Quantitative evaluation of shale brittleness based on statistics: Taking shale of Niutitang formation in western Hubei as an example
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摘要:研究目的
页岩储层的脆性是反映页岩气储集层压裂品质的参数之一，对压裂难易程度和压裂缝网形态有着重要的影响。
研究方法为准确评价鄂西地区牛蹄塘组页岩储层脆性特征，对鄂西黄陵背斜南翼五口钻井进行了系统采样和全岩矿物及黏土含量测试、主微量元素含量测试、声学力学联测实验等分析测试，利用聚类分析和主成分分析法对页岩脆性进行了定量评价。
研究结果矿物与岩石脆性密切相关，利用聚类分析方法可以定量表征泥页岩中有效脆性矿物成分和非有效脆性矿物成分；利用主成分分析法建立了基于岩石力学、矿物组分和元素成分的脆性指数综合定量评价公式，克服单一方法的局限性，形成鄂西地区牛蹄塘组页岩层段的脆性指数剖面。
结论微地震监测及压裂施工结果显示，新建立的脆性指数剖面能准确指示页岩的高脆性层段，与压裂结果吻合较好。
创新点:克服单一方法的局限性，利用多种地质因素数学模型建立脆性指数模型；对页岩脆性进行定量评价并在实践中检验。
Abstract:This paper is the result of geological survey engineering.
ObjectiveThe brittleness of shale reservoir is one of the parameters reflecting the fracturing quality of shale gas reservoir, which has an important influence on the degree of difficulty of fracturing and the shape of fracture network.
MethodsIn order to accurately evaluate the brittleness characteristics of Niutitang Formation shale reservoir in western Hubei, systematic sampling, whole rock mineral and clay content test, main and trace element content test, acoustic mechanics joint test and other analytical tests were carried out on five wells in the south wing of Huangling anticline in western Hubei. The quantitative evaluation of shale brittleness was carried out by cluster analysis and principal component analysis.
ResultsThere is a close relationship between minerals and rock brittleness, and the cluster analysis method can quantitatively characterize the effective brittle mineral composition and non−effective brittle mineral composition in shale; The comprehensive quantitative evaluation formula of brittleness index based on rock mechanics, mineral composition and element composition is established by using principal component analysis method, which overcomes the limitation of single method and forms the brittleness index profile of Niutitang Formation shale section in Western Hubei.
ConclusionsThe results of microseismic monitoring and fracturing show that the newly established brittle index profile can accurately indicate the high brittle layer of shale, and the fracturing effect is good.
Highlights:Overcoming the limitation of a single method, a brittleness index model is established by using a variety of geological factor mathematical models. The shale brittleness is quantitatively evaluated and tested in practice.
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图 1 鄂西黄陵背斜周边地质略图（据陈孝红等, 2018修改）
1—第四系；2—古近系—新近系；3—白垩系；4—侏罗系；5—三叠系；6—泥盆系—二叠系；7—志留系；8—寒武系—奥陶系；9—南华系—震旦系；10—新元古代花岗岩；11—向斜轴线；12—正断层；13—性质不明断层；14—构造单元边界；15—取样钻井
Figure 1. Structural outline of the Huangling anticline in western Hubei and surrounding areas (modified from Chen Xiaohong et al., 2018)
1–Quaternary; 2–Paleogene–Neogene; 3–Cretaceous; 4–Jurassic; 5–Triassic; 6–Devonian–Permian; 7–Silurian; 8–Cambrian–Ordovician; 9–Nanhua–Sinian; 10–Neoproterozoic granite; 11–Syncline axis; 12–Normal fault; 13–Unknown fault; 14–Tectonic unit boundary; 15–Sampling wells
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图 2 岩心在不同温压条件下声学（纵波速度、横波速度及其波形）测试结果（20 ℃，80 MPa）
Figure 2. Results of acoustic (P-wave velocity, shear wave velocity and wave shape) of core samples under different temperature and pressure conditions (20 ℃, 80 MPa)
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图 3 静态杨氏模量（a）和体积模量（b）随温度和压力变化关系曲线
Figure 3. Changing curves of the Static Young's modulus (a) and the bulk modulus (b) versus temperature and pressure
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图 4 鄂西地区下寒武统牛蹄塘组矿物成分聚类分析树状图
Figure 4. Dendrogram of mineral composition cluster analysis of Niutitang Formation in Lower Cambrian, western Hubei
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图 5 基于矿物组分的脆性指数与石英和黏土矿物的变化关系
Figure 5. Relationship between brittleness index and quartz and clay mineral based on mineral compositions
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图 6 基于元素成分的脆性指数与石英和黏土的变化关系
Figure 6. Relationship between brittleness index and quartz and clay mineral based on element compositions
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图 8 EYY1HF井压裂参数及微地震监测缝网图
Figure 8. Fracture parameters and micro seismic monitoring fracture network of EYY1HF well
表 1 鄂西地区部分页岩样品矿物组分
Table 1 Mineral compositions of the shale samples in western Hubei
井名 矿物组分体积分数/% 采样深度/m 样品数量 石英 长石 碳酸盐矿物 黄铁矿 黏土矿物 YY1 (26~64)/45.38 (11~23)/17.62 (8~31)/18 (3~7)/4.9 (8~21)/13.92 2936~3064 13 YC2 (29.6~92.1)/43.22 (0~7.3)/4.17 (7.9~40)/16.33 (0~8.8)/6.0 (0~42.9)/28.31 2440~2490 10 YD1 (38.6~71.8)/49.71 (5.6~16)/8.49 (6.3~16)/10.18 (2.2~9.2)/5.99 (3.1~39.2)/25.07 1061~1182 9 ZD2 (29.3~90.6)/52.6 (0~13.3)/5.87 (2.2~22.9)/11.28 (0~7.6)/3.7 (2.9~50.4)/25.36 700~835 13 WD1 (36.4~90.9)/50.28 (0~9.9)/7.34 (0~16.5)/5.46 (0~5.5)/2.2 (2.9~48.9)/33.52 670~1315 25 注：表中矿物组分表示：(最小值~最大值)/平均值。 
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表 2 鄂西地区部分页岩样品元素含量
Table 2 Element compositions of the shale samples in western Hubei
井名 元素含量/% Si Ca Mg K Al YY1 (20.41~33.42)/28.12 (1.62~17.20)/4.79 (0.46~2.58)/1.07 (1.44~3.01)/2.09 (3.48~9.16)/5.74 YC2 (18.48~39.05)/24.48 (2.66~17.32)/8.89 (0.56~2.45)/1.8 (0~3.62)/2.48 (0.16~9.26)/6.98 YD1 (18.11~29.03)/24.58 (2.09~9.03)/4.34 (0.54~2.52)/1.51 (1.1~2.8)/2.06 (3.67~8.14)/6.37 ZD2 (21.17~36.67)/28.73 (1.21~14.83)/7 (0.51~2.32)/1.77 (1.12~3.22)/2.44 (2.05~9.83)/6.68 WD1 (3.99~33.92)/26.56 (0.26~25.47)/4.64 (0.42~10.46)/2.09 (0.15~3.5)/1.97 (1.02~11.26)/7.22 注：采样深度和样品数量、及元素含量表示方法见表1。
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表 3 解释的总方差
Table 3 Total variance explained
成分 初始特征值 提取平方和载入 合计 方差% 累积% 合计 方差% 累积% 1 1.375 45.843 45.843 1.375 45.843 45.843 2 1.317 43.896 89.739 1.317 43.896 89.739 3 0.308 10.261 100
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