Abstract Gas storage in shale (organic-rich mudstone) consists of three different states: free gas in pores and natural fractures; adsorbed gas on organic and inorganic pore walls; and absorbed gas into organic matter (kerogen). Since it is difficult to differentiate absorbed gas from adsorbed gas, most current studies combine the adsorbed gas with absorbed gas and call the combination as gas sorption. In this study, a conceptual model of gas sorption is proposed to account for the contributions from adsorption and absorption to gas storage in shale, respectively. Methane sorption capacity of Barnett and Eagle Ford shale core samples is measured by magnetic suspension sorption system. Regression analysis is performed on the measured data by Simplified Local-Density model coupled with modified Peng-Robinson Equation of State (SLD-PR). Absolute sorption capacity of these two shale core samples is estimated based on the density profile of SLD-PR model. Additionally, the absorbed gas, which is regarded as the gas molecules dissolving/diffusing into the bulk of solid kerogen, is distinguished from the adsorbed gas through interpreting the results of gas expansion measurements using Fick’s law of diffusion. Moreover, methane diffusion coefficients for the two shale core samples are determined, which range from 10−22 m2/s to 10−21 m2/s. The percentage of absorbed gas accounting for total sorbed gas increases with pore pressure. When the pore pressure increases, more gas molecules attempt to adsorb on the surface of kerogen and create a larger gas concentration gradient for gas diffusing into the kerogen. The gas molecules, which adsorb on the pore walls of kerogen and then diffuse into the solid lattice of kerogen, may lead to the swelling of kerogen and thus reduce the pore width for free gas transportation. Therefore, the accurate prediction of the gas absorption capacity of kerogen is significant to understand gas storage mechanism and characterize original gas in place (OGIP) in shale gas reservoirs.

中文翻译：

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页岩干酪根中甲烷吸收的实验测量和分析估计

摘要 页岩（富含有机质泥岩）储气由三种不同状态组成：孔隙和天然裂缝中的游离气；吸附在有机和无机孔壁上的气体；并将气体吸收为有机物（干酪根）。由于很难区分吸附气体和吸附气体，目前大多数研究将吸附气体与吸附气体结合起来，称为气体吸附。在这项研究中，提出了气体吸附的概念模型，以分别说明吸附和吸收对页岩储气的贡献。Barnett 和 Eagle Ford 页岩岩心样品的甲烷吸附能力是通过磁悬浮吸附系统测量的。通过简化局部密度模型结合修正的彭-罗宾逊状态方程 (SLD-PR) 对测量数据进行回归分析。这两个页岩岩心样品的绝对吸附能力是根据 SLD-PR 模型的密度剖面估算的。此外，被认为是气体分子溶解/扩散到大部分固体干酪根中的吸收气体通过使用菲克扩散定律解释气体膨胀测量的结果与吸收气体区分开来。此外，确定了两个页岩岩心样品的甲烷扩散系数，范围从 10-22 m2/s 到 10-21 m2/s。吸附气体占总吸附气体的百分比随着孔隙压力的增加而增加。当孔隙压力增加时，更多的气体分子试图吸附在干酪根表面，并为气体扩散到干酪根中产生更大的气体浓度梯度。气体分子，吸附在干酪根孔隙壁上，然后扩散到干酪根固体晶格中，可能导致干酪根膨胀，从而减小自由气输送的孔隙宽度。因此，准确预测干酪根的吸气能力对于了解储气机制和表征页岩气藏原始气（OGIP）具有重要意义。