Effect of fractures on mechanical behavior of sand powder 3D printing rock analogue under triaxial compression

In practical engineering applications,rock mass are often found to be subjected to a triaxial stress state.Concurrently,defects like joints and fractures have a notable impact on the mechanical behavior of rock mass.Such defects are identified as crucial contributors to the failure and instability of the surrounding rock,subsequently impacting the engineering stability.The study aimed to investigate the impact of fracture geometry and confining pressure on the deformation,failure characteristics,and strength of specimens using sand powder 3D printing technology and conventional triaxial compression tests.The results indicate that the number of fractures present considerably influences the peak strength,axial peak strain and elastic modulus of the specimens.Confining pressure is an important factor affecting the failure pattern of the specimen,under which the specimen is more prone to shear failure,but the initiation,expansion and penetration processes of secondary cracks in different fracture specimens are different.This study confirmed the feasibility of using sand powder 3D printing specimens as soft rock analogs for triaxial compression research.The insights from this research are deemed essential for a deeper understanding of the mechanical behavior of fractured surrounding rocks when under triaxial stress state.

Near-wellbore fracture propagation physical simulation based on innovative interlaced fixed perforation

An innovative perforation method of interlaced fixed perforation was put forward based on the analysis of the characteristics of fractures in various periods of perforation and conventional perforation modes.By conducting a large-scale perforation shooting experiments,we investigated the morphology,propagation mechanism and propagation law of the near-wellbore fractures generated during perforating processes under different fixed angle and interlaced angle combinations,and discussed the control method of near-wellbore fractures in different types of unconventional oil and gas reservoirs.The experimental results show that:(1)The interlaced fixed perforation strengthens the connectivity between the perforation tunnels not only in the same fixed plane but also in adjacent fixed planes,making it likely to form near-wellbore connected fractures which propagate in order.(2)Three kinds of micro-fractures will come up around the perforation tunnel during perforation,namely typeⅠradial micro-fracture,typeⅡoblique micro-fracture and typeⅢdivergent micro-fracture at the perforation tip,which are interconnected into complex near-wellbore fracture system.(3)Different types of perforation bullets under different combinations of fixed angles and interlaced angles result in different shapes of near-wellbore fractures propagating in different patterns.(4)By using the interlaced perforation on fixed planes,arranging fixed planes according to the spiral mode or the continuous"zigzag"shape,the desired near-wellbore fractures can be obtained,which is conducive to the manual control of main fractures in the fracturing of unconventional or complex conventional reservoirs.

Tensile and tear-type fracture toughness of gypsum material:Direct and indirect testing methods

In this context,four specimens,i.e.(i)circumferentially notched cylindrical torsion(CNCT),(ii)circum-ferentially notched cylindrical direct tension(CNCDT),(iii)edge notch disc bend(ENDB)and(iv)three-point bend beam(3PBB),were utilized to measure the modesⅠandⅢfracture toughness values of gypsum.While the CNCT specimen provides pure modeⅢloading in a direct manner,this pure mode condition is indirectly produced by the ENDB specimen.The ENDB specimen provided lower KⅢc and a non-coplanar(i.e.twisted)fracture surface compared with the CNCT specimen,which showed a planar modeⅢfracture surface.The ENDB specimen is also employed for conducting pure modeⅠ(with different crack depths)and mixed modeⅠ/Ⅲtests.KIc value was independent of the notch depth,and it was consistent with the RILEM and ASTM standard methods.But the modeⅢfracture results were highly sensitive to the notch depth.While the fracture resistance against modeⅢwas significantly lower than that of modeⅠ,the greater work of fracture under modeⅢwas noticeable.

Permeability evaluation for artificial single rock fracture according to geometric aperture variation using electrical resistivity

A convenient approach was proposed by which to evaluate and monitor the permeability of a rock fracture by verifying the quantitative correlation between the electrical resistivity and permeability at laboratory scale.For this purpose,an electrical resistivity measurement system was applied to the laboratory experiments using artificial cells with the shape of a single rock fracture.Sixty experiments were conducted using rock fractures according to the geometry,aperture sizes,wavelengths,and roughness amplitudes.The overall negative relationship between the normalized electrical resistivity values and the aperture sizes directly linked with the permeability,was well fitted by the power-law function with a large determination coefficient(≈0.86).The effects of wavelength and roughness amplitude of the rock fracture on the electrical resistivity were also analyzed.Results showed that the electrical resistivity was slightly increased with decreasing wavelength and increasing roughness amplitude.An empirical model for evaluating the permeability of a rock fracture was proposed based on the experimental data.In the field,if the electrical resistivity of pore groundwater could be measured in advance,this empirical model could be applied effectively for simple,quick monitoring of the fracture permeability.Although uncertainty may be associated with the permeability estimation due to the limited control parameters considered in this research,this electrical resistivity approach could be helpful to monitor the rock permeability in deep underground facilities such as those used for radioactive waste repositories or forms of energy storage.

