| Glauberite is an important sulfate deposit and one of the sources of glauberite,an important chemical raw material.In order to improve the mining efficiency of glauberite and reduce the mining cost,the research team proposed the scheme of in-situ modified fluidization mining of glauberite.However,glauberite belongs to insoluble rock salt.In the early stage,complex fracture network needs to be formed in the ore bed through hydraulic fracturing in order to realize large-scale commercial mining.The geological conditions of glauberite reservoir in China are complex and changeable.At present,it is impossible to effectively monitor the deep fracture propagation process,coupled with the lack of understanding of the fracture propagation mechanism,resulting in a certain waste of resources.Therefore,it is of great significance to study the fracture propagation mechanism and analyze the influencing factors of fracture propagation under the condition of glauberite hydraulic fracturing.This paper will evaluate the compressibility of glauberite through the combination of experiment and numerical simulation,and analyze the influence of rock brittleness,in-situ stress difference,natural fracture,fracturing fluid temperature,displacement,fracturing well location and other factors on hydraulic fracturing fracture propagation.The main research contents and results are as follows:(1)Mechanical experiment of glauberite: uniaxial and triaxial compression and tensile experiments were carried out on glauberite.The experimental results show that the average uniaxial compressive strength of glauberite is 16.36 Mpa,the average elastic modulus is 2.65 Gpa,the average Poisson’s ratio is 0.25,and the average compressive strength of glauberite is 37.404 Mpa,41.079 MPa,49.627 MPa,56.424 MPa and65.232 MPa when the confining pressure is 4MPa,5MPa,6Mpa,7MPa and 8Mpa respectively.The average elastic modulus is 8.507 gpa,9.560 GPA,11.912 GPa,15.347 GPa and 16.835 GPa respectively,and the average tensile strength of glauberite is 2.0Mpa.(2)Rock brittleness: using the mechanical parameters of glauberite obtained in Chapter 2,the brittleness of glauberite is evaluated based on the stress-strain curve and strain energy density.It is found that the brittleness of glauberite decreases with the increase of confining pressure.Then the effects of different elastic modulus,Poisson’s ratio,compression tension ratio and residual stress on rock brittleness are explored by using RFPA simulation software.Finally,it is found that the greater the elastic model and compression tension ratio of rock,the higher the brittleness of rock and the more prone to failure;The greater the Poisson’s ratio,the lower the brittleness of the rock and the higher the ability to resist failure.(3)Mechanism of glauberite hydraulic fracturing: the whole fracturing process of glauberite hydraulic fracturing is analyzed step by step,and the mathematical control equation suitable for rock salt hydraulic fracturing is established.(4)Glauberite hydraulic fracturing experiment: use the large true triaxial seepage simulation device in the laboratory to explore the effects of different in-situ stress,fracturing fluid temperature and displacement on glauberite hydraulic fracturing.The experimental results show that the greater the in-situ stress is,the more difficult it is for glauberite to crack,and the cracks generally develop along the direction of the maximum horizontal principal stress.With the increase of glauberite buried depth,not only the in-situ stress will change,but also the temperature of glauberite reservoir will rise.Under the condition of consistent in-situ stress,compared with the water temperature at 80℃,60℃ and normal temperature,the cracking pressure at 80℃ is 2.8% and 5.7% lower than that at 60 ℃ and normal temperature respectively,and the fracture opening increases,which further indicates that the dissolution also has a certain impact on the cracking pressure of glauberite.At the same time,the fracturing effect of glauberite reservoir is closely related to the hydraulic displacement.Maintaining a low displacement is conducive to the extension and expansion of fractures and the formation of more complex fractures.Higher displacement can reduce the fracturing time.(5)Numerical simulation experiment: in order to explore the influence of brittleness on glauberite hydraulic fracturing,rfpa-flow numerical simulation software is used to conduct hydraulic fracturing on glauberite under different confining pressure,elastic modulus,Poisson’s ratio and pressure pull ratio.The research results show that the higher the brittleness of glauberite,the more complex the effective hydraulic fracturing network formed after hydraulic fracturing,which is more conducive to the effect of hydraulic fracturing.At the same time,the influence of natural fractures,non-uniform water pressure and multi well simultaneous fracturing on fracture propagation is also explored.The research shows that the activation of closed natural fractures is related to the dip angle of hydraulic fractures and natural fractures.When the dip angle is large(close to 90 °),the formation has a higher fracturing pressure,and the easier it is to form two-way branches at both ends of natural fractures,which is more conducive to the formation of fracture network.When multiple wells are fractured at the same time,if the two wells are close,the crack propagation of one well will be restrained by the other well due to the stress superposition effect.When the distance is large,the inhibition effect will weaken or disappear... |
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