水平井中多条裂缝同步扩展时裂缝竞争机制

水平井中多条水力裂缝间的应力干扰行为,造成了压裂液排量的非均匀分配,影响了水力裂缝的几何形态。采用边界元法研究岩体在压裂液作用下的变形程度,以幂律流体泊肃叶平板流动方程来研究水力裂缝内部的压裂液流场,考虑了多条裂缝间应力干扰和压裂液流量分配,建立了流-固耦合的水平井多条水力裂缝同步扩展模型。模型可模拟水平井多条水力裂缝几何形态、应力干扰情况和压裂液排量的分配情况,可解释水力裂缝之间的竞争机制。多条裂缝同步扩展时,压裂液排量并非均等地分配到各个裂缝之中,进入到内部裂缝的压裂液流量最小,内部裂缝宽度最小;内部的水力裂缝增长一定长度后,停止增长,并且在应力干扰下逐渐闭合。     

基于ABAQUS平台的水力裂缝扩展有限元模拟研究

随着扩展有限元理论的深入研究,利用扩展有限元方法模拟水力压裂具有了一定的可操作性。相比于常规有限元方法,XFEM方法具有计算结果精度高和计算量小的优点。但是,如何模拟射孔孔眼、如何模拟流体与岩石相互作用以及分析水力裂缝的扩展规律仍然是难题。以研究水力压裂裂缝扩展规律为目的,建立了岩石多孔介质应力平衡方程、流体渗流连续性方程和边界条件。通过有限元离散化方法对耦合方程矩阵进行处理。通过富集函数定义初始裂缝（射孔孔眼）,选择最大主应力及损伤变量D分别作为裂缝起裂和扩展判定准则,利用水平集方法模拟水力裂缝扩展过程。数值模拟结果显示：增加射孔方位角、压裂液排量和减小水平地应力差,起裂压力上升;黏度对起裂压力无明显影响。增加射孔方位角、压裂液排量、黏度和减小水平地应力差值有助于裂缝宽度的增加。增加水平地应力差值、压裂液排量和减小射孔方位角以及压裂液黏度有助于裂缝长度增加,反之亦然。基于ABAQUS的水力裂缝扩展有限元法可对不同井型和诸多储层物性参数及压裂施工参数进行分析,且裂缝形态逼真,裂缝面凹凸程度清晰,结果准确。此研究可作为一种简便有效研究水力压裂裂缝扩展规律的方法为油田水力压裂设计与施工提供参考与依据。     

危岩水力劈裂分析的扩展有限元法

危岩是三峡库区典型的地质灾害类型之一,而主控结构面受荷断裂扩展是危岩发育成灾的关键核心。将危岩主控结构面类比为宏观裂纹,利用扩展有限元法在模拟裂纹扩展方面的优势,基于考虑裂纹面水压力作用的虚功原理推导出了采用扩展有限元法分析水力劈裂问题的控制方程,给出了危岩主控结构面水力劈裂问题的扩展有限元实现方法,对重庆万州太白岩危岩主控结构面的水力劈裂进行了数值模拟分析。计算结果表明：暴雨是威胁危岩稳定性的最敏感因素,随着裂隙水压力上升,裂端拉应力会急剧升高,危岩的稳定性降低;I型裂纹扩展是危岩主要的结构面扩展形式,结构面一旦发生开裂,将处于非稳定扩展状态。     

煤岩水力压裂裂缝扩展形态试验研究

通过室内水力压裂物理模拟试验系统,对大尺寸原煤进行了水力压裂模拟试验,根据水力裂缝的空间展布形态分析了煤岩储层水力裂缝的延伸规律,揭示了网状裂缝的形成机制。结果表明：水力裂缝易在弱层理处分叉和转向,发育的层理和裂缝系统等结构面为压裂形成裂缝网络提供了前提条件。泵压曲线呈现出的频繁波动是煤岩内产生网状裂缝的一个显著特征。水力裂缝的起裂与延伸有4种基本模式,裂缝网络的形成多为这4种基本模式的组合。地应力差异系数和泵注排量对煤层水力裂缝形态有较大影响。较小的地应力差异系数更利于网状裂缝的形成;较高的压裂液排量易形成相对简单的裂缝形态,导致压裂改造效果较差。该试验方法和试验结果可为现场水力压裂参数设计和优化提供参考和依据。     

Numerical Simulation of 3D Hydraulic Fracturing Based on an Improved Flow-Stress-Damage Model and a Parallel FEM Technique

