| In order to cope with the problem that the single energy structure cannot meet the urgent requirements of the development concept at the current stage,it is urgent to carry out research on the dry hot rock resources in light of the fact that China was late to the hot dry rock research and was achieved little in its development.The development and utilization of hot dry rock involves interdisciplinary studies.The related scientific issues and engineering technology are very complicated.Reservoir stimulation and thermal energy extraction are the two key issues in the energy exploitation of dry hot rock.The reservoir stimulation requires the mastery of the fracture morphology and permeability of the rock during the fracturing process,which requires an accurate understanding of the rock mechanics properties.The exploitation of thermal energy involves the heat exchange between rock mass and water,which determines the final production capacity of EGS.This requires an accurate understanding of the thermal properties of rock.When rocks are under high temperature and high pressure,the physical and mechanical properties are different from those under the condition of no-static pressure at normal temperature.The effects of temperature and confining pressure on the physical and mechanical properties of the rock cannot be ignored.This involves temperature-stress-Fluid(THM)multi-field coupling problem.Therefore,studying the physical and mechanical properties of rock under high temperature and high pressure is the key to solve the two core issues of EGS system.It is of great significance to the development and utilization of EGS engineering.This article takes the research of dry hot rock as the background,mainly carried out the work including the analysis of the effect of the temperature and pressure on the physical and mechanical properties based on indoor experiment,hot dry rock reservoir stimulation and heat production capacity analysis.The indoor physical and mechanical tests and numerical simulations were performed on the shallow core of ZR2 well in Guide area in 3600 m.The variation rules of various parameters under the conditions of temperature and confining pressure were summarized,and the equation was obtained by fitting the change curve.Each parameter change equation was programmed with the hot-pressure change program HeaPre.f90 and embedded in the TOUGHREACT-FLAC3 D program developed by our group.The heat output simulation was performed to determine the optimal production rate and the heat generation performance under the conditions.Firstly,indoor experiments were conducted on the rock sample structure,chemical composition and basic physical properties in the Guide Basin,Qinghai Province,and the experimental results were analyzed.It was found that the rock samples were mainly composed of quartz,potash feldspar,hornblende,muscovite,and black Mica and sericite.The texture of the rock is diversified and the structure of the rock are single.The structures are mainly massive structure,and very few are almond-like structures.The thin rock sheets are dense and have few pores which are mostly fine pores with relatively uniform distribution.The content of thorium and potassium in granite is consistent with that of normal granites.The content of deep uranium between 3100-3600 m is 4.896-12.09?g / g,and the average is 7.57?g / g which is far higher than the content of uranium in normal granite.The difference between them is 37.2%,and the decay of high levels of radioactive uranium gave the rich geothermal energy resources in the Guide Basin.The rock masses in the deep underground are always kept compact under the action of the ground stress,and the depth change affects the density state fundamentally.The results of laboratory tests showed that the density of rock samples varied from 2.378 to 2.843 g/cm3 and increased with the deepening of the formation.With the deepening of the stratum,the thermal conductivity of rock samples fluctuates and the amplitude decreases.The variation range of thermal conductivity is 2.5-3.6 W/(m·K)in the shallow low-temperature strata of 100-130 °C,which is extremely scattered and irregular.In the strata above 130°C-150°C,the thermal conductivity of rock samples tends to be stable and its value is 3.2 W/(m·K).The specific heat capacity changes with the deepening of formation depth and the fluctuation ranged from 0.511 to 0.972 kJ/(kg·K).During the fluctuation process,the specific heat capacity value increased slightly and finally stabilized at 0.823 kJ/(kg·K).The porosity of rock varies in a jump with depth,and there is no obvious regularity;The permeability of rock gradually decreases with depth,and it rises and falls at a shallow curve amplitude before 2000 m.After 2000 m,the rock gradually becomes stable to 0.289 md.The porosity of rock varies greatly within the range of 130 °C,and it tends to stabilize to 3.290% within the range of 130-150 °C.Both the shear wave velocity and the longitudinal wave velocity of rocks increase with the deepening of stratum depth.The S-wave velocity increases from 2.718 km/s at 500 m depth to 3.020 km/s at 3600 m depth,which increases by 10.0% downwards at 3100 m,and the longitudinal wave speed increases from 5.102 km/s at 500 m to 5.814 km/s at 3600 m,which increases by 12.2% downwards at 3100 m.The curve equations of rock density,thermal conductivity,specific heat capacity,porosity,permeability and wave velocity with formation temperature were fitted respectively.The rock samples were then subjected to uniaxial(20-140°C)and triaxial(30-150°C,0.1-50 MPa)rock tests.The results of uniaxial rock tests showed that the compressive strength and tensile strength of rock samples increase with the temperature increase ranged from 20 to 80 °C and decreases with temperature increase in the interval of 80-140 °C.The range is 131.22-143.33-134.33 MPa and 10.58-11.55-7.88 MPa respectively.The cohesion and internal friction angle of rock tend to decrease with increasing temperature.The maximum deformation of rock increases with the increase of confining pressure.The peak strength of rock increase linearly with the increase of confining pressure.When the confining pressure is low,the temperature has a positive correlation with the peak intensity.When the confining pressure is higher,the temperature has a negative correlation with the peak intensity.The elastic modulus varies discretely with temperature,and the variation law is not obvious.The confining pressure and elastic modulus are in a positive proportion relationship.The higher the confining pressure is,the higher the elastic modulus is.The Poisson's ratio varies irregularly with increasing confining pressure,and the Poisson's ratio variation is small but the rate of change is large,indicating that the confining pressure also contributes to the Poisson's ratio.The change rule of Poisson's ratio varies with the temperature is similar to those with the confining pressure.According to the law we obtained,the variation of each parameter with temperature under different confining pressure was fitted respectively.The fitting equations related to the above physical-mechanical parameters were programmed with the hot-pressure change program HeaPre.f90 and embedded in the TOUGHREACT-FLAC3 D program developed by our group to simulate the heat transfer performance of the rock with changes in temperature and pressure.In the 3200-3400 m of ZR2 well,Stoneley has obvious “W” reflection fringes,with high fissure density and fissure development.From 3210 m,the resistivity value of the formation,the natural gamma value and the geothermal gradient increase.The magmatic rock tends to meet the dry hot rock standard and is set as a fracturing section.Fracturing simulations of different proppant concentrations and displacements were carried out in the fracturing section to obtain the optimal solution: half length 185.7 m,height 101.3 m,opening 5.4 mm,conductivity 255.2 mD·m,and permeability 47.3 D.With constant proppant concentration,the fracture half length and height increase significantly with the increase of construction displacement.The crack opening decreases,the diversion capacity weakens,and the permeability does not change much with the increase of construction displacement.The change of construction displacement significantly affects the hydraulic cracks,geometry and conductivity.When the construction displacement kept constant,as the propellant concentration increases,the half-length and height of the cracks almost do not change,the crack opening increases,the conductivity is doubled,the permeability increases significantly.The concentration of proppant significantly affects the conductivity,opening and penetration rate of hydraulic fracturing.The maximum production flow rate was determined to be 6 kg/s,the power generation during the 20-year period was 0.009-0.039 MW,and the flow resistance was 4 MPa/(kg/s). |
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