Research On The Influence Factors And Evaluation Methods Of Tight Sandstone Fracability In Qinghai Oilfield

At present,as the focus of global oil and gas exploitation has shifted from conventional oil and gas to unconventional oil and gas,tight oil and gas development,a key component of the development and utilization of unconventional oil and gas resources,has become a research hotspot at home and abroad in recent years.It is of vital importance for the feasibility evaluation of early fracturing in actual engineering to evaluate rock fracability of compact sandstone.In the final analysis,the evaluation of fracability can be regarded as the evaluation of the possibility and capacity of complex fracture network.In this work,based on the previous studies taking compact sandstone in a block of Qinghai Oilfield as the research object,the factors affecting rock fracability are analyzed,combining with laboratory experimental analysis and numerical simulation.A new comprehensive evaluation method of pressure of compact sandstone is established.The results show that:（1）The factors influencing the fracability of rock are tested in laboratory,and the law of the factors influencing the fracability is analyzed.The results show that under single factor conditions,the higher the content of quartz and carbonate minerals is,the stronger the brittleness of the rock is;the larger the elastic modulus,the smaller the Poisson’s ratio and the stronger the brittleness;the larger the sine value of the internal friction angle,the better the brittleness of the rock;the lower the energy consumption before the peak strength,and the less obvious the plastic yield stage,that is,the easier the rock fracturing,the higher fracability;the smaller the energy after the peak provided by the outside,the lower the fracture energy and the higher the fracability.（2）The numerical simulation on various factors affecting the fracability of rock is carried out.The results show that under single factor conditions,the larger the elastic modulus,the longer the fracture length;the larger the Poisson’s ratio is,the smaller the width of the fracture.Namely with not obvious change of length,the Poisson’s ratio has little effect on crack length;based on the method of elastic mechanics parameter,the larger the brittleness index,the longer the crack length;the larger the minimum horizontal principal stress,the smaller the crack length is and the maximum horizontal principal stress has less influence on the crack length;the smaller the horizontal stress difference is,the easier the direction of crack propagation is to turn,which will develop along the direction of natural fractures,thus forming a complex fracture network;the larger the horizontal stress difference is,the easier it is for the cracks to directly pass through the natural fractures and continue to develop forward;fracturing fluid displacement has a greater impact on fracture propagation.The larger the fracturing fluid displacement is,the longer the fracture length is;the larger the fracturing fluid viscosity,the smaller the fracture length;when hydraulic fractures and natural fractures are combined,with larger fracture intersection angle,it is easier for fractures to pass through the natural fracture and continue to develop forward;with smaller fracture intersection angle,it is easier for fractures to extend along the direction of natural fractures,thus activating the natural fracture to form a complex fracture network.（3）The comprehensive fracability index is calculated based on the AHP（analytic hierarchy process）-entropy method.Meanwhile,the influence weight of rock fracability is determined,affected by the following factors,including the brittleness index B₁ based on the mineral composition method,the brittleness index B₂ based on the elastic mechanics parameter method,the brittleness index B₃ based on the internal friction angle,the fracability index B₄based on the the whole process of energy evolution of stress-strain curve and geostress difference coefficient B₅.The values are 0.0727,0.1716,0.0512,0.4148 and 0.2896,respectively.The grey correlation analysis method is used to analyze the influence of the relevant parameters in the numerical simulation on the fracture propagation.The length of the fracture is used as the objective function to determine the weight of each factor,and the gray correlation method is used to calculate the comprehensive fracability index.The results show that the comprehensive fracability index calculated by the AHP-entropy method is basically the same as the one calculated by numerical simulation verification,and the comprehensive fracability index calculated is consistent with the correlation coefficients among the indicators.In this work,given that the influence of various factors on rock fracability from perspectives of rock mineral composition,rock physical and mechanical characteristics,geostress characteristics,energy characteristics,natural fractures,etc.a new comprehensive evaluation system for compact sand and rock fracability is established.The method solves the problem of excessive subjectivity in determining the influence weight in the existing techonology,providing a new technical method and evaluation system for the comprehensive evaluation of fracability.