储层非均质性对水力压裂的影响

从岩石细观非均质性的特点出发,采用RFPA2D-Flow软件对单孔和双孔数值模型进行压裂计算,研究岩石非均质性对水力压裂的响应,重点探讨双孔模型孔间吸引效应对裂纹演化形态的影响。岩石细观单元的力学、水力学特性由统计分布生成以体现岩石的随机不均质性,水力压裂过程中流体压力传递通过单元渗流-损伤耦合迭代来实现。数值计算结果表明:(1)岩石非均质性影响裂缝的扩展形态,导致水力裂纹尖端微裂纹的分支。随着均质度的增加,水力裂纹的扩展形态变得更加平直光滑,单孔模型两侧裂纹更加对称,双孔间裂纹的连通性变差。(2)岩石的非均匀性对于岩石的起裂压力和地层破裂压力影响较大。随着均质度的增大,起裂应力和地层破裂应力增大,并且两者间的差值逐渐变小,在储层为均质的条件下,两者几乎相等。(3)相同的边界条件下,均质模型的应力分布曲线光滑连续,非均质模型的应力分布曲线呈现出明显波动,井眼对称剖面上的应力分布不尽相同,反映了细观单元强度非均匀性及裂缝扩展形态对应力分布的影响。(4)双孔模型孔间存在孔隙水压力增加带,孔间产生吸引效应,双孔方位影响临界压力。研究结果对水压裂试验设计和现场压裂施工具有一定的参考意义。     

超临界CO2压裂起裂压力模型与参数敏感性研究

超临界CO2流体具有低黏度、无表面张力的特性,若应用于非常规油气储层压裂可望实现一种无污染的新型无水压裂方法。通过建立流体在井筒内的增压速率模型,得到了考虑超临界CO2流体黏度、压缩性及增压速率的裂缝起裂压力预测模型,并与水力压裂、液态CO2压裂起裂压力进行了对比分析。结果表明,超临界CO2流体的起裂压力比液态CO2流体低20.5 %,比常规水力压裂起裂压力低75.5%;超临界CO2流体的黏度、压缩性及增压速率对裂缝起裂压力影响显著。模型与文献中试验数据对比,误差在3%以内,可为超临界CO2压裂起裂压力预测提供指导。     

Influence of Heterogeneity of Mechanical Properties on Hydraulic Fracturing in Permeable Rocks

Summary Numerical simulations of circular holes under internal hydraulic pressure are carried out to investigate the hydraulic fracture initiation, propagation and breakdown behavior in rocks. The hydraulic pressure increases at a constant rate. The heterogeneity of the rocks is taken into account in the study by varying the homogeneity index. In addition, the permeability is varied with the states of stress and fracture. The simulations are conducted by using a finite element code, F-RFPA^2D, which couples the flow, stress and damage analyses. The simulation results suggest that the fracture initiation and propagation, the roughness of the fracture path and the breakdown pressure are influenced considerably by the heterogeneity of rocks. The hole diameter elongation and the stress field evolution around the fracture tip during the fracture propagation can also provide useful information for the interpretation of the hydraulic fracturing behaviour.     

Capturing the two‐way hydromechanical coupling effect on fluid‐driven fracture in a dual‐graph lattice beam model

Fluid‐driven fractures of brittle rock is simulated via a dual‐graph lattice model. The new discrete hydromechanical model incorporates a two‐way coupling mechanism between the discrete element model and the flow network. By adopting an operator‐split algorithm, the coupling model is able to replicate the transient poroelasticity coupling mechanism and the resultant Mandel‐Cryer hydromechanical coupling effect in a discrete mechanics framework. As crack propagation, coalescence and branching are all path‐dependent and irreversible processes, capturing this transient coupling effect is important for capturing the essence of the fluid‐driven fracture in simulations. Injection simulations indicate that the onset and propagation of fractures is highly sensitive to the ratio between the injection rate and the effective permeability. Furthermore, we show that in a permeable rock, the borehole breakdown pressure, the pressure at which fractures start to grow from the borehole, depends on both the given ratio between injection rate and permeability and the Biot coefficient.     

