一种用于水压致裂试验的水压供给装置研制及应用

为研究水压致裂过程中水力裂缝的起裂、扩展规律及机制,研制了一种新型水压供给控制装置。该装置由水泵、增压泵、空气压缩机和导水钢垫板等组成。该装置的主要特点是易于与目前广泛存在的压力伺服机相配合,共同完成水压致裂试验,具有操作简便、效率高、成本低、组装方便等优点,使水压致裂试验更易大面积推广。利用该试验装置对类岩石试块进行了水压致裂试验,研究了不同围压下含预置裂隙试件的水力裂缝扩展规律,并与无预置裂隙试件水力裂缝扩展规律进行了对比。试验结果表明,对于含预置裂隙试块,当水平应力差较小时,预置裂隙裂尖对水力裂缝扩展影响较大,水力裂缝起裂后向预置裂隙裂尖方向扩展;随着水平应力差的增大,水力裂缝向最大水平应力方向偏转。试验证明该装置可靠性高,试验过程稳定。该研究对水压致裂试验具有促进意义。     

单孔岩样水压致裂的数值分析

水压致裂是改变岩体结构的一种天然行为和人为手段。采用F－RFPA2D软件,对水压致裂过程、裂缝扩展形态及注水孔形状和大小、应力条件和岩样强度等影响因素进行了研究。将开始出现声发射的水压称为微裂压力,将声发射急剧增多、裂缝非稳定扩展直至岩样破坏的水压称为破裂压力。岩样尺寸一定时,微裂压力和破裂压力随内孔面积增加而降低,方形孔岩样的微裂压力和破裂压力均小于同面积的圆形孔。微裂压力和破裂压力随围压或岩样强度增加而增加,且其差值随岩样强度增加而增加,理论破裂压力与模拟值趋势基本一致。方形孔的宏观裂纹起裂位置多在角点附近,而圆形孔比较随机。无围压时,宏观裂纹的延伸方向随机;有围压时,宏观裂纹扩展方向大致与主应力方向一致,且沿较大主应力方向的宏观裂纹扩展至岩样破坏,较小主应力方向宏观裂纹不完全发育。研究结果对水压致裂试验和工程实践有一定参考意义。     

单轴应力下带钻孔花岗岩注入高温蒸汽破坏特征研究

大型水力压裂是干热岩地热能开发中人工储留层建造的最有效手段,其核心力学问题为高温、高压下岩石的水力破岩机制。通过单轴应力下带钻孔花岗岩注入高温蒸汽破坏试验,研究固-热耦合作用下花岗岩的水力破岩机制。结果表明:高温对花岗岩破裂有很大的促进作用,热效应导致强度弱化,降低破裂压力。高速率注入430℃和350℃蒸汽破坏试验中,破裂压力比常温水压裂至少降低58%;低速率注入400℃和450℃蒸汽破坏试验中,花岗岩破裂压力比常温水压裂降低75%。注蒸汽破坏过程可分为热破裂损伤和宏观裂缝扩展两个阶段。高温蒸汽产生的热应力在钻孔周围随机发生热破裂,随着注入蒸汽时间的增加,热破裂范围由钻孔附近逐渐向远处扩展,热破裂分布密度增大,为宏观裂缝的产生提供便利条件。初始宏观裂缝首先出现在钻孔两侧,沿着最终形成的宏观裂缝轨迹扩展,直到试样破坏。与常温水压裂相比,低速率注蒸汽破坏是一个缓慢的延性拉破坏过程,裂缝相对钻孔不对称扩展,宽度小于水力压裂裂缝宽度。     

孔隙水压力对岩石裂纹扩展影响的数值模拟

应用岩石破坏过程渗流-应力-损伤(FSD)耦合分析软件F-RFPA2D,通过对孔隙水压作用下岩石试件加载破坏过程的数值模拟,对孔隙水压力大小和梯度对岩石试样中裂纹的萌生和扩展进行了数值模拟研究。模拟结果再现了孔隙水压力作用下裂纹萌生扩展的全过程,表明孔隙水压力大小和梯度对岩石中裂纹的萌生和扩展模式都有很大的影响。     