Estimation of Fracture Geometry Parameters and Characterization of Rock Mass Structure for the Beishan Area,China

The accurate estimation of fracture geometry parameters and the characterization of rock mass structure are two important topics in the geological disposal system of high-level radioactive waste(HLW).The Beishan area,as the current preselected area for China’s HLW disposal,has three subareas considered to be the key survey area at the stage of site selection.In this paper,a comprehensive survey method conducted on the outcrop is developed to estimate fracture geometry parameters.Results show that fracture occurrence obeys a Fisher distribution,fracture trace length obeys a normal distribution,and the distribution of spacing obeys a negative exponential distribution.An evaluation index,Rock Mass Structure Rating(RMSR),is proposed to characterize rock mass structure for the three subareas.The results show that the Xinchang area is more suitable to act as China’s HLW disposal repository site.At the same time,the index can also be applied to characterize surface rock mass structure and rock mass integrity at the site selection phase of HLW disposal.

Performance-based fractal fracture model for complex fracture network simulation

The paper presents a novel hydraulic fracturing model for the characterization and simulation of the complex fracture network in shale gas reservoirs. We go beyond the existing method that uses planar or orthogonal conjugate fractures for representing the ＇＇complexity＇＇ of the network. Bifurcation of fractures is performed utilizing the Lindenmayer system based on fractal geometry to describe the fracture propagation pattern, density and network connectivity. Four controlling parameters are proposed to describe the details of complex fractures and stimulated reservoir volume（SRV）. The results show that due to the multilevel feature of fractal fractures, the model could provide a simple method for contributing reservoir volume calibration. The primary-and second-stage fracture networks across the overall SRV are the main contributions to the production, while the induced fracture network just contributes another 20% in the late producing period. We also conduct simulation with respect to different refracturing cases and find that increasing the complexity of the fracture network provides better performance than only enhancing the fracture conductivity.

A simple method for determining independent fracture toughness and tensile strength of rock

This paper develops a model that only requires two sets of small-size rock specimens with the ratio of the structural geometry parameter maximum to minimum ae,max:ae,min≥3:1 to determine the rock fracture and strength parameters without size effect and predict the actual structural performance of rock.Regardless of three-point-bending,four-point-bending,or a combination of the above two specimen types,fracture toughness KICand tensile strength ftof rock were determined using only two sets of specimens with ae,max:ae,min≥3:1.The values KICand ftwere consistent with those determined using multiple sets of specimens.The full structural failure curve constructed by two sets of small-size specimens with ae,max:ae,min≥3:1 can accurately predict large-size specimens fracture failure,and±10%upper and lower limits of the curve can encompass the test results of large-size specimens.The peak load prediction curve was constructed by two sets of specimens with ae,max:ae,min≥3:1,and±15%upper and lower limits of the peak load prediction curve can cover the small-size specimen tests data.The model and method proposed in this paper require only two sets of small-size specimens,and their selection is unaffected by the specimen type,geometry,and initial crack length.

A FRACTAL MODEL AND ENERGY DISSIPATION FOR EN ECHELON FRACTURES

observations from the field and the taboratory show that en echelon fractures withinfracture zones have a foede1 within Ricdel structure’ The tensile fallure mecbanism of en echelonfractures can be described by the pile-ups of sbear crack-dislocations. A fractal model can be used tosimulate the Riedel within Xiedel geometry, allowing the direct rneasarement of tbe ftactal dimen sions of en echelon fractare systems. The energy dissipation of tbe en ccbe1on fracture system canbe deduced using a fracil damage evo1ution model which exptalns tbe evo1ution process of en eche lon fracture svstems. The fractal nature of the fractures can be used to dcrive an accurate estimateof total energy dissipation.

