Experimental And Model Study On Heat Transfer Mechanism Of Water Flowing Through Deep Fractured Rock Mass With High Temperature

Energy is an important basis for maintaining economic development and social stability.With the improvement of world prospersity and the rapid population growth,the demand for energy is growing continuously.The world's energy structure is still dominated by conventional fossil fuels,with oil,coal,and natural gas accounting for85%of the total in the past 10 years.Undoubtedly,excessive use of conventional energy has a devastating consequence on the environment.Therefore,under the double challenges of increasing energy demand and environment protection,countries all over the world focus on the development and utilization of renewable energy.Geothermal resources,as a kind of clean and renewable energy,has huge reserves,little environmental impact,and can be continuously and stably exported,which has broad application prospects.No matter in conventional geothermal reservoirs or in enhanced geothermal system,the heat stored in rock reservoir are extracted by fluid flowing through the fractured rock mass with high temperature.The heat extraction process involves the participation of hydraulic field,mechanical field,thermal field and chemical field,which is a complex problem,leading it is very challenging to predict the heat production of geothermal system accurately.It is of great significance for the development and utilization of geothermal resources to explore the characteristics and mechanism of fluid flow and heat transfer in fractured rock mass under multi field coupling condition.At the same time,it has important theoretical and practical significance for the safety evaluation of radioactive waste geological disposal,thermal oil recovery engineering,deep mining engineering,water conservancy and hydropower engineering and many other fields involving underground resource exploitation and underground space utilization.In this paper,aiming at the scientific problems involved in the development of deep underground resources and the utilization of underground space,such as the THCM coupling process when fluid flowing through fractured rock mass and the simulation evaluation of geothermal reservoir,relevant research was conducted by combing the method of theoretical analysis,laboratory experiments and numerical simulation.The specific research contents and conclusions are summarized as follows:Firstly,based on the Newton's cooling formula and previous research,the calculation formula of convective heat transfer coefficient is derived,which is more practical.Through 3D printing technology and cement mortar pouring method,samples with different seepage paths are prepared.Seepage heat transfer tests are carried out on the test device developed by our research group indemendently.The influence of flow path tortuosity,aperture,initial temperature on the heat transfer characteristics is analyzed and discussed.The experimental results show that the overall convective heat transfer coefficient anc heat transfer rate of fracture with different flow path decrease to some extent compared with the sommth fracture sample.The larger the tortuous of the seepage path,the greater of the overall heat transfer coefficient.The increase of the initial temperature will raise the heat transfe rate.The characteristics number equation in power function has good fitting results for the test results.Combining with the tortuosity of the sample seepage path,the correlation formula of C value,n value and tortuosity in the characteristic number equation is proposed,which provides important parameters for the numerical simulation of T-H-M coupling in fractured rock mass.Then,the granite samples with one fracture,two intersecting fractures and three intersecting fractures are obtained by wire cutting method,and the seepage heat transfer tests under different confining pressures are carried out.The calculation formula of the convective heat transfer coefficient in multi fracture channels is derived.The relationship between the seepage characteristic parameters and confining pressure in the process of pressure increasing and pressure releasing is mainly analyzed and discussed.The mechanism of seepage and heat transfer in the multi fracture channels is discussed.The characteristics of heat transfer when water flowing through a single fracture and multi fracture channels are compared and analyzed.The research results show that the injection pressure and the injection flow rate are linearly positively correlated,and the rate of injection pressure increasing with the injection flow rate is positively correlated with the confining pressure,and the equivalent hydraulic width is negatively correlated with the confining pressure;The permeability of intersecting multiple fractures is better than that of single fractures.Compared with single fractures,the injection pressure of intersecting two fractures is 64.46%?75.86%lower than that of single fractures,and the equivalent hydraulic width is increased by 2.14 times to 2.22times.The increase in permeability coefficient and permeability is between 2.29 times and 2.87 times;The injection pressure of the intersecting three fracturs is 86.90%-96.26%lower than that of the single fissure.Compared with the single fracture,the equivalent hydraulic fracture is increased by between 3.60 times to 4.59 times,and the increase in permeability coefficient and permeability is between 4.31 times and 7.03 times.Compared with the single fracture rock sample,the heat exchange area of the rock sample with multi fracture is increase immensely,which causes the heat transfer path to be significantly shortened,leading the heat transfer intensity to be improved and the final water outlet temperature to be increased.The data show that,compared with the single fracture rock sample,the convective heat transfer coefficient of intersecting two fractures rock increases by 6.16%?20.93%,and the convective heat transfer coefficient of intersecting three fractures increases by 21.56%?31.27%.To study the influence of fracture distribution characteristics on heat transfer characteristics,a three-dimensional numerical model was established to conduct thermal-fluid-solid coupling numerical simulation of fractured rock mass.The temporal and spatial evolution of production temperature,rock temperature field and pore pressure field are obtained and analyzed.Based on the simulation result,the influence of different fracture distribution on the seepage and heat transfer characteristic are analyzed,providing a basis for the site selection of geothermal system and the reservoir stimulation plan.Numerical simulation results show that after the model runs for a period,a cold front will form in the fractured rock mass reservoir.As time increases,the cold front gradually expands to the water outlet.The range of low-temperature areas in the fractured rock mass and the positions of the two fractures,inclination,and inclination are closely related.the larger the fracture angle,the faster the water outlet temperature drops,indicating that when the fractures are more evenly distributed in the reservoir,the heat extraction rate of the flowing water from the surrounding reservoir is faster.Finally,heat production characteristics of a geothermal heating system in naturally fractured reservoirs of Guide Basin were investigated.This work conducted numerical simulation in Guide Basin considering the heterogeneity and anisotropy which combines the advantages of both DFN model theory and continuum medium model.Firstly,DFN models was established based on fractures information obtained from the outcrops,reservoir structure,core description and borehole images.Then,the model was cut into hundreds blocks with size of 100m×100m×100m.The permeability of each block in x,y and z direction were calculated using 3DEC mode.Finally,the calculated permeability was put into the model established using TOUGH2 and the heat production forecast was numerically investigated.Numerical simulation results show that the production temperature remained stable during the first 10 years of operation.During the 30 years after operation,the production temperature dropped from 194.87°C to 184.22°C,a decrease of about 5.47%,reaching the commercial standard.During model operation process,the range of flow impedance is 0.46?0.54MPa/(kg/s),which is slightly higher than the commercial standard(0.1?0.2MPa/(kg/s)).Therefore,in actual engineering,corresponding measures need to be taken to reduce flow resistance.The cumulative power generation has reached 8225GWh during the operation of 40years,which is equivalent to saving 1.48×10~8kg of coal,reducing 2.81×10~8kg of carbon dioxide emissions and 3.61×10~6kg of sulfur dioxide emissions.