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。     

基于最大周向应变断裂准则定向射孔水力裂缝扩展研究

利用最大周向应变断裂准则,基于裂纹扩展增量法模拟水力裂缝扩展过程。数值模拟结果表明,上述裂缝扩展方法能有效模拟水力裂缝扩展过程。在此基础上利用该方法分析了定向射孔水力裂缝初始起裂水压、扩展路径和裂缝"偏转角"受射孔长度与倾角、地应力差、泊松比和注水压力的影响。通过预测裂缝初始起裂水压和扩展路径,可以优化选择合适的射孔参数。减小地应力差有助于降低裂缝起裂水压,同时可以使裂缝在扩展过程中沟通更大的有效储层。泊松比在临界荷载条件下对裂缝起裂特性产生显著影响,但是在高水压条件下对裂缝扩展路径无明显影响。敏感性分析表明,射孔倾角是影响压裂效果的关键性因素。对上述多因素的影响分析可以为定向射孔水力压裂设计与施工提供重要的参考依据。     

水平井多段压裂应力场计算新模型

为了准确认识水平井多段压裂过程中井筒周围应力场变化,指导裂缝起裂和延伸机制研究,综合考虑井筒内压、地应力、压裂液渗流、热应力、射孔和先压开裂缝等因素的影响,根据应力叠加原理,建立了水平井多段压裂应力场计算模型。该模型利用位移不连续法,重点考虑了先压开裂缝在水平井筒周围水平面上产生的诱导应力大小,克服了裂缝诱导应力解析模型只能计算裂缝高度平面诱导应力大小的局限。结果表明,通过应力场模型进行的破裂压力预测与实际破裂压力值吻合度较高,验证了建立模型的准确性;人工裂缝形成后会对水平井筒周围应力场造成很大影响,在裂缝尖端出现应力集中,造成应力分量降低,在裂缝周围一定区域造成应力分量有较大的升高;多段压裂压开人工裂缝越多,应力之间干扰越严重,水平井筒周围应力分布越复杂。该研究对认识多段压裂存在先压裂缝干扰下后续裂缝的起裂和延伸机制,指导压裂施工具有一定意义。     

水压致裂试验过程中诱发的声发射

为了研究水压致裂过程中裂缝的扩展机制,在北京房山花岗岩体中开展了大型水压致裂试验。试验是在一个深 301 m的岩石新鲜完整的FR钻孔中进行的,其周围半径约40 m的范围内布置了4个120 m深的声发射观测孔。在FR孔中深度60 m至140 m的范围内选取了7段没有天然节理的部位进行了水压致裂。在所有的试验中,发现水压致裂形成的裂缝通过天然节理与AE孔连通,只在深度118.5 m进行的试验,观测到由水压致裂产生的AE事件。由声发射的震源机制解得到的P轴和T轴的方向与水压致裂应力测量的方向一致。     

高水压下岩体裂纹扩展的渗流-断裂耦合机制与数值实现

采用渗流力学、断裂力学理论结合Monte Carlo方法描述岩体裂纹的随机分布,研究高水压作用下岩体原生裂纹的变形和翼形裂纹的萌生、扩展、贯通的渗流-断裂耦合作用机制,建立高水压作用下岩体裂纹的渗流-断裂耦合数学模型,给出该数学模型的求解策略与方法,在Fortran95平台下开发高水压下岩体裂纹扩展的渗流-断裂耦合分析程序HWFSC.for。高水压下岩体裂纹扩展的渗流-断裂耦合体现在岩体裂纹网络和渗流初始条件都随渗流时步变化。对高压注水岩体裂纹扩展过程进行渗流-断裂耦合分析。结果表明,高压注水条件下,岩体裂纹扩展存在起动水压力,当水压力大于起动水压力时,裂纹尖端开始萌生翼形裂纹,随着裂纹水压力的增加,翼形裂纹扩展,进而与其他裂纹搭接贯通,停止扩展。渗流-断裂耦合分析考虑了裂纹动、静水压力对裂纹产生的法向扩张效应及翼形裂纹的扩展而形成新的渗流通道两方面的影响,连通裂纹数随渗流的发展而增加。岩体裂纹的渗流-断裂耦合分析,能较真实地再现岩体裂纹的水力劈裂现象,描述岩体裂纹的扩展、贯通过程及与之相耦合的渗流响应。     

