基于Ono-Kondo格子模型的超临界页岩气吸附试验研究

Investigation of supercritical shale gas adsorption in shale based on the Ono-Kondo lattice model

页岩储层普遍埋藏较深,地层温度和压力较高,相当一部分页岩气在储层中处于超临界吸附状态。超临界吸附与传统亚临界吸附在机理上有着较大区别,将建立在亚临界条件的吸附模型运用到超临界页岩气吸附领域仍然面临一些挑战。因此,准确表征超临界页岩气吸附是精准评估页岩气储量和制定合理开发方案先决条件。针对页岩中超临界甲烷吸附问题,借助高精度磁悬浮天平重量法吸附仪,开展了高温高压甲烷吸附实验,分别测定了超临界甲烷在40,60,80和100℃的气体相密度和等温吸附曲线。实验结果表明:磁悬浮天平能同时测定气体相甲烷密度和过剩吸附量;超临界甲烷过剩吸附量随压力先增加后减小,过剩吸附量随温度升高而降低,最大过剩吸附量对应的压力随温度升高而升高。在不对吸附相密度和体积做任何假设的前提下,利用Ono-Kondo格子模型分别分析了页岩气的单层和多层吸附。Ono-Kondo格子模型分析结果表明:超临界甲烷吸附只能形成单层吸附,其中吸附相密度和绝对吸附量均随压力单调递增,吸附相体积近似保持不变。随着压力的升高,吸附相密度与气体相密度的差值先增加后减小,而吸附相体积近似保持不变,因此会导致过剩吸附量出现极大值。若将吸附相密度假定为定值会低估高压力段的绝对吸附量。

Shale formations are typically buried deeper with higher temperature and pressure than conventional reservoirs,resulting in a significant proportion of shale gas being adsorbed under the supercritical state.Because the mechanism of supercritical adsorption is substantially different from that of conventional subcritical adsorption,there are still some challenges in applying the adsorption model established in subcritical conditions to the field of supercritical shale gas adsorption.Consequently,an accurate representation of supercritical shale gas adsorption is a prerequisite for the accurate assessment of gas in place and formulation of reasonable development plans.To address the aforementioned issues of supercritical CH_(4)adsorption in shale,the adsorption isotherms and bulk densities of supercritical CH_(4)at various temperatures(40,60,80 and 100℃)were determined.The gravimetric adsorption experiments used the gravimetric apparatus equipped with a high-precision magnetic suspension balance.The results indicate that the gravimetric method can simultaneously determine the bulk methane density and the excess adsorption.The excess adsorption increases with pressure and then decreases;the excess adsorption decreases with increasing temperature,and the pressure corresponding to the maximum excess adsorption increases with increasing temperature.The Ono-Kondo lattice model was used to analyze monolayer and multilayer adsorption without any assumptions about the density and volume of the adsorbed phase.The results of the Ono-Kondo lattice model show that the supercritical CH_(4)adsorption in shale can only form monolayer,in which the adsorbed phase density and absolute adsorption volume increase monotonically with pressure,and the adsorbed phase volume remains approximately constant.As the equilibrium pressure increases,the difference between the density of adsorbed phase and the density of bulk phase first increases and then declines,which can result in a maximum amount of excess adsorption.Assuming a constant value for the density of the adsorption phase would underestimate the absolute adsorption in the high-pressure range.