Study On The Transport And Plugging Regularity Of Temporary Plugging Agent In Hot Dry Rock Fracture Under Heat Flow

As a kind of clean and renewable energy with wide distribution and stable energy supply,hot dry rock geothermal resources are of great significance to the transformation of China's energy structure and the construction of clean energy utilization system.Artificial heat exchange system constructed by hydraulic fracturing technique is the key to enhance the heat recovery ability of dry hot rock.However,due to the high temperature of bedrock and strong anisotropy of in-situ stress in hot dry rock reservoirs,the artificial fracture structures formed are often relatively single,and temporary plugging diversion fracturing has become one of the most promising heat recovery technologies to improve the complexity of fracture network.At present,there are few researches on temporary plugging diversion fracturing technology of hot dry rock under high temperature at home and abroad,and the research on the influence of temperature on the physical and mechanical properties of hot dry rock,the migration regularity and temporary plugging characteristics of temporary plugging agent in artificial fractures is still in the exploratory stage,so it is urgent to carry out research on the migration and plugging regularity of temporary plugging agent in fractures.The physical and mechanical properties of hot dry rock at high temperature,the migration regularity of temporary plugging agent in fractures,and the temporary plugging law of temporary plugging agent in fractures are systematically studied using the core test,numerical simulation and experimental in this paper.The main results are as follows:(1)Experimental study on the evolution law of physical and mechanical properties of hot dry rock at high temperature.The results of core test on the physical properties of hot dry rock such as porosity and permeability and thermal conductivity show that With the increase of temperature,the micro-cracks caused by thermal damage in hot dry rock gradually increase,the porosity and permeability gradually increase.On the contrary,the thermal conductivity and thermal diffusion coefficient gradually decrease with the increase of temperature,and there is a temperature threshold,which makes the thermodynamic parameters of hot dry rock obviously attenuates;Meanwhile,mechanical deformation and failure characteristics of hot dry rock under high temperature were studied by using the triaxial experimental system of high temperature and high pressure rock mechanics with acoustic emission as monitoring,and it was found that with the increase of temperature,the acoustic emission events at the early stage of loading increase continuously,and the rock mechanics parameters such as fracture toughness,compressive strength,elastic modulus and Poisson's ratio all showed a trend of decline,and the artificial fracture morphology also becomes more complex due to the high temperature.(2)Numerical simulation of the migration regularity of temporary plugging agent in artificial fractures based on CFD-DEM bidirectional coupling method.Based on Eulerian-Lagrange description method,a CFD-DEM bidirectional coupling mathematical model was established to simulate the transport process of granular temporary plugging agent in complex artificial fractures of hot dry rock under different temperatures,so as to accurately capture the migration process of temporary plugging agent in fractures and the interaction between particles at high temperatures.The results show that the main factors affecting the migration process of temporary plugging agent in fracture are the temperature of hot dry rock,the mass concentration of temporary plugging agent,the viscosity and flow state of carrying liquid,etc.With the increase of temperature,the movement velocity of temporary plugging agent particles increases and the interaction force between particles increases,and the amount of temporary plugging agent entering complex branch fracture decreases.With the increase of fracture width and injection rate,the distribution quantity of temporary plugging agent particles in the branch fracture increases.(3)Experimental study on fracture temporary plugging characteristics under heat flow.The temporary plugging characteristics of hot dry rock artificial fractures under the action of heat flow were studied by preparing the model of hot dry rock artificial fractures and building the experimental platform for the evaluation of temporary plugging performance of hot dry rock artificial fractures.The results show that under the condition of high temperature,the process of temporary plugging of hot dry rock artificial fractures will have obvious pressure fluctuations,and the number of pressure fluctuations will increase with the increase of temperature.At the same time,there is an obvious relationship between the number of pre-plugging and the pressure fluctuation under the action of high pressure in the fracture.The time required for complete fracture plugging increases gradually with the increase of temperature,on the contrary,it basically shows a linear downward trend with the increase of injection rate.The distribution distance of temporary plugging agent in fracture increases with the increase of temperature and fracture width.The research results of this paper will provide theoretical guidance and technical support for the optimization of temporary plugging agent dosage and the regulation mechanism of artificial fracture network for temporary plugging diversion fracturing of hot dry rock.