Отправлено: 21.10.13 02:46. Заголовок: 2D and 3D imaging resolution trade-offs in quantifying pore throats for prediction of permeability

2D and 3D imaging resolution trade-offs in quantifying pore throats for prediction of permeability
L.E. Beckinghama, C.A. Peters,W.Umb, K.W. Jones, W.B. Lindquist
Advances in Water Resources 62 (2013) 1–12

abstract
Although the impact of subsurface geochemical reactions on porosity is relatively well understood,
changes in permeability remain difficult to estimate. In this work, pore-network modeling was used to
predict permeability based on pore- and pore-throat size distributions determined from analysis of 2D
scanning electron microscopy (SEM) images of thin sections and 3D X-ray computed microtomography
(CMT) data. The analyzed specimens were a Viking sandstone sample from the Alberta sedimentary basin
and an experimental column of reacted Hanford sediments. For the column, a decrease in permeability
due to mineral precipitation was estimated, but the permeability estimates were dependent on imaging
technique and resolution. X-ray CT imaging has the advantage of reconstructing a 3D pore network while
2D SEM imaging can easily analyze sub-grain and intragranular variations in mineralogy. Pore network
models informed by analyses of 2D and 3D images at comparable resolutions produced permeability estimates with relatively good agreement. Large discrepancies in predicted permeabilities resulted from
small variations in image resolution. Images with resolutions 0.4 to 4lm predicted permeabilities differing by orders of magnitude. While lower-resolution scans can analyze larger specimens, small pore
throats may be missed due to resolution limitations, which in turn overestimates permeability in a
pore-network model in which pore-to-pore conductances are statistically assigned. Conversely, high-resolution scans are capable of capturing small pore throats, but if they are not actually flow-conducting
predicted permeabilities will be below expected values. In addition, permeability is underestimated
due to misinterpreting surface-roughness features as small pore throats. Comparison of permeability predictions with expected and measured permeability values showed that the largest discrepancies resulted
from the highest resolution images and the best predictions of permeability will result from images
between 2 and 4lm resolution. To reduce permeability underestimation from analyses of high-resolution images, a resolution threshold between 3 and 15lm was found to be effective, but it is not known
whether this range is applicable beyond the samples studied here