| Hot dry rock reservoirs are widely distributed and have huge reserves,which are of great practical significance for improving China's energy reserves.However,due to the extremely low permeability of the reservoir rock,it is difficult to obtain economically beneficial heat recovery only by relying on its own permeability,and manual intervention is required to generate fractures.As a new type of waterless fracturing method,liquid nitrogen fracturing technology has extremely superior potential for fracturing high-temperature reservoirs due to the ultra-low temperature characteristics of the fluid used.Therefore,studying the fracture mechanism of liquid nitrogen fracturing high-temperature granite has exploratory theoretical research and guiding significance for the development and utilization of deep dry hot rock geothermal energy.In this paper,the change of stress state around the wellbore during the injection of liquid nitrogen into high-temperature reservoirs,the damage characteristics of high-temperature granites under the action of cryogenic liquid nitrogen,and the fracture characteristics and crack morphology of granites under the combined action of cryogenic and pressure liquid nitrogen are studied respectively.The specific research contents are as follows:(1)In order to reveal the change law of the stress distribution around the wellbore during the process of liquid nitrogen injection into the hot dry rock reservoir,a thermalhydraulic-mechanical coupling numerical model of the stress and heat transfer around the wellbore was established,and the surrounding area of the wellbore was studied during liquid nitrogen injection.The law of temperature field and stress field change.The study found that the liquid nitrogen injected into the wellbore will cause the temperature of the rock around the wellbore to decrease,forming a cryogenic zone.Due to the formation of the temperature difference inside the rock,a huge tensile stress exceeding the tensile strength of the granite will be generated around the wellbore.As the initial temperature increases,the thermal stress around the wellbore increases,and the tensile stress region inside the reservoir also increases.The inlet pressure and ambient pressure have little effect on the tensile stress produced by liquid nitrogen cooling.The thermal stress generated in the process of liquid nitrogen injection helps to generate thermal fractures around the wellbore in the direction of minimum in-situ stress,and to a certain extent promotes the propagation of some fractures in the direction of minimum in-situ stress during fracturing,which is conducive to the formation of complex fractures during fracturing..Compared with hydraulic fracturing,liquid nitrogen fracturing can effectively reduce the injection pressure and enhance the fracturing effect of high-temperature reservoirs.(2)In order to reveal the damage characteristics of high-temperature granite caused by cryogenic action of liquid nitrogen,the effects of cryogenic cooling of liquid nitrogen and freezing of liquid nitrogen on mechanical parameters and micro-crack propagation of high-temperature granite were studied by using ultrasonic,Brazilian splitting and acoustic emission monitoring.The results show that the cryogenic action of liquid nitrogen can increase the damage degree of high-temperature granite,that is,further cryogenic damage is generated on the basis of thermal damage,and the deterioration of rock mechanical properties is aggravated.Therefore,the wave velocity,tensile strength,absorption energy and elastic energy of the rock sample after liquid nitrogen treatment are lower than those after heating treatment.In addition,liquid nitrogen treatment can reduce the difficulty of microcrack initiation of high-temperature granite,and even the phenomenon of microcrack propagation occurs inside the rock sample at the initial stage of loading.When the heating temperature is 25?300 ?,the process of heating will cause less damage to the granite.The cryogenic state of liquid nitrogen will increase the cryogenic cracking effect of liquid nitrogen on the hightemperature granite,which will help reduce the mechanical strength of the rock and the difficulty of microcrack initiation.When the heating temperature increases to300?600 ?,the thermal damage caused by the heating process to the rock sample increases.The cryogenic state of liquid nitrogen will make the pore structure of the high-temperature granite more compact,and the cryogenic cracking effect of liquid nitrogen will be weakened.(3)The self-developed liquid nitrogen simulated fracturing test system is used to conduct fracturing tests on granite samples with pore.Under the combined effect of cryogenic and high pressure of liquid nitrogen,the rock sample fractured along the axis of the borehole,forming a double-wing-shaped fracture.At the same time,a liquid nitrogen fracturing test was carried out on the rock sample under high temperature.Due to the combined effect of thermal damage during heating and thermal expansion deformation of minerals,the fracture pressure of rock samples decreases first and then increases and then decreases with the increase of heating temperature.When the heating temperature of the rock sample is lower than 150 ?,the fracture pressure of hightemperature granite during liquid nitrogen fracturing first decreases and then increases.When the heating temperature exceeds 220 ?,the fracture pressure begins to decrease significantly with the increase of the heating temperature,and is significantly lower than that of the rock sample after the heating treatment at the same heating temperature.When the rock sample is heated to 220 ? and 300 ?,the fracture pressure is reduced by 32.32% and 40.63% respectively compared with the original rock sample.The thermal shock caused by liquid nitrogen is expected to produce thermal cracks around the perforations of the rock sample.The higher the initial temperature of the rock sample,the farther the thermal cracks extend,and the more complex cracks are formed in the high-temperature granite during the fracturing process.(4)The cryogenic action of liquid nitrogen will further increase the damage of high-temperature granite,increase the number of micro-cracks in the granite,and improve the connectivity of the rock microstructure.The fracture pressure of the rock sample after liquid nitrogen treatment decreases with the increase of heating temperature,and is lower than that of the rock sample after heating treatment.In addition,three macroscopic fractures appeared in the fracture mode of the rock sample with liquid nitrogen treatment after liquid nitrogen fracturing,and the complexity of the fractures began to increase.In the cryogenic state of liquid nitrogen,the original microfractures are opened due to the cold shrinkage of the rock matrix.During fracturing,the permeability of liquid nitrogen in the granite is greatly improved,which is conducive to the full expansion of rock micro-fractures during the fracturing process.The fracture mode of the rock sample is mainly composed of randomly distributed tiny cracks and holes.Liquid nitrogen fracturing can have multiple fracturing effects on thermal fracturing(thermal shock,cryogenic damage and freezing cracking)and pressure fracturing of hot dry rock reservoirs.The thermal fracturing effect generated by liquid nitrogen helps to reduce the difficulty of fluid entering into the dry-hot rock,reduce the reservoir fracture pressure,promote the overall expansion of micro-pore fractures in dry-hot rock reservoirs,and thus improve the complexity of fractures.The paper has 60 figures,6 tables and 132 references... |
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