大宁—吉县区块深层煤层气产能影响因素与生产制度优化

Factors affecting production capacity and production system optimization of deep coalbed methane in Daning-Jixian area

  • 摘要:
    目的 深层煤层气“特低孔、特低渗、超饱和”等特性使其不能照搬浅层煤层气与页岩气的产能模型,且深层煤层气不同井段的生产效果差异大,不同因素对产能的影响程度不清。针对大宁—吉县区块地质特征开展深层煤层气产能数值模拟分析,探究其关键产能因素的影响程度,尤其是确定适宜的簇间距与科学排采制度至关重要。
    方法 以鄂尔多斯盆地东缘大宁—吉县区块深层煤层气地质特征为基础,综合考虑煤层气赋存状态转变、多相流体渗流及储层应力应变本构关系,针对排采过程中煤储层变形与流体运移的多物理场耦合机制,通过引入游离气渗流扩散与吸附气解吸扩散的双路径运移模式,建立考虑气—水两相流动与煤体变形耦合作用的产能数学模型;利用COMSOL Multiphysics软件对模型进行求解,并将模拟结果与研究区现场生产实际数据进行比对,充分验证模型的准确性;运用灰色关联法分析产能主控因素,根据产能预测结果,揭示不同井间距和降压速率对产能的影响规律。
    结果和结论 模拟结果显示:初始渗透率比值k0、Langmuir体积VL、Langmuir应变系数εL、弹性模量E与吸附气产量成正比,初始含水饱和度Sw0、Langmuir压力pL、泊松比v与吸附气产量成反比;k0pLεLE与游离气产量成正比,Sw0VLv与游离气产量成反比,其中,VL对游离气产量的影响较小;影响研究区产能的主要因素为煤层的εLk0;通过对不同簇间距条件下的吸附气产量、游离气产量、产水量进行分析,得出大宁—吉县区块的最优簇间距为20 m,最优降压速率为0.18 MPa/d。

     

    Abstract: The deep coalbed methane has the characteristics of "ultra-low porosity, ultra-low permeability and super-saturation", which makes it impossible to copy the productivity models of shallow coalbed methane and shale gas, and the production effects of different sections of deep coalbed methane are quite different, and the influence degree of different factors on productivity is unclear. Moreover, numerical simulation analysis of deep coalbed methane productivity is carried out according to the geological characteristics of Daning-Jixian block on the eastern margin of the Ordos Basin to explore the influence degree of its key productivity factors, especially to determine the appropriate cluster spacing and scientific drainage system.Based on the geological characteristics of deep coalbed methane in Daning-Jixian block, this paper comprehensively considers the change of coalbed methane occurrence state, multiphase fluid seepage and stress-strain constitutive relation of reservoir, and establishes a productivity mathematical model considering the coupling effect of gas-water two-phase flow and coal deformation by introducing the dual-path migration model of free gas seepage diffusion and adsorption gas desorption diffusion in view of the multi-physical field coupling mechanism of coal reservoir deformation and fluid migration during drainage and production. The model is solved by COMSOL Multiphysics software. And the accuracy of the model is fully verified by comparing the simulation results with the actual on-site data in the study area, and the main factors affecting the production capacity of the study area are analyzed by using the grey correlation method, and the effects of different well spacings and bucking rates on the production capacity are revealed according to the results of the production capacity prediction. The simulation results show that the initial permeability ratio k0, Langmuir volume VL, Langmuir strain coefficient εL and elastic modulus E are directly proportional to the adsorption gas production, while the initial water saturation Sw0, Langmuir pressure pL and Poisson ratio v are inversely proportional to the adsorption gas production. The initial permeability k0, Langmuir pressure pL, Langmuir strain coefficient εL and elastic modulus E are directly proportional to the free gas production, while the initial water saturation Sw0, Langmuir volume VL and Poisson's ratio v are inversely proportional to the free gas production, among which Langmuir volume VL has little influence on the free gas production. The grey relational analysis shows that the main factors affecting the productivity of the study area are εL and k0 of coal seam. By analyzing the adsorbed gas production, free gas production and water production under different cluster spacing conditions, it was concluded that the optimal cluster spacing was 20 m and the optimal pressure reduction rate was 0.18 MPa/d in the Daning-Jixian block.

     

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