Research on evolution characteristics of shale pore-fracture and permeability under freeze-thaw cycles
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摘要:
页岩气储层具有超低渗特性，液氮(LN2)致裂作为一种很有前景的储层增透技术备受关注。以四川南部龙马溪组页岩为研究对象，研究了含水页岩岩芯在受低温流体——液氮冻融循环下的物理响应。针对含水状态下的页岩进行LN2冻融循环处理，运用扫描电子显微镜(SEM)对LN2冻融循环前后的页岩样品进行微观孔裂隙结构的定点观察，采用数字图像处理技术和分形理论对同一位置的孔隙变化进行了定量化分析，测试了孔隙度和渗透率，运用计算机断层扫描(CT)展示了页岩样品随LN2冻融循环的宏观断裂破坏过程，探讨了液氮冻融的致裂机理。结果表明，液氮冻融循环处理可以有效促进孔隙、裂隙的萌生和扩展。液氮冻融时页岩在热应力和冻胀力的作用下产生新裂纹，随冻融循环次数的增多页岩孔裂隙稳定发展，冻融循环下页岩孔隙度累积增长幅度为54.6%，渗透率的提高非常显著(高达3个数量级)。
Abstract:Shale gas reservoirs have ultra-low permeability, and liquid nitrogen fracturing has attracted much attention as a promising reservoir permeability enhancement technology. This paper took the Longmaxi Formation shale in southern Sichuan as the research object, and studied the physical response of the water-bearing shale core under the freezing-thawing cycle of liquid nitrogen (LN2), which was a cryogenic fluid. The LN2 freeze-thaw cycle treatment was carried out for the shale in the water-bearing state, scanning electron microscope (SEM) was used to observe the microscopic pore and fracture structure of shale samples before and after the LN2 freeze-thaw cycle, digital image processing technology and fractal theory were used to quantitatively analyze the pore-fracture changes at the same location, and then porosity and permeability tests were performed, computer tomography (CT) was used to show the macroscopic fracture failure process of shale samples with the LN2 freeze-thaw cycle, finally discussed the cracking mechanism of liquid nitrogen freezing and thawing. The results showed that the liquid nitrogen freeze-thaw cycle treatment could effectively promote the initiation and expansion of pores and cracks. When the liquid nitrogen froze and thawed, the shale generated new cracks under the action of thermal stress and frost heave force, and the pores and cracks developed steadily increased with the number of freeze-thaw cycles. The cumulative increase in shale porosity under the freeze-thaw cycle was 54.6%, and the increase in permeability was very significant (up to 3 orders of magnitude).
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Keywords:
- shale /
- liquid nitrogen freeze-thaw cycle /
- pore structure /
- permeability /
- fractal theory
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图 3 岩石区域1、区域2在低倍数下的电镜图片
Figure 3. Electron microscope pictures of rock area 1 and area 2 at low magnification
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图 4 岩石区域1在冻融一次后微观结构变化
Figure 4. The microstructure of rock zone 1 changes after one freeze-thaw cycle
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图 5 岩石区域2微观结构随LN2冻融循环次数变化
Figure 5. The microstructure of rock zone 2 changes with the number of freeze-thaw cycles of liquid nitrogen
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图 7 岩石面孔隙度、分形维数随LN2冻融循环次数变化
Figure 7. The porosity and fractal dimension of rock face change with the number of freeze-thaw cycles of liquid nitrogen
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图 8 页岩孔隙度随LN2冻融循环次数变化
Figure 8. Shale porosity changes with the number of liquid nitrogen freeze-thaw cycle
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图 9 页岩渗透率随LN2冻融循环次数变化
Figure 9. Shale permeability changes with the number of liquid nitrogen freeze-thaw cycle
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图 10 CT扫描图像下页岩随LN2冻融循环次数的变化
Figure 10. The changes of shale with the number of freeze-thaw cycles of liquid nitrogen in CT scan images
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图 11 冻融循环下页岩孔裂隙结构演化及致裂机制
Figure 11. Shale pore fracture structure evolution and fracture mechanism under freeze-thaw cycle
表 1 试样基本物理参数
Table 1 Basic physical parameters of samples
试件编号 直径/mm 高度/mm 质量/g 密度/（g·cm−3） D0 10 3 0.58 2.453 W1 24.12 49.0 54.2 2.450 W2 24.21 50.7 56.4 2.456 
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表 2 页岩矿物成分组成
Table 2 Mineral composition of shale
矿物
成分石英 钠长石 方解石 白云石 黄铁矿 高岭石 伊利石 热膨胀系数
/
10−6℃−124.3 9.6 −3.2 3.2 27.3 9.13 9.13 质量分数/% 60 4 7.8 7.5 2.6 2.2 15.9
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表 3 页岩面孔隙度、分形维数
Table 3 Shale surface porosity and fractal dimension
冻融
次数电镜下
图像面积
/μm2孔隙面积
/μm2面孔隙度
/%分形维数
拟合公式分形
维数R2 0 2 920.78 189.67 6.49 y=−1.258x+11.01 1.258 0.997 3 2 803.19 309.582 11.04 y=−1.364x+11.62 1.364 0.998 5 2 994.11 336.90 11.44 y=−1.378x+11.71 1.378 0.998 7 3 146.64 436.475 13.87 y=−1.424x+11.95 1.424 0.998
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表 4 页岩孔隙度、渗透率
Table 4 Shale porosity and permeability
冻融次数 孔隙度/% 增长率/% 渗透率/% 相对增长率/% 0 4.62 — 6.61×10−19 1 6.63 43.54 1.32×10−16 13 012.66 3 6.70 45.06 1.61×10−16 13 100.61 5 6.94 50.18 2.38×10−16 21 383.47 7 7.14 54.60 6.48×10−16 81 865.77
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