Pore space analysis

Thin sections from a sandstone (so-called “Weißer Mainsandstein”) treated with a shape charge were investigated by image analysis (using the free software ImageJ) based on optical and scanning electronic microscopy for their porosity, pore size and permeability. We focussed on the crushed zone around the perforation channel in comparison with the untouched sandstone.

Porosity of the untouched sandstone is roughly 9 area-%. Within the crushed zone around the perforating channel porosity is significantly higher: approximately 15 area-%.

Pore size distribution also differs significantly for the untouched sandstone and the crushed zone: thin sections from the untouched sandstone show their maximum number of pores at a pore size of 5*102 to 5*103 µm2. Pores in the crushed zone tend to be taller than in the untouched zone: here the maximum lies in the pore size class 5*103 to 5*104 µm2.

Average pore shapes are very similar for the untouched sandstone and for the crushed zone: in both cases the average pore is slightly elongated. According to Anselmetti et al. (1998) from pore shapes permeabilities can be derived. They range between 60 and 100 mD, for both the treated and the untreated sandstone. Consequently, permeability is not increased by the treatment with shape charges. However, the crushed zone is much more enriched in elongated micro cracks than the untouched rock. These elongated cracks are parallel with the perforation channel and the bedding planes.

From the porosity investigations, specimens from the untouched sandstone and from the crushed zone can be distinguished easily.

Quartz grains in the specimens affected by the application of the shape charges (BSE images 03 and 04 to the right) are considerably fractured and disintegrated. Moreover, diagenetically grown clay minerals within the pore space can be observed in the high magnification detail BSE images 02 and 04. Feldspar and quartz can be distinguished by their different grey shades (feldspar: light, quartz: dark) which are the result of the mass dependent, so-called Z-contrast. Bright spots in the overview BSE image 03 are the result of electric charging due to the lack of carbon sputtering. In contrast, the detail BSE images 02 and 04 were photographed and investigated with carbon sputtering which avoided electrical charging of the thin sections.

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