Three-scale analysis of the permeability of a natural shale.

The macroscopic permeability of a natural shale is determined by using structural measurements on three different scales. Transmission electron microscopy yields two-dimensional (2D) images with pixels smaller than 1 nm; these images are used to reconstruct 3D nanostructures. Three-dimensional focused ion beam-scanning electron microscopy (5.95- to 8.48-nm voxel size) provides 3D mesoscale pores of limited relative volume (1.71-5.9%). Micro-computed tomography (700-nm voxel size) provides information on the mineralogy of the shale, including the pores on this scale which do not percolate; synthetic 3D media are derived on the macroscopic scale by a training image technique. Permeability of the nanoscale, of the mesoscale structures and of their superposition is determined by solving the Stokes equation and this enables us to estimate the permeabilities of the 700-nm voxels located within the clay matrix. Finally, the Darcy equation is solved on synthetic 3D macroscale media to obtain the macroscopic permeability which is found in good agreement with experimental results obtained on the centimetric scale.

Two-scale analysis of a tight gas sandstone.

Tight gas sandstones are low porosity media, with a very small permeability (i.e., below 1 mD). Their porosity is below 10%, and it is mainly composed of fine noncemented microcracks, which are present between neighboring quartz grains. While empirical models of permeability are available, their predictions, which do not compare well with macroscopic measurements, are not reliable to assess gas well productivity. The purpose of this work is to compare the permeability measured on centimetric plugs to predictions based on pore structure data. Two macroscopic measurements are performed, namely dry gas permeability and mercury intrusion porosimetry (MIP), together with a series of local measurements including focused ion beam and scanning electron microscopy (FIB-SEM), x-ray computed microtomography (CMT), and standard two-dimensional (2D) SEM. Numerical modeling is performed by combining analyses on two scales, namely the microcrack network scale (given by 2D SEM) and the individual 3D microcrack scale (given by either FIB-SEM or CMT). The network permeability is calculated by means of techniques developed for fracture networks. This permeability is proportional to the microcrack transmissivity, which is determined by solving the Stokes equation in the microcracks measured by FIB-SEM or CMT. Good correlation with experimental permeability values is only found when using transmissivity from 3D CMT data.

Effect of gas pressure on the sealing efficiency of compacted bentonite-sand plugs.

This research relates to the assessment of the sealing ability of bentonite/sand plugs when swollen in presence of both water and gas pressures, in the context of deep underground radioactive waste storage. Compacted bentonite/sand plugs are placed inside a constant volume cell, and subjected to swelling in presence of both water and gas: swelling kinetics and effective swelling pressure Pswell are identified. Secondly, the gas breakthrough (GB) characteristics of swollen plugs are assessed to determine their ability for gas migration, which has to be minimal for sealing radioactive waste repositories. We show that gas pressure Pg does not affect significantly Pswell until a threshold Pg>2MPa. When swelling occurs inside a tube with a smooth (turned) inner surface, continuous GB occurs when Pg is equivalent to the effective Pswell (obtained without gas pressure, at 7.32MPa±0.11). When the plug swells inside a grooved tube, continuous GB does not occur up to Pg≥10.5MPa: smooth interfaces are a preferential gas migration pathway rather than grooved interfaces, and rather than water-saturated bentonite-sand plugs. With smooth tubes, in presence of Pg≥2MPa, although Pswell is not affected, gas passes through the sample at significantly lower values than Pswell, due to partial sample saturation. It is concluded that GB pressure is a more accurate indicator of partial sample saturation than swelling pressure Pswell alone.

Endogenous superoxide dismutase and catalase activities and radiation resistance in mouse cell lines.

The relationship between the endogenous cytoplasmic levels of the enzymes superoxide dismutase and catalase and the inhibition of cell proliferation by radiation has been studied in 11 mouse cell lines. The resistance of these mouse cell lines to radiation was found to vary by over 25-fold. No correlation was found between the cytoplasmic level of CuZn-superoxide dismutase or catalase and the resistance to radiation as measured by extrapolation number (EN), quasi-threshold dose (Dq), or DO. None of the cell lines had detectable cytoplasmic Mn-superoxide dismutase. The apparent Ki of potassium cyanide for mouse CuZn-superoxide dismutase was determined (Ki = 6.5 mumol dm-3).