【作者】 李玮；

【导师】 闫铁；

【作者基本信息】 大庆石油学院 ， 油气井工程， 2010， 博士

【摘要】 自上个世纪四十年代末期,水力压裂第一次成功应用以来,就广泛应用于低渗透储层的改造中,并成为一项有效的低渗透油田增产增注措施。为了提高压裂施工的成功率和进一步改善压裂效果,学者们逐渐开展了以岩石断裂力学为基础的水力压裂断裂机理研究,逐渐形成了压裂裂缝沿垂直于最小主应力的单一平面延伸和扩展的结论。该结论成为水力压裂裂缝形态分析的根本,并为水力压裂数值模拟奠定了理论基础。随着油田开发的深入,需要改造的储层逐渐复杂化,学者们也渐渐认识到用单一平面裂缝无法在理论上圆满解释一些复杂情况下的压裂施工。大量现场实践表明,压裂储层存在多条同时延伸的裂缝,且裂缝形状也未必是平面的。这种情况在裂缝性储层表现的尤为明显。针对这一问题,本文将以研究储层孔隙介质特征为出发点,以分形理论为研究手段,在室内实验及现场施工资料的基础上,对不同储层下水力压裂的造缝机理展开研究,深入分析水力压裂的裂缝形态及其影响因素。在理论研究方面,首先应用分形方法分析储层存储介质的分形特征,并由孔隙介质特征的分形描述成功建立孔隙介质有效应力分形模型,为重新分析井壁围岩应力状态奠定基础,然后根据新的井壁围岩应力状态方程分析孔隙性储层和裂缝性储层的水力压裂造缝机理。在实验分析方面,首先通过扫描电镜和压汞法分析和测定砂岩孔隙结构的分形维数和对裂缝性储层取心测定了裂缝结构的分形维数,为用分形方法分析储层存储介质的分形特征奠定数据基础,然后根据水力压裂破裂过程模拟实验来分析孔隙性储层和裂缝性储层中水力压裂的造缝机理,以作为理论研究的实验基础。论文的主要研究内容及研究成果如下：1.应用分形几何方法分析了储层孔隙介质的微观特征。根据孔隙结构特征的分形模型建立了基于分形特征参数的孔隙介质有效应力模型,并给出有效应力模型的简化式,使之能够适用不同现场情况。该模型定量地描述了孔隙介质结构特征在骨架颗粒和孔隙流体支撑总应力时起到的协调关系,且将孔隙介质的微观参数和宏观统计参数联系在一起。2.基于孔隙介质有效应力的分形模型重新推导了井壁围岩应力状态方程,并进而推导了裸眼完井、射孔完井情况下含有分形参数的压裂起裂判别准则。新模型中引入了描述孔隙介质结构特征的分形参数,为应用分形方法分析储层水力压裂的造缝机理奠定了基础。3.应用分形几何方法分析了储层裂缝介质的分布特征。建立VB.Net & OpenGL三维可视化平台,根据裂缝介质系统的分形分布规律通过确定性离散建模方法,模拟了布达特储层裂缝介质系统三维分布。模拟结果证明,分形方法能很好地描述单一裂缝的随机性和整体裂缝系统的复杂性,是裂缝性储层建模的有效方法。4.应用分形几何方法分析了岩石断裂面的延伸特性。使用分形方法描述裂纹的曲折扩展形态,将分形特征量引入到岩石断裂韧性中,由此建立了基于分形特征参数的Ⅰ型、Ⅱ型和混合型岩石断裂韧性的计算方法。5.应用了模拟水力压裂过程的装置,进行孔隙性储层和裂缝性储层水力压裂模拟实验,根据实验结果分析影响水力压裂造缝机理的因素。实验结果表明,储层中的天然裂缝对水力压裂裂缝的起裂压力和裂缝延伸形态有重要影响。天然裂缝的存在降低了岩石的破裂压力,使压裂裂缝的形态变得更加复杂。6.应用分形方法分析天然裂缝的分布规律,确定孔隙度和渗透率随分形参数的变化关系。由此,根据二维的PKN、KGD模型分析了分形参数对压裂裂缝缝长、缝宽的影响。水力压裂造缝过程是一个岩石断裂过程。多方面的影响因素使这个断裂过程变得复杂。本文的研究明确了不同因素对岩石断裂过程的影响情况,尤其是天然裂缝存在的情况下各影响因素之间的转变。通过本文的研究加深了对水力压裂造缝机理的认识,为进一步分析水力压裂造缝机理和指导现场压裂施工提供了理论指导和技术支持。更多还原

