Experimental study on vertical propagation of fractures of multi-sweet of spots shale oil reservoir
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摘要: 鄂尔多斯盆地延长组长7段发育丰富砂岩和页岩多薄互层叠置的页岩油资源,勘探开发评估资源量达数十亿吨以上。但页岩油储层多甜点层叠置,层理、裂缝发育、各向异性和非均质性显著,压裂改造时缝高垂向延伸距离短,体积压裂改造难度大。基于室内真三轴室内压裂物模试验,针对长庆长7段井下连续储层段获取的全直径致密砂岩和页岩全直径岩心,利用混凝土包裹全直径井下岩心测试真三轴环境下的水力裂缝起裂和垂向延伸形态,揭示长7段页岩油储层多甜点岩性压裂裂缝垂向扩展机理。试验发现：页岩油储层层理呈“千层饼”状且胶结弱,压裂液易沿层理渗滤,在垂向应力与最小水平主应力之差小于12 MPa时,水力裂缝形态多呈现水平缝,压裂液沿层理逐层渗滤。排量增大到30 mL/min时,页岩易发生剪切滑移破坏,形成高倾角水平缝或跳跃台阶缝。垂向应力与最小水平主应力之差曾加到14 MPa时,会产生明显的垂向穿层缝,纵向沟通多套甜点层。无论排量高低（30,15 mL/min）,垂直缝均沟通多个层理,压裂液会在沿垂直缝上下延伸过程中,当被弱胶结层理面捕获时,沿层理缝渗滤扩展,产生“十”字型或“丰”字型的复杂缝。压裂施工时,优选垂向应力与最小水平主应力差值大的层位射孔,有利于水力裂缝穿层扩展增加复杂裂缝体积,提高页岩油储层体积改造效果。Abstract: Shale oil resources are developed richly in the Yanchang Formation of the Ordos Basin, which is deposited with multiple layers of sandstone and shale, and the results of exploration and development in recent years have shown that its conservatively assessed resources can reach more than billions of tons. The shale oil reservoirs are stacked with developed bedding and natural fractures and obvious anisotropy and heterogeneity, so the fractures height of in the longitudinal extension distance is usually short, making it difficult to accomplish reservoir reconstruction. Based on indoor true triaxial fracturing physical experiments on the full-diameter shale and sandstone cores obtained from the downhole reservoir section of Changqing 7, the full-diameter core is wrapped by concrete to test the initiation and vertical propagation of hydraulic fracturing in a true triaxial environment. Experiment are carried out to reveal the vertical propagation mechanism of fractures of multi-sweet spots with different lithologic reservoirs in 7 shale oil formation of Changqing. It is found that the shale oil reservoir bedding is in the shape of a "thick cake", and the bedding is cemented weakly. Fracturing fluid is easy to percolate along the bedding. When the difference between the vertical stress and the minimum horizontal stress is less than 12 MPa, the shape of the hydraulic fracture generally exhibits a horizontal fracture, and the fracturing fluid is percolated along the bedding. If the displacement is large (30 mL/min), the samples will be caused to produce shear slip damage, resulting in a high-inclination horizontal fracture or a jumping step fracture. When the difference between the vertical stress and the minimum horizontal stress reaches 14 MPa, the obvious vertical fracture will be produced, and it will connect multi-sweet spots. At this time, regardless of whether the flowing rate is high or low, the vertical fractures will communicate with multiple stratification fractures, and the fracturing fluid will be captured by the weakly cemented stratification surface during the expansion of the vertical fractures, then fracturing fluid is percolated and expands along the bedding fractures, resulting in a complex cross shape or road-network complex fractures. During the fracturing construction, it is recommended to perforate a layer with a large difference between the vertical stress and the minimum horizontal principal stress, which is conducive to create vertical hydraulic fractures, then the complex fractures will beformed to enhance the effect of reservoir reconstruction.
