Prediction method of surface subsidence due to underground coal gasification under thermal coupling
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摘要:
煤炭地下气化是煤炭低碳绿色开采技术体系的重要组成，煤炭行业“双碳”目标的实施使得煤炭地下气化迎来良好的发展机遇。然而煤炭地下气化也会引起岩层移动及地表变形，导致利用地下气化回收井工难以开采的“三下”压煤时，严重威胁地面建（构）筑物安全。如何兼顾煤炭地下气化特点准确的预测其地表沉陷，已成为制约煤炭地下气化产业化应用的重要瓶颈之一。基于此，结合“条采−面采”后退式地下气化工艺特点，探究了热力耦合作用下煤炭地下气化引起地表沉陷的诱因，得出地下气化产生地表沉陷的根源是岩层挠曲与焦化隔离煤柱压缩变形。在此基础上，建立了热力耦合作用下煤炭地下气化顶板挠曲变形计算方法以及基于D-P准则的气化煤柱屈服模型与压缩计算方法，进而根据等效下沉空间原理构建了热力耦合作用下煤炭地下气化地表沉陷精准预测模型，并通过乌兰察布煤炭地下气化地表沉陷的实测数据验证了新方法的有效性和准确性。
Abstract:Underground coal gasification (UCG) is an essential part of the low-carbon green coal mining technology system. The implementation of the “double carbon” goal of the coal industry has brought excellent development opportunities for UCG. However, UCG will also cause rock movement and surface deformation, resulting in serious threat to safety of ground buildings (structures) when use UCG to recover the “three under” coal that is difficult to mine by underground mining methods. How to accurately predict the subsidence considering characteristics of UCG has become one of the critical bottlenecks limiting the industrial application of UCG. Based on this, combined with the characteristics of ‘strip mining-surface mining’ backward UCG process, this paper explores the causes of surface subsidence caused by UCG under the thermal coupling, and concludes that the root of surface subsidence caused by UCG is the deflection of rock strata and the compression deformation of coking barrier coal pillar. Further, the calculation method of deflection deformation of UCG roof under thermal-mechanical coupling is established, and the yield model and compression calculation method of gasification coal pillar based on D-P criterion are proposed. Then, according to the principle of equivalent subsidence space, an accurate prediction model of surface subsidence of UCG under thermal coupling is constructed, and the effectiveness and accuracy of the new method are verified by the measured data of UCG in Ulanqab. The research results have important practical significance for promoting the recovery of difficult-to-mine “three under” coal resources and the industrialization for UCG.
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图 1 煤炭地下气化覆岩移动与等效下沉空间
Figure 1. Overburden rock movement and equivalent subsidence space of underground coal gasification
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图 2 煤炭地下气化岩板力学模型
Figure 2. Mechanical model of underground coal gasification rock plate
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图 4 不同因素对煤柱屈服区宽度的影响
Figure 4. Influence of different factors on width of coal pillar yield zone
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图 5 “条采−面采”气化炉后退式控制注气地下气化工艺示意
Figure 5. Schematic diagram of “strip mining-regional mining” gasifier backward controlled gas injection underground gasification technology
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图 7 煤炭地下气化地表下沉等值线
Figure 7. Surface subsidence contour of underground coal gasification
表 1 岩层分布及其力学参数
Table 1 Rock strata distribution and its mechanical parameters
岩性 厚度/m 泊松比 平均重力密度/（kg·m3） 抗拉强度/MPa 粉砂岩 15 0.26 2 205 1.12 泥岩 2 0.28 2 147 0.83 粉砂岩 1 0.26 2 205 1.12 泥岩 9 0.28 2 147 0.83 粉砂岩 2 0.26 2 205 1.12 泥岩 2 0.28 2 147 0.83 细粒砂岩 1 0.32 2 311 0.83 泥岩 4 0.28 2 147 0.83 细粒砂岩 1 0.32 2 311 0.83 泥岩 5 0.28 2 147 0.83 煤层 3 0.23 1 400 0.50 泥岩 3 0.28 2 147 0.83 砂质泥岩 1 0.25 2 316 0.71 砂质泥岩 3 0.25 2 316 0.71 泥岩 4 0.28 2 147 0.83 煤层 5 0.23 1 400 0.50 
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表 2 岩层热物理参数
Table 2 Thermal physical parameters of rock strata
岩性 导热系数λ/(W•m−1•K−1) 比热容c/(J•kg−1•K−1) 时间t/s 密度/（kg•m−3） 热扩散系数a/(m2•s−1） 泥岩 1.202 890 7776000 2316 5.83×10−7 砂岩 1.2 1010 7776000 2205 5.39×10−7
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表 3 岩层弹性模量随温度的变化规律
Table 3 Variation law of elastic modulus of rock strata with temperature
岩性 拟合函数 公式编号 泥岩 $E\left[ {T(x)} \right] = 4 \times {10^{ - 8} }{\left[ {T(x)} \right]^3} - 5 \times {10^{ - 5} }{\left[ {T(x)} \right]^2} + 2.14 \times {10^{ - 2} }\left[ {T(x)} \right] + 1.085\;6$ （40） 砂质泥岩 $E\left[ {T(x)} \right] = 2 \times {10^{ - 8} }{\left[ {T(x)} \right]^3} - 3 \times {10^{ - 5} }{\left[ {T(x)} \right]^2} + 1.41 \times {10^{ - 2} }\left[ {T(x)} \right] + 0.918\;6$ （41）
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