碎软煤层间接压裂井产能预测方法及敏感性分析

Prediction and Sensitivity Analysis of Productivity for Indirectly Fractured Wells in Broken Soft Coal Seams

  • 摘要: 为解决碎软煤层间接压裂井产气量动态预测困难的问题,建立了碎软煤层间接压裂井产能预测模型。该模型耦合了煤层气解吸、扩散、渗流及层间流动等多物理过程,并通过室内层间流动试验,得到了煤岩与砂岩之间的层间流动阻力系数,设计了间接压裂储层渗流试验,并验证了模型。模拟结果表明,间接压裂可促进界面附近、远离裂缝区域煤层气的解吸,从而显著提升产气量;相邻岩层渗透率、相邻岩层与煤层厚度比均与产气量正相关,且当厚度比大于1.6时,增益效应减弱,邻层裂缝高度对产气量的影响较小,但裂缝长度增长,可显著提升产能;产能敏感性影响因素从大到小依次为裂缝长度、砂岩层渗透率、砂岩层厚度和砂岩层裂缝高度。沁水盆地某试验井现场实测日产气量与预测值拟合的决定系数为0.850,均方根差为0.0265×104 m3/d,进一步验证了模型的可靠性。研究结果为碎软煤层间接压裂井的层位选择及排采制度优化提供了理论依据。

     

    Abstract: To address the challenge of dynamically predicting gas production for indirectly fractured wells in broken soft coal seams, a productivity prediction model for such wells is established. The model couples multiple physical processes, including coalbed methane (CBM) desorption, diffusion, seepage, and interlayer flow, etc. Through laboratory interlayer flow experiments, the interlayer flow friction coefficient between coal rock and sandstone is obtained. An indirectly fractured reservoir seepage experiment is designed to validate the model. Simulation results indicate that indirect fracturing could promote CBM desorption near the interface and in areas far from the fractures, thereby significantly enhancing gas production. The permeability of adjacent layers and the thickness ratio of adjacent rock layers to the coal seam are both positively correlated with gas production. However, when the thickness ratio exceeds 1.6, the enhancement effect diminishes. The fracture height in adjacent layers has little influence on gas production, whereas increasing fracture length can substantially improve productivity. The factors influencing productivity sensitivity, in descending order, are fracture length, sandstone layer permeability, sandstone layer thickness, and fracture height in sandstone layers. The coefficient of determination for fitting between the field-measured daily gas production and the predicted values from a test well in the Qinshui Basin was 0.850, and the root mean square error was 0.0265×104 m3/d further verifying the reliability of the model. The findings provide a theoretical basis for layer selection, and dewatering and production system optimization of indirectly fractured wells in broken soft coal seams.

     

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