Research on CT image segmentation and identification methods for micro/nanoscale pores and fractures in sandstone
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摘要:
致密砂岩微−纳米孔裂隙的精准识别是揭示岩石内部多相介质作用机制的重要基础。传统图像分割方法处理复杂孔裂隙结构时分割精度存在明显短板,且不同分割方法对孔隙三维重建、渗流分析结果的影响尚缺少系统评估。采用微焦点 X 射线计算机断层扫描(μCT)技术,构建致密砂岩微−纳米孔裂隙 CT 数据集;以灰度阈值分割、分水岭算法作为传统对照,选取 U-Net、SegNet、DeepLabv3-ResNet50、DeepLabv3-ResNet101共4种深度学习模型开展孔裂隙分割,同时引入公开微米砂岩数据集开展跨尺度迁移测试,综合对比不同方法二维分割精度、三维孔隙重建效果与渗流模拟结果。深度学习方法整体优于传统分割算法,其中 U-Net 模型综合性能最优,相较于传统方法最优结果,交并比(IoU)与F1分数分别提升 18.70%,16.47%,准确率达 99.03%,表现出较好的孔裂隙识别能力;U-Net 可较好保留纳米孔裂隙边界完整性与细节连续性,三维重建孔隙率为 2.408%,与原始孔隙率 2.785% 接近,重建的孔隙网络模型(PNM)孔喉参数与真实样本匹配度高。数据增强会破坏裂隙原生拓扑,造成模型精度下降;DeepLabv3 系列存在过分割问题,传统方法易出现微小孔隙漏检或虚假连通,进一步造成渗透率与压力场模拟偏差。相较于传统分割方法,U-Net 分割得到的孔隙网络拓扑与真实结构高度吻合,能够显著提升孔隙率、孔喉及渗透特性的定量分析精度,可为致密砂岩微−纳米孔裂隙自动识别与建模提供参考依据。
Abstract:ObjectiveAccurate characterization of micro/nanoscale pores and fractures in tight sandstone is fundamental for revealing the mechanisms of multiphase interactions within rocks, and it provides support for hydrocarbon exploitation, geothermal development, and geohazard assessment. Nevertheless, conventional image segmentation techniques suffer from obvious accuracy limitations when handling intricate pore-fracture structures. Moreover, few studies have systematically evaluated how segmentation algorithms affect subsequent three-dimensional pore reconstruction and seepage simulation results.
MethodsIn this study, micro-focus X-ray computed tomography (μCT) was adopted to build a self-established CT dataset containing micro/nanoscale pores and fractures of tight sandstone. Two conventional segmentation approaches—grayscale thresholding and the watershed algorithm—were employed as baseline comparisons. Four convolutional-neural-network-based deep learning models including U-Net, SegNet, DeepLabv3-ResNet50, and DeepLabv3-ResNet101 were implemented for pore-fracture segmentation. A publicly available micrometer-scale sandstone dataset was further utilized to conduct cross-scale transfer validation. Model performance was comprehensively compared and evaluated in terms of two-dimensional segmentation accuracy, three-dimensional pore network model (PNM) reconstruction, and single-phase seepage simulation.
ResultsThe experimental results showed that deep learning models outperformed traditional segmentation methods in general. Among all tested models, the U-Net achieved optimal comprehensive performance. Compared with the best results obtained using conventional segmentation methods, U-Net increased the intersection over union (IoU) and F1-score by 18.70% and 16.47%, respectively, with an overall accuracy of 99.03%, demonstrating favorable capability for pore and fracture identification. U-Net could effectively preserve the boundary integrity and fine-scale continuity of nanoscale pores and fractures. Its three-dimensional reconstructed porosity reached 2.408%, which was close to the original porosity of 2.785%, and the pore-throat parameters of the reconstructed pore network model (PNM) exhibited good consistency with the actual sample. It was noteworthy that data augmentation operations could destroy the inherent topological features of natural fractures and reduce model segmentation accuracy. The DeepLabv3 series models suffered from obvious over-segmentation artifacts, while traditional algorithms tended to produce missed detection of tiny pores or spurious pore connectivity. These defects further induced non-negligible deviations in permeability calculation and pressure-field distribution during seepage simulation.
ConclusionCompared with traditional segmentation approaches, the pore-network topology derived from U-Net segmentation matches well with real pore-fracture structures. The U-Net model substantially improves the quantitative analysis accuracy of porosity, pore-throat geometry, and permeability properties. This study provides a feasible reference for automatic identification and digital modeling of micro/nanoscale pore-fracture systems within tight sandstone.
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Key words:
- pore-fracture segmentation /
- deep learning /
- CT images /
- tight sandstone /
- micro/nanoscale
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表 1 主要矿物组成及其密度
Table 1. Major mineral composition and density
矿物名称 wB/% 密度/(g·cm−3) 矿物名称 wB/% 密度/(g·cm−3) 蒙脱石 36.60 2.00~3.00 石英 22.70 2.65 伊利石 2.79 方解石 16.80 2.71 高岭石 2.61~2.68 斜长石 15.30 5.54 表 2 各分割模型评价指标统计
Table 2. Statistics of evaluation indicators for each segmentation model
分割方法 评价指标/% 交并比IoU 准确率A 召回率R 精确率P F1分数F1 交互式阈值 40.74 98.27 42.96 89.08 57.70 分水岭 39.73 97.17 51.17 65.85 56.47 U-Net 59.44 99.03 70.84 79.98 74.17 SegNet 55.54 98.88 70.27 73.92 70.95 DeepLabv3-ResNet50 48.06 98.52 67.93 61.95 63.93 DeepLabv3-ResNet101 48.34 98.60 65.26 64.18 64.12 表 3 引入图像增强后各模型评价指标
Table 3. Evaluation indicators of different models with image enhancement
分割方法 评价指标/% 交并比IoU 准确率A 召回率R 精确率P F1分数F1 U-Net 53.68 98.18 61.99 69.96 65.13 SegNet 49.31 98.09 60.39 62.73 62.37 DeepLabv3-ResNet50 40.35 97.75 52.97 51.54 52.94 DeepLabv3-ResNet101 40.41 97.43 51.62 51.23 53.46 表 4 公开数据集下各模型性能评价
Table 4. Performance evaluation of different models on public dataset
分割方法 评价指标/% 交并比IoU 准确率A 召回率R 精确率P F1分数F1 U-Net 98.78 99.07 99.41 99.37 99.39 SegNet 99.18 98.61 99.10 99.06 99.08 DeepLabv3-resnet50 83.65 85.92 94.92 87.58 91.10 DeepLabv3-resnet101 83.60 85.90 94.68 87.72 91.06 表 5 孔隙网络模型(PNM)定量参数统计
Table 5. Quantitative parameter statistics of pore network model (PNM)
分割方法 连通孔
隙率/%孔 喉 数量 等效半径/μm 数量 吼道长度/μm 标签 0.67 440 2.07 1006 6.17 交互式阈值 0.21 100 2.12 174 7.81 分水岭 1.36 274 2.32 596 7.97 U-Net 0.61 387 2.03 630 6.32 SegNet 0.44 282 1.77 894 5.29 DeepLabv3-ResNet50 0.70 200 2.24 396 8.60 DeepLabv3-ResNet101 0.70 184 2.43 365 9.04 -
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