In-situ stress calculation for coal reservoirs based on log parameters: A case study of the southern Yanchuan block
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摘要:
地应力对煤储层渗透率的影响贯穿整个煤层气开采过程，进而影响煤层气的运移和产出效果。为了探讨煤层地应力的有效计算方法，以鄂尔多斯盆地东缘延川南区块为例，基于研究区主力煤层测井、试井资料，及煤岩泊松比、弹性模量、储层压力等参数计算结果，构建适用于该区煤储层地应力的计算模型，并对研究区地应力分布特征进行剖析。结果表明：(1)相比于组合弹簧模型，修改后的葛氏模型计算的煤层水平主应力平均相对误差更小，更适用于延川南区块地应力预测；(2)研究区2号煤层最小水平主应力(
$ {\sigma }_{\mathrm{h}} $ )为9.23~29.36 MPa，平均16.87 MPa，最大水平主应力($ {\sigma }_{\mathrm{H}} $ )为10.74~44.71 MPa，平均25.49 MPa，垂向主应力($ {\sigma }_{\mathrm{v}} $ )为15.64~39.51 MPa，平均27.07 MPa；(3)平面上，研究区以西掌断裂带为分界线，煤储层地应力呈现西北高、东南低的特征；垂向上，随煤层埋深的增大，3个方向的应力及侧压系数均呈现递增的趋势，应力场状态也由正断型($ {\sigma }_{\mathrm{v}} $ >$ {\sigma }_{\mathrm{H}} $ >$ {\sigma }_{\mathrm{h}} $ )逐渐向走滑型($ {\sigma }_{\mathrm{H}} $ ≈$ {\sigma }_{\mathrm{v}} $ >$ {\sigma }_{\mathrm{h}} $ )转变，且区内煤储层侧压系数及应力场状态的变化与小型逆断层的分布密切相关。修改后的葛氏模型预测精准度达92.15%，且可通过修改构造应力系数应用到其他区块，对研究区深部煤层气乃至对其他区块深部煤层气开发具有重要意义。Abstract:In-situ stress affects the permeability of coal reservoirs throughout the extraction of coalbed methane (CBM) and further influences the migration and output of CBM. To explore the effective methods for calculating the in-situ stress in coal seams, this study investigated the southern Yanchuan block at the eastern margin of the Ordos Basin. Based on the logs and well test data of the major coal seams in the study area, as well as the calculation results of parameters including the coal rock Poisson's ratio, modulus of elasticity, and reservoir pressure of coals, this study built a calculation model for the in-situ stress in the coal reservoirs of the study area and analyzed the distribution of in-situ stress in the study area. The results show that: (1) Compared with the combined spring model, the modified Ge’s model is subjected to a lower average relative error of the horizontal principal stress of coal seams and is more applicable to the prediction of the in-situ stress in the southern Yanchuan block; (2) The No.2 coal seam in the study area has a minimum horizontal principal stress (
$ {\sigma }_{\mathrm{h}} $ ) of 9.23‒29.36 MPa (average: 16.87 MPa), a maximum horizontal principal stress ($ {\mathrm{\sigma }}_{\mathrm{H}} $ ) of 10.74‒44.71 MPa (average: 25.49 MPa), and a vertical principal stress (${\sigma}_{ \mathrm{v}}$ ) of 15.64‒39.51 MPa (average: 27.07 MPa); (3) Planarly, the in-situ stress in the coal reservoirs of the study area is high in the northwest and low in the southeast with the Xizhang fault zone as the boundary. Vertically, with an increase in the burial depth of coal seams,${\sigma }_{ \mathrm{v}}$ ,${\sigma }_{ \mathrm{H}}$ ,${\sigma }_{ \mathrm{h}}$ , and the lateral pressure coefficient all show an upward trend, and the stress field state gradually transforms from the normal fault type ($ {\sigma }_{\mathrm{v}} $ >$ {\sigma }_{\mathrm{H}} $ >$ {\sigma }_{\mathrm{h}} $ ) to the strike-slip type ($ {\sigma }_{\mathrm{H}} $ ≈$ {\sigma }_{\mathrm{v}} $ >$ {\sigma }_{\mathrm{h}} $ ). Moreover, the changes in the lateral pressure coefficient and stress field state of the coal reservoirs in the study area are closely related to the distribution of small reverse faults. The modified Ge's model has a prediction accuracy of 92.15% and can be applied to other blocks by modifying the tectonic stress coefficients. Therefore, this model holds great significance for the extraction of deep CBM in the study area and even other blocks.-
Keywords:
- in-situ stress /
- logs /
- Ge’s model /
- lateral pressure coefficient /
- reverse fault /
- southern Yanchuan block
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图 1 延川南区块2号煤层构造纲要
Fig. 1 Geological structure outline map of No.2 coal seam in the southern Yanchuan block
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图 2 地应力预测值与实测值对比
注：0.03为相对误差，其他同。
Fig. 2 Comparison of predicted and measured values of in-situ stress
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图 3 延川南区块2号煤层地应力平面分布
Fig. 3 Planar distribution of in-situ stress in the No.2 coal seam of the southern Yanchuan block
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图 4 延川南煤层地应力与深度关系
Fig. 4 In-situ stress vs. depth of coal seams in the southern Yanchuan block
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图 5 延川南煤层侧压系数与深度关系
Fig. 5 Lateral pressure coefficient vs. depth of coal seams in the southern Yanchuan block
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图 6 延川南侧压系数分布现象分析
Fig. 6 Distribution of lateral pressure coefficient in the southern Yanchuan block
表 1 地应力建模数据及预测结果
Table 1 Modeling data and prediction results of in-situ stress
类型 不同井号建模数据 A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 最大水平主应力/MPa 实测 19.14 25.31 16.13 18.76 26.45 29.39 15.30 9.54 14.66 35.92 32.91 最小水平主应力/MPa 实测 13.02 15.42 14.54 11.46 18.55 17.32 14.74 8.99 12.21 21.41 21.83 类型 不同井号预测结果 A12 A13 A14 A15 A16 A17 最大水平主应力/MPa 实测 26.81 25.34 19.14 20.54 17.15 26.63 预测 27.75 22.30 17.31 24.62 18.93 27.40 最小水平主应力/MPa 实测 20.34 15.02 12.55 16.70 13.67 21.22 预测 21.06 16.02 12.51 18.63 15.96 20.25 
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