水压致裂法测量地应力理论与应用

针对水压致裂测量地应力理论与工程研究现状,对水压致裂测量地应力理论、方法、技术以及特点进行了分析,并对水压致裂测量地应力的测量结果进行了论述。     

水压致裂原地应力测试井下水压力数据采集系统及其应用研究

中国地震局地壳应力研究所首次研发了水压致裂原地应力测量井下压力数据采集系统。该系统的使用可以对井下压裂段的水压力进行较为精确的测量,克服了水压致裂测试系统柔性以及测段孔壁孔隙压力的不均匀性等因素对测试结果的影响,大大提高了水压致裂应力测试的精度。现场测试表明,井下压力采集系统运行的稳定性、环境适用性都达到了应用的要求。笔者课题组运用该系统成功的进行2个测段的原地应力测试,并分别对比分析各测段的井下和地面采集系统得到的数据。结果显示,测试深度越大,测试结果受系统柔性和其他不确定性因素的影响也越大,最终的计算结果误差也越大。压裂过程中,地面采集压裂曲线在升压阶段的压力梯度和井下压力采集曲线基本一致;降压阶段,地面采集曲线的压力梯度要明显大于井下数据采集曲线的压力梯度;地面压力采集计算得到各个压力特征值尤其重张压力值,以及原地应力值比井下数据采集得到的准确值要偏小。     

水压致裂法在大巴山深埋隧道地应力测试中的应用

大巴山地区地质构造复杂，目前建设的隧道多为深埋隧道。了解地应力条件对隧道施工设计参数的选择以及岩爆的风险预测具有重大意义。选择水压致裂法，以大巴山区域旗杆山隧道为例，介绍了水压致裂法测试深埋隧道地应力的方法，并对测试结果数据进行分析，从而对该隧道施工期间发生岩爆的可能性进行预测。实践表明，运用水压致裂法，能较为准确地测量到深埋隧道相关施工区域的地应力分布特征，同时具有施工周期短，施工操作简单的特点。     

水压致裂法地应力测量的数值模拟

由于测量深度深及其他突出优点,水压致裂法在地应力测量中已在国内外得到了广泛使用。基于大型通用有限元软件平台 Abaqus,建立了二维应力-损伤演化模型,对不同水压及初始裂隙下的多工况进行数值模拟试验,并与理论解进行比较以验证数值模拟结果的可靠性。分析结果表明:数值模拟试验的裂纹开展方位、致裂压力最小值与理论解吻合;初始裂隙的存在将影响致裂最小水压力值和裂纹扩展方向及模式,其结果可对原位地应力测试和水压致裂工程设计提供借鉴和指导。     

一种新型绳索取芯钻杆内置式双管水压致裂地应力测试方法及其应用

综合论述水压致裂地应力测量理论和技术的发展过程,讨论基于推拉阀控制的单管加压系统或由外置式高压软管与钻杆组成的传统双管加压测试的水压致裂技术及适用条件。针对滇中引水工程勘察阶段大量采用绳索取芯钻探技术进行复杂地质条件地质勘探的实际情况,以绳索取芯钻杆条件的岩体地应力测试为研究对象,研究一种新型绳索取芯钻杆内置,并分段固定式双管水压致裂地应力测试方法,研发相应试验测试装置,对测试过程控制以及相应管道压力损失估算等进行研究和讨论。利用该项技术成功进行5个测孔56个测段的地应力测试,最大测试深度为724 m。该测试方法解决了传统水压致裂地应力测量方法不能直接应用绳索取芯钻杆的难题,对复杂条件下的工程岩体地应力测试问题提供了一种新的测试技术手段。     