水力压裂扩展特性的数值模拟研究

采用ABAQUS建立了水力压裂计算模型,模拟了地应力、岩石力学特性、压裂液流体特性等各种复杂因素对水力压裂扩展的影响。通过计算分析得到一些有益结论：（1）在注入压力一定的情况下起裂压力与最小水平地应力、临界应力、初始孔隙压力成正比,而与压裂液黏度、最大水平地应力、弹性模量无关;（2）裂缝扩展长度和最大缝宽与最小水平地应力、初始孔隙压力、弹性模量成反比,而与最大水平地应力无关;（3）水力压裂作业中,缝长的扩展过程可分为无扩展阶段、快速扩展阶段、稳定扩展阶段以及缓慢扩展阶段等4个阶段。研究结论对于水力压裂作业优化具有参考价值。     

青海共和盆地干热岩地热储层水力压裂物理模拟和裂缝起裂与扩展形态研究

水力压裂是青海共和盆地干热岩地热资源开发的难点技术问题之一。本文基于升级改造的大尺寸真三轴水力压裂物理模拟实验系统模拟干热岩储层高温高压环境,利用青海共和盆地露头岩心进行水力压裂物理模拟实验,揭示干热岩储层水力裂缝的起裂和扩展规律。通过物理模拟实验发现：干热岩储层裂缝起裂可以通过文中提出的起裂模型判断起裂方式和预测起裂压力;水力裂缝在岩石基质中的扩展形态简单,仅沿最大主应力方向延伸;但是水力裂缝会受到岩石中弱面的影响,发生转向沿弱面延伸,形成较复杂的裂缝形态。因此,建议在干热岩储层实际施工中,在天然裂缝发育较丰富的层段开展水力压裂,以实现复杂裂缝网络提取地热能。     

Onset of Hydraulic Fracture Initiation Monitored by Acoustic Emission and Volumetric Deformation Measurements

In this paper, the results of laboratory studies of fracture initiation, early propagation and breakdown are reported. Three experiments were conducted on a low permeability sandstone block, loaded in a polyaxial test frame, to representative effective in situ stress conditions. The blocks were instrumented with acoustic emission (AE) and volumetric deformation sensors. In two experiments, fluids of different viscosity were injected into the wellbore, fluid injection was interrupted soon after the breakdown pressure had been reached. This allowed us to investigate hydraulic fracture initiation. In the third test, fracture initiation criteria were applied to stop hydraulic fracture propagation significantly earlier, prior to breakdown, and as it propagated a short distance from the wellbore. The analysis of AE results shows an increase in AE activity and a change in the AE spatial correlation, during the fracture initiation. This early stage of fracturing correlates strongly with the onset of rock volumetric deformation, and is confirmed by the analysis of ultrasonic transmission monitoring. The rock microstructure, after the test, was investigated by analysis of scanning electron microscope images. These indicated the development of leak-off zone near the wellbore and a dry hydraulic fracture at the farther distance from the wellbore.     

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.      

Experimental Investigation on the Basic Law of Hydraulic Fracturing After Water Pressure Control Blasting

