Characterization and reconstruction of meso-structure of gas-bearing soils at different storage pressures
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摘要: 含气土的储藏气压与细观结构表征是研究浅层气地质灾害的关键因素。利用工业CT扫描测试系统，采用立式旋转扫描，微焦点X射线光源的位置固定，样品沿XY平面方向匀速旋转1周，设定旋转步长为0.3°/s，对反应釜内含气样品注气加压至2 MPa、4 MPa、6 MPa，充分考虑样品成像最佳分辨率、最佳探测范围等因素的影响。结果表明，CT扫描获得的切片图像与重构图像具有良好的实验效果；加压注气到2 MPa时，小气泡灰度值增加；加压到6 MPa时气体整体灰度值增加明显；增压过程中气泡数量随着气泡半径增加而减少；加压注气过程会导致固−液−气三相物质局部变化，表现为孔隙气、孔隙水的体积变化幅度整体大于土骨架，微观局部位置会有较大的升高或降低。当不同位置的气体含量上升且占据主导地位时，会驱动着孔隙水的减少与土骨架的移动。Abstract: The storage pressure and reconstruction of the mesostructure of gas-bearing sediments are key factors in the study of shallow gas geohazards. Using an industrial CT scanning test system, vertical rotational (SR) scanning is used, with the position of the microfocus X-ray source fixed and the sample rotated at a constant speed of 360° along the XY plane, with a set rotation step of 0.3° per second, the gas-bearing samples in the reactor are pressurized to 2 MPa, 4 MPa, and 6 MPa, taking into account the best resolution of the sample imaging, the best detection range and other. The results show that the slices and reconstructed images obtained from the CT scan have good experimental results; the greyscale values of small bubbles increase when pressurized to 2 MPa; the overall greyscale values of gas increase significantly when pressurized to 6 MPa; the number of bubbles decrease with increasing bubble radius during the pressurization process; the pressurization process lead to local changes in the solid-liquid-gas phase, which show that the volume of pore gas and pore water changes more than that of the soil skeleton. The overall change is greater than that of the soil skeleton, and the microscopic local location will have a greater rise or decrease. When the gas content at different locations rises and dominates, it will drive the reduction of pore water and the movement of the soil skeleton.
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Key words:
- gas bearing sediments /
- gas reservoir pressure /
- microstructure /
- CT scan
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图 1 制样所用的土样、甲烷和反应釜
Fig. 1 Soil sample, methane and reactor used for sample preparation
图 4 2 MPa压力下含气土CT图像切片
Fig. 4 CT image slices of gas bearing sediments under 2 MPa pressure
图 5 不同注气压力下的土在X-Z平面CT图像比较
Fig. 5 Comparison of X-Z plane CT images of sediments under different gas injection pressures
图 6 不同气藏压力下的样品切片灰度值比较（高度：2.17 mm）
Fig. 6 Comparison of grey degree of sample slices under different gas reservoir pressures (height: 2.17 mm)
图 7 气藏压力值与气泡体积分数非线性拟合曲线
Fig. 7 Non-linear fitting curve of gas reservoir pressure values to bubble volume
图 8 4 MPa下含气土固、液、气三相分布
Fig. 8 Solid-liquid-gas distribution of gas-bearing soils at 4 MPa
图 9 不同等效半径范围的气泡数量与气泡体积分布
Fig. 9 Distribution of bubble number and bubble volume in different equivalent radius range
图 10 不同气藏压力下气泡数量与等效半径的关系
Fig. 10 The relationship between bubble number and equivalent radius under different gas reservoir pressures
图 12 0～2 MPa的表观含水率与孔隙率分布曲线
Fig. 12 Apparent water content and porosity distribution curve from 0 MPa to 2 MPa
图 13 0～6 MPa的固、液、气含量变化值比较
Fig. 13 Comparison of changes of solid, liquid and gas contents between 0‒6 MPa
表 1 实验砂土基本物理性质指标
Tab. 1 Basic physical properties of experimental sand
粒径/mm 比重/（g·cm−3） 含水率/% 孔隙度/% 饱和度/% <0.7 2.62 40 45 88.9 
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表 2 不同气藏压力下表观含气量
Tab. 2 Apparent gas content under different gas reservoir pressures
气藏压力值/MPa 气泡数量/个 气泡体积分数/% 0 11 499 4.93 2 16 391 4.65 4 10 164 4.97 6 11 966 5.66
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