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0. 引言
冰-岩碎屑流是一种在启动时携带或在运动时铲刮裹挟冰屑的特殊碎屑流，相较于一般碎屑流拥有更快的运动速度与更长的冲出距离，因而其发生往往具有巨大的致灾力[1]。近年来，随着岩土体性质受冻融循环及温度变化的影响日益显著以及青藏高原独特的地形地貌特征和发育的活动断裂带，在地震和冰川侵蚀作用下产生了大量的松散固体物质，为冰－岩碎屑流的发生提供了丰富的物源条件。例如2000年的西藏易贡崩滑体导致下游450 km范围内受灾[2-3]；位于雅鲁藏布江左岸的色东普沟自2014年起共发生8次冰-岩碎屑流事件，导致雅鲁藏布江4次大规模堵塞[4]。然而冰-岩碎屑流往往发生在人烟稀少的山区，精确识别和定位此类灾害具有重要的科学价值和实际防灾减灾需求。
近年来，对高寒山区的冰-岩碎屑流的研究逐渐吸引了众多学者的目光，研究大多侧重于冰-岩碎屑流的运动特征、堆积形态及动力学参数等，如杨情情等[5-6]基于2000年易贡滑坡的灾史资料和斜槽试验，分析了冰-岩碎屑流的运动特性并进一步揭示了其运动机理；李昆仲等[7]利用DAN3D软件对2018年色东普沟冰-岩碎屑流建立三维数值模型，通过反演得到了碎屑流的堆积特征、滑体运动速度和铲刮深度等动力特征参数；师璐璐等[8]则以云南玉龙雪山南坡一起较大规模的冰-岩碎屑流型高速远程滑坡为例，通过遥感影像和现场调查阐释了冰-岩碎屑流的成因机制和运动过程。尽管目前针对冰-岩碎屑流的研究已取得一些阶段性成果，但研究始终存在对灾害过程的直接观测数据不足的困扰。
随着环境地震学的发展，基于地震动信号对冰-岩碎屑流进行精准识别成为一种新兴的研究方向，灾害过程中激发的地震动信号能够直观反映冰-岩碎屑流在运动中的相关信息。此外，由Huang等[9-11]提出的Hilbert-Huang变换方法在分析和处理瞬态信号方面展现出了极大的优越性，如Fan等[12-13]则通过HHT方法对地震作用下含软弱夹层岩质边坡的震损过程进行了精准识别。HHT作为一种新兴的时频分析方法，是目前对冰岩崩灾害产生的地震动信号分析效果最好的技术手段。
对此，本研究共设计了5组大型斜槽试验来模拟不同工况下的冰-岩碎屑流运动过程，通过现场布设的动态信号采集仪实时获取地震动信号，基于HHT方法对比分析了5种工况下冰-岩碎屑流运动过程中激发的信号的频谱特征，对含冰率和冰所处位置与冰-岩碎屑流冲击力的相关性进行了论证，研究可为此类灾害的精准识别和远程监测提供技术参考。
1. 试验方案
1.1 试验装置
为探究坡度变化对冰－岩碎屑流冲击力的影响，试验中布设有两段式斜槽，加速段为38°，缓坡段为15°。斜槽横截面为等腰梯形，底宽50 cm，顶宽120 cm，腰长50 cm。斜槽右侧布设钢化玻璃，以便观察冰－岩碎屑流的滑动状态。在滑槽下方支架上布设3个传感器，采集频率为500 Hz。试验安排在11月下旬进行，试验时现场气温为10°左右，以阴天为主，较低的气温与较弱的光照尽可能地减小了冰块因外界因素而产生的损耗。具体试验装置如图 1所示。
1.2 试验材料
以西藏高原地区采集的土壤为样本，按筛分出的颗粒级配进行本试验所用材料的物料配比，配比参数见表 1。试验中采用粒径为1.5 cm×1.5 cm×1.5 cm的方形冰块，所用物料如图 2所示。
表 1 物料组成Table 1. Material compositions物质 冰 砾粒 砂粒 黏土 状态 粒径/mm 10~20 10~20 5~10 2~5 1~2 — 百分比/% / 54 49 32 9 10 质量/kg 1 25 40.35 16.92 11.19 3.00 3.54 混合 2 50 26.90 11.28 7.46 2.00 2.36 3 100 0.00 0.00 0.00 0.00 0.00 4 25 40.35 16.92 11.19 3.00 3.54 冰在上 5 50 26.90 11.28 7.46 2.00 2.36 1.3 试验工况
本试验以含冰率和冰所在位置两个因素为变量，共设置5种工况（表 1），每次试验的物料总质量均为100 kg。受冰块融化及块体间相互摩擦的影响，试验结束后岩冰质量有所减小，耗损程度大致为2%~3%。试验过程如图 3所示。
2. 地震动信号频谱分析
