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基于变边界分段模型的页岩损失气量和解吸气量评价方法

A NOVEL METHOD FOR EVALUATING SHALE LOST GAS AMOUNT AND DESORPTION GAS AMOUNT BASED ON SEGMENTED VARIABLE BOUNDARY MODEL

  • 摘要: 储层含气量的准确评估是目前制约非常规天然气高效开发的重要因素, 直接法采用损失气估算模型结合解吸曲线估算储层含气量, 但现有损失气估算模型均基于煤层气的常压边界条件和球形颗粒假设, 如美国矿业局提出的USBM方法, 为埋藏深、柱状岩心的页岩气藏含气量的估算带来较大误差. 本文基于扩散理论, 采用时变压力边界条件和柱坐标系求解一维扩散方程获得解析解, 从而提出了新的损失气估算模型, 即变边界分段模型, 该模型能够反演出提钻和解吸两个阶段气体逸散的不同特征. 结果表明: 在提钻阶段, 环境压力不断降低, 岩心内外压差增大, 气体逸散速率加快, 从而是下凸函数; 在解吸阶段, 环境压力恒定, 岩心内压力随气体逸散而下降, 内外压差减小, 气体逸散速率减慢, 因而是上凸函数. 进一步为证明模型的准确性, 基于相似原理在实验室搭建了损失气−解吸气复原实验系统, 采用圆柱状页岩岩心复现提钻过程和解吸过程的气体逸散情况, 得到的实验结果与变边界分段模型吻合, 而已有的USBM方法不能进行准确预测, 验证了本文提出的变边界分段模型正确性. 根据川南地区Y151井现场测试数据, 采用变边界分段模型进行拟合预测, 所得结果良好, 验证了变边界分段模型的适用性.

     

    Abstract: The accurate evaluation of gas content is significant for the efficient exploration of unconventional natural gas reservoir, direct method adopts lost gas model and combines with desorption curve to evaluate reservoir gas content. However, the classical lost gas model is derived from constant pressure condition and spherical particle assumption in the estimation of coal bed methane, such as USBM method proposed by US Bureau of Mine, it brings a lot of errors for the deep-depth shale gas reservoir in which core sample is cylindrical shape. Based on diffusion theory, this work used time-varying pressure condition and cylindrical coordinate to solve one-dimensional diffusion equation and obtained analytical solution, then proposed a novel lost gas model (segmented variable boundary model). This model is enabled to describe two processes, i.e., drifting process and desorption process, with different gas-diffusion features. The result of segmented variable boundary model shows that the pressure drop between core sample and boundary increases in the drifting process when core sample is drifted from bottom hole to ground, due to the decreases of boundary pressure, lead to that the diffusion rate is accelerating when gas is escaping from core sample to environment, thus the diffusion curve of drifting process is concave. In the desorption process, core sample is placed in desorption canister and boundary pressure is constant, pressure drop between core sample and boundary is decreasing along the gas escaping from core sample, the diffusion rate is moderative and thus the curve of desorption process is convex. For further validating this model, a lost gas-desorption gas simulating experiment system was set up in lab based on the principle of similitude, and we conducted simulating experiment using cylindrical shale samples to obtain the diffusion curve in drifting process and desorption process, through comparing the experimental data with segmented variable boundary model and USBM model, demonstrated the validation of segmented variable boundary model. Moreover, the segmented variable boundary model is applied to fit the experiment data from the Y151 well in South Sichuan Basin, the fitting results have good consistency with experimental data, which indicates that the segmented variable boundary model is suitable for practical engineering condition.

     

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