Evaluation of coal structure based on machine learning logging inversion: A case from NO.8 coal of Benxi Formation in Yulin area of Ordos Basin
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摘要:
煤体结构直接影响煤储层孔裂隙发育,其准确判识对煤层压裂及煤层气开采具有重要指导价值。以鄂尔多斯盆地榆林地区本溪组8号煤为例,其煤体结构复杂,引入机器学习方法解决煤层气储层测井中的非线性问题。采用区内已完成预处理的取心井数据,采用BP神经网络、随机森林以及XGBoost算法进行训练,进行全区煤体结构反演,并结合区内煤层顶底板及煤厚,剖析构造控制下的煤体结构发育特征。结果表明:①随机森林以及XGBoost算法相较于BP神经网络,对目标煤层煤体结构的反演结果更接近于岩心观测的真实情况,准确度更高;②榆林地区8号煤从NW向SE,煤体破碎程度逐渐加剧;③区内中部至东南部发育构造带,在构造带影响下煤厚减小且原生结构煤逐渐转变为糜棱结构煤。本研究可为实际煤层气生产中的煤体结构识别以及构造带分析提供参考价值。
Abstract:Objective Coal structure directly affects the pore and fracture system of coal reservoirs. Therefore, accurate identification of coal structure is crucial for guiding coal seam fracturing and coal bed methane extraction. Taking No. 8 coal of the Benxi Formation in the Yulin area of the Ordos Basin as an example, the coal structure is complex, necessitating the use of machine learning methods to address the nonlinear challenges in logging data interpretation.
Methods In this study, Back Propagation Neural Network (BP), Random Forest, and XGBoost algorithms are trained on pre-processed core well data from the study area to perform coal structure inversion across the region. The analysis also considers the top and bottom plates of the coal seams and the coal thickness to explore the development of coal structure under tectonic control.
Results The results indicate that: (1) Random Forest and XGBoost algorithms provide more accurate inversion results than the BP neural network, aligning more closely with real core observations. (2) The degree of coal structure fragmentation in No. 8 coal in the Yulin area increases progressively from northwest to southeast. (3) Tectonic zones, developed from the central to southeastern part of the study area, cause a reduction in coal thickness, with the coal structure transitioning from primary coal to mylonitic coal under tectonic influences.
Conclusion The study can provide valuable insights for coal structure identification and tectonic zone analysis in coalbed methane production.
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Key words:
- Coal structure /
- machine learning /
- Back Propagation Neural Network /
- Random Forest /
- XGBoost /
- structural control /
- Ordos Basin /
- Yulin area
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表 1 随机森林算法主要参数设置
Table 1. Random forest algorithm main parameter settings
环节 参数名 含义 设定值 样本数据划分 random_state 算法随机种子 1 test_size 验证集样本比例 0.3 决策树建立 max_depth 决策树树深 5 random_state 算法随机种子 42 随机森林建立 n_estimators 决策树数量 40 表 2 XGBoost算法主要参数设置
Table 2. XGBoost algorithm main parameter settings
参数类型 参数名 参数含义 设定值 通用参数 booster 弱学习器类型 gbtree silent 缄默运行判定 1 nthread 线程数 4 Tree Booster参数 eta 学习率 0.1 gamma 损失函数下降阈值 0.1 max_depth 树深 6 min_child_weight 最小建模样本数 3 subsample 决策树采样样本比例 0.7 colsample_bytree 决策树特征采样比例 0.7 Linear Booster参数 lambda L2正则化的惩罚系数 2 alpha L1正则化的惩罚系数 1 学习任务参数 objective 算法目标 multi:softmax num_class 目标类型数 3 seed 算法随机种子 1000 表 3 3种算法基于不同样本容量的适用情况
Table 3. Applicability of the three algorithms based on different sample sizes
算法类型 样本容量 小容量 中容量 大容量 BP神经网络 表现不佳,易导致过拟合或欠拟合 性能有所提升,但仍存在过拟合的风险 充分识别数据特征,反演准确度较高 随机森林 集成算法优势明显,反演准确度较高 性能稳定,准确度略微下降 性能稳定程度变差,泛化能力提高 Xgboost 正则化能力强大,反演准确度较高 充分发挥迭代优势,反演准确度较高 并行处理数据,保持反演高效性及准确性 -
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