The Study Of Fracture Surface Morphology Characteristic And Seepage Characteristic Of Fracture Based On Fractal Theory

The essential purpose of hydraulic fracturing is to create seepage channel with high permeability for tight oil and gas reservoir.The physical properties of artificial fractures system are very complex,fracture aperture was controlled by a lot of factors such as fracture closure stress,mechanical properties of rock,and fracture surface morphology and so on.Through the study of single crack,the physical properties of fractures system can be better understanded and evaluated.The change of fracture conductivity during the exploitation in reservoir,is the result of the change of mesoscopic fracture surface contact state with the increase of closure stress.In this process,the effective closure stress and the mechanical properties of the rock becomes decisive key,so it is very important to study fracture surface morphology and the change rules.At present,the fracture surface morphology was studied by use of two kinds of parameters: geometry parameters and fractal dimension,most of them are to analyze the morphological characteristics of the one-dimensional profile,which is difficult to fully reflect the characteristics.therefore,to establish a new characterization parameter for the analysis of the fracture surface morphology and study the change of fracture aperture before and after crack slumping are very necessary.and then deeply analyze the relationship between the flow conductivity and the above parameters,To achieve accurate evaluation of the change of flow conductivity in the end.This paper mainly study the fracture surface morphology in laboratory scale,through digital image processing,the height point cloud data of the 3D fracture surface was obtained,on that basis,the fracture surface morphology was deeply analyzed by use of single fractal and multifractal theory.Fracture aperture was assumed as a rough surface,the characterization of which before and after crack slumping are studied using the same measures,and the corresponding characteristic parameters are proposed.Considering the height distribution characteristics of the composite surface,the fracture closure model is established based on the fundamental conception of fractal.Comprehensive application of percolation and fractal theory,the law of fracture conductivity change is revealed.Several research works is conducted as follows:1.The computational method for fracture surface fractal dimension is improved,combined with the fracture plane height parameters and fractal dimension,a new characteristic parameter Rn is proposed to describe the fracture surface morphology in laboratory scale.which overcome the shortcoming that the numerical distribution range of fractal dimension of rough surface is too small,and highlight the large scale asperity's influence on the mechanical properties of fracture.Besides that,the characterization of fracture surface is analyzed by use of multi-fractal theory,the results are in agreement with those provided by above methods2.Slip the rough surfaces along the horizontal direction,the law of fracture aperture change with various state combinations are analyzed,combined with the fracture aperture distribution parameters and fractal dimension,a new characteristic parameter Rn'is proposed to describe the characteristics of fracture aperture.The calculating result show that there is linear relationship between Rn' and the rate of fracture conductivity change.3.The characterization of fracture aperture are studied by use of multi-fractal theory,the result show that with the increase of slip value,the multifractional spectrum parameter show a set of change character : ?min and f(?min)increases gradually,at the same time,the left half width gradually narrowed.It turned out that when the fracture suface slip in a certain range,fractal subset with high probability will increase,in other words,there is an increasing trend for,especially the number of cloud points which have a large fracture aperture.4.Considering the height distribution characteristics of the composite surface,the fracture closure model is established based on the fundamental conception of fractal.Based on that,the change features of actual contact stress and real contact area under different closuer stress are obtained,the influences of model parameters such as scale coefficient,fractal dimension,average height of equivalent asperity and its variance are analysed.The result show that with the increase of scale coefficient,fracture closure increase gradually,the other parameters produced the opposite effects.The dependence of fracture closure and real contact area on the relative closure stress is obtained,the result show that the parameters ?0,Rn' and D associates with closuer negatively.5.By the view of percolation probability and fracture aperture characteristics,the law of fracture conductivity change is analysed.Under relatively high stress condition(10~20MPa),the real contact area reached about 30%,in this process,the maximal connected clique decrease rapidly,Circulation branchs are break up into a lot of unconnected clique by new contact points,which cause the conductivity decrease rapidly.There is a negative correlation between the feature parameters of the initial fracture aperture(?? and Rn')and the change rate of conductivity.6.Based on the fractal theory,The fracture conductivity is also carried on.The fluid channels within fracture are assumed to be a large number of capillaries,which are of fractal features.Using this premise,a novel predictive model for fracture conductivity that considering closure stress is developed.The calculating result of model show that the law of conductivity change is in good agreement with experimental data.