| The mechanical properties and rupture failure characteristics of brittle rocks under the influence of external environment have very important research value in the field of rock mechanics and petroleum engineering.Unconventional reservoirs such as shale gas and tight oil/gas usually require large-scale hydraculic fracturing to achieve higher productivity,and the brittleness of reservoir rock is closely related to the initiation and propagation behaviors of hydraulic fractures.However,most of the existing studies only focus on evaluation methods of rock brittleness.The mechanism of rock brittleness affecting the initiation and propagation of hydraulic fracture has not been studied thoroughly and meticulously.In this study,a new rock brittleness evaluation index was established from the viewpoint of energy evolution during rock failure process.Based on this index,the fracture initiation model and deflection model considering rock brittleness were deduced,and the influence of rock brittleness on the initiation and deflection of hydraulic fractures were studied.The specific research works in this paper are as follows:(1)Based on the analysis of the applicable conditions and advantages and disadvantages of the existing brittleness indices,the evolution of elastic energy,dissipated energy and post-peak rupture energy during the whole process from pre-peak deformation to post-peak failure of different brittle rocks were analyzed.On this basis,the key energies affecting the pre-peak deformation and post-peak failure stage of rock were determined,and the brittleness evaluation index which can comprehensively reflect the mechanical characteristics of rock before and after failure were established.(2)Laboratory mechanical experiments were carried out based on bedded shale and tight sandstone.The anisotropic characteristics of shale brittleness and the effect of condining pressure on brittleness of tight sandstone were studied respectively.The results showed that the brittleness of shale showed the change of increase-decrease-increase with the increasing of bedding planes' inclination angles,and the increasing of confining pressure would weaken the brittleness characteristics of tight sandstone.The calculation results based on the brittleness index established in this paper were in good agreement with the experimental results.At the same time,the comparison with other existing brittle indices showed that the new brittleness index was a function of some existing indices,which can comprehensively reflect the brittleness characteristics of rock during the whole process from pre-peak deformation to post-peak failure.(3)Based on the test data in this paper,the fitting relationship between tensile-compressive strength ratio and Poisson's ratio and rock brittleness were obtained.Then,the influence of rock brittleness on Micro-crack Ggeneration Zone(CGZ)at the tip of fracture was studied based on the Unified Strength Theory.The hydraulic fracture initiation model considering rock brittleness characteristics was deduced,and the variation of initiation pressure and initiation angle of hudralic fracture under different factors and rock brittleness were studied by theoretical calculation.The results showed that the initiation of fractures in the rock with high brittleness had three characteristics: high energy accumulation,low initiation pressure and large initiation angle.Under the combined influence of these three aspects,multi-directional branching fractures were easy to form during the initiation process of hydraulic fractures.(4)The existing intersection criteria between hydraulic fracture and natural frature oversimplify the stress distribution at the tip of hydraulic fracture,and are not suitable for quasi-brittle reservoir rock materials.In this paper,a new criterion for the drfection of hydraulic fracture considering rock's brittleness characteristics was deduced based on the Unified Strength Theory,which can predict various propagation patterns before and after the intersection of hydraulic fracture and natural fracture under different factors and rock brittleness.(5)Two series of hydraulic fracturing experiments were carried out based on the natural tight sandstone specimens with rough artificial fractures and artificial cement mortar specimens with different brittleness respectively.The effects of in-situ stress difference,intersection angle between fractures,roughness of artificial fracture and rock brittleness on the propagation and defection patterns of hydraulic fracture were studied.The experimental results showed that hydraulic fracrures were easier to penetrate through preset fractures with the increase of in-situ stress difference and intersection angle.With the increase of rock brittleness,hydraulic fracures tended to defect into and propagate along the preset fractures,thus the complex fracture patterns similar to Z-shaped,?-shaped and fishbone-shaped can be formed in rock specimens.The experimental results were in good agreement with the theoretical results.At the same time,for rock with lower brittleness,the propagation of hydraulic fracture in rock matrix usually shows a flat and single fracture pattern,while for rock with higher brittleness,the multiple branching propagation pattern of hydraulic fracture appeared in rock matrix.This paper established a new method for evaluating rock brittleness from the viewpoint of energy evolution of rock deformation and failure,and innovatively took rock brittleness as an important mechanical parameter into account in the study of hydraulic fracture initiation,propagation and defection mechanism,and obtained the new fracture initiation and defection model which can reflect the effect of rock brittleness.The study in this paper can reveal the influence of rock brittleness on hydraulic fracture's initiation and propagation mechanism more clearly.The results of this paper can provide new ideas and directions for the study of hydraulic fracturing mechanism of unconventional oil and gas reservoirs such as shale gas and tight oil and gas. |
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