Study Of Fracture Surface Morphology Characteristics By Using Fractal Method

In the development of domestic reservoirs, many low-permeability oil and gas wells generally no natural capacity after completion, so it must form an artificial fracture with enough length through hydraulic fracturing, or connect with natural fractures to achieve commercial development. Fractures greatly improved permeability of reservoir, so methods to study fractures are increasingly diverse. However, due to the distribution complexity, poor regularity and limited observation, detection means and research methods of fractures, it is still not fully solved to describe and predict the fine fractures. Previous studies of stress sensitivity of fractured reservoirs discussed more start from the relationship between stress and fracture conductivity, but closure stress and surface morphology of fracture also have a very close relationship. Analysis of fracture morphology and its variation under different stresses is in favor of predicting fracture width and fracture conductivity under reservoir conditions, thus helping to protect the fractured reservoir. Closure of artificial fracture width has very close relationship with fracture morphology.This paper means to analyze the contact relations of rock fracture surfaces through studying the method of digitizing micromorphology of rock fracture and using a combined methodology of fractal geometry and traditional digitization of rock fracture. Firstly, 8 groups of flow conductivity experiments are carried out under a closure pressure of 10MPa、20MPa、30MPa、40MPa. And 32 pictures of rock fracture interval are made. Secondly, the basic mathematical model is deducted by use of essential features of fractal geometry and the contact relations of fractal dimension and rock fracture plane is researched. Thirdly, study previous closure law of rock fracture conductivity prediction model under different closure pressures, deduce the relation between the fractal dimension and the coincidence and the relation between the fractal dimension and the fracture aperture, and use this equation to deduce fracture conductivity prediction model under different closure pressures. Finally,summary experimental results, and compare the predictive calculated value from model and experimental values, analyze the causes of error.Main understandings and achievements in this paper are:(1) Summarize predecessors’ conventional research method to rock fracture surfaces morphology. Combining with fractal geometry method, use digital method to calculate the fractal dimension representing contact relationship of fracture surfaces, and study relationship between contact relation and conductivity.(2) Analyze advantages and disadvantages of different methods to study fracture morphology, and four models to calculate fractal dimension, puts forward the development trend to study fracture morphology.(3) Introduction of the latest three-dimensional scanner to obtain high accurate fracture surfaces spacing diagrams, using digital image box-counting dimension calculation program from MATLAB to calculate the fractal dimension.(4)Taking advantage of the characteristics of fractal dimension, induce the relational expression of contact area and the fractal dimension. Use fractal dimension to represent fracture surface spacing diagram, elaborate relationship between fractal dimension and the contact distribution of two fracture surfaces.(5)Through the concept of coincidence and calculating method, derive the relationship expression between fractal dimension and flow conductivity and fracture aperture.(6)Based on predecessors’ research achievements, build improved new rock fracture closure model about fractal dimension.(7)Combining with the expression of the fractal dimension, fracture aperture and special cubic law to solve the calculation of conductivity prediction model under different closure pressures.This paper aims to predict fracture aperture, fracture conductivity and supportive relationship of fracture surfaces through studying microstructure of fracture and change law of fractal dimension under different closure pressures. Introduction of fractal geometry to perfect conductivity prediction model under different closure stress, hope to study the influence of fracture spacing distribution on conductivity more effectively, and enrich research methods to basic theory of fracturing technology industry. At the same time, analysis all above is also practical in dynamic prediction for production of fractured reservoir, and providing helps to field production and management.