| Because of complex geological structure,strong heterogeneity and discontinuity of oil and gas storage of carbonate reservoir,acid fracturing has become a key technology for production,stabilization and stimulation.Achieving a high and stable conductivity of acid-etched fractures is of great importance for the reconstruction of high-temperature and high-pressure carbonate reservoirs in deep layers.The shape and size of seepage space directly determine the level of conductivity;therefore,an accurate characterization of seepage space is significant for conductivity evaluation.Benefiting from heterogeneous dissolution of acid solution and slipping effect of fracture surface,acid etching fracture can still construct effective seepage space under closure stress and maintain high conductivity.Due to the complex surface morphology of fracture surface,it becomes very difficult to research the problem of fracture contact during the closure process and the deformation mechanism of asperity on fracture surface under different contact modes needs to be improved.Simultaneously,the surface morphology of acid etching fractures is mainly studied based on single 2D parameter,but neglecting the quantitative description of spatial morphology,so that the spatial morphological characterization parameters of acid etching fractures are lacking.Therefore,it is necessary to clarify the formation mechanism of acid-fractured slipping fractures and to further analyze the closure mechanism of acid-etched fracture surface for spatial reconstruction of acid-etching fractures seepage and the establishment of fracture space quantitative characterization methods which can reveal evolution law of the fracture space under slipping effect.Natural fracture slipping effect in stress disturbance zone is numerically simulated by finite element software,natural fracture slipping effect theory in fracture connection zone was expounded,artificial fracture slipping mechanism was evaluated and the effect of acid etching softening on slip fracture was summarized.The different angles between artificial and natural fractures were selected to simulate natural fracture slip effect in stress interference zone.The results show that induced stress of artificial fracture and squeezing of injected fluid on fracture wall lead to the slipping of natural fracture wall in stress interference zone.No matter the angle between artificial and natural fractures,there will be slip effect and exist difference in formation mechanism.The slipping effect has certain universality in acid fracturing process.Natural fracture slipping theory shows that the natural fracture in fracture connection area will slip first and then be acid etched.The slipping degree is mainly affected by Young’s modulus,Poisson’s ratio,geostress difference,angle,natural fracture length and friction coefficient of fracture surface.On the one hand,artificial fracture slips when crack is initiated.On the other hand,it forms a geometrically equivalent slip caused by differential etching on both sides of fracture.Under the effect of acid etching softening,there is a power function between reduction of width and closure stress.Because of slipping effect,the asperities on both sides of fracture in zone II has both positive and side contact,thus more effective seepage channels can be formed.Based on micro-mechanics,considering the three deformation stages of asperity,i.e.complete elasticity,elastoplasticity,complete plasticity,interpolation method is introduced in the elasto-plastic deformation stage to establish the micro-convex body normal contact model,which overcomes the shortcomings of conventional model.Through Gaussian distribution law,the whole fracture surface was mapped to form a fracture surface contact model.This model is highly reliable since it can precisely describe the closed behavior of fracture when the normal stress increases.As for side contact,an improvement is made in this study on the contact mechanism of asperity in three deformation stages in the light of classical KE model,and the fracture surface side contact model is formed according to the law of Gauss distribution.In this research,the digital method was used to reconstruct acid etching fracture space,and the evolution law under slipping was investigated by means of self-written program.It can be concluded that with the increase of slip distance,the spatial morphology of equivalent acid etching fracture changes greatly compared with initial morphology.The maximum of equivalent space height is positively correlated with slip distance,and the proportion of non-contact has no obvious rule with slip distance.The curvature concept was introduced to construct 7×7 mesh model to describe features in four directions of a certain point in space,and different weights were given to establish a complex space body curvature calculation method,which effectively characterized the variation characteristics of acid etching fractures space.That is,the average curvature of equivalent acid etching fracture space has a quadratic function relationship with slip distance.The larger the slip distance and average curvature.,the more complicated the etching space is,and the more effective the seepage channel appears to be.The study of the average width of acid-etched fracture shows that slipping effect significantly increases the average fracture width.The more complex the fracture space shape before slipping,the larger the increase ratio of fracture width after slipping.When it comes to the relationship between average fracture width and average curvature,this research,taking the degree of fracture surface closed(satisfaction),closure stress and rock mechanics parameters into consideration,builds a conductivity prediction model based on curvature of seepage in acid etching fracture.In addition,it also establishes the conductivity prediction model of acid-etching slipping fracture under multi-factors,in accordance with softening effect,closure deformation characteristics,and evolution law of acid etching fracture under slipping effect. |
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