The failure mechanism of hydraulic fractures in heterogeneous geological materials is an important topic in mining and petroleum engineering. A three-dimensional (3D) finite element model that considers the coupled effects of seepage, damage, and the stress field is introduced. This model is based on a previously developed two-dimensional (2D) version of the model (RFPA2D-Rock Failure Process Analysis). The RFPA3D-Parallel model is developed using a parallel finite element method with a message-passing interface library. The constitutive law of this model considers strength and stiffness degradation, stress-dependent permeability for the pre-peak stage, and deformation-dependent permeability for the post-peak stage. Using this model, 3D modelling of progressive failure and associated fluid flow in rock are conducted and used to investigate the hydro-mechanical response of rock samples at laboratory scale. The responses investigated are the axial stress–axial strain together with permeability evolution and fracture patterns at various stages of loading. Then, the hydraulic fracturing process inside a rock specimen is numerically simulated. Three coupled processes are considered: (1) mechanical deformation of the solid medium induced by the fluid pressure acting on the fracture surfaces and the rock skeleton, (2) fluid flow within the fracture, and (3) propagation of the fracture. The numerically simulated results show that the fractures from a vertical wellbore propagate in the maximum principal stress direction without branching, turning, and twisting in the case of a large difference in the magnitude of the far-field stresses. Otherwise, the fracture initiates in a non-preferred direction and plane then turns and twists during propagation to become aligned with the preferred direction and plane. This pattern of fracturing is common when the rock formation contains multiple layers with different material properties. In addition, local heterogeneity of the rock matrix and macro-scale stress fluctuations due to the variability of material properties can cause the branching, turning, and twisting of fractures.     

基于扩展有限元法的碳酸盐岩地热储层岩体裂缝扩展行为

水力压裂作为一种主要的地热能开采手段,其压裂效果除与岩体基本物理力学性质有关外,还与裂隙分布、地应力状态、压裂工程参数等密切相关.为了探究以上因素对水力压裂过程中裂缝扩展行为的影响,以冀中坳陷碳酸盐岩储层岩体为研究对象,基于扩展有限元法,建立裂缝扩展流固耦合模型,分析了水平应力差、射孔方位角、注入液排量和压裂液黏度等参数对裂缝扩展行为的影响.结果表明:单裂缝扩展时,射孔方位角越小、注入量越大、越有利于裂缝扩展;双裂缝扩展时,水平应力差增大,裂缝偏转程度变小;水力裂缝与天然裂缝相交时,较小水平应力差有利于天然裂缝开启.      

基于连续损伤的水平井射孔-近井筒三维破裂模拟

射孔作为井筒与储层之间的液流通道,是水力压裂过程中的重要可控性参数。为研究水平井射孔-近井筒破裂机制,采用岩层变形-流体渗流方程描述应力状态变化,应用连续损伤破裂单元表征三维破裂位置与形态演化,并开发有限元求解程序模拟分析了射孔对水平井初始破裂压力、破裂位置及近井筒裂缝复杂性的调控作用。通过与解析模型及射孔压裂物理模型试验结果对比,验证了模型及有限元程序的有效性;水平井破裂压力数值分析结果与现场测试数据吻合较好。研究表明：射孔可调控水平井破裂压力与初始破裂位置,同时对近井筒区域裂缝扩展形态影响显著。通过优化射孔参数可以引导初始破裂向最优破裂面扩展、有效降低破裂压力,减小由于螺旋射孔空间排布引起的水平井近井筒裂缝迂曲与复杂程度,提高致密油气藏压裂改造效果。     

An explicitly coupled hydro‐geomechanical model for simulating hydraulic fracturing in arbitrary discrete fracture networks

Modeling hydraulic fracturing in the presence of a natural fracture network is a challenging task, owing to the complex interactions between fluid, rock matrix, and rock interfaces, as well as the interactions between propagating fractures and existing natural interfaces. Understanding these complex interactions through numerical modeling is critical to the design of optimum stimulation strategies. In this paper, we present an explicitly integrated, fully coupled discrete‐finite element approach for the simulation of hydraulic fracturing in arbitrary fracture networks. The individual physical processes involved in hydraulic fracturing are identified and addressed as separate modules: a finite element approach for geomechanics in the rock matrix, a finite volume approach for resolving hydrodynamics, a geomechanical joint model for interfacial resolution, and an adaptive remeshing module. The model is verified against the Khristianovich–Geertsma–DeKlerk closed‐form solution for the propagation of a single hydraulic fracture and validated against laboratory testing results on the interaction between a propagating hydraulic fracture and an existing fracture. Preliminary results of simulating hydraulic fracturing in a natural fracture system consisting of multiple fractures are also presented. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical Modeling of Pore Pressure Influence on Fracture Evolution in Brittle Heterogeneous Rocks

