双井筒试验下页岩水力裂缝扩展与机制研究

水力压裂是页岩气开采的主要技术,研究水力裂缝的扩展规律有利于促进复杂缝网的形成。为了研究裂缝扩展规律和破裂机制,对页岩进行了真三轴双井筒水力压裂模型试验。结果表明:(1)竖向应力越大,裂缝形态越单一,多条水力裂缝间越不易沟通,页岩破裂压力越大;(2)水平应力差越大,双井筒水力裂缝与天然层理沟通越明显,越易形成垂直于水平最小主应力的裂缝;(3)双井筒出口在不同层理面时,两条水力裂缝形态相似,各自会沟通层理和天然裂缝;(4)双井筒出口在同一层理面时,两条主裂缝容易相交,与天然裂缝的沟通较少,不易产生缝网;(5)通过对上升斜率RA和频度FA值综合分析得到,页岩水力压裂以剪切破坏为主,在不同层理压裂更易产生复杂缝网。     

定向射孔对水力裂缝起裂与延伸的影响

 射孔孔眼是沟通井筒和地层的通道,压裂施工中射孔参数影响施工效果。采用大尺寸真三轴水力压裂物理模拟实验系统,研究定向射孔方位角、水平应力差和微环隙对裂缝起裂、延伸、转向、破裂压力及形态的影响,并建立了射孔直井在地应力条件下产生垂直裂缝的破裂压力预测模型。实验结果表明：利用定向射孔压裂技术可以在地层中形成双翼弯曲水力裂缝;随着定向射孔方位角的增大,破裂压力越来越高,转向距离也越来越大;且水平最大主应力与水平最小主应力之间的应力差对裂缝的转向距离有很大影响;微环隙对裂缝形态及破裂压力也有影响,其使得破裂压力较从射孔孔眼起裂所需压力大幅降低,且裂缝形态与裸眼井起裂类似。结论对于实际的射孔参数优化设计和水力压裂施工具有参考意义。     

酸液预处理对深部裂缝性页岩储层压裂的影响机制

深层裂缝性页岩油气藏具有高温、高应力、高破裂压力以及低渗透的特点,极大提高了现场压裂施工难度,如何有效改善近井筒岩石学性质及降低施工压力是安全高效压裂的关键。从岩石力学性质的影响因素入手,分析了酸化过程中酸与矿物的化学反应,从宏观和微观两方面阐述了酸液预处理改善近井筒岩石性质、降低破裂压力的力学机理。选取川东南地区龙马溪组页岩露头开展真三轴水力压裂物理模拟试验,采用酸液预处理压裂试件的裸眼井段,分析酸岩反应对水力裂缝起裂及扩展规律的影响。试验结果表明,酸液浸泡裸眼井段能够显著降低页岩破裂压力,酸岩反应会引起天然裂缝面性质的变化,容易导致近井筒附近复杂多裂缝的形成;裂缝扩展穿透酸化区时会引起二次憋压,酸化区内的天然裂缝越发育,酸岩反应越剧烈,近井筒裂缝形态越复杂,压裂曲线波动越频繁。室内试验和现场施工结果吻合程度良好,证实了通过酸液预处理提高深部页岩地层改造效果的有效性。     

浅部地层破裂压力预测研究

擅要地层的破裂压力对钻井液密度确定、井身结构设计和水力裂缝起裂形态预测有着重要的指导作用。井壁的破裂压力不仅取决于地应力，还与井眼形态和地层的强度各向异性有关。从地应力与岩石力学参数研究入手，分析井周应力场分布，根据起裂准则，建立了浅层破裂压力预测模型，并在现场得到了成功的应用。     