超声波钻孔电视在地应力测量研究中的应用

通过一个300m和一个800m典型钻孔的实际测试结果,揭示超声波钻孔电视在地应力测量研究领域中的几个应用实例,如:岩层裂缝的检测、地应力测试段的选定、岩芯定向,特别是对水压致裂法形成的诱发裂缝和孔壁崩落的测定及分析,在地应力测量研究中将可显示出较强的优势,尤其是最大水平主应力方向的获取,更优于其它方法。并将在两个钻孔中所得结果与常用的定向印模测试结果进行了对比,取得了较为一致的成果。关于超声波钻孔电视对水压致裂法诱发裂缝的测试还未曾见过报道。最后简述了地应力数值大小的推估研究与展望。     

拉日线峡谷区地应力分布特征的分析

介绍了拉日线峡谷区地应力分布特征的概况,分析了雅鲁藏布江峡谷区地应力的特征,进行了峡谷区水压致裂原地应力测量,探讨了高地应力对工程的影响,提出了高地应力的应对措施和选线原则具有一定的参考价值。     

某水源地工程水压致裂地应力测试研究

介绍了该水源地建设工程的基本地质概况,依据水压致裂法基本原理,通过水压致裂地应力现场测试,研究该水源地建设工程地应力场特征。分析现场测试结果得到了水平主应力量值和最大水平主应力方向,σHσhσZ,地应力方向在浅部符合地形地貌应力分布一般规律,深部主要受区域构造应力场影响。     

水压致裂应力测量在广州抽水蓄能电站设计中的应用研究

在高压输水隧洞及地下厂房围岩中，遇水膨胀的蚀变岩带十分发育。但根据水压致裂原地应力的测量结果及最小主应力设计准则，广州抽水蓄能电站的高压输水隧洞甚至高压岔管段均取消了钢衬。电站建成以来的安全运行表明，该设计是完全合理的。实践表明：在充分利用岩体自身的承载能力，优化工程设计方面，水压致裂原地应力测量具有十分重要的意义。     

水压致裂法测试地应力简析

高地应力对地下工程危害大,对地应力测量十分必要。文章结合工程实例对现行公路规范推荐的"水压致裂法"地应力测试作简要的分析,具有一定的借鉴意义。     

Determination of the natural stress state in a Brazilian rock mass by back analysing excavation measurements: a case study

The Serra da Mesa Hydroelectric Power Plant, located in the Tocantins river, 210 km north of Brasilia, Brazil, has been completed and power (1200 MW) has been generated since 1998. This project includes one of the largest underground structures in Brazil, totalling 550,000 m³ of underground excavations in rock for the hydraulic circuit which was excavated in very high quality granite. Geotechnical investigations, laboratory tests and geological mapping showed that the rock mass could be considered as a continuous, homogeneous, isotropic and linearly elastic (CHILE) material.

In situ tests, for obtaining the natural stress tensor, namely hydraulic fracturing and small flat jack tests (SFJ), were executed. The hydraulic fracturing tests were performed in two boreholes, at the planned position of the future underground structures. SFJ were executed in a test gallery especially constructed for the purpose. These latter tests confirmed the in situ rock stress data obtained from the hydraulic fracturing tests.

This paper presents a new technique for interpretation of the SFJ results. This is achieved by inputting the SFJ measurements into a 3D program that compiles the influence matrix of the excavated rock mass domain and then, via the least square technique, the determination of the stress tensor. All the equations are fully developed and the methodology is presented in its entirety. The successful application of the methodology is also presented, with comparisons between the results obtained and the in situ stress tensor determined by other methods.     