Rock is a heterogeneous geological material. When rock is subjected to internal hydraulic pressure and external mechanical loading, the fluid flow properties will be altered by closing, opening, or other interaction of pre-existing weaknesses or by induced new fractures. Meanwhile, the pore pressure can influence the fracture behavior on both a local and global scale. A finite element model that can consider the coupled effects of seepage, damage and stress field in heterogeneous rock is described. First, two series of numerical tests in relatively homogeneous and heterogeneous rocks were performed to investigate the influence of pore pressure magnitude and gradient on initiation and propagation of tensile fractures. Second, to examine the initiation of hydraulic fractures and their subsequent propagation, a series of numerical simulations of the behavior of two injection holes inside a saturated rock mass are carried out. The rock is subjected to different initial in situ stress ratios and to an internal injection (pore) pressure at the two injection holes. Numerically, simulated results indicate that tensile fracture is strongly influenced by both pore pressure magnitude and pore pressure gradient. In addition, the heterogeneity of rock, the initial in situ stress ratio (K), the distance between two injection holes, and the difference of the pore pressure in the two injection holes all play important roles in the initiation and propagation of hydraulic fractures. At relatively close spacing and when the two principal stresses are of similar magnitude, the proximity of adjacent injection holes can cause fracturing to occur in a direction perpendicular to the maximum principal stress.     

含有硬包裹体分布的非均质岩石水力压裂特性研究

基于完全流-固耦合的弹塑性理论给出了水力压裂数值计算的弥散裂缝模型,其中材料的弹性部分采用线弹性本构关系,塑性部分采用摩尔-库仑破坏准则及强化准则。依据当前的有效应力状态修正渗透系数来模拟压裂液在裂缝中的流动。渗透系数的修改使用双曲正切函数,并采用平均有效应力作为水力裂缝的起裂判据。在ABAQUS软件中通过用户自定义程序添加了该模型。根据岩石的切面照片建立了含有硬包裹体分布的非均质岩石的有限元计算模型,模拟了中心点注水条件下的水力压裂传播过程,讨论了在常应力状态下非均质岩石中开裂区域、典型位置的应力路径变化和裂缝传播范围随时间变化的特点。进行了多种条件下含有硬包裹体分布的岩石材料的数值试验,得出了基岩材料的弹性模量、凝聚力和渗透系数以及注水速率对峰值注水压力、平均注水压力和裂缝开度的影响规律。     

Numerical modeling of stress-permeability coupling in rough fractures

A numerical model is described for coupled flow and mechanical deformation in fractured rock. The mechanical response of rock joints to changes in hydraulic pressure is strongly influenced by the geometric characteristics of the joint surfaces. The concept of this work is to combine straightforward finite element solutions with complex and realistic fracture surface geometry in order to reproduce the non-linear stress-deformation-permeability coupling that is commonly observed in fractures. Building on the numerous studies that have expanded the understanding of the key parameters needed to describe natural rough-walled fractures, new methods have been developed to generate a finite element mesh representing discrete fractures with realistic rough surface geometries embedded in a rock matrix. The finite element code GeoSys/Rockflow was then used to simulate the coupled effects of hydraulic stress, mechanical stress, and surface geometry on the evolving permeability of a single discrete fracture. The modeling concept was experimentally verified against examples from the literature. Modeling results were also compared to a simple interpenetration model.     

地应力对破裂压力和水力裂缝影响的试验研究

为研究水力压裂时地应力对破裂压力和水力裂缝的影响规律,采用水力压裂试验系统,以相似材料作为研究对象,进行了水力压裂物理模拟试验,并借助煤岩断面三维扫描系统、位移计提取了水力裂缝信息。试验结果表明,随着主应力差的增大,破裂压力逐渐降低,破裂时间也逐渐缩短;随着主应力差的增大,破裂面的表面积逐渐增加,破裂面变得逐渐粗糙;围压相近时水力裂缝易出现转向、分叉,压裂液的动力效应越明显;水力裂缝张开度随着主应力差的增大而逐渐减小。研究结果可为水力压裂试验的进一步研究、裂缝网络系统的建立提供参考。     

Linear Elastic and Cohesive Fracture Analysis to Model Hydraulic Fracture in Brittle and Ductile Rocks

