Numerical simulation on development effect of tar-rich coal through in-situ conversion by convective heating in Huangling Mining Area
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摘要: 有效开发富油煤提取石油对缓解我国油气紧张具有重要的现实意义，地下原位开采对流加热技术是富油煤有效开发的主要技术之一。以黄陵矿区延安组富油煤层为例，将高温蒸汽和氮气作为加热介质，采用地下原位转化开采对流加热的技术手段，分别研究2种加热介质在不同注入压力下的开采效果与能量回报率。结果表明：当注入气体的温度为400℃时，注蒸汽开采的加热效率更高，相同注入压力下注蒸汽开采将地层干酪根开采完所需要的时间小于注入高温氮气，二者比例为0.52~0.68，节省时间效果明显，注蒸汽开采与注氮气开采的天然气总产量比例为1.07~1.11，油总产量比例为0.82~0.85，二者差值随注入压力的增加而逐渐减小。注蒸汽开采与注氮气开采的能量回报率比例为1.754~2.363，注蒸汽开采注入压力为6 MPa时能量回报率在4.99 a达到峰值1.796，注氮气开采均未达到能量正收益。无论哪种加热介质，增加注入压力都能够缩短加热反应时间，有利于提高注蒸汽能量回报率。注氮气开采的地层流体有效渗透率较高，利于油气在地层中流动。注蒸汽开采在富油煤清洁开采方面具有优越性，为富油煤清洁开采的具体生产过程提供一定的数据参考依据。Abstract: The effective development of tar-rich coal to extract oil is of great practical significance to alleviate the oil and gas tension in China, and the convective heating technology of underground in-situ mining is one of the main technologies for the effective development of tar-rich coal. Herein, the mining effect and energy return rate of the two heating media (high-temperature steam and nitrogen) under different injection pressures were studied by convective heating technology for underground in-situ conversion mining based on the tar-rich coal seam of Yan'an Formation in Huangling Mining Area. The results show that: the heating efficiency of steam injection mining is higher when the injected gas is at 400℃. Besides, the time required for steam injection mining to finish the extraction of stratum kerogen is less than that of high-temperature nitrogen injection under the same injection pressure, and the ratio of the two is 0.52‒0.68, with obvious time saving effect. Meanwhile, the ratio of total gas production by steam injection mining to that by nitrogen injection mining is 1.07‒1.11, while the ratio of total gas production is 0.82‒0.85, and the difference between the two values decreases with the increase of injection pressure. In addition, the ratio of energy return rate of steam injection mining and nitrogen injection mining is 1.754‒2.363. Specifically, the energy return rate of steam injection mining reaches a peak of 1.796 at 4.99 a at an injection pressure of 6 MPa, but no positive energy return is obtained by nitrogen injection mining. Regardless of the heating medium, increasing the injection pressure could shorten the heating reaction time, which is conducive to improving the energy return rate of steam injection. However, the effective permeability of formation fluids in nitrogen injection mining is high, which could facilitate the flow of oil and gas in the formation. Generally, steam injection mining has superiority in clean mining of tar-rich coal, and provides some data reference basis for the specific production process for clean mining of tar-rich coal.
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图 1 黄陵矿区延安组含煤地层综合柱状图
Fig. 1 Comprehensive column chart of coal-bearing strata of Yan’an Formation in Huangling mining area
图 2 原位注采井网分布及原理
Fig. 2 In-situ injection and extraction well network distribution and principle
图 4 不同注入压力下天然气产出随时间变化曲线
Fig. 4 Natural gas output versus time curve under different injection pressure
图 5 注入压力为4 MPa时地层流体饱和度分布
Fig. 5 Formation fluid saturation distribution at injection pressure of 4 MPa
参数 赋值 地层初始压力/kPa 650 地层初始温度/℃ 35 基质的总孔隙率/% 0.305 8 基质的原始有效孔隙率/% 0.173 2 基质渗透率/10−3 μm2 0.05 裂缝渗透率/10−3 μm2 10 主裂缝渗透率/10−3 μm2 15 000 基岩比热容/(J·m−1·K−1) 6.5×106 孔中干酪根浓度/(mol·m−3) 3.14×104 岩石热传导率/(W·m−1·K−1) 1.84×105 岩石热膨胀系数/K 7.2×10−6 平均总有机碳含量/% 6 岩层基质密度/(kg·m−3) 2 400 基质中初始流体饱和度/% 100 
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表 2 拟组分的化学模型
Table 2 Chemical modeling of the proposed components
拟组分 分子式 标况下相态 分子量/(g·mol−1) 干酪根 CH1.45O0.04N0.02S0.01 固相 14.7 重油 C27.17H56.34 液相 382.4 轻油 C15.26H32.52 液相 215.7 烃类气 C3.16H8.33 气相 46.3 预焦 － 固相 12.7 焦炭 C 固相 12.0
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表 3 干酪根热解的反应模型(BB模型，1992)
Table 3 Reaction model of kerogen pyrolysis (modified by Braun and Burnham, 1992)
名称 化学式 频率因子/d−1 活化能/(kJ·mol−1) 干酪根热解 kerogen→0.010699 HO+0.009722 LO+0.007131 HC GAS+0.641083 prechar 2.592×1018 213.5 重油热解 HO→0.661282 LO+1.503765 HC GAS+ 13.4175prechar 8.64×1017 226.09 轻油热解 LO→3.237828 HC GAS+5.182242 prechar 4.32×1016 226.09 预焦热解 prechar→0.017177 HC GAS+0.99021 char 8.64×1017 226.09
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表 4 地下原位开采注采参数
Table 4 Injection production parameters of underground in-situ mining
序号 气体种类 注入压力/MPa 1 高温蒸汽 3 2 4 3 5 4 6 5 高温氮气 3 6 4 7 5 8 6
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表 5 注蒸汽开采与注氮气开采效果比较
Table 5 Comparison of the effect of steam injection and nitrogen injection
压力/MPa 生产时间/a 比例 天然气产量/(m3·d−1) 比例 油产量/(m3·d−1) 比例 蒸汽 氮气 蒸汽 氮气 蒸汽 氮气 4 18.54 27.39 0.68 12 559.14 11 305.46 1.11 99.08 120.38 0.82 5 11.75 19.85 0.59 12 248.73 11 368.94 1.08 102.19 120.06 0.85 6 7.92 15.14 0.52 12 311.80 11 458.77 1.07 103.59 120.02 0.86 注：比例的计算方法是注蒸汽开采相关值与注氮气开采的比值。
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表 6 水蒸汽蒸发焓
Table 6 Enthalpy of water evaporation
压力/MPa 凝点/℃ 蒸发焓/ (J∙kg−1) 0.2 120.24 2 201.7 3 233.89 1 794.9 4 250.39 1 713.4 5 263.96 1 639.5 6 275.63 1 570.5 注：0.2 MPa为统一设置的生产井压力，相当于生产井基本无回压。
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表 7 注水蒸汽开采与注氮气开采能量回报率对比
Table 7 Comparison of energy return rate between steam injection recovery and nitrogen injection recovery
注入压力/MPa 能量回报率 比例 到达时间/a 比例 蒸汽 氮气 蒸汽 氮气 4 1.486 0.847 1.754 19.19 26.81 0.716 5 1.650 0.799 2.065 9.52 19.10 0.498 6 1.796 0.760 2.363 4.99 14.62 0.341 注：比例的计算方法是注水蒸汽开采相关值与注氮气开采的比值。
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