Because of the advantages of integrating water pressure blasting and hydraulic fracturing, the use of hydraulic fracturing after water pressure control blasting is a method that is used to fully transform the structure of a coal-rock mass by increasing the number and range of hydraulic cracks. An experiment to study hydraulic fracturing after water pressure blasting on cement mortar samples (300 × 300 × 300 mm³) was conducted using a large-sized true triaxial hydraulic fracturing experimental system. A traditional hydraulic fracturing experiment was also performed for comparison. The experimental results show that water pressure blasting produces many blasting cracks, and follow-up hydraulic fracturing forces blasting cracks to propagate further and to form numerous multidirectional hydraulic cracks. Four macroscopic main hydraulic cracks in total were noted along the borehole axial and radial directions on the sample surfaces. Axial and radial main failure planes induced by macroscopic main hydraulic cracks split the sample into three big parts. Meanwhile, numerous local hydraulic cracks were formed on the main failure planes, in different directions and of different types. Local hydraulic cracks are mainly of three types: local hydraulic crack bands, local branched hydraulic cracks, and axial layered cracks. Because local hydraulic cracks produce multiple local layered failure planes and lamellar ruptures inside the sample, the integrity of the sample decreases greatly. The formation and propagation process of many multidirectional hydraulic cracks is affected by a combination of water pressure blasting, water pressure of fracturing, and the stress field of the surrounding rock. To a certain degree, the stress field of surrounding rock guides the formation and propagation process of the blasting crack and the follow-up hydraulic crack. Following hydraulic fracturing that has been conducted after water pressure blasting, the integrity of the sample is found to be far lower than after traditional hydraulic fracturing; moreover, both the water injection volume and water injection pressure for hydraulic fracturing after water pressure blasting are much higher than they are for traditional hydraulic fracturing.     

Investigation of thermal-hydro-mechanical coupled fracture propagation considering rock damage

Thermal-hydro-mechanical (THM) coupled fracture propagation is common in underground engineering. Rock damage, as an inherent property of rock, significantly affects fracture propagation, but how it influences the THM coupled fracturing remains stubbornly unclear. A pore-scale THM coupling model is developed to study this problem, which combines the lattice Boltzmann method (LBM), the discrete element method (DEM), and rock damage development theory together for the first time. This model can more accurately calculate the exchanged THM information at the fluid-solid boundary and fluid conductivity dependent on fracture and rock damage. Based on the developed model, the synergistic effect of injected temperature difference (fluid temperature below rock temperature) and rock damage (characterized by the parameter “critical fracture energy”, abbreviated as “CFE”) on fracture propagation of shale are investigated particularly. It is found that: (1) the generation of branched cracks is closely related to the temperature response frontier, and the fracture process zone of single bond failure increases in higher CFE. (2) through the analysis of micro failure events, hydraulic fracturing is more pronounced in the low CFE, while thermal fracturing displays the opposite trend. The fluid conductivity of fractured rock increases with a higher injected temperature difference due to the more penetrated cracks and wider fracture aperture. However, this enhancement weakens when rock damage is significant. (3) in the multiple-layered rock with various CFEs, branched cracks propagating to adjacent layers are more difficult to form when the injection hole stays in the layer with significant rock damage than without rock damage.

     

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。     

低温气体辅助煤层气压裂中的冷冲击机理

液N₂等气体辅助煤层气压裂的常规机理已较为清楚,但其低温特征对煤层物性的影响以及对压裂效果的改善机理尚未引起重视,缺乏理论认识.归纳总结了低温气体对煤岩的冷冲击作用机制,评价了地层水的结冰条件和低温气体的热物性,通过室内实验和数值模拟等手段验证和预测了液N₂对煤岩及近井煤层的冷冲击效果,并进行了冷冲击机理应用潜力分析.研究结果表明,煤层气压裂过程中注入N₂等低温气体对煤层进行冷冲击,可引起煤岩基质收缩和地层水结冰膨胀,使煤岩产生大量微裂缝和力学强度降低(10%~30%),有利于实现冰晶暂堵和改善煤层气压裂效果.以沁水盆地樊庄区块3#煤层为例,当液N₂注入总量为30~120 m3时,可在近井周围3~5 m内形成低温区(＜-20 ℃),煤层孔隙度将平均增大约1.5倍,渗透率增大4倍,还可造成煤岩的拉伸和挤压破坏.利用低温气体的冷冲击机理及其产生的冰晶暂堵可以作为改进煤层气和页岩气压裂工艺的新方向.      

Two－Dimensional Model of Hydraulic Fracturing in Geosciences：Effects of Fluid Buoyancy

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