Hydraulic fracturing technology is being widely used within the oil and gas industry for both waste injection and unconventional gas production wells. It is essential to predict the behavior of hydraulic fractures accurately based on understanding the fundamental mechanism(s). The prevailing approach for hydraulic fracture modeling continues to rely on computational methods based on Linear Elastic Fracture Mechanics (LEFM). Generally, these methods give reasonable predictions for hard rock hydraulic fracture processes, but still have inherent limitations, especially when fluid injection is performed in soft rock/sand or other non-conventional formations. These methods typically give very conservative predictions on fracture geometry and inaccurate estimation of required fracture pressure. One of the reasons the LEFM-based methods fail to give accurate predictions for these materials is that the fracture process zone ahead of the crack tip and softening effect should not be neglected in ductile rock fracture analysis. A 3D pore pressure cohesive zone model has been developed and applied to predict hydraulic fracturing under fluid injection. The cohesive zone method is a numerical tool developed to model crack initiation and growth in quasi-brittle materials considering the material softening effect. The pore pressure cohesive zone model has been applied to investigate the hydraulic fracture with different rock properties. The hydraulic fracture predictions of a three-layer water injection case have been compared using the pore pressure cohesive zone model with revised parameters, LEFM-based pseudo 3D model, a Perkins-Kern–Nordgren (PKN) model, and an analytical solution. Based on the size of the fracture process zone and its effect on crack extension in ductile rock, the fundamental mechanical difference of LEFM and cohesive fracture mechanics-based methods is discussed. An effective fracture toughness method has been proposed to consider the fracture process zone effect on the ductile rock fracture.     

页岩水平井多段分簇压裂起裂压力数值模拟

页岩储层孔隙度和渗透率极低,天然裂缝和水平层理发育,常规压裂增产措施无法满足页岩气的开发要求,水平井多段分簇压裂是页岩气开发的关键技术之一,该技术能够大幅度提升压裂改造的体积、产气量和最终采收率。为确定页岩储层水平井多段分簇射孔压裂的起裂点和起裂压力,采用有限元方法建立了水平井套管完井（考虑水泥环和套管的存在）多段分簇射孔的全三维起裂模型。数值模型的起裂压力与室内试验结果吻合较好,证明了数值模型的准确性和可靠性。利用数值模型研究了页岩水平井多段分簇射孔压裂的起裂点和起裂压力的影响因素,研究发现：射孔孔眼附近无天然裂缝或水平层理影响,起裂点发生在射孔簇孔眼的根部;射孔簇间距越小,中间射孔簇的干扰越大,可能造成中间的射孔簇无法起裂;射孔密度和孔眼长度增大,起裂压力降低;天然裂缝的存在,在某些情况能够降低起裂压力且改变起裂位置,主要与天然裂缝的分布方位及水平主应力差有关;水平层理可能会降低起裂压力,但与垂向主应力与水平最小主应力的差值有关。获得的起裂压力变化规律,可作为进一步研究水平井多段分簇射孔条件下的裂缝扩展规律的基础,可以为压裂设计和施工的射孔参数确定及优化给出具体建议。     

Finite element simulations of 3D planar hydraulic fracture propagation using a coupled hydro-mechanical interface element

Two-dimensional hydraulic fracturing simulations using the cohesive zone model (CZM) can be readily found in the literature; however, to our knowledge, verified 3D cohesive zone modeling is not available. We present the development of a 3D fully coupled hydro-mechanical finite element method (FEM) model (with parallel computation framework) and its application to hydraulic fracturing. A special zero-thickness interface element based on the CZM is developed for modeling fracture propagation and fluid flow. A local traction-separation law with strain softening is used to capture tensile cracking. The model is verified by considering penny-shaped hydraulic fracture and plain strain Kristianovich‑Geertsma‑de Klerk hydraulic fracture (in 3D) in the viscosity- and toughness-dominated regimes. Good agreement between numerical results and analytical solutions has been achieved. The model is used to investigate the influence of rock and fluid properties on hydraulic fracturing. Lower stiffness tip cohesive elements tend to yield a larger elastic deformation around the fracture tips before the tensile strength is reached, generating a larger fracture length and lower fracture pressure compared with higher stiffness elements. It is found that the energy release rate has almost no influence on hydraulic fracturing in the viscosity-dominated regime because the energy spent in creating new fractures is too small when compared with the total input energy. For the toughness-dominated regime, the released energy during fracturing should be accurately captured; relatively large tensile strength should be used in order to match numerical results to the asymptotic analytical solutions. It requires smaller elements when compared with those used in the viscosity-dominated regime.     

