| To ensure the smooth implementation of the national "dual carbon" policy,the development of clean and efficient dry hot rock has emerged as a crucial focus of energy reform.Since the dry-hot rocks are found in deep underground granite that has low porosity,low permeability,and high strength,it is necessary to introduce the Enhanced Geothermal System(EGS)technology to generate numerous through-fracture networks in the granite reservoir by hydrofracturing.This will enable the development of dry-hot rock resources through heat exchange.Geological factors such as temperature fluctuations during hydrofracturing,the meso-structural characteristics of rocks,and the stress of deep surrounding rock,have a significant impact on the formation of fissure networks within reservoirs.In view of this,in order to explore the law of granite micro-structure characteristics affecting the development of fracture network under the action of temperature and pressure,the indoor test and theoretical analysis are used to carry out the research on temperature damage characteristics and fracture development law of granite with different micro-structures;Secondly,in order to overcome the difficulty of quantitative study on mesoscopic characteristics of granite in laboratory test,based on the spatial correlation function,the granite PFC numerical model is established and the relevant test research is carried out to comprehensively analyze the granite damage characteristics and fracture development law under the action of temperature and pressure;Finally,the fractal theory analysis method is used to study the change law of fracture network under the influence of temperature,rock structure and surrounding rock stress.This work provides a new understanding of the formation mechanism of reservoir fracture network in the dry hot rock development under complex geological conditions.Specific research contents and results are as follows:(1)Conducting laboratory tests on the damage characteristics and micro-fissure development rule of granite with different micro-characteristics under various temperatures,this study was based on the damage theory of inhomogeneous materials.The research helped to verify the damage mechanism of micro-fissure under temperature and gain knowledge about the laws of temperature action and changes in physical and mechanical properties of granite.The study also clarified the mechanism of temperature damage caused by microinhomogeneity and revealed the influence of micro-inhomogeneity on temperature damage.By utilizing damage tensor and crystal expansion theory,this study derived the temperature damage equation of granite with meso-inhomogeneity parameters and optimized the calculation method of temperature damage of granite.(2)Considering the difference of meso-heterogeneous characteristics of natural granite,a granite crystal model(SCGM)with quantitative changes in fabric is constructed by combining spatial correlation function and Voronoi crystal model.The validity of the SCGM model is verified by mineral distribution and crystal structure characteristics respectively.Compared with the traditional homogenization model and GBM crystal model,the new SCGM model highlights the characteristics of different mineral aggregation and distribution in granite,while taking into account the characteristics of crystal scale distribution.On the basis of this model,the influence of meso-heterogeneous characteristics such as mineral content and distribution on the physical and mechanical properties of granite and the law of crack development is studied.The results show that the proportion of intergranular crack is affected by mineral accumulation characteristics,and the law of mineral content affecting the mechanical properties of granite is revealed.(3)Based on the quantitative characterization of the heterogeneity of granite,the heterogeneity coefficient H of comprehensive mineral distribution and crystal scale is constructed.Combining with the theory of crystal expansion and the criterion of crack expansion,the mechanism of the influence of the heterogeneity coefficient H on the thermal damage characteristics and the law of micro-crack expansion of granite is analyzed from the theoretical point of view.Based on the mechanical-thermal coupling theory in PFC software,a temperature damage model is constructed,and the mechanical properties and temperature damage characteristics of granite under different heterogeneity coefficient H are studied.The law of the effect of the heterogeneity factor H on the temperature damage of granite is revealed,and the reason why the micro-cracks exhibit different developmental characteristics is explained.The two-stage damage change characteristic with 400 ℃ as the dividing line is obtained.(4)A hydraulic fracturing test model was established considering the distribution of insitu stress,temperature variation and heterogeneous characteristics of granite.The numerical test of hydraulic fracturing under different influencing factors was carried out,and the variation law of crack development Angle was analyzed by introducing the side pressure coefficient λ,which determined that the microcrack was more likely to develop along the direction of high stress.The variation law of micro-crack under different temperature effects and heterogeneity coefficient H was analyzed,and the conclusion was obtained that increasing the heterogeneity coefficient H would improve the temperature damage effect.The fractal dimension algorithm was used to analyze the influences of different factors on the fracture development results from the perspective of fracture distribution pattern.It was proposed that there were two different rules of microfracture changes before and after the main fracture coalescence,and the threshold effect characteristics of microfracture changes were obtained,which provided a basis for the use of damaged microcracks in hydraulic fracturing engineering. |