试验共截取8 s的数据，对采集的原始地震动信号进行无限脉冲带通滤波，滤波范围为0.01~200 Hz，选择该滤波范围能够保留试验产生的绝大部分有效信号，且能在一定程度上降低噪音的干扰。以垂向地震动信号为例，经预处理后5种工况下的信号如图 4所示。
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图 4 5种工况下记录的冰岩崩激发的垂向地震动信号Figure 4. Vertical ground motion signals induced by ice-rock avalanches recorded under 5 conditions以信号振幅的突然增大和趋近于噪音为节点，可以大致划分出试验过程中冰-岩碎屑流激发的地震动信号的持续时程。从图 4可以看出，5种工况下的地震动信号特征基本一致，信号振幅均先在噪音基础上明显增大，以较为平稳的振动持续一段时间后再次突增至峰值状态，此时信号振幅为前期的2~3倍，此后信号开始逐渐衰减，振幅缓慢恢复至噪音水平。为便于各个工况间的对比分析，本研究将信号的整个持续时程划分成3个阶段：平稳期、峰值期和衰弱期，其中峰值期采用红色矩形框在图中标出。在信号平稳区冰-岩碎屑流处于加速下滑状态，与加速段滑槽发生摩擦碰撞从而激发明显的地震动信号。在物料由加速段进入缓坡段时，由于坡度改变，碎屑流对缓坡面造成巨大冲击，信号进入峰值区。当物料冲出滑槽末端与河岸发生冲击碰撞时，持续激发强烈的地震动信号。此后冰-岩碎屑流逐渐沿河道上下游堆积，信号不断衰减直至恢复噪音水平，试验停止。
通过图 4（a）~（c）对比发现，当冰土混合时，随着含冰率的增加，信号的整个持续时间不断减小，峰值区域更加集中，且信号振幅更大。这是由于冰与滑脱面间的摩擦系数明显比土与滑脱面的摩擦系数小，随着冰-岩碎屑流中冰含量的不断提高，滑体整体的摩擦系数将不断减小，冰-岩碎屑流会受到更大的下滑力，以更快的速度冲下滑槽，冲击状态更加倾向于整体运动而非被滑道拉长截留。在极端工况即含冰率100%的纯冰状态下，激发的信号峰值振幅显著高于其余四种工况，且激发信号的峰值区间最为集中，峰值不再缓慢减低，而是平稳维持在较高振幅一段时间后突然下降，这与纯冰的下滑速度更快且冰体难以被滑道截留拉长，以整体形态迅速撞击滑槽并散落堆积在河道中的情况相符。
通过图 4（a），（d）和图 4（b），（e）两组对照试验，可以看出冰在土体上层时冰岩崩激发的地震动信号的持续时间均略大于冰土混合时的工况，且工况4和工况5的信号峰值区间更长而振幅更低。造成这一现象的原因是当冰在上层时，冰-岩混合物开始下滑时与滑道间的摩擦基本由下层土体承担，由于土体摩擦系数较大，在下滑过程中冰-岩混合物速度较低且滑动形态被逐渐拉长，位于上层的冰体先于土体冲击缓坡段，下层土体再陆续到达缓坡段并发生碰撞，在试验结束后仍有小部分土体残留在滑道中。
在从信号的时域角度进行初步分析后，采用Huang等[9-11]提出的HHT方法进一步对地震动信号进行处理，得到的Hilbert时频谱如图 5所示。由于工况3的振幅较为特殊，仅对其余4种工况的时频谱在时间轴上进行积分，获得的Hilbert边际谱如图 6所示。
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图 5 不同工况下冰岩崩激发的垂向地震动信号Hilbert时频谱Figure 5. Hilbert-spectra of vertical ground motion signals excited by rock-ice avalanches recorded under different working conditions![]()
图 6 不同工况下的Hilbert边际谱Figure 6. Hilbert marginal spectra under different working conditions4种工况下冰-岩碎屑流产生的地震动信号的频谱特征基本一致，信号频率主要分布在0.06~45 Hz范围内，以低频振动为主，峰值均出现在0.8~0.9 Hz，高频成分集中出现在信号的峰值期，即冰-岩碎屑流与缓坡段及河道两岸发生强烈冲击碰撞时期。
3. 结论
本文通过开展5次模型试验，基于地震动信号分析了高寒山区冰-岩碎屑流在不同工况下的运动特性，得到如下结论：
（1）信号的整个持续时程可划分为3个阶段：平稳期、峰值期和衰弱期，随着含冰率的增加以及冰与滑脱面的接触面积增大，信号的持续时间不断减小，峰值区域更加集中，且信号振幅更大。
