扩展有限元法应用于页岩气藏水力压裂数值模拟研究

发布时间：2018-05-10 23:24

本文选题：页岩 + 水力压裂　；参考：《中国科学技术大学》2017年博士论文

【摘要】：页岩气是储藏于页岩基质中的一种非常规天然气资源。随着常规资源日益减少以及能源需求不断增加,页岩气得到全世界越来越广泛的关注。页岩气藏由于具有低孔隙度低渗透率特点,需要进行水力压裂等增产措施改造储层才能获得可观经济产量。水平井多段压裂是一种能有效提高页岩储层产量的重要技术手段。多段压裂一般分为顺序压裂和同时压裂,施工设计要求人工裂缝能够尽量生成横向直裂缝以延伸到较远地层区域,沟通更大面积储层以提高导流率。由于裂缝之间存在应力阴影效应导致裂缝偏离其预设方向生成非平面裂缝,降低了压裂效果。增大裂缝间距能降低应力干扰强度,但是过大的裂缝间距也会使整个储层导流率降低。合理的裂缝间距是水平井多段压裂设计的关键因素。页岩作为一种典型的沉积岩,在垂直于层理方向和平行于层理方向呈现显著的各向异性力学特征,可以简化为正交各向异性材料处理。水力裂缝在各向异性岩石中的起裂和扩展规律与在各向同性岩石中显著不同。考虑岩石各向异性对水力裂缝扩展形态的影响是压裂施工设计的关键力学问题之一。页岩地层中包含大量天然裂缝,水力裂缝在扩展过程中与地层中天然裂缝相交形成复杂缝网,能显著提高地层导流率。地层中天然裂缝可以分为两类:摩擦型天然裂缝和粘结型天然裂缝。水力裂缝与这两种类型天然裂缝相互作用机理不同。研究水力裂缝与不同类型天然裂缝相交行为对预测页岩地层中复杂缝网形成过程具有重要意义。水力压裂是涉及岩石骨架变形、裂缝起裂和扩展、裂缝中流体流动、压裂液滤失和岩石基质中孔隙渗流的多场耦合复杂力学问题,受限于理论研究和实验研究的局限性,数值模拟成为研究水力压裂的一种有效手段。扩展有限元法(XFEM)通过在传统有限元位移插值函数中引入增强函数来描述裂纹等不连续位移场,可以模拟裂纹沿任意路径扩展而不需要网格重构,极大减少计算量。本文基于扩展有限元法,建立了二维非线性流固耦合水力压裂数值模型,研究水平井多段压裂、正交各向异性岩石中裂缝扩展、水力裂缝与天然裂缝相交等问题:(1)研究了水平井多段压裂裂缝之间应力干扰问题,对比了顺序压裂和交替压裂方法应力干扰强度和最优裂缝间距。结果表明,地应力差对应力干扰区域大小有显著影响;地应力差较小时,需要增大裂缝间距才能避免应力阴影效应的影响。裂缝附近的应力分布会随着裂缝的扩展不断变化,这是在预测裂缝扩展路径时必须要考虑的重要因素。交替压裂通过在两条已压裂完成的裂缝中间压裂第三条裂缝,可以利用应力干扰相互抵消的效应缩短生成横向直裂缝所需压裂间距。(2)研究了正交各向异性岩石的材料角和杨氏模量比对水力裂缝扩展路径的影响。结果表明,当水力裂缝初始方向和正交各向异性岩石材料主轴之间有夹角时,裂缝扩展过程中将偏离其预设方向而生成非平面裂缝。水力裂缝在第一个扩展步会发生明显的转向,初始偏转角随材料角呈现周期性变化。水力裂缝偏转程度随杨氏模量比增大而增加,水力裂缝倾向于沿着杨氏模量更小的材料主轴方向扩展。地应力差较小时,裂缝扩展方向主要由材料正交各向异性特征决定;地应力差较大时,裂缝扩展方向主要由地应力决定。(3)研究了水力裂缝与天然裂缝相交行为,对比了摩擦型天然裂缝和粘结型天然裂缝对水力裂缝扩展行为和复杂缝网形成过程的影响。结果表明,当水力裂缝与摩擦型天然裂缝相交时,水力裂缝能否穿透天然裂缝取决于裂缝相交角、地应力差、裂缝摩擦系数和岩石抗张强度。当水力裂缝与粘结型天然裂缝相交时,水力裂缝能否穿透天然裂缝取决于裂缝相交角、粘结裂缝断裂韧度和岩石基质断裂韧度。水力裂缝与粘结型天然裂缝相交常形成L型裂缝,而与摩擦型天然裂缝相交常形成T型裂缝:因此,对于含有相同初始几何构型天然裂缝的地层,水力压裂在摩擦裂缝地层形成的缝网结构比在粘结裂缝地层形成的缝网结构更复杂。(4)研发了二维扩展有限元水力压裂程序Matlab-XFEM,可以模拟各向同性岩石和正交各向异性岩石中裂缝的起裂和扩展,多条裂缝同时扩展,水力裂缝与天然裂缝相互作用以及复杂缝网的形成过程。本文建立的数值模型、分析方法和计算结果,能为页岩气藏水力压裂施工设计提供一定的技术指导,以期获得最优压裂效果,提高地层导流率以实现整个储藏产量的增加。
[Abstract]:Shale gas is a kind of unconventional natural gas resources stored in shale matrix. With the increasing reduction of conventional resources and increasing energy demand, shale gas has been paid more and more attention all over the world. Shale gas reservoirs need water pressure cracking and other measures to reconstruct reservoir because of low porosity and low permeability. Multistage fracturing in horizontal wells is an important technical means to effectively improve the production of shale reservoirs. Multi section fracturing is usually divided into sequential fracturing and simultaneous fracturing. The construction design requires artificial fractures to be able to generate transverse straight fractures as far as possible to extend to the farther formation area, and to communicate a larger area of reservoir to improve the conductivity. The existence of stress shadow effect between cracks causes the crack to deviate from its preset direction to generate non plane cracks and reduce the fracturing effect. Increasing the crack spacing can reduce the stress interference intensity, but the excessive gap spacing will also reduce the whole reservoir conductivity. The reasonable gap spacing is the key factor for the multi section fracturing design of the horizontal well. For a typical sedimentary rock, it can be simplified as orthotropic material in the direction of bedding and parallel to the bedding direction. It can be simplified as orthotropic material treatment. The crack initiation and expansion of hydraulic fractures in anisotropic rocks is significantly different from that in isotropic rocks. The influence of the slit expansion form is one of the key mechanical problems in the fracturing design. In the shale formation, a large number of natural cracks are included in the shale formation, and the hydraulic cracks intersected with the natural cracks in the formation to form complex seams in the process of expansion. The formation conductivity can be greatly improved. The natural cracks in the strata can be divided into two types: friction natural cracks and bonded days. The interaction mechanism between hydraulic fractures and these two types of natural fractures is different. It is of great significance to study the intersecting behavior of hydraulic fractures and different types of natural fractures. The hydraulic fracturing involves the deformation of the rock skeleton, the cracking and expansion of the cracks, the fluid flow in the cracks, and