岩石类非均质脆性材料破坏过程的数值模拟

用梁–颗粒模型BPM2模拟了岩石的破坏过程。梁–颗粒模型是在离散单元法基础上,结合有限单元法提出的用于模拟岩石类材料破坏过程的数值模型。在模型中,采用3种类型梁单元随机分布来模拟岩石类材料力学参数的空间变异性,并通过自动生成的非均质材料模型对岩石类材料的破坏机理进行了研究。岩石类非均质脆性材料在单轴受压状态下破坏过程细观数值模拟结果显示,岩石材料宏观破坏是由于其内部细观裂纹产生、扩展、贯通的结果。与实际矿柱破坏形态的对比分析表明,梁–颗粒模型是计算和模拟岩石类脆性材料破坏问题的有力工具。     

自然营造力作用下岩石单裂纹水力劈裂数值仿真模型

考虑水利水电工程中自然营造力作用下岩石水力劈裂特点,建立裂纹内初始含水量饱和而外界水压增大情况下,裂纹进一步发生水力劈裂的数值仿真模型。在数值模型中,通过半解析半数值方法建立裂纹内水压分布梯度与裂纹张开位移间的耦合关系,不仅简化耦合迭代分析,而且提高计算精度;断裂力学模型采用以Hillerborg黏滞区裂纹模型为基础的COD准则,并引入损伤变量对其开裂准则进行修正;裂纹的扩展采用预置零厚度接触单元的方法巧妙地解决耦合与非线性迭代中裂纹的前进与后退问题。最后,对简单的岩石试件进行水力劈裂过程数值仿真分析,得到的裂纹内水压分布规律与已有的试验结果吻合,证明所建模型是合理的,计算结果是可靠的。     

水力压裂三维数学模型研究

根据渗流力学、岩石力学、传热学、断裂力学知识，建立了油水井三维水力压裂模拟计算的数学模型。模型考虑了压裂液的流变性、支撑剂在压裂缝中的运移、压裂液与储层岩石的热交换以及压裂过程中的流固耦合作用，给出了裂纹扩展和流固耦合计算的数值解法，通过算例验证了所提出数学模型的合理性。     

储层非均质性对水力压裂裂缝扩展的影响研究

为了研究滤失和储层非均质性对水平井多簇射孔水力压裂裂缝扩展形态的影响,在前期建立的水力压裂流固耦合模型基础上,增加考虑多孔介质内流体流动,裂缝-孔隙间的流体交换和射孔流量分配的物理过程,形成了用于多簇射孔水力压裂裂缝扩展的裂缝-孔隙流固耦合模型,并数值模拟研究了致密砂岩气储层性质均质、非均质下水平井多簇射孔水力压裂裂缝扩展。研究发现:(1)滤失对中间簇射孔水力压裂裂缝扩展影响较大;(2)在储层均质情况下,多裂缝呈对称扩展,而在储层杨氏模量、抗拉强度、基质渗透率等非均质分布情况下,多裂缝呈非对称、非均匀扩展;(3)在储层复杂非均质情况下,四簇和三簇射孔水力压裂存在射孔簇过度改造、欠改造、甚至未起裂扩展的情况,尤其是在小间距条件下的四簇射孔。而两簇射孔水力压裂即使在间距较小时裂缝也能正常起裂扩展。因而,为了裂缝更多地有效扩展,在实际工程中,如果水力压裂密集施工,建议采取两簇射孔压裂;如果水力压裂不密集施工,兼顾施工成本建议采取三簇或四簇射孔压裂。     

Simulating hydraulic fracturing processes in laboratory-scale geological media using three-dimensional TOUGH-RBSN

In this context, recent developments in the coupled three-dimensional (3D) hydro-mechanical (HM) simulation tool TOUGH-RBSN are presented. This tool is used to model hydraulic fracture in geological media, as observed in laboratory-scale tests. The TOUGH-RBSN simulator is based on the effective linking of two numerical methods: TOUGH2, a finite volume method for simulating mass transport within a permeable medium; and a lattice model based on the rigid-body-spring network (RBSN) concept. The method relies on a Voronoi-based discretization technique that can represent fracture development within a permeable rock matrix. The simulator provides two-way coupling of HM processes, including fluid pressure-induced fracture and fracture-assisted flow. We first present the basic capabilities of the modeling approach using two example applications, i.e. permeability evolution under compression deformation, and analyses of a static fracturing simulation. Thereafter, the model is used to simulate laboratory tests of hydraulic fracturing in granite. In most respects, the simulation results meet expectations with respect to permeability evolution and fracturing patterns. It can be seen that the evolution of injection pressure associated with the simulated fracture developments is strongly affected by fluid viscosity.     