（2）冰-岩碎屑流激发的地震动信号以低频为主，信号频率主要分布在0.06~45 Hz，峰值出现在0.8~0.9 Hz，高频成分集中出现在冰-岩碎屑流与缓坡段及河道两岸发生强烈冲击碰撞时期。
（3）基于连续波形数据可以较为准确地识别冰-岩碎屑流运动过程中的一些动态信息，研究可为此类灾害的远程监测和灾后救援工作提供参考，也可为后续冰-岩碎屑流的动力学特征参数反演奠定基础。
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图 3 垂向应力与最小水平应力之差对裂缝扩展形态影响
Figure 3. Influences of difference between vertical stress and minimum horizontal stress on propagation morphology of fractures (15 mL/min)
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图 4 垂向应力与最小水平应力之差对裂缝扩展形态影响
Figure 4. Influences of difference between vertical stress and minimum horizontal stress on propagation morphology of fractures (30 mL/min)
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图 5 三向应力22—18—16 MPa下试样裂缝扩展形态
Figure 5. Propagation morphologies of fractures under triaxial stress conditions of 22-18-16 MPa
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图 6 三向应力28—22—16 MPa下试样裂缝扩展形态
Figure 6. Propagation morphologies of fractures under triaxial stress conditions of 28-22-16 MPa
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图 7 三向应力22—18—16 MPa下试样#6、#7压裂曲线
Figure 7. Fracturing curves of sample No. 6 and No. 7 under triaxial stress conditions of 22-18-16 MPa
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图 8 三向应力28—22—16 MPa下试样#2、#3压裂曲线
Figure 8. Fracturing curves of sample No. 2 and No. 3 under triaxial stress conditions of 28-22-16 MPa
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图 9 三向应力30—22—16 MPa下试样#4、#5压裂曲线
Figure 9. Fracturing curves of sample No. 4 and No. 5 under triaxial stress conditions of 30-22-16 MPa
表 1 致密砂岩–页岩水力压裂模拟试验参数
Table 1 Parameters for hydraulic fracturing experiments on shale and sandstone
试样编号 三向应力/MPa 排量/(mL·min-1) (σV−σh) /MPaσV σH σh #1 28 22 16 15 12 #2 28 22 16 15 12 #3 28 22 16 30 12 #4 30 22 16 15 14 #5 30 22 16 30 14 #6 22 18 16 15 6 #7 22 18 16 30 6 
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