CBM well drainage data-based dynamic inversion study of reservoir gas content
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摘要: 煤储层含气量是煤层气开发的核心参数，但实测煤储层含气量与煤储层的真实含气量之间往往存在误差。基于窑街矿区海石湾井田煤层气井不同时段的产气量，以煤储层含气量“定体积”降低为基础，反演煤储层实时含气量，研究煤层气井排采过程煤储层实时含气量的变化规律。结果表明：煤储层含气量随排采时间呈线性下降趋势，不同步长煤层气井产气量与煤储层含气量降低幅度一致，遵循“定体积”产气特征，即煤层气单井产气量是煤基质“定体积”产出；煤层气井的产气量与含气量降低速率有关，而与煤储层原始含气量无关。煤储层为隔水层，水力压裂难以改变煤基微孔隙通道的结合水状态，CH4产出过程受水–煤界面作用控制，煤层气产出是“CH4·煤·水”三相界面传质作用的结果，水–煤界面作用中水的湍动提供并传递能量，激励块煤中CH4解吸与产出。Abstract: The gas content of coal reservoir is the core parameter of coalbed methane production. There is an error between the measured gas content and the real gas content of coal reservoir. In this paper, based on the gas production of Haishiwan coalbed methane wells in different periods and the decrease of the "constant volume" of the gas content of coal reservoirs, the real-time gas content of coal reservoirs is inverted, and the change rule of the real-time gas content in the process of coalbed methane well drainage is explored. The results show that: (1) The gas content of coal reservoir decreases linearly with the drainage time t. the gas production of different long coalbed methane wells is consistent with the decrease of coal reservoir gas content, and follows the characteristics of "constant volume" gas production, that is, the gas production of single coalbed methane well is the "constant volume" production of coal matrix; (2) The gas production of coalbed methane wells has nothing to do with the original gas content of coal reservoir, but is related to the reduction of gas content; (3) The coal reservoir is a water-resistant layer, and hydraulic fracturing is difficult to change the combined water state of coal-based micropore channels. The CH4 production process is controlled by the water-coal interface. Coalbed methane production is the result of mass transfer at the three-phase interface of "CH4, coal and water", in which water turbulence provides and transfers energy to stimulate the desorption and production of CH4 in block coal.
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图 2 海石湾HSW05煤层气井群排采曲线
Fig. 2 Coalbed methane drainage and production curves of well group HSW05 in Haishiwan
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图 3 不同原始含气量步长为1 d实时煤储层含气量变化曲线
Fig. 3 Real-time gas content change in different original gas contents in 1 d step size
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图 4 不同原始含气量不同步长实时含气量变化曲线
Fig. 4 Real-time gas content change in different original gas content in different step size
表 1 HSW05不同煤层气井煤二层参数
Table 1 Parameters of the second coal seam in different wells of well group HSW05
井号 煤厚/m 压裂缝长设计/m 视密度ρ/(t·m–3) 产气煤体积V/m3 HSW05-1D 3.50 120 1.4 158 256 HSW05-2D 1.25 56 520 HSW05-3V 17.20 777 715 HSW05-4D 9.30 420 509 HSW05-5D 22.10 999 274 
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表 2 HSW05井群不同排采阶段曲线斜率
Table 2 Straight slope of HSW05 wells in different drainage stages
井号 上升阶段 下降阶段 HSW05-1D –0.001 8 –0.003 2 HSW05-2D –0.011 6 –0.013 0 HSW05-3V –0.000 1(缓慢) –0.001 6 –0.001 2(快速) HSW05-4D –0.000 7 –0.001 9 HSW05-5D –0.000 5(缓慢) –0.001 2 –0.001 0(快速)
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