Original and pyrometamorphical altered Bentheimer sandstone

Abstract

Bentheimer sandstone is a quartz-rich permeable hard sedimentary rock used for core flooding experiments. When fired to stabilize clays (subjected to high temperatures), pyrometamorphical phase changes induce texture and pore framework alteration. As a consequence the new dielectric response may influence wettability. The literature regarding pyrometamorphical behavior during and after thermal treatment is ambiguous, so we evaluate desirable effects (fixation of clay minerals) and undesirable effects (dielectric surface changes) in the matrix. Porosity, permeability, surface charge, specific surface area and dielectric respond were measured before and after firing of samples up to View the MathML source∼1000°C under oxidizing and non-oxidizing conditions. The matrix properties were determined using X-ray diffraction and X-ray fluorescence, scanning electron microscope imaging, and thermomechanical-, and thermogravimetric analysis with differential scanning calorimetry.

Firing causes dehydration, dehydroxylation and irreversible transformation of original clays, organic matter, and carbonates to glass, oxides and feldspars. During heating quartz transfers from α- to β -quartz and back during cooling. This changes the grain volumes and consequently reduces the matrix integrity. The sandstone has a slight porosity and permeability increase (∼5%∼5%). Further, a shift in the point of zero charge toward a higher pH may result in wettability alteration from strongly water-wet to oil-wet. Additionally, a decrease in the permittivity value and marginal dispersion of the dielectric constant (∼5%∼5%) between the high and the low frequencies was observed. Due to firing and related dispersion of the iron oxides within the matrix framework, Bentheimer sandstone becomes a weaker insulator.