limited because of the single fracture network of geothermal system.In practical production,the following countermeasures should be adopted comprehensively:thermal insulation of well shaft,sealing of shortest seepage flow path,proper back pressure production,even distribution of production system for preventing imbalance of ground temperature,and enrichment of fracture network with hydraulic fracturing technology. |
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