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.