Prediction of fracture distribution and evaluation of shale gas preservation conditions in Longmaxi Formation in Dongxi area
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摘要: 页岩气的商业性开发证实成熟页岩具有较大的勘探潜力，川东南地区是我国南方页岩气勘探开发的战略先导区。裂缝发育情况是页岩气保存和开发部署的关键因素，对于构造环境复杂特殊的盆缘地区，在加大页岩裂缝研究难度的同时，也给页岩气勘探开发提供了新的方向。页岩气勘探作为我国油气勘探的新领域，尚未形成成熟的裂缝评价体系。以川东南东溪地区为例，基于三轴岩石力学实验结果，首先采用有限元数值模拟手段进行应力场模拟，而后利用岩石力学参数进行模型构建和模拟结果的反复试算，以获取东溪地区最大、最小主应力和应力差分布图，最后运用莫尔–库伦准则计算岩体破裂系数预测裂缝分布规律。结果表明：研究区裂缝发育情况主要分为4个级别，其中，Ⅰ级裂缝呈条带状分布在东西两侧断裂附近，Ⅱ级裂缝在Ⅰ级裂缝周围发育，而Ⅲ级裂缝分布在北部背斜核部和西部背斜翼部；优选页岩气埋深、距剥蚀区距离、距齐岳山断裂距离、断裂作用、应力差及压力系数等10个保存条件参数，采用组合权重法确定保存条件参数权重，其中一级参数权重分别为埋深(0.2)、距剥蚀区距离(0.1)、距齐岳山断裂距离(0.1)、断裂作用(0.25)、应力差(0.15)、压力系数(0.2)，以此建立相对完善的页岩气评价体系与评价标准。优选出2类页岩气勘探目标有利区，其中A类、B类有利区分别分布在研究区南部背斜核部及翼部和中部宽缓褶皱两翼。研究成果为川东南地区的页岩气勘探提供重要的参考价值。Abstract: Commercial development of shale gas confirmed that mature shale has great potential for exploration, southeast Sichuan is the strategic pilot area of shale gas exploration and development in southern China, cracks are the key factor of shale gas preservation and development deployment, for the structural environment complex and special basin edge area, increasing the difficulty of shale crack research, but also provides a new direction for shale gas exploration and development. As a new field of oil and gas exploration in China, shale gas exploration has not yet developed a mature crack evaluation system. Taking Dongxi area of the southeast of Sichuan as an example, the stress field was simulated in the region by finite element numerical simulation. The model was constructed and calculated by rock mechanical parameters, the diagrams of maximum principal stress, minimum principal stress and differential stress distribution in Dongxi area were obtained through repeated debugging of the simulation results. Mohr-Coulomb criterion was used to calculate rock fracture coefficient and to predict the law of crack distribution in the study area, The prediction shows that the crack distribution of the study area is divided into four levels, GradeⅠcracks are distributed in bands near the east and west sides, grade cracks develop around gradeⅠcracks, the grade cracks are distributed in the northern dorsal nucleus and the western dorsal wing, respectively. At the same time, 10 preservation condition parameters, such as shale gas burial depth, distance from the ablative area, fracture distance from Qiyun Mountain, fracture action, differential stress and pressure coefficient, were clarified, and were used to determine the primary and secondary parameter weight by the combined weight method. Among them, the first stage parameters are buried depth(0.2), distance from the exfoliation area(0.1), fracture distance(0.1), fracture action(0.25), differential stress(0.15), and pressure coefficient(0.2). A relatively sound shale gas evaluation system and evaluation standards have been established. The two types of shale gas exploration targets are preferred, the favorable areas of the typeⅠand the typeⅡare located in the core and wings of the anticline core in the south of the study area and wings of the gentle folds in the middle of the study area, This study provides an important reference for shale gas exploration in southeast Sichuan.