页岩水力压裂水力裂缝与层理面扩展规律研究

在对含天然层理弱面页岩进行水力压裂过程中,水力主裂缝的起裂、扩展及层理面的扩展对缝网的形成有重要影响。为研究水力主裂缝的起裂、扩展规律和层理面对水力裂缝扩展的影响,开展真三轴试验条件下的水力压裂试验,采用声发射系统监测水力压裂过程,并在试验后对试样进行剖切与CT扫描;同时进行定量的理论分析,并通过试验结果验证。研究表明:(1)起裂方向由初始角度转至最大水平主应力方向;垂向应力与水平最大主应力相差极小时,各个方向起裂压力相差极小,裂缝很快转向最大水平主应力方向。(2)水力主裂缝整个扩展过程中所需水压区间与裂缝长度、断裂韧性值相关。(3)形成由层理面与主裂缝构成的网状的裂缝系,层理面在主裂缝的靠近过程中张开区的长度极小,主要在主裂缝接触到层理面后产生较大的张开区与剪切区,层理面的剪切区域长度远大于张开区域长度,剪切区域提供主要的导流通道;剪切区的长度对层理面黏聚力c和水力裂缝与层理面交角?参数敏感性很高。研究结果可以为压裂模型的建立提供几何参数,并对施工参数的设计有指导意义。     

Initiation of a secondary hydraulic fracture and its interaction with the primary fracture

Laboratory and field experiments have shown that secondary fracture treatment along a different direction to the primary hydraulic fracture can increase the recovery of gas from tight sands. Success of a restimulation treatment (secondary fracture treatment) is, however, highly dependent on transient stress distribution around the well. In this paper a numerical poroelastic reservoir model is used to provide a more realistic picture of stress field around a stimulated well during post-fracture production. Initiation of a re-oriented hydraulic fracture and its interaction with the primary hydraulic fracture are also studied using a wellbore re-pressurization model that includes a poroelastic formation, a wellbore and a pair of finite conductive hydraulic fractures. Numerical results have shown that during post-fracture production the directions of maximum and minimum horizontal stresses are reversed within a large area around the wellbore and the hydraulic fractures. We have also shown that relatively small areas exist at the wellbore wall where the original directions of horizontal principal stresses remain unchanged. It is shown that the presence of these small areas could act against initiation of a secondary hydraulic fracture along the most desired directions in terms of recovery enhancement. This study also shows that oriented perforations prior to refracture treatment help initiate secondary fractures in a preferred direction before the primary hydraulic fractures start propagation during re-pressurisation of the wellbore.     

Exploitation of Hot Dry Rock Geothermal Energy in the United Kingdom

There exists within the crust of the earth a vast thermal resource of rock which is at elevated temperature but which lacks significant fluid flow to be exploited naturally. Measurements of heat flow made throughout the United Kingdom have identified the Cornubian batholith as such a structure, with an abnormally high geothermal gradient.

Shallow depth field research has been conducted into the mechanics of linking wells together to engineer the required heat transfer area. This has shown that a combination fracturing technique, involving explosive stimulation and hydraulic pressurisation, has considerable advantages over the conventional hydraulic fracturing technique. The experimental design of a second phase of research is presented: the results of which are expected to provide definite guidelines for the engineering requirements of a system capable of producing super-heated steam for the generation of electrical power.     

