Study On The Loading Failure Mode Of High Temperature Granite And Influence On Reservoir Permeability

With the development of the economy,human demand for energy is increasing year by year.The energy structure dominated by traditional fossil energy has brought huge pressure to the environment.It is urgent to find and develop stable,sustainable and renewable clean energy.Geothermal energy has become the third largest renewable energy in the world due to its advantages of green,clean and pollution-free,widely distribution,high efficiency and stability,sustainable regeneration,and flexible development.According to the burial depth,geothermal energy can be divided into shallow geothermal energy,hydrothermal geothermal energy and hot dry rocks geothermal energy.Among them,hot dry rocks geothermal resources are considered to be the most potential resources in the 21^stcentury because of their huge reserves.However,hot dry rocks geothermal resources generally occur in low porosity and low permeability metamorphic rocks or crystalline rocks.Due to the characteristics of high temperature,high in-situ stress and high rock mechanical strength,such high-temperature hard rock have caused enormous damage to hot well drilling and reservoir construction.Therefore,its degree of development and utilization is the lowest among geothermal resources.In order to increase the exploitation efficiency,experts propose the method of artificially forming geothermal reservoirs to economically exploit deep geothermal energy from low-permeability rocks,namely the enhanced geothermal system(EGS).The key to the effectiveness of EGS is the construction of an artificial fracture network that enables water circulation between injection wells,artificial fracture network and production wells.The effective method of reservoir stimulation is hydraulic shearing,which injects low temperature and high pressure fluids into fractured reservoirs.This method can reduce the effective stress of the reservoir and failure the rock mass,thereby increasing the permeability of the fractured reservoir.However,in the geothermal reservoir,the rock is in a high temperature and high in-situ stress environment.During the process of geological evolution,the natural fractures in the reservoir have different modes,and the mechanical characteristic of the rock are constantly changing.Under the action of thermal-hydraulic-mechanical interaction,when the fluid through the fracture,the self-supporting ability and hydraulic conductivity significantly differ,which makes the fluid flow path in the fracture network unclear.In the process of hydraulic fracturing,the connectivity of fractures and the characteristic of the rock have a great impact on the construction of geothermal reservoirs and the improvement of the injection-production efficiency of EGS.To solve the above problems,this paper select the Matouying heating site in Hebei as the research area,and collects granite samples in the area to carry out laboratory experiments to analyze the basic physical properties of granite and the influence of high temperature on rock mechanical properties.Thermal triaxial compression experiments were conducted to discuss the changes of rock mechanical characteristics and failure modes with confining pressure under the effect of high temperature.The fracture surface was determined in three dimensions and the fracture topography parameters were calculated.After the triaxial experiment,the fractured rocks were collected,and rocks with different failure modes were selected to carry out the fracture seepage experiment.According to the experiment results,the fracture under high temperature,the effect of the three-dimensional shape,pressure and flow rate on the seepage capacity are analyzed.The reservoir temperature in the EGS is generally between 150-200?.The triaxial test results show that the compressive strength and fracture toughness of the rock do not change in this temperature range,the cohesion is slightly reduced,and the internal friction angle slightly increased.Under high temperature conditions,with the increase of confining pressure,the Poisson's ratio remains almost unchanged,otherwise the elastic modulus increases with the increase of the confining pressure;the failure mode of the rock changes from complex to simple.The rock converts from tension-shear mixed failure to shear failure;after the rock failure,the inclination angle of the fracture surface and the width of the fracture gradually increase,the roughness of the fracture surface gradually increases,and a large number of debris appear on the fracture surface.The results of the fracture seepage experiment show that the rock has the highest permeability in the multiple composite failure mode(mode I),followed by the Y-shape shear failure mode(mode II),and the single-shear failure mode(mode III).Compared with the first two failure modes,the pressure difference was the largest in the single-shear failure mode.When the confining pressure was increased from 5 MPa to 30 MPa,the permeability of the three failure modes decreased by 88.8%,88.4%,and 89.9%,and the hydraulic aperture decreased by 66.5%,65.9%,and 68.3%,respectively.After the pressure reaches 25 MPa,it is difficult for further crush damage at the tip on the fracture surface,and the permeability decline slowed.Compared with the first two failure modes,the confining pressure has the greatest effect on the single-shear failure mode.With the increase of the injection flow rate,in the process of increasing the confining pressure from 5 MPa to 30 MPa,although the increase of permeability and hydraulic aperture decreased,but the decrease was small.This phenomenon indicated that increasing the flow rate could effectively improve seepage capacity of fractures.The flow rate has the greatest effect on single-shear failure and the least effect on Y-shape shear failure.Increasing the injection flow rate can significantly increase the fracture permeability.The influence of flow rate on the seepage capacity of fractures mainly includes two parts:(1)liquid pressure can effectively inhibit fracture closure;(2)the scouring effect of low temperature fluid on high temperature rock to promote fracture closure.Based on the drilling data and laboratory experimental results of the Matouying heating site,a random fracture model was established by TOUGH2-Biot simulator.The evolution characteristics of temperature,pressure and seepage field during the reservoir stimulation process were analyzed to evaluate the fracturing effect.Sensitivity analysis of injection rate and injection temperature combined with the original model.The simulation results show that the reservoir stimulation mainly includes three stages.In the first stage,the increase in permeability is the extend of primary fractures,and the main reason for the increase of permeability in the second stage is the opening of non-conducting fractures and the extend of water-conducting fractures to form a fracture network.In the third stage is the shear failure of the rock in the reservoir,which generates new fractures leading to permeability increased.Before the reservoir stimulation,the fracture equivalent permeability was in the order of 1md.After the first stage,the permeability at the injection point increased by17 times;after the second stage,it increased by 256 times;after the third stage,it increased by 818 times.After the third stage,the influence range of the horizontal maximum principal stress and the minimum principal stress direction is 34 m and 30m,and the vertical direction is 30 m.The permeability enhancement volume is about33,000 m~3.High flow rate will accelerate the extend of primary fractures and the opening process of non-conductive fractures,but the reservoir will occur strong shear fracture at high flow rate.The low temperature fluid has a strong heat exchange capacity with the reservoir,which will cause the temperature of the fractured rock decreases,thereby enhance its hydraulic aperture;at the same time,a lower injection temperature will generate greater thermal stress to accelerate the expansion and permeability of the reservoir,but the injection temperature is too low will affect the subsequent exploitation and utilization of geothermal energy.Therefore,the reservoir stimulation effect is better with moderate water injection flow rate and injection temperature.