Characterization of pore space in sandstone using the anisotropy of magnetic susceptibility

The pore space in siliciclastic rocks is one of the most important petrophysical properties in reservoir rock characterization. Of particular interest is the 3D distribution of pore space and permeability for the purpose of reservoir model development. We used a magnetic technique to determine the preferred orientation of the pore space. The approach is based on the injection of a magnetic ferrofluid, which is a stable colloidal suspension of approx. 10 nm-sized magnetite particles, into a rock specimen. The anisotropy of magnetic ferrofluid susceptibility (AMFFS) is then measured for its AMS and the orientation was compared with the rock’s AMS. We used red Permo-Triassic sandstones of different Buntsandstein and Rotliegend facies, which represent Europe’s highest geothermal water and hydrocarbon reservoir potential. The used porosity methods were helium pycnometry, mercury injection porosimetry, computer tomography and ferrofluid injection. Although the used methods show a strong deviation for single samples, which is related to the different pore size ranges for which each method provide reliable information, the trends are comparable. The computer tomography showed pore space network to be parallel with the bedding of the sandstone. The AMFFS is also mostly similar to the rock’s AMS with principal minimum susceptibility axis normal to the bedding. However, small deviations of AMS and AMFFS axis orientation occur. This deviation could indicate that the ferrofluid fabric is related to a special size of pores. Further investigations are needed to verify this hypothesis.