基于辛几何方法测定Ⅰ型应力强度因子

为得到高精度的Ⅰ型应力强度因子,提出了一种基于辛几何方法测定裂纹尖端Ⅰ型应力强度因子的方法。通过在裂纹尖端构建扇形区域,分离辛几何方法下裂纹尖端的全状态向量,结合裂纹尖端的位移场,计算出裂纹的应力强度因子。利用Ⅰ型裂纹尖端位移场的理论解,研究了扇形区域的特征点个数、圆心角大小以及半径长度对应力强度因子精确度的影响。研究结果表明:特征点个数对该方法求解Ⅰ型应力强度因子的精度的影响较大,特征点个数大于37时,相对误差稳定趋向于0;扇形圆心角对应力强度因子的精度影响较小,但是相同精度要求下,不同的扇形圆心角需要的特征点个数不同;扇形半径过小时,由于断裂过程中裂纹尖端塑性区域的影响,应力强度因子的解不稳定且相对误差较大。对比相对误差可以发现该方法计算得到的Ⅰ型应力强度因子的精度较高,且该方法只需得到裂纹尖端的位移场就可以精确得到应力强度因子,解决了以往求解体系在复杂应力等情况下求解步骤复杂,导致无法精确得到Ⅰ型应力强度因子的难题。

准噶尔盆地吉木萨尔凹陷页岩水平井水力压裂裂缝形态

准噶尔盆地吉木萨尔凹陷、四川盆地南部等地区页岩水平井的微地震侧视数据云图中均出现了平行于层理方向数据点的密度与扩展程度都远远大于垂直方向的现象,与传统的水力压裂裂缝几何形态解释结果矛盾,但是目前没有明确的三维水力压裂裂缝几何形态对这种现象做出解释。建立微地震数据反演方法,并对反演结果进行了二次重构,给出了页岩水平井三维水力压裂裂缝几何形态,并通过真三轴压裂物理模拟实验对裂缝形态以及微地震数据反演方法进行了验证。研究结果表明:反演得到的裂缝在三维空间中主要呈现出1条主裂缝与多条次生裂缝相互交错的形态,结合真三轴压裂物理模拟实验结果,说明了页岩水平井水力压裂裂缝几何形态呈水平垂直交错;通过对比声发射实验结果和吉木萨尔凹陷露头样品真三轴压裂物理模拟后呈现出的裂缝形态,验证了微地震数据反演方法的可靠性以及页岩水平井水力压裂裂缝的复杂性。

真三轴应力状态下海相与陆相页岩射孔压裂裂缝扩展规律对比研究

页岩油气高效开发需要对储层进行大规模压裂改造,水力裂缝垂向扩展激活纹层弱面是形成复杂压裂缝网的关键。然而,由于海相页岩与陆相页岩沉积环境、矿物成分和岩体结构存在显著差异,导致水力压裂效果差异较大,亟需对水力裂缝扩展规律进行深入探索。本文选取了龙马溪海相页岩和长7段陆相页岩,开展了力学特性试验和真三轴应力状态下水力裂缝扩展规律研究。分析了海相页岩和陆相页岩力学各向异性、注入压力曲线、压裂缝网形态与裂缝扩展过程的声发射能量。结果表明:(1)海相页岩力学强度和弹性模量明显高于陆相页岩,抗压强度相差61.4~102.2 MPa,抗拉强度差值约为1.7~2.6 MPa,弹性模量差值约为7.0~25.7 GPa,表明层理弱面对陆相页岩力学特性和破裂模式影响更为显著。(2)海相页岩破裂压力和声发射能量高于陆相页岩,水力裂缝起裂和扩展特征明显。陆相页岩注入压力曲线明显不同于海相页岩,压裂液沿层理缝滤失,造成注入压力曲线非线性增长。陆相页岩水力裂缝起裂和主破裂特征不明显,声发射能量较低。(3)海相页岩水力裂缝形态较为平直,射孔起裂后形成两条主裂缝。陆相页岩水力裂缝形态曲折,垂向扩展受到层理弱面抑制,裂缝高度较小。当压裂液黏度由60 MPa·s增大至240 MPa·s时,陆相页岩水力裂缝垂向扩展高度增大,裂缝形态更加复杂。

