Study On The Mechanical Properties And Hydraulic Fracture Propagation Laws Of Pre-damaged Hot Dry Rock

Hot dry rock fracturing is the core procedure in Enhanced Geothermal Systems（EGS）projects,which require the formation of a complex network of cracks in the thermal reservoir connecting injection and extraction wells.However,high breakdown pressure,single crack development,and unpredictable fracture extension direction are common problems in hydraulic fracturing of hot dry rock.To address these issues,the assisted hydraulic fracturing method,which pre-damages the hot dry rock before fracturing,can effectively reduce the breakdown pressure during fracturing and improve the crack network size and density after fracturing,making it an indispensable technical tool for the future development and utilization of hot dry rock.This paper investigated the mechanical properties and hydraulic fracture propagation laws of pre-damaged hot dry rock through experimental investigations,theoretical derivations,and numerical analyses,and the main research methods and results are as follows:（1）Four lithologies were collected from stratigraphic outcrops of hot dry rocks in the Zhangzhou Basin,Fujian,and the specimens were heated to different temperatures before being pretreated by microwave irradiation,chemical stimulation,or blasting,and the response patterns of hot dry rock with different lithologies and temperatures were analyzed in each treatment.The results show that changes in the fractions and proportions of minerals susceptible to microwave and solution treatments in hot dry rock with different lithologies resulted in differences in heating rates and chemical reaction levels in microwave and chemical treatments.Variations in the wave impedance of hot dry rock with different lithologies resulted in discrepancies in the peak circumferential stresses in the hole wall to which they were subjected during blasting.In addition,temperature influenced the behavior of hot dry rock in each treatment by altering its pore structure and wave impedance.（2）Employing scanning electron microscopy and nuclear magnetic resonance techniques,the effects of various pretreatment methods on microstructural damage and pore structure alterations in hot dry rocks of various lithologies and temperatures were investigated.The variation law of tensile strength was obtained based on ring compression tests at high temperatures in real time.The results show that the more intensively the hot dry rock responded to different pretreatment methods,the more severe the microstructural damage,the more complex the pore structure,the larger the porosity,and the greater the pore connectivity.Moreover,the microwave or chemical pretreatment mainly induced the large pores to form pre-damage,and the micro-pores contributed less to the pre-damage;after blasting treatment,the pre-damage was mainly developed by the micro-pores and macro-pores together.The ring compression test results reveal that the variance in tensile strength after pretreatment corresponded to the pre-damage pattern of hot dry rock with different lithologies and temperatures.（3）The real-time,high-temperature,true triaxial hydraulic fracturing test of pre-damaged hot dry rock and the fracture test during hydraulic fracturing were carried out to obtain the crack initiation pressure,breakdown pressure,and fracture pressure of pre-damaged hot dry rock.Meanwhile,the 3D laser scanner technology was utilised to obtain the 3D morphology of the hydraulic fracture surface.By constructing the fracture toughness theoretical model based on fractal theory,the pre-damaged hot dry rock fracture toughness considering fractal cracks was obtained,and the influence law of fractal dimension on it was analysed.The results show that the more severe the pre-damage,the greater the reduction in initiation pressure,breakdown pressure,and fracture pressure of the hot dry rock compared to the undamaged condition.Additionally,the fractal crack fracture toughness was consistently larger than the linear crack fracture toughness for each specimen,and the ratio of both was positively correlated with fractal dimension,both increasing with pre-damage.（4）Based on the theoretical model of hydraulic fracturing initiation pressure,the initiation pattern of different pre-damaged hot dry rocks and the variation law of fracturing crack extension pattern were analysed in conjunction with the test results.The hydraulic fracture surface characteristics were used to calculate the fractured crack surface’s unit surface area and fine morphological characteristics.Develop a theoretical model of reopening pressure during hydraulic fracturing of hot dry rock,validate the model using fracture test results,and investigate the reopening pressure impacting parameters.The results show that hot dry rock with different lithologies and temperatures exhibited different fracturing initiation patterns and fracturing crack extension morphologies after different pre-treatment methods,as well as differences in the fracture surface unit surface area and fine morphological characteristics.Compared to the traditional reopening pressure theory,the predicted values of the fracture mechanics-based reopening pressure theory model are closer to the measured values and more in line with engineering practice.（5）Based on four parameters:unit surface area,root mean square height S_a,kurtosis S_ku,and maximum height difference S_z,a principal component analysis model was developed to obtain a unique parameter to characterize the fine-grained morphology of the hydraulic fracture surface.This unique parameter was then employed as the dependent variable,along with the initiation pressure,breakdown pressure,and reopening pressure,to generate the pre-damage feasibility evaluation score index by constructing a TOPSIS evaluation model.Finally,different machine learning algorithms were selected to establish optimal microwave,chemical,and blasting pre-damage models based on the data characteristics of various hot dry rock hydraulic fracturing.The comprehensive score indexes obtained from different pre-damage models were compared to achieve a comprehensive prediction and evaluation of the feasibility of hydraulic fracturing for hot dry rocks with different temperatures and lithologies after pre-damage by microwave,chemical,or blasting methods.