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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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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