【Abstract】 Hydraulic fracturing has been widely used in the reconstruction of low-permeability reservoirs, and becomes an effective method of stimulation for low-permeability oilfield since its first successful application in the late 1940’s. In order to increase the success ratio of fracturing operation and further improve the efficiency of hydraulic fracturing, researches on fracture mechanism which based on rock fracture mechanics have been done by scholars. We can draw a conclusion that induced fracture’s direction of extending and expanding which is perpendicular to single plane cracks of minimum principal stress. This conclusion has become the basis of shapes analysis and numerical simulation of hydraulic fracturing.Along with further development of oilfield, reservoirs needing to reform become more complex, and scholars have gradually recognized that single-plane cracks theorem can not give a perfect explanation on fracturing operation under complex situations in theory. The considerable field practices show that fractured reservoir has multi-fracture extending at the same time and the fracture shape may not be flat. And this situation is especially obvious in fractured reservoir. This paper, by means of fractal theory study, analyzes fracture shape of hydraulic fracturing and its influential factors via the study on the fracture initiation mechanism under different reservoirs by laboratory experiments and field data.To ensure dissertation, this study use the approach of combining theoretical research with experimental analysis. In the aspect of theory, first, analyzing fractal features of reservoir media with the method of fractal theory and setting up the effective stress fractal model.Then, analyzing fracture initiation mechanism of fractured and porosity reservoirs according to new stress condition equations of rock around borehole.In the aspect of experimental analysis, firstly, fractal dimensions of pore texture are determined by means of scanning electron microscope (SEM), mercury porosimetry method (MPM) and fractal dimensions of fractured texture are measured by coring.That provides data basis of fractal features for analysis of reservoir storage medium in fractal method. Then, it analyze the fracture initiation mechanism of fractured and porosity reservoirs on the basis of large dimension hydraulic fracturing simulation experiments as the experimental base on theoretical investigation.The main contents and outcomes are as following:1. Analyzing the microscopic feature of porous media using fractal geometric methods. To meet the filed requirement, effective stress fractal model is built according to fractal model of pore structure characteristics and simplified effective stress model is introduced. Coordinating connections between pore fluid and skeletal grains of pore structure on support total stress are quantitatively described by this model as well as the micro-parameters of porous medium are linked with the macro-parameters.2.Fractal model based on effective stress of porous medium derives new stress condition equation of rock around borehole and gets fracturing discrimination criteria with fractal parameters in the condition of casingless completion and perforation completion on further derivation.New model introduces fractal parameters describing the structure characteristics of porous medium.It is the basis of analysing fracture initiation mechanism of reservoir by use of fractal model.3. Analyzing the structural feature of porous media by use of fractal geometric methods. Establishing 3-D visualization platform such as VB.Net & OpenGL.3-D distribution of Budate formation’s fracture system is simulated on the base of distribution rules of fracture medium by use of deterministic scattered modeling approach. In terms of simulation results, fractal theory can reflect randomness of single fracture and complexity of fracture system and it is an effective modeling method.4. Analyzing fracture criterion (rock fracture toughness) according to fractal geometry model. By using fractal method, shape and propagation direction of fracture are given. The fractal character is introduced into the fracture toughness, as a result, the fracture toughness values of type-Ⅰ, type-Ⅱand mixed rocks are obtained.5. Using large dimension hydraulic fracturing simulant installation, the study analyze factors of fracture initiation mechanism according to the results of hydraulic fracturing simulation experiment. Experiments show that natural fracture has a significant influence on initiation pressure and fracture propagation shapes. The existence of natural fracture reduces the fracture pressure of rock, and makes the fracture geometry more complex.6. Analyzing the distribution rules of natural fracture by use of fractal methods, and ascertaining the relationship between porosity fractal parameters and permeability fractal parameters.Thus, Analyzing fractal parameters’influence on fracture length and fracture width according to PKN and KGD model which are 2-D.Fracture creation of hydraulic fracturing is a rock fracture process.Several different factors make the process more complex. This dissertation briefly concerns the effects of various factors on rock fracture, especially when natural fractures exist. This research deepened our understanding of fracture initiation mechanism and can be used for further research as well as direction of field fracturing practice in theory and technology.更多还原