水力压裂脉冲频率对煤岩预制孔密封效果影响

脉冲水力压裂技术是改造低渗储层的一种重要手段,通过水楔效应和脉冲疲劳损伤双重作用沟通裂隙网络,提高低渗储层导流能力。在脉冲水力压裂室内试验中,试样中预制孔的密封问题是决定水力压裂试验成败的关键,而起裂压力又是评价脉冲封孔段密封效果的重要指标。通过煤岩脉冲水力压裂室内试验,建立了封孔段薄弱结合面与煤岩基体力学性质的关联,研究不同频率对煤岩起裂压力的影响,最终拟合相关数据得到:基于煤岩脉冲作用下起裂压力的预制孔封孔压力经验公式。研究结果为脉冲水力压裂室内试验的试样预制孔密封提供依据。      

Study on Breakdown Pressure in Hydraulic Fracturing Process of Hard Rock Based on Numerical Simulation

Pre-conditioning by hydraulic fracturing is a vital work in block caving mining of hard-rock metal mine, and breakdown pressure is one of the important parameters in rock hydraulic fracturing process, which determines the equipment selection and job-parameter control. Based on numerical simulation method, a pre-holed cylindrical hard rock sample was built for simulation of hydraulic fracturing process to investigate the effect of stress condition, rock properties and water injection rate on breakdown pressure. The results show that the breakdown pressure is increased with the confining stress, and the breakdown pressure is 2.28 times confining stress based on rock tension strength. The variation of rock tension strength affects the breakdown pressure and a positive linear relation exists. Pre-formed notch could obviously reduce the breakdown pressure and control the orientation of hydraulic fracture. And the borehole diameter has a reverse influence on rock breakdown pressure.

     

倾斜煤层水力压裂起裂压力计算模型及判断准则

针对目前倾斜煤层起裂机制不明确,导致水力压裂时盲目升高压力或增大注水量来增加煤层透气性的问题,根据最大拉应力理论,分析真实环境下倾斜煤层压裂钻孔周围应力状态,建立压裂钻孔周围煤岩体起裂压力计算模型及判断准则,并在重庆松藻煤电公司同华煤矿进行了验证,结果表明,根据实测压裂区域地应力状态,起裂压力随煤层倾角增大而增大,钻孔起裂位置随煤层倾角增大逐渐向走向方向偏转;现场试验起裂压力与理论计算相符,随煤层倾角增大而增大,从而验证了计算模型的正确性。     

FDEM-flow3D: A 3D hydro-mechanical coupled model considering the pore seepage of rock matrix for simulating three-dimensional hydraulic fracturing

A hydro-mechanical coupled model that can simultaneously consider the pore seepage of a rock matrix and the fracture seepage is proposed to simulate three-dimensional hydraulic fracturing. This model appropriately takes into account the fluid leak-off into the surrounding rock matrix from the fracture. Several examples are given to validate the seepage algorithms and the coupled model. The results suggest that this model can solve problems involving pore seepage and fracture seepage through simple pure fracture seepage. Moreover, it can reproduce the fluid pressure distribution and the crack initiation and propagation and consider the fluid loss during hydraulic fracturing.     

A numerical investigation of the hydraulic fracturing behaviour of conglomerate in Glutenite formation

Rock formations in Glutenite reservoirs typically display highly variable lithology and permeability, low and complex porosity, and significant heterogeneity. It is difficult to predict the pathway of hydraulic fractures in such rock formations. To capture the complex hydraulic fractures in rock masses, a numerical code called Rock Failure Process Analysis (RFPA2D) is introduced. Based on the characteristics of a typical Glutenite reservoir in China, a series of 2D numerical simulations on the hydraulic fractures in a small-scale model are conducted. The initiation, propagation and associated stress evolution of the hydraulic fracture during the failure process, which cannot be observed in experimental tests, are numerically simulated. Based on the numerical results, the hydraulic fracturing path and features are illustrated and discussed in detail. The influence of the confining stress ratio, gravel sizes (indicated by the diameter variation), and gravel volume content (VC) on the hydraulic fracturing pattern in a conglomerate specimen are numerically investigated, and the breakdown pressure is quantified as a function of these variables. Five hydraulic fracturing modes are identified: termination, deflection, branching (bifurcation), penetration, and attraction. The propagation trajectory of the primary hydraulic fractures is determined by the maximum and minimum stress ratios, although the fracturing path on local scales is clearly influenced by the presence of gravels in the conglomerate, particularly when the gravels are relatively large. As the stress ratio increases, the fractures typically penetrate through the gravels completely rather than propagating around the gravels, and the breakdown pressure decreases with increasing stress ratio. Furthermore, the breakdown pressure is affected by the size and volume content of the gravel in the conglomerate: as the gravel size and volume content increase, the breakdown pressure increases.     