热应力影响下干热岩水压致裂数值模拟

干热岩水压致裂过程中低温诱导热应力与注入水压共同影响裂缝的萌生与扩展。首先通过THM耦合分析了低温压裂液注入过程中注入水压与热应力的相互作用及其对裂缝萌生的影响,随后建立描述岩石细观结构的THMD耦合模型对热应力影响下高温岩石水压致裂过程进行初探。结果表明：低温压裂液注入高温岩石产生的热应力包括岩石自身温度梯度形成的热应力与岩石颗粒非均匀膨胀导致的热应力,并在井筒周围呈现为拉应力。高注入压力将抑制热应力导致的多裂缝萌生,井筒附近热应力的存在对注入压力也具有削弱作用。基岩温度升高,裂缝萌生阶段更多裂缝在井筒附近起裂,缝网沿最大地应力方向的扩展速度减慢,但改造规模增加,同时多裂缝的存在也使得裂缝延伸压力增加。     

用流固耦合方法研究油藏压裂后应力应变和孔渗特性变化

油藏压裂后将引起地应力场发生变化，使岩石变形，导致孔隙度和渗透率变化，进而影响产量，为研究这一问题，作者建立了油藏压裂后流－固耦合渗流模型，考虑了以下因素：油藏岩石变形，地应力，孔隙度和渗透率变化，人工裂缝，流体渗流与岩石应变耦合，储藏渗流与裂缝渗流耦合，非达西效应等。较详细地给出了耦合方程及推导过程，控制方程包括的未知变量有压力，饱和度及位移，11个变量，和11个方程，用有限差分方法将流体渗流和岩石应变方程离散成主对角占优的七对角矩阵，可在修改已有三维二相渗流和三维固体力学程序的基础上，采用隐式迭代方法求解，示例分析表明，用此模型可以研究储层应力变变，孔隙度和渗透率随时间和空间变化规律，为开发方案制定，整体压裂设计，压后生产管理等方面提供定量分析技术。     

煤层气井水力压裂同层多裂缝分析

多裂缝理论是水力压裂理论的前沿理论,特别是对裂隙发育、物理力学性质有别于均质砂岩的煤岩,其研究难度较大,面临问题很多。根据弹性力学、岩石力学、断裂力学和流体力学等学科基本理论,结合多裂缝理论与井底压力协同理论,对在天然裂隙发育的煤层同时开启的多裂缝模型进行了分析与计算。结果显示,综合滤失系数、流量、主应力差等均不同程度影响了多裂缝在近井筒区域的汇合相连概率与延伸方向。      

不同水力加载条件下非均质储层缝网扩展规律研究

水力压裂可显著提高页岩气等致密储层岩体的渗透性以增加油气产量,然而受多因素影响,水力压裂形成缝网结构的机理和压裂优化设计一直是研究的焦点和难点。本研究基于渗流-应力-破坏耦合计算模拟方法,对不同水力加载条件下的非均质储层水力压裂过程进行了模拟和对比研究。研究结果表明:水力压裂过程中起始注水压力和增量大小对水力压裂缝网扩展和改造区域形态有着显著的影响。当起始注水压力小于等于模型材料体抗拉强度,并缓慢增压致裂时,压裂过程可近似视为稳态应力-破坏-渗流耦合作用过程的不同阶段,这种情况下仅在压裂井孔周围形成两组对称式的伞状水力裂缝带。当对模型体施加高于模型材料体破裂压力的注水压力时,相当于对压裂孔快速施加高动水压力,水力裂缝沿压裂孔全方位迅速萌生并快速扩展,当注水压力值高于破裂压力一定幅值时,压裂改造可形成围绕压裂井全方位的放射状裂缝网络,使压裂储层得以最大范围改造。在拟静力和拟动力两种加载条件下,不同水岩相互作用机理是造成不同水力加载条件出现不同缝网结构的力学机制,而对于实际的页岩气储层改造,压裂产生围绕压裂井全方位放射状的缝网结构则是一种最优的体积压裂改造。     

页岩气藏缝网压裂裂缝间距优化研究

页岩气藏一般具有低孔隙、低渗透等特征,对其实施缝网压裂是高效开发页岩气的最佳途径。采用位移不连续法建立线弹性二维均质地层诱导应力场分布数学模型,通过水平应力差异系数对顺序压裂和交替压裂的裂缝间距进行优化研究。结果表明,水平应力差异系数受到裂缝净压力、裂缝缝长、原地应力场等因素的影响;裂缝净压力越大、缝长越长,水平应力差异系数越小;随着与裂缝距离的增加,水平应力差异系数呈现先减小后增加的趋势,因此,存在后续裂缝形成复杂网络的最佳裂缝间距;顺序压裂裂缝间距不宜过大,且后续压裂裂缝间距应适当减小;交替压裂裂缝间距最优时,缝间水平应力差异系数最小,对中间裂缝形成缝网最有利。