the fracturing fluid filtration. The multi field coupled complex mechanics problem in the porous rock matrix is limited to the limitations of theoretical and experimental research. Numerical simulation is an effective method to study hydraulic fracturing. The extended finite element method (XFEM) is used to describe the discontinuous displacement fields, such as the crack, by introducing an enhancement function into the traditional finite element displacement interpolation function. In this paper, a two-dimensional nonlinear fluid solid coupling hydraulic fracturing numerical model is established based on the extended finite element method, which is based on the extended finite element method, and studies the multi section fracturing of horizontal wells, the crack propagation in the orthotropic rock, and the intersecting of hydraulic fractures and natural fractures. (1) study The stress interference between fractured fractures in multi section of horizontal wells is discussed, and the stress interference intensity and optimal gap between sequential and alternate fracturing methods are compared. The results show that the stress difference has a significant influence on the size of the force interfering region. The gap between the stress and the stress difference should be increased to avoid the effect of the stress shadow effect. The stress distribution in the vicinity will change with the expansion of the crack. This is an important factor that must be considered when predicting the crack propagation path. Through the fracturing of third fractures in the middle of the two fractured fracture, the fracture spacing of the fracture can be shortened by the effect of stress interference cancellation. (2) The effect of the material angle and the young's modulus of the orthotropic rock on the expansion path of the hydraulic fracture is studied. The results show that when the initial direction of the hydraulic fracture and the axis of the orthotropic rock material have a angle, the fracture propagation will deviate from the preset direction and become non plane cracks. The deflecting degree of the hydraulic crack increases with the ratio of Young's modulus, and the hydraulic crack tends to expand along the direction of the material spindle with smaller Young's modulus. The crack propagation direction is mainly determined by the orthogonal anisotropy of the material. When the stress difference is large, the direction of crack propagation is mainly determined by ground stress. (3) the intersecting behavior of hydraulic cracks and natural fractures is studied, and the effects of natural cracks and natural cracks on the expansion behavior of hydraulic fractures and the formation of complex seams are compared. Whether the force cracks can penetrate the natural cracks depends on the intersecting angle of the fracture, the difference of ground stress, the friction coefficient of the crack and the tensile strength of the rock. When the hydraulic crack intersects with the natural crack of the bond, it depends on the intersecting angle of the fracture, the fracture toughness of the bond crack and the fracture toughness of the rock matrix. Natural fracture intersecting often forms L type cracks, and intersects with friction natural fractures to form T type cracks. Therefore, the fractured net structure formed by hydraulic fracturing in the frictional fractured stratum is more complex for the stratum containing the same initial geometric configuration natural fissure. (4) the two dimensional extended finite element is developed. The hydraulic fracturing procedure Matlab-XFEM can simulate the cracking and expansion of cracks in isotropic rock and orthotropic rock, multiple cracks spread simultaneously, the interaction of hydraulic cracks and natural fractures and the formation of complex seams. The numerical model, analysis method and calculation results established in this paper can be used for hydraulic fracturing in shale gas reservoirs. The construction design provides some technical guidance for achieving the best fracturing effect and improving the formation diversion rate, so as to realize the increase of the whole storage production.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O346.1

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