| In order to achieve the two-carbon goal of "carbon peak" and "carbon neutral" as soon as possible,the development and utilization of geothermal energy becomes particularly important.In our country,dry hot rock geothermal reserves are sufficient and widely distributed,so efficient development of dry hot rock geothermal energy is of practical significance to achieve the double carbon goal.Hot and dry rock reservoirs are characterized by high rock strength,high grillability and low permeability,so they need to be fractured.When liquid nitrogen enters the reservoir as fracturing fluid,there will be a temperature difference with the high temperature reservoir,resulting in the temperature stress inside the reservoir and promoting the cracking of the reservoir.Therefore,it is particularly important to understand the damage characteristics and mechanism of the high temperature rock caused by the low temperature effect of liquid nitrogen.The liquid nitrogen jet assisted fracturing technology increases the impact of liquid nitrogen jet on the original low temperature effect,so it is necessary to further study the damage characteristics and mechanism of liquid nitrogen jet impact on high temperature rock.The main research contents of this thesis are as follows:(1)The Brazilian splitting test was carried out on granite treated with air cooling and liquid nitrogen cooling at different temperatures,and the basic mechanical parameters of the rock samples after different treatments were obtained.The acoustic emission signal parameters of the rock samples during the loading process were recorded by acoustic emission apparatus,and the influence law of the low temperature effect of liquid nitrogen on the physical and mechanical properties of granite and the change law of acoustic emission characteristic parameters were summarized.According to the energy evolution calculation formula,the elastic energy,absorbed energy and dissipated energy of the rock sample during the Brazilian splitting test were calculated,and the energy evolution law of the rock sample during the loading process was revealed.The test results show that the heating process will cause damage to the rock sample,and the low temperature effect of liquid nitrogen will further strengthen the damage effect.The wave velocity of liquid nitrogen treated rock samples is lower than that of natural treated rock samples at all heating temperatures;with the increase of heating temperature,the ring counts of liquid nitrogen cooled rock samples are significantly higher than those of natural treated rock samples;at the same heating temperature,the dissipation energy of natural cooled rock samples is greater than that of liquid nitrogen treated rock samples.Under the same stress level and heating temperature,the absorbed energy of natural cooling treated rock samples is larger than that of liquid nitrogen treated rock samples.(2)Using ANSYS finite element analysis software,a downhole model is established to analyze the distribution law of the flow field and heat transfer characteristics of the bottom hole under the impact of liquid nitrogen jet,as well as the distribution law of the non-uniform stress field and the scope of the damage area.On this basis,the influences of confining pressure,rock elastic modulus,rock temperature,outlet pressure and fluid temperature on rock stress are analyzed.The simulation results show that the maximum velocity of liquid nitrogen jet is greater than that of water jet and the pressure generated at the bottom hole is greater than that of water jet.The impact effect of liquid nitrogen jet on the bottom hole is better than that of water jet.Compared with water jet,the impact of liquid nitrogen jet can make the rock receive tension damage more easily,so as to crack more easily.The damage effect of liquid nitrogen jet on rock decreases with the increase of confining pressure.When the impact pressure drop of the jet is 20 MPa and the outlet pressure is 45 MPa,the impact effect of the liquid nitrogen jet is the best.(3)Brazilian splitting tests were carried out on high-temperature rock samples after liquid nitrogen injection cycle and liquid nitrogen immersion cycle to reveal the influence of liquid nitrogen jet impact on the physical and mechanical properties of high-temperature granite.Acoustic emission signal parameters of rock samples during loading were recorded by acoustic emission instrument,and the variation rules of acoustic emission characteristic parameters of granite rocks at different temperatures after liquid nitrogen injection cycle and liquid nitrogen immersion cycle were summarized.The variation law of elastic energy of rock sample during loading is analyzed.The experimental results show that with the increase of the number of cycles,the damage effect of liquid nitrogen jet impact on rock samples is enhanced,while the damage effect of liquid nitrogen immersion is weakened.The tensile strength of the 400°C injection samples was higher than that of the immersion samples until the 7th cycle,when the tensile strength of the injection samples decreased significantly and was lower than that of the immersion samples.200°C tensile strength of the samples treated with liquid nitrogen injection cycles was always higher than that of the samples treated with liquid nitrogen immersion.The thesis has 83 figures,7 tables and 105 references... |
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