Response Of Fracture-filled Hot Dry Rock Mass To Thermo-Hydro-Mechanical Coupling And Its Application

As a high-quality and undeveloped resource with huge reserves,the efficient development and utilization of hot dry rock(HDR)geothermal energy has become a hot research topic worldwide.Using the natural fracture system of deep HDR mass to construct reservoir will greatly simplify the reservoir construction process,reduce the construction difficulty and development cost,and improve the efficiency of water-rock heat exchange.Therefore,a new research direction is put forward to build HDR reservoir and exploit geothermal energy based on the structural characteristics of fracture-filled granite.Aiming at this brand-new issue,we conducted a series of studies and came to the following conclusions.(1)The characteristics of the natural fracture system of deep HDR were studied through field observations,it was found that structural fractures in deep granite bodies filled by magma or hydrothermal fluid are common phenomena.Through observations with a polarising microscope,it was found that under the high-temperature and hydrothermal fluid action of the backfill,changes in the number of thermal-cracking fractures of fracture-filled granite versus the distance from the cementation interface can be divided into three zones:a zone in which the number of fractures in the backfill fluctuates gently,a zone showing a sharp increase in the number of fractures in the parent rock,and a zone in which the fluctuation in the number of fractures in the parent rock decreases.The position of the maximum number of thermal-cracking fractures is located in the parent rock at a certain distance away from the cementation interface.This maximum number has an exponential relation with the thickness of the backfill,and its position is also related to the thickness of the backfill.In addition,the distribution of thermal-cracking fractures in the granite parent rock conforms to the fractal law.The universality of the fracture backfill and its large-range influence on the granite parent rock caused by thermal action result in the formation of a huge weak-plane structure in the granite parent rock.This weak-plane structure is very likely to become a natural reservoir in HDR geothermal development or an easy-to-rupture position when an artificial reservoir is constructed by hydraulic fracturing,which is beneficial for the construction of artificial reservoirs.(2)The difference in thermal and mechanical properties between the granite in Luya Mountain,Shanxi Province,China(coarse-grained granite)and the granite in Shandong Province,China(fine-grained granite)under high-temperature(100-400?)triaxial stress was studied by using high temperature high pressure rock mechanics triaxial testing machine and the effect of grain size on the properties of granite was discussed.The results show that the thermal expansion coefficient of the coarse-grained granite increases linearly with increasing temperature,and the thermal expansion coefficient of the coarse-grained granite is 1.52 times that of the fine-grained granite on average,and the difference reaches a maximum at 400?.The elastic modulus of the coarse-grained granite increases slowly first and then decreases sharply with increasing temperature,and its threshold temperature varies with temperature is approximately 300?.The elastic modulus of the fine-grained granite is 1.4-2.6times that of the coarse-grained granite,and the difference between them increases with increasing temperature and confining pressure.According to the failure test under 25 MPa hydrostatic pressure and at 400?,for the coarse-grained granite,the peak strength,elastic modulus and the threshold temperature of elastic modulus change with temperature are smaller,the peak strain is larger and the elasto-plastic transition occurs easier than those for the fine-grained granite.At200-400?and 4 MPa hydrostatic pressure,the permeability of coarse-grained granite is 2.2-4.3 times of that of fine-grained granite,and the difference between them increases with the increase of temperature.Micro-observation shows that the larger crystal particles and the extreme heterogeneity of the coarse-grained granite lead to larger thermal deformation and greater deterioration of the mechanical properties,compared with those of the fine-grained granite,further leading to higher permeability of the coarse-grained granite under high temperature and high pressure.(3)The deformation and permeability tests of the parent rock(type?granite),hydrothermal fluid backfill(type?granite),cementation interface between the backfill and the parent rock laterally positioned through the specimen(type ? granite),and cementation interface between the backfill and the parent rock longitudinally positioned through the specimen(type ? granite)at high temperature(maximum temperature 500?)and high pressure(confining pressure 25 MPa)are carried out by using the high temperature and high pressure rock mechanics triaxial testing machine.The results of the deformation experiment of four kinds of fracture-filled granite show that the thermal expansion coefficient of four kinds of granites can be divided into three stages:1)the fluctuates slowly section at low temperature;2)the rapid increase section at middle-low temperature;3)the rapid decrease section at middle-high temperature.Besides,the threshold temperature of elastic modulus changes(T_cE)of four types of granite are 300?,200?,250?and300?,respectively.Taking T_cE as the boundary temperature,the