水劈裂过程中岩体渗透性规律及机理分析

岩体的结构及其透水性直接关系到建筑物围岩的稳定及安全。通过水力劈裂试验,可以真实地反映高水压作用下岩体的结构和渗透性的变化规律。以某水电站工程坝址区岩体所作的水力劈裂试验资料为基础,分析了在水力劈裂过程中,岩体的结构和渗透性发生的变化及其规律以及在该过程中岩体裂隙形成的机理。     

Experimental and theoretical study on the effect of unsteady flow on the fracturing pressure in hydraulic fracturing test

Reasonable determination of formation fracturing pressure concerns the stable operation of underground fluid injection projects. In this work, we studied the effect of unsteady flow on fracturing pressure. Hydraulic fracturing tests on low permeable sandstone were conducted with the injection rate between 0.1 and 2.0 ml/min. Then, the fracturing pressure prediction models for hollow cylinder under both unsteady flow and steady flow conditions were deduced. Finally, the effect of unsteady flow on the fracturing pressure was studied based on the experimental result and several influence factors. It was shown that fracturing pressure increased with the elevated pressurization rate in the tests, while the slope of the variation curve decreases. The model considering unsteady flow can reflect the variation tendency of fracturing pressures in experiments, while fracturing pressures from the model considering steady flow are invariant with different pressurization rates. Fracturing pressure decreases with the elevated rock permeability and increases with the elevated fluid viscosity, and these two effects are actually generated by the unsteady flow. Whether to consider the unsteady flow has no significant influence on the effect of rock tensile strength on fracturing pressure when the tensile strength is very low. However, when the tensile strength is high, the effect of unsteady flow cannot be neglected.     

The role of pore pressure during hydraulic fracturing and implications for groundwater outbursts in mining and tunnelling

Water outbursts from the floor during underground mining, and those from the surrounding rock mass of tunnels, involve the basic principle of hydraulic fracturing. Based on the hydraulic-fracturing mechanism, considered to be dependent on the coupling between seepage and damage, it is deemed that the variation of the pore-fluid pressure coefficient must be taken into account during this coupled process, in order to correctly establish the crack propagation mechanism during hydraulic fracturing. The coupled seepage-damage model is validated using numerical simulations of hydraulic fracturing around one hole and three holes; the model may also enable scientific and reasonable explanation of the dominance of hydraulic gradient on the crack propagation path in permeable rock. Finally, the water outburst from the floor at a coal mining site in Hebei Province, China, is numerically simulated, and the coupled seepage and damage mechanism during the mining-induced rock failure is clarified. The numerical simulation implies that the seepage-damage is the main mechanism for controlling the water outburst. Therefore this mechanism should be considered in the numerical simulation to understand the essence of water outburst induced in mines.     

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。     

Impact of water and nitrogen fracturing fluids on fracturing initiation pressure and flow pattern in anisotropic shale reservoirs

A numerical model is proposed to investigate the impact of water and nitrogen fracturing fluids on the fracturing initiation pressure and the flow pattern in anisotropic shale reservoirs. This model considers the anisotropy of shale deformation and permeability, the compressibility of fracturing fluid, and the fluid moving front. A crack initiation criterion is established with the stress intensity factor of mode I crack. Both crack initiation pressure and seepage area are verified and analyzed. These results show that shale deformation and permeability anisotropy, fracturing fluid compressibility, viscosity, and pressurization rate have significant impacts on fracturing initiation pressure and seepage area.     