设计定量几何结构试件预测混凝土结构断裂特性

该文采用描述试件尺寸、裂缝长度、前后边界、加载型式等耦合影响的几何结构参数a_(e),替代传统尺寸效应理论所用的单一试件尺寸W,阐释了混凝土强度与断裂特性尺寸效应的细观物理机制。不拘泥于实验室条件下试件尺寸、裂缝长度、加载型式等条件的选取限制,发展建立了由任意两组a_(e)比值为3的设计试件,来确定混凝土无尺寸效应的真实断裂与强度参数的模型与方法。基于所提方法,由a_(e)比值为3的设计试件,建立起混凝土不同断裂模式的转换曲线,其±15%即可涵盖实验室条件下的试件测试结果;将实际结构的等效面积代入所提解析公式,可预测大尺寸实际结构的峰值荷载。该文的研究内容搭建了实验室条件下与现场条件下确定混凝土真实材料性能的纽带;实现了由定量的不同几何结构的实验室试件,准确预测现场混凝土结构真实断裂性能的目的。

不同干燥温度条件下非饱和红黏土裂隙演化及抗剪强度变化规律研究

温度作为重要外界因素,对红黏土的强度特性和裂隙性具有重要影响。为研究干燥温度、含水率和干密度对非饱和红黏土强度特性和裂隙发育的影响,对不同蒸发温度条件下不同初始状态的非饱和红黏土进行直剪试验和干燥试验。分析了不同温度下红黏土的强度特性,结合图像处理技术对裂隙的发育规律进行了阐述,并对抗剪强度指标(黏聚力、内摩擦角)和裂隙形态参数(裂隙率)的关系进行了探讨。试验结果表明:裂隙几何参数(裂隙总长度、节点数和裂隙率)随着温度升高而增大。干密度越低,含水率越大,裂隙指标随温度的变化幅度越大,裂隙也越发育。张拉应力的存在是裂隙产生的主要力学原因。黏聚力随温度升高先增大后减小,衰减的幅度和干密度有关。随着温度升高,内摩擦角先缓慢增长,而后增速加快,含水率越低,变化越明显。随着裂隙率的增加,黏聚力先增多后减小,内摩擦角则不断增长。

Comparison of Proximal Femoral Geometry and Risk Factors between Femoral Neck Fractures and Femoral Intertrochanteric Fractures in an Elderly Chinese Population

Background： Few studies have investigated the differences in proximal femoral geometry and risk factors between patients with different types of hip fracture, especially in elderly Chinese. This study aimed to assess the differences in proximal femoral geometry parameters between patients with femoral neck fractures and patients with intertrochanteric fractures to provide guidance for individualized customized prosthesis and accurate reconstruction of proximal femurs in elderly Chinese patients. Methods： We retrospectively studied the electronic medical records of 198 elderly patients over 65 years of age who were admitted to the orthopedic department with hip fractures between January 2017 and December 2017 in The Third Hospital, Hebei Medical University. Age, fracture site, gender, and proximal femoral geometry parameters （.neck shaft angle [NSA], center edge angle [CEA], femoral head diameter [FHD], femoral neck diameter [FND], femoral neck axial length [FNAL], hip axial length [HAL], and femoral shaft diameter [FSD]） were recorded. Student＇s t-test was used to compare the continuous variables, Chi-square test was used to analyze categorical variables, and multiple logistic stepwise regression analysis was used to evaluate the influencing factors of hip fracture type. Results： Statistically significant differences in NSA （137.63 ± 4.56° vs. 132.07 ± 4.17°, t = 1.598, P 〈 0.001）, CEA （37.62 ± 6.77° vs. 43.11 ±7.09°, t = 5.597, P 〈 0.001 ）, FND （35.21 ± 3.25 mm vs. 34.09 ±3.82 mm, t = 2.233, P = 0.027）, and FNAL （99.30 ± 7.91 mm vs. 103.58± 8.39 ram, t = 3.715, P 〈 0.001 ） were found between the femoral neck fracture group and femoral intertrochanteric fracture group. FHD, FND, FSD, HAL, and FNAL were different between sexes （all P 〈 0.001 ）. The greater NSA was the risk factor for femoral neck fractures （,odds ratio [OR]： 0.70, P 〈 0.001 ）, greater CEA and longer FNAL were risk factors for femoral intertrochanteric fractures （OR： 1.15, 1.17, all P 〈 0.001）, and greater FND was a protective factor for femoral intertrochanteric fractures （OR： 0.74, P 〈 0.001）. Conclusions： We demonstrate differences in geometric morphological parameters of the proximal femur in different hip fracture types, as well as an effect of sex. These differences should he considered in the selection of prostheses for fracture internal fixation and hip replacements. These data could help guide the design of individualized customized prostheses and improve the accurate reconstruction of the proximal femur for elderly Chinese hip fracture patients.

