Study On The Mechanism Of Fracture Propagation In Thermal Reservoir Fracturing

Geothermal energy is a kind of renewable and clean energy,which has abundant reserves,especially dry hot rock geothermal resources.However,it is difficult to use because of the characteristics of large buried depth,low porosity and low permeability.Enhanced Geothermal System（EGS）refers to the high permeability artificial Geothermal reservoir formed by the hot dry rock between the injection well and the production well through the fracturing technology of water medium or other fluid medium（supercritical carbon dioxide）.However,because the reservoir is in a high-temperature environment,the low-temperature induced thermal stress generated by the temperature gradient has an important effect on fracture propagation when the low-temperature fluid is injected into the high-temperature formation during fracturing.However,the mechanism of fracture initiation and propagation in dry hot rock under the influence of low temperature induced thermal stress is still unclear,which makes it difficult to effectively guide the design of EGS fracturing construction scheme and accurately predict the production capacity.For three different rock samples（granite,sandstone and shale）,the physical and mechanical properties of rocks were tested and the brittleness analysis was carried out.In the experiment,different initial temperatures of rock samples were set to simulate the actual environment of the hot reservoir.In addition,different injected fluids（slickwater,guanidine gum,supercritical CO₂）,different in-situ stress conditions and displacement rates were used to clarify the fracture propagation mechanism of the hot reservoir.The coupled temperature-seepage stress-damage（THMD）model was established,and its correctness was verified by physical model experiments.The fracture propagation mechanism of fractured high-temperature rock fracturing was further studied.The results show that the low temperature induced thermal stress is generated due to the temperature gradient between the rock and the injected fluid during the high temperature fracturing experiment.Due to the existence of low-temperature induced thermal stress,the higher the initial temperature of the rock,the lower the fracture pressure of the rock.Taking the granite sample as an example,the fracture pressure of the rock is 25.4 MPa when the temperature is 100℃and 16.3 MPa when the temperature is 300℃.In the high-temperature fracturing process,low-temperature induced thermal stress can lead to a more complex fracture grid,and the greater the temperature difference,the stronger the heterogeneity,the more abundant the natural fractures,and the more obvious the low-temperature induced thermal stress effect.Compared with conventional fracturing media,supercritical CO₂ also leads to more complex fracture morphology and can act synergistically with low-temperature induced thermal stress during high-temperature fracturing.