近井眼裂缝控制技术研究现状

对目前国内外水力压裂技术近期进展作了调研，分析了高能气体压裂技术、水力喷射压裂技术、剪切压裂技术等压裂技术在近井眼区域控制裂缝起裂延伸的机理、应用范围及效果，对这些技术存在的问题进行了研究，并结合存油气田生产中的应用，指出了进一步发展的方向及需要解决的主要问题。     

裂缝性地层水力裂缝张性起裂压力分析

 室内实验和矿场压裂实践表明,裂缝性地层水力裂缝在近井区域可能扩展为复杂的径向缝网,这与均质地层水力压裂产生的平面对称双翼裂缝具有巨大差异。由于水力裂缝延伸形态受起裂和延伸2个过程的控制,为此,研究裂缝性地层水力裂缝起裂机制是认识径向缝网扩展的前提。基于弹性力学和岩石力学理论,考虑天然裂缝与射孔孔眼相交,结合张性起裂准则,建立裂缝性地层水力裂缝沿天然裂缝张性起裂的起裂压力计算模型。计算结果表明：天然裂缝与孔眼相交点越靠近孔眼指端,张性起裂压力越小;天然裂缝与孔眼相交点越靠近孔眼顶部,张性起裂压力越小;受天然裂缝和水平地应力方位的影响,井眼周向上不同方位孔眼的张性起裂压力差可能急剧减小,这种作用效应将导致水力裂缝从井眼周向上不同方位的孔眼同时起裂延伸,产生径向缝网。实例计算表明,文中建立的起裂压力模型计算精度高,计算结果可靠,可用于裂缝性地层水力裂缝起裂压力计算和径向缝网扩展分析。     

高水平应力差下水平井螺旋射孔参数优化研究

在压裂施工中,射孔是一个重要环节。射孔能够降低岩石破裂压力,减少近井筒缝网结构的复杂性,有利于水力压裂之后的油井增产。通过开展大尺寸真三轴室内实验,研究水平应力差大小以及在高水平应力差下射孔相位、射孔密度和射孔簇间距对于裂缝的形态、裂缝面的扩展和岩石破裂压力的影响。实验证明:在高水平应力差下,由于地层的各向异性,导致地层的能量不稳定性,有利于相邻裂缝扩展的相互影响,减少近井筒附近的裂缝的复杂性,同时减少岩石的破裂压力;在高水平应力差下,射孔相位增大时,射孔间距变大,不利于相邻裂缝的沟通,增加了近井筒裂缝的复杂的可能性。通过物模实验表明,与射孔相位为90°相比,射孔相位为60°形成的单一平整裂缝扩展面更有利于油井增产,同时岩石破裂压力更小,有利于水力压裂;射孔簇间距适当增加,能够减少两簇之间的相互影响,能够增加多簇同时起裂的可能性,同时还能增加射孔过程中总的射孔起裂数量;射孔密度增加还能减少孔间距,减少相邻射孔之间相互影响的能量,减少近井筒裂缝的复杂性,同时还能够有效地减少岩石破裂压力。     

Assessing fracturing mechanisms and evolution of excavation damaged zone of tunnels in interlocked rock masses at high stresses using a finitediscrete element approach

Deep underground excavations within hard rocks can result in damage to the surrounding rock mass mostly due to redistribution of stresses.Especially within rock masses with non-persistent joints,the role of the pre-existing joints in the damage evolution around the underground opening is of critical importance as they govern the fracturing mechanisms and influence the brittle responses of these hard rock masses under highly anisotropic in situ stresses.In this study,the main focus is the impact of joint network geometry,joint strength and applied field stresses on the rock mass behaviours and the evolution of excavation induced damage due to the loss of confinement as a tunnel face advances.Analysis of such a phenomenon was conducted using the finite-discrete element method(FDEM).The numerical model is initially calibrated in order to match the behaviour of the fracture-free,massive Lac du Bonnet granite during the excavation of the Underground Research Laboratory(URL)Test Tunnel,Canada.The influence of the pre-existing joints on the rock mass response during excavation is investigated by integrating discrete fracture networks(DFNs)of various characteristics into the numerical models under varying in situ stresses.The numerical results obtained highlight the significance of the pre-existing joints on the reduction of in situ rock mass strength and its capacity for extension with both factors controlling the brittle response of the material.Furthermore,the impact of spatial distribution of natural joints on the stability of an underground excavation is discussed,as well as the potentially minor influence of joint strength on the stress induced damage within joint systems of a non-persistent nature under specific conditions.Additionally,the in situ stress-joint network interaction is examined,revealing the complex fracturing mechanisms that may lead to uncontrolled fracture propagation that compromises the overall stability of an underground excavation.     