A test of different stress measurement methods in two orthogonal bore holes in Äspö Hard Rock Laboratory (HRL), Sweden

Within the scope of works to provide the necessary rock mechanics support for site investigations for a radioactive waste repository, the Swedish Nuclear Fuel and Waste Management Co (SKB) has studied some of the available equipment for in situ stress measurements in deep boreholes. A project with the objective of comparing three different types of equipment for in situ stress measurement under similar conditions has been carried out. The tests were performed in orthogonal boreholes at the Äspö Hard Rock Laboratory (HRL), Oskarshamn, Sweden. The two most common methods to measure in situ stress are the overcoring and hydraulic fracturing methods. Among the three tests, two are carried out using the overcoring method, the third uses the hydraulic fracturing method. The overcoring equipment's are the AECL's Deep Doorstopper Gauge System (DDGS) and SwedPower's Triaxial Strain Cell (Borre Probe). The measured results could be verified against known conditions at the Äspö HRL. The results from the three in situ stress measurement methods illustrate the complexity of determining the in situ stresses in a rock mass. To understand the difference in results and answer the associated questions, it was necessary to undertake deeper investigations involving laboratory tests and theoretical calculations.     

Determination of three dimensional in situ stress from core discing based on analysis of principal tensile stress

To determine all of the components of in situ stress from core discing, both the directions and magnitudes of the principal in situ stresses must be determined for a disc of a given thickness. In this study, we analyzed the direction and magnitude of tensile stress below an HQ core stub for 11 core lengths using stress conditions under which core discing is likely to occur. First, based on an analysis of the direction of tensile stress below the core stub, we propose a method for determining directions of in situ stress from the height distribution at the periphery of the end surface of a disc. This method can be used with a disc of any thickness. Next, based on an analysis of the magnitude of tensile stress in the central part of a core, we propose a linear criterion for core discing, which can be applied to a core of any length. This criterion was in good agreement with an empirical formula obtained previously in laboratory experiments. By combining information on the direction of in situ stress and the linear criterion for core discing, we propose a method for determining all of the components of in situ stress from core discing under the assumption that vertical stress is given by the overburden stress. Finally, these methods were applied to discs obtained from a field where hydraulic fracturing was performed to measure horizontal stresses. The results showed that the azimuths of the principal stresses estimated from core discing were consistent with those of the principal horizontal stresses determined by hydraulic fracturing and that while the magnitudes of the principal horizontal stresses estimated from core discing showed a large scatter, they were similar to those determined by hydraulic fracturing.     

Uncertainty in the maximum principal stress estimated from hydraulic fracturing measurements due to the presence of the induced fracture

The classical theory for hydraulic fracturing stress measurements assumes an ideal case with a linear elastic, homogenous, and isotropic medium; and a fracture that reopens distinctly when the minimum tangential borehole stress is exceeded. The induced fracture disturbs this ideal picture in several aspects, which are important for the evaluation of the maximum horizontal principal stress using the fracture reopening pressure. This disturbance can be attributed to the fracture normal stiffness and the initial hydraulic fracture permeability. In this paper, the hydraulic fracturing reopening test is studied by coupled hydromechanical modeling that takes into account an induced fracture that is incompletely closed. The result shows that with realistic equipment compliance, the apparent fracture reopening evaluated from the well-pressure is close to the magnitude of the minimum horizontal principal stress with little or no correlation to the maximum horizontal principal stress. This observation suggests that the determination of maximum principal stress by hydraulic fracturing using the reopening pressure is very uncertain.     

钙芒硝盐岩水力压裂裂纹端面特征试验研究

为研究钙芒硝水力压裂作用下裂纹扩展-溶解机理,对钙芒硝矿层的流体化开采提供参考,同时填补水力压裂中只有物理作用而无化学作用的空白,丰富水力压裂理论,作者采用室内试验的方法,分别进行了钙芒硝试件在轴压/围压为5/4 MPa、7/4 MPa、9/4 MPa三种应力条件下的水力压裂实验,并采用3D形貌扫描仪对破坏断面的裂纹形态进行扫描,结合裂纹粗糙度表征公式对试验数据进行计算分析。研究发现:钙芒硝水力压裂是应力和溶解的共同作用,压裂过程大致可以分为水压升高阶段、裂纹扩展阶段、水压下降阶段3个阶段;3个阶段的划分与前人研究成果相同,但裂纹扩展阶段的压力波动剧烈,与其他岩石(例如,煤,页岩)水力压裂试验结果相差较大,其为钙芒硝溶解现象影响所致;不同应力下,钙芒硝水压致裂后裂纹形态不同;当垂向应力差异系数(k)≤0.75时,钙芒硝盐岩产生横向裂纹,k≥1.25时,钙芒硝盐岩将产生纵向裂纹;同一应力状态下钙芒硝清水致裂后裂纹端面粗糙度大于饱和盐水致裂后的粗糙度,不同应力状态下,水压致裂后粗糙度随轴压的增大而增大。研究成果对钙芒硝矿层的原位溶浸流体化开采具有重要的工程价值。     