基于粗糙离散裂隙网络模型的煤层气流动特性研究

煤体内随机分布的裂隙网络是煤层气的主要流动通道,而研究煤层气流动特性的关键在于准确模拟煤体中错综复杂的裂隙网络。基于像元概率分解理论,介绍了一种新的离散裂隙网络建模方法,相比传统方法多假设裂隙为平滑裂隙这一不足,该方法可充分考虑裂隙的粗糙特征,并结合数值模拟手段分析了不同裂隙特征参数对煤层渗透性的影响规律。结果表明:裂隙率(0.1~0.25)、分形维数(1.55~1.85)、最大裂隙长度(0.011~0.026 m)以及比例系数(0.006~0.08)与裂隙网络出口流速之间呈现较好的幂律增长关系,而迂曲度分形维数(1.0~1.02)、裂隙倾角(10°~85°)与裂隙网络出口流速之间呈现幂律降低的关系;其中,比例系数、裂隙倾角和最大裂隙长度对裂隙网络出口流速的影响最为显著。

An optimal fracture geometry design method of fractured horizontal wells in heterogeneous tight gas reservoirs

In this work, the unified fracture design （UFD） is extended for the first time to the fractured horizontal wells in heterogeneous closed box-shaped tight gas reservoirs. Utilizing the direct boundary element method and influence function, the dimensionless fracture productivity index is obtained and expressed in the function of proppant volume and fracture geometry at the pseu- do-steady state. With the iterative method, the effectively propped permeability, kfe, is corrected using the i^-situ Reynolds number, NRe. The goal of this paper is to present a new UFD extension to design the proppant volume and the optimal fracture geometry. The results show that there exists an optimal proppant volume for a certain reservoir. The small aspect ratio （yJXe） and high permeability reservoirs need short and wide fractures to diminish the non-Darcy effect. On the contrary, long and narrow fractures are required for the large aspect ratio and low permeability reservoirs. A small proppant volame is prone to creating long fractures, while a relatively large proppant volume creates wide fractures. The new extension can be used to evaluate the previous fracture parameters and design the following fracture parameters of the fractured horizontal well in heterogeneous tight gas reservoirs, with the non-Darcy effect taken into account.

面向深层页岩气“甜点”预测的地震勘探技术进展——以DX探区全方位三维地震为例

深层页岩气效益勘探开发需要落实易于压裂求产的“甜点”区,对地震资料成像品质、信息丰度有针对性需求,一是要对与构造保存评价密切相关的断层、断洼以及低序级断裂实现高品质成像,二是要为可压裂性“甜点”预测提供高品质、高精度、高信息丰度的叠前五维地震成像数据。考虑到四川盆地深层页岩气效益勘探对地震资料的针对性需求,重点介绍了地质目标导向的观测系统设计论证与叠前全方位信息保持处理技术及其在綦江高陡构造带DX探区的应用效果。地质目标导向的观测系统设计论证以真地表三维建模及地震正演模拟为基础,通过多维度照明和退化处理分析,设计出满足地质目标探测需求的最优化观测系统。叠前全方位信息保持处理以偏移距向量片地震数据划分为基础,融入山地地震数据规则化和成像道集时差校正技术,获得高品质叠前五维地震成像数据。DX探区应用结果表明,炮道采样对称均匀的宽(全)方位、高覆盖、强耦合的高精度三维采集与OVT域全方位处理,基本能够实现山地探区深层页岩气地质目标的叠前方位偏移距地震信息的充分采集与保留,能够获得高品质、高精度、高信息丰度的地震成像数据,有效支撑深层页岩气构造保存评价、水平井轨迹设计与控制以及可压裂性预测等工作。

不同岩性地层水力压裂裂缝扩展规律的模拟实验

利用真三轴模拟压裂实验系统对玄武岩、巨砾岩、泥灰岩岩心进行了水力压裂裂缝起裂及裂缝扩展模拟实验,得到了压后裂缝几何形态和压裂过程中压力随时间的变化规律。研究结果表明,玄武岩中天然裂缝发育程度较低,抗拉强度较高,裂缝起裂会导致明显的压降,压后能够形成比较理想的双翼缝;巨砾岩中天然裂缝较为发育,裂缝起裂不会导致明显的压降,高排量压裂后形成的裂缝为多组复杂裂缝,裂缝扩展摩阻很大,裂缝延伸压力几乎与破裂压力相当;泥灰岩抗拉强度较低,部分发育有天然裂缝,破裂压力较低,裂缝起裂后延伸压力与最小水平地应力相当。

宽方位角地震勘探技术评述

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