水力压裂裂缝穿层及扭转扩展的三维模拟分析

 应用并行有限元程序对水力压裂过程进行真三维数值模拟,实现对压裂裂缝起裂、扩展及扩展中的穿层、扭转行为的全过程分析;数值计算中无需假定压裂裂缝的起裂位置和扩展路径,即可根据实际岩体水力学模型的力学、水力学等边界条件,自动标定出压裂裂缝的三维扩展模式,并显示出该并行有限元程序对复杂地质力学条件下水力压裂过程三维模拟分析的适用性。通过对压裂裂缝扩展过程中孔隙压力分布、裂缝几何形状和尺寸的演化进行了解读,显示出压裂裂缝的扩展模式与地层分布密切相关。当生产层很薄或生产层与上下阻挡层岩性差别较大时,裂缝穿层现象突出,可能会出现压裂实际缝长远远小于设计缝长的现象;近场地应力差异、地层分布特征及岩体细观非均匀性都有可能诱发压裂裂缝扭转扩张。分析结果对水力压裂施工设计具有一定的参考价值。     

Fracture toughness of a soft sandstone

Fracture toughness of a rock/material is important in the design of rock drilling/boring equipment, rock bursting, prediction of rock drilling forces, hydraulic fracturing, wellbore stability and stability of jointed rock masses. A series of fracture toughness tests under mode-I, mode-II and mixed-mode (I–II) loading conditions were conducted on straight-edge notch Brazilian disk (SENBD) specimens in an attempt to develop an empirical failure envelope under both tension-shear and compression-shear conditions. This series of tests is the first of its kind on a naturally weakly-cemented Antler sandstone. Even though there have been published data on artificially cemented sandstones, there are no data on mixed-mode fracture toughness of such soft formations. The SENBD specimen configuration used herein provides consistent failure patterns and development of a failure envelope in the compression-shear zone, as well. Microscopic studies using thin sections of tested and untested specimens were carried out to study the fracture propagation at micro-level.     

2D-numerical analysis of hydraulic fracturing in heterogeneous geo-materials

A geo-material failure process analysis (F-RFPA2D), considering the coupling of stress distribution, fluid flow, and element damage evolution, is used to investigate the mechanisms of crack initiation and propagation around a 2-D cylindrical cavity in heterogeneous stiff soils during hydraulic fracturing. A large number of numerical analysis on hydraulic fracturing in stiff soil with pre-existing injection cavity have been carried out to study the mechanism of hydraulic fracturing in stiff soil. In addition, the characteristic of acoustic emission (AE) due to hydraulic fractures are studied by numerical simulations. The results provide a better understanding of the crack initiation and propagation mechanisms during hydrofracturing. The simulation software package can be a powerful tool for study of soil behavior during hydraulic fractures.     

二维裂隙岩体渗流传热的离散裂隙网络模型数值计算方法

 针对裂隙岩体渗流传热问题,用解析方法,比较2种不同岩石基质与裂隙水界面热交换假设下的计算结果,对一般裂隙岩体,2种假设下的计算结果相同。基于离散裂隙网络模型的思想,在商业有限元软件COMSOL中实现一种计算已知裂隙网络的裂隙岩体渗流和传热过程的数值方法,该方法可以同时计算岩石基质与裂隙中的渗流和传热过程及二者间的交换,并与解析解比较进行验证。用该方法对一随机生成的二维裂隙岩体进行计算,得到的出口温度曲线,可以反映裂隙岩体渗流传热的早期热突破和长尾效应等特点,并分析岩石基质渗透率、热传导系数的不同取值对裂隙岩体渗流和传热过程的影响。