薄互层储层水平井压裂裂缝延伸规律模拟分析

为尝试研究非均匀层状岩层中的复杂水力压裂裂缝延伸行为,以胜利油田某典型砂泥薄互层储层的水平井压裂为背景模型,利用基于有限元的数值模拟软件,开展了一系列水平井压裂裂缝延伸行为的数值模拟研究。在模拟中,表征远场地应力的模型围压、层理(界面)的强度和地层岩性是考虑的主要因素。数值模拟表明,水力压裂裂缝的延伸模式是远场地应力、界面性质及地层岩性的综合作用结果。裂缝延伸过程中遇到层间界面会出现偏转、分叉和钝化终止行为,总体可分为4类延伸模式:直接穿透、钝化、T型扩展、偏转。这些非平面、复杂裂缝并非体积压裂改造裂缝,因此,不但不利于油气的流动,而且也是压裂压力畸高、有效主裂缝长度减少甚至压裂失败的原因之一。数值模拟初步解释了裂缝延伸过程中应力演化及其与层间界面的相互作用机制,可望为水力压裂改造工程相关的设计与机理分析提供参考。     

Acoustic emission monitoring of hydraulic fracturing in laboratory and field

In the hydraulic fracturing tests, water was injected into four kinds of granitic rock specimens with different grain sizes. The fault plane solutions of acoustic emission (AE) indicated that the shear fracturing is dominant in the specimens with larger grains whereas the tensile fracturing is dominant in those of smaller grains. In addition, the experiments were conducted in granitic rock specimens of the same kind, under three different conditions: injection of water, injection of viscous oil and pressurization via a urethane sleeve. The shear fracturing was dominant due to water injection and the pressurization whereas the tensile fracturing was dominant due to viscous oil injection. Further in a field, AE was monitored by a borehole sonde in the hydraulic fracturing to measure rock stress. The AE count rates increased with closing and reopening hydraulically induced cracks. The result suggested that AE monitoring could improve the reliability and the accuracy of hydraulic fracturing stress measurements.     

Experimental study on slurry-induced fracturing during shield tunneling

Facial support in slurry shield tunneling is provided by slurry pressure to balance the external earth and water pressure. Hydraulic fracturing may occur and cause a significant decrease in the support pressure if the slurry pressure exceeds the threshold of the soil or rock material, resulting in a serious face collapse accident. Preventing the occurrence of hydraulic fracturing in a slurry shield requires investigating the effects of related influencing factors on the hydraulic fracturing pressure and fracture pattern. In this study, a hydraulic fracturing apparatus was developed to test the slurry-induced fracturing of cohesive soil. The effects of different sample parameters and loading conditions, including types of holes, unconfined compressive strength, slurry viscosity, and axial and circumferential loads, on the fracturing pressure and fracture dip were examined. The results indicate that the fracture dip is mainly affected by the deviator stress. The fracturing pressure increases linearly with the increase in the circumferential pressure, but it is almost independent of the axial pressure. The unconfined compressive strength of soil can reflect its ability to resist fracturing failure. The fracturing pressure increases with an increase in the unconfined compressive strength as well as the slurry viscosity. Based on the test results, an empirical approach was proposed to estimate the fracturing pressure of the soil.     

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。