A two‐dimensional coupled hydromechanical discontinuum model for simulating rock hydraulic fracturing

Within the framework of our discontinuous deformation analysis for rock failure algorithm, this paper presents a two‐dimensional coupled hydromechanical discontinuum model for simulating the rock hydraulic fracturing process. In the proposed approach, based on the generated joint network, the calculation of fluid mechanics is performed first to obtain the seepage pressure near the tips of existing cracks, and then the fluid pressure is treated as linearly distributed loads on corresponding block boundaries. The contribution of the hydraulic pressure to the initiation/propagation of the cracks is considered by adding the components of these blocks into the force matrix of the global equilibrium equation. Finally, failure criteria are applied at the crack tips to determine the occurrence of cracking events. Several verification examples are simulated, and the results show that this newly proposed numerical model can simulate the hydraulic fracturing process correctly and effectively. Although the numerical and experimental verifications focus on one unique preexisting crack, because of the capability of discontinuous deformation analysis in simulating block‐like structures, the proposed approach is capable of modeling rock hydraulic fracturing processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Mesoscale numerical study on the evolution of borehole breakout in heterogeneous rocks

Inherent heterogeneity of a rock strongly affects its mechanical behavior. We numerically study the mechanisms governing the initiation, propagation, and ultimate pattern of borehole breakouts in heterogeneous rocks. A two-dimensional finite element model incorporating material heterogeneity is established to systematically examine the effects of several key factors on borehole failure, including borehole diameter, far-field stress, and rock heterogeneity. The inherent heterogeneity of a rock is explicitly characterized by prescribing the rock mechanical properties of mesoscale elements statistically obeying the Weibull distribution. Elastic damage mechanics is used to represent the constitutive law of the mesoscale element. We find that borehole diameter reduction remarkably changes the crack failure from tensile to shear and elevates the critical hydrostatic pressure. Far-field stress anisotropy strongly affects the shape of the borehole breakout. Rock heterogeneity dictates the location of the preferred crack under the hydrostatic stress, which leads to local stress concentration, and determines the types of breakouts around the borehole. Our findings facilitate in-depth understanding of the classic borehole stability problems in heterogeneous rocks.     

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。     

基于孔壁应变发展规律的压裂孔三阶段起裂特征试验研究

利用自行研制的煤岩体水力压裂试验系统,开展了配比型煤与原煤水力压裂试验,测试并分析了水力压裂过程中压裂孔孔壁应变-水压曲线,并基于孔壁应变的发展规律,分析了压裂孔的三阶段起裂特征。结果表明,在压裂孔起裂过程中,钻孔孔壁呈现拉伸与压缩应变两种类型,并呈现拉伸破裂区与压缩变形区,其中压缩型应变具有较好的可恢复性,其应变恢复比远大于拉伸型应变;钻孔起裂过程分为3个阶段,即水气作用诱导微损伤形成阶段,孔壁内形成气流通道并产生初始损伤;局部损伤带形成阶段,孔壁形成拉伸破裂区和压缩变形区;试件失稳破坏阶段,裂缝不断延伸直至试件破裂,拉伸破裂区依然保持拉伸变形并较好地保持残余变形,而压缩变形区则由于作用力转向而得到一定程度恢复。研究成果对于揭示钻孔起裂行为及能量的演化规律具有重要理论意义。     

多级循环泵注水力压裂模拟实验研究

超深储层地层起裂压力较高,水力压裂受现场泵注设备的限制严重,文中重点研究了大尺度水力压裂物模实验水泥样品尺寸（762 mm×762 mm×914 mm）在循环和常规两种泵注条件下的起裂扩展和声发射规律。实验结果显示,（1）相对于普通泵注,采用循环泵注方式进行水力压裂可以有效降低起裂压力,类似于单轴和三轴循环加载岩石力学行为,都是由于循环加载引起疲劳损伤;（2）对于螺旋射孔完井方式,水力压裂裂缝只从最薄弱射孔处起裂,一旦起裂后其他射孔孔眼很难再开启,水力压裂现场应合理选择分段距离和簇间距,实现储层改造效率最优化;（3）循环泵注水力压裂存在Kaiser效应（当加载应力到前次加载最高应力值时出现的声发射信息）。因孔隙流体扩散到岩石并导致孔隙压力的局部上升,破坏模式仍然可以由摩尔圆表示。研究成果对循环泵注条件的裂缝扩展规律研究以及发展新型压裂改造技术具有重要意义。