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图 3 东溪地区构造应力场模拟边界条件
Fig. 3 Structural stress field simulated boundary conditions in Dongxi area
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图 5 东溪地区应力应变模拟分布
Fig. 5 Distribution diagram of stress and strain simulation in Dongxi area
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图 6 岩石综合破裂准则破坏接近程度图解
Fig. 6 Illustration of the damage proximity of the rock integrated fracture criterion
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图 11 东溪地区龙马溪组压力系数预测平面图
Fig. 11 Prediction plan of pressure coefficient of Longmaxi Formation in Dongxi area
表 1 DYS1井岩石力学参数
Table 1 Rock mechanical parameters of well DYS1
岩石类型 泊松比μ 弹性模量E/MPa 黏聚力c/MPa 内摩擦角φ/(°) 密度ρ/(g·cm–3) 黑色含粉砂质泥岩 0.209 39 576.6 53.43 16.73 2.58 黑色炭质页岩 0.254 38 162.7 43.67 38.94 2.60 灰白色灰岩 0.237 35 497.1 2.82 36.78 2.65 
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表 2 不同单元类型岩石力学参数
Table 2 Rock mechanical parameters of different unit types
单元类型 弹性模量E/MPa 泊松比μ 黏聚力c/MPa 内摩擦角φ/(°) 一级断裂 36 729 0.251 43 35 二级断裂 37 781 0.248 45 34 构造高区 39 576 0.207 44 36 构造低区 38 649 0.198 42 34 正常沉积区 38 000 0.213 40 35
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表 3 东溪地区龙马溪组岩石裂缝预测的η值标准
Table 3 Predicted-η criteria for rock cracks in Longmaxi Formation in Dongxi area
η值 破裂程度 裂缝发育级别 < 0.737 不发育区 Ⅳ 0.737~1.053 发育临界区 Ⅲ > 1.053~1.368 较发育区 Ⅱ > 1.368~1.684 发育区 Ⅰ > 1.684 破坏区 断裂带
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表 4 东溪地区断裂系统及断裂特征统计
Table 4 Statistical table of fracture system and fracture characteristics in Dongxi area
断层名称 断层性质 级别 走向 延伸距离/km 垂直断距/m 断开层位 形成时间 倾角/(°) 与现今最大主应力倾角/(°) DX-F2 逆断层 三级 NE 32.2 100 TS-TЄ 燕山晚期
Ⅰ幕55 75 DX-F3 逆断层 三级 NE 25.6 200 TP-TЄ 45 DX-F7 逆断层 四级 NE 20.1 50 TS-TЄ 65 DX-F1 逆断层 三级 NNW 32.0 700 TP-TЄ 燕山晚期Ⅱ幕 50 85 DX-F4 逆断层 四级 NNW 6.7 20 TS-TЄ 60 DX-F5 逆断层 四级 NNW 8.8 30 TS-TЄ 55 DX-F6 逆断层 四级 NNW 17.2 200 TS-TЄ 60 注释：TS表示时间域志留系；TP表示时间域二叠系；TЄ表示时间域寒武系。
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表 5 东溪地区龙马溪组页岩气评价参数
Table 5 Shale gas evaluation parameters in Longmaxi Formation in Dongxi area
因素(一级权重) 评价参数(二级权重) 评分等级 好(1.0) 较好(0.75) 一般(0.5) 差(0.25) 埋深/m(0.2) 4 500~5 000 4 000~4 500 > 5 000 < 4 000 距剥蚀区距离/km(0.1) > 15 10~15 5~10 < 5 距齐岳山断裂距离/km(0.1) > 10 5~10 5~2 < 2 断裂作用(0.25) 次级断裂规模—断裂垂直断距/m (0.2) < 60 60~100 100~200 > 200 断裂分级分期(0.2) 早期Ⅲ、Ⅳ级 晚期Ⅰ、Ⅱ级 距离较大规模次级断裂(断距 > 150 m)水平距离/km (0.3) > 2 1~2 0.5~1 < 0.5 次级断裂倾角(0.2) < 45 45~55 55~65 > 65 裂缝发育程度(0.1) Ⅱ级发育区 Ⅲ级发育区 Ⅰ级发育区或欠发育区 应力差(0.15) 低 中 高 压力系数(0.2) > 1.4 1.2~1.4 0.9~1.2 < 0.9
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