基于气体扩散模型的页岩气体散失规律研究

针对不同页岩损失气量计算方法计算结果差异大的问题，分析了传统单孔扩散模型的特点及不足，建立了耦合更贴近实际的时变气体扩散系数和时变岩芯边界气体浓度的修正单孔扩散模型。研究表明，原始气体浓度、气体扩散系数、岩芯边界气体浓度和岩芯尺寸是影响页岩气体散失的关键参数。当气体扩散系数较大或损失时间过长时，USBM直线法对页岩损失气量的计算不够准确。与分段线性递减相比，岩芯边界气体浓度指数递减模式可能更贴近实际情况。岩芯边界气体浓度递减速率主要控制岩芯内气体扩散完成的总时长，而气体扩散系数变化速率主要影响气体扩散前期的特征。建立的修正单孔扩散模型可较好地从气体扩散角度揭示页岩的气体散失特征，但该模型仍需耦合气体在压差下的流动过程并基于实测数据优化其关键参数，进而提高页岩损失气量的计算精度。

Study on Gas Loss Characteristics of Shale Based on Gas Diffusion Model

The commonly used lost gas calculation models of shale use different assumptions or simplified conditions, and result in the uncertainty of the lost gas content of shale. In this paper, the characteristics and deficiencies of the conventional unipore diffusion model are specified. A modified unipore diffusion model is established considering the time-dependent gas diffusion coefficient and time-dependent gas concentration at core boundary. The analysis indicates that the initial gas concentration, gas diffusion coefficient, gas concentration at core boundary, and core size are the key parameters affecting the shale gas loss process. USBM method may misestimate the lost gas content when the gas diffusion coefficient is too large, or the lost time is too long. Compared with the piecewise linear decline mode, the exponential decline mode of boundary gas concentration may be closer to the actual situation. The decline rate of boundary gas concentration mainly controls the total time of gas diffusion in the core, while the change rate of gas diffusion coefficient mainly affects the characteristics of gas diffusion at the early stage. The modified unipore diffusion model established in this paper can better reveal the gas loss characteristics of shale from the gas diffusion perspective, but the gas flow process under pressure difference should be further integrated into this mathematical method and its key parameters still need to be optimized based on the experimental data, so as to improve the calculation accuracy of lost gas content of shale.