Study On The Seepage Equivalent Of Fractures Surrounding Rock In Underground Water Seal Reservoir

The underground water-sealed oil reservoir is the best type of oil reserves.With the advantages of small area,high security,large storage capacity and environmental protection,the underground water-sealed oil storage has been greatly developed in our country.Underground water-sealed oil storage are often built in hard granite rock,the sealed effect of water is mainly affected by the spatial distribution characteristics of joints and fractures.Because the seepage of fractured rock in underground water-sealed reservoir is a large-scale engineering geological problem,the fracture network has strong size effect and anisotropy.And three dimensional random fracture model is complex,difficult to seepage calculation applied to large scale model.Under the condition that the permeability and permeability coefficient are basically the same before and after the equivalence,this paper proposes a simplified fracture network equivalent method.This method simplifies actual complicated fracture network,and preliminarily realized the seepage field calculation of large scale underground water sealed oil depot considering the fracture characteristics of rock mass.The main research contents and results are as follows:(1)Combined with the drilling data of the reservoir site area,the 3D model of the underground water seal reservoir is established to facilitate the visualization of the reservoir site area model.In the 3D model,a typical section is cut to determine the relative position of main tunnel chamber,water curtain system,construction roadway and stratum.Then according to the borehole data of the repository site and the statistical data of the exposed fractures,the fractures in the reservoir area are divided.the characteristics of fracture space development in different regions are studied.Then,the mathematical statistics of the fractures in the typical region are analyzed,and the mathematical distribution forms of the fracture,trace length and density are obtained,and the two-dimensional stochastic fissure network is generated by the Monte Carlo method.(2)This paper analyzes the trend of permeability of two-dimensional random fractures with the trend of model size and seepage direction,and studies the anisotropic seepage characteristics and size effect of fractured surrounding rock.Then the seepage ellipse of different sizes is fitted by the calculation results,and the main value and main direction are calculated.When the size effect of the rock mass is more than 120 m × 120m(REV),and the size effect can be ignored.(3)By using the single fracture seepage theory and the two-dimensional fractured seepage equation,the equivalent method of a single group of parallelfractures,two groups of cross fractures and two-dimensional random fractures is derived.Combined with the field water extraction test,the opening of the fracture after the equivalent fracture network is retrieved under the condition of the same seepage coefficient and the direction of infiltration.The permeability REV is taken as the equivalent basic unit of fractured rock mass,which makes the equivalence representative to some extent.(4)The simplified fracture network model was established by using the discrete element software,and the water pressure distribution and flow velocity change rule are studied under the condition of water curtain or no water curtain,and the water seal mechanism and the change law of the seepage field were studied.Comparing the results of the traditional equivalent continuum model with the discrete fracture network seepage model,the advantages and disadvantages of the equivalent fracture method are analyzed.