Table 2 - CO2 accumulation parameters

An analytical solution to the equations describing the flow of a buoyant fluid released into a porous medium below a horizontal impermeable boundary is used to model the growth of CO2 accumulations beneath thin mudstone beds in the Utsira sand reservoir at Sleipner in the North Sea. Here supercritical CO2 has been injected at a rate of ab. 1 MT/yr since 1996 and imaged by time-lapse seismic data in 1999, 2001 and 2002. The CO2 rises as a narrow plume and is partially trapped by a number of thin mudstones before reaching the caprock to the reservoir. The radii of the individual layers of trapped CO2 increase as the square root of time since initiation as predicted by the modelling for constant input flux. However apparent negative initiation times for horizons low in the reservoir suggests that net input fluxes for these layers have decreased with time, most probably as the spreading layers have increased their leakage rates. Accumulation of CO2 in the layers higher in the reservoir was initiated up to 3 yr after injection started. Modelling of the thickness profiles across three of the higher layers suggests that their net input fluxes have increased with time. The observation that the central thicknesses of the deeper layers have remained approximately constant, or have slightly decreased since first imaged in 1999, is consistent with the model predictions that the central thickness is directly proportional to net input flux. However, estimates of the permeability of the reservoir from the rate of increase of the radii of the CO2 accumulations are an order of magnitude less than measured permeabilities on the reservoir sandstone. Permeabilities estimated from the modelling of layer thickness changes scatter in the same range. These discrepancies may arise from, 1) approximations in the model not being valid, 2) the measured permeabilities not being representative of the permeability for two-phase flow on the scale of the reservoir or, considered less likely, 3) that much less CO2 is being stored in the imaged CO2 accumulations than estimated from the seismic reflection profiles. The most probable cause of the discrepancy is that the relative permeability for the CO2 phase is significantly reduced at lower CO2 saturations.本研究采用解析解法求解描述水平不透水边界下方多孔介质中浮力射流流动的控制方程，以此对北海斯莱普涅尔（Sleipner）地区乌季拉砂岩储层中薄泥岩层下方二氧化碳（CO₂）聚集体的生长过程进行建模。该区域自1996年起以约1百万吨/年的速率注入超临界二氧化碳（supercritical CO₂），并分别于1999年、2001年及2002年通过时移地震（time-lapse seismic）数据完成成像监测。二氧化碳以窄羽状流形式向上运移，在抵达储层盖层前，会被多套薄泥岩层部分截留。被截留于各泥岩层中的二氧化碳，其分布半径随注入启动后的时间平方根呈正比增长，这与恒定输入通量下的模型预测结果一致。但储层深部层位出现了表观负启动时间，这表明该类层位的净输入通量随时间逐渐降低，最可能的原因是展布范围扩大的岩层漏失速率有所提升。储层上部层位的二氧化碳聚集过程，最晚可滞后于注入启动3年启动。对三套上部层位的厚度剖面进行建模分析后发现，其净输入通量随时间呈上升趋势。储层深部层位的中心厚度自1999年首次成像以来基本保持稳定，或仅出现小幅下降，这与模型预测的"中心厚度与净输入通量呈正比"的结论一致。但通过二氧化碳聚集体半径增长速率估算得到的储层渗透率（permeability），比储层砂岩的实测渗透率低一个数量级。通过岩层厚度变化建模得到的渗透率估算值，其离散分布也处于相同的数值区间内。上述偏差可能由以下原因导致：1）模型中采用的近似假设不成立；2）实测渗透率无法代表储层尺度下两相流（two-phase flow）的有效渗透率；3）成像监测到的二氧化碳聚集体实际储气量远低于地震反射剖面的估算值——第三种可能性被认为较低。该偏差最可能的成因是：在较低的二氧化碳饱和度条件下，二氧化碳相的相对渗透率（relative permeability）出现了显著降低。