elastic moduli of the four types of granite first increase slightly and then decrease rapidly.The honeycomb-type dissolution pore structure in the hydrothermal fluid backfill(type?granite)yields the lowest elastic modulus of the constituent materials,but under triaxial stress at 500?,the compressive strengths of type?,?granite are lower than that of type?granite due to their cementation interface structure.Additionally,all types have a lower compressive strength than type?granite.Then,the failure model of fracture-filled granite under high-temperature triaxial stress was determined.In addition to the conventional shear failure surface,the failure surface of fracture-filled granite also occurs at the boundary of coarse crystal particles in the parent rock,the dissolution pores in the backfill and the cementation interface.The failure surface at the cementation interface propagates along the direction of the cementation interface,and the failure surface has the greatest influence on the compressive strength of the granite body.The results of the permeability experiment of four kinds of fracture-filled granite show that the threshold temperature of the permeability changes for the four types of granite are 300?,200?,300?and 250?,respectively.From the threshold temperature to the maximum test temperature,the permeability of type ?,?,? and ? granite increases exponentially by 1,2,2 and 3 orders of magnitude,reaching 10^-6 D,10^-4 D,10^-5 D and 10^-4 D,respectively.Micro-observation results of the micro-structures show that thermal cracking of the heterogeneous rock mass is the main reason for the increase in permeability of type ? and ? granite;the low strength and deteriorated mechanical properties caused by dissolution are the main reasons for the permeability of type ? granite to significantly exceed that of type ? and ? granite;the extreme heterogeneity and the deteriorated mechanical properties of the backfill together indicate that type ? granite has the best permeability.During reservoir construction by hydraulic fracturing in fracture-filled granite,the flow of the fracturing water along the direction perpendicular to the cementation interface is limited.However,when the reservoir temperature exceeds 250?,the backfill strengthened by the cementation near the cementation interface restores the weak-plane structure characteristics;when the reservoir temperature exceeds 400?,the thermal cracking fractures near the cementation interface interconnect with each other along the cementation interface to form a permeability channel.Therefore,the fracturing water can easily flow along the cementation interface.(4)The hydraulic fracturing test of fracture-filled granite at high temperature(maximum temperature 400?)and high pressure(maximum hydrostatic pressure 25 MPa)was carried out by using the high temperature-high pressure rock mechanics triaxial testing machine.In addition,the acoustic emission system is used to monitor the test process in real time,and the fracture characteristics of hydraulic fracturing process are qualitatively analyzed.Under 25 MPa hydrostatic pressure and within 400?,regardless of the strike of cementation interface between backfill and parent rock,the crack initiation position of fracture-filled granite will always at the cementation interface,and hydraulic fractures will expand along the cementing interface and inside the backfill body.In addition,the initiation pressure of fracture-filled granite changes with increasing temperature in a negative logarithm.When the temperature exceeds 300?,the thermal shock in the hydraulic fracturing process will cause the granite to form a fracturing fluid seepage channel along the direction of the cementation interface,which greatly reduces the initiation pressure of the granite.Under the conditions of 25 MPa axial pressure and 10 MPa confining pressure,the propagation direction of hydraulic fracture is not perpendicular to the direction of the minimum principal stress but along the direction of the cementation interface,which indicates that the existence of the backfill makes the fracture-filled granite no longer subject to stress conditions limits.In the process of hydraulic fracturing,acoustic emission(AE)signals with high frequency and high persistence will be generated in the backfill body.At room temperature,only when the pressure reaches the initiation pressure,can the obvious AE signal be produced.Within 200-400?,the AE signal will go through four stages:1)a small sudden increase stage of AE signal;2)a quiet stage of AE signal;3)a rapid increase stage of AE signal;4)a slow weakening stage of AE signal.When the fracturing fluid pressure reaches the initiation pressure,the intensity of AE signal reaches the maximum value synchronously,and the ringing number and duration of AE signal at different temperatures are basically the same,however,the higher the temperature,the lower the energy of AE signal.(5)Based on the in-depth study on the structural characteristics of natural HDR mass,the geothermal development plan of fracture-filled granite is proposed.By using the natural weak-plane structure such as natural fracture-filled zones and areas with an increased number of fractures in granite parent rock to build large volume reservoir,the horizontal well construction is canceled,the construction cost and the difficulty of extracting geothermal energy of HDR are greatly reduced,which provides a new theory and technology for large-scale,high-efficiency and low-cost construction of high-permeability HDR reservoir.