Pore fabric characterization in sandstones using magnetic anisotropy methods

Abstract

Determining directional properties of pore networks in potential reservoirs is indispensable to develop models and to better understanding the directional characteristics of fluid flow. Specifically, the direction of preferred orientation of pore long axes will exert a strong control on the direction of maximum permeability, and therefore the anisotropy of porosity is crucial for predicting fluid paths. We describe and test a method to determine the directional properties of pore fabric in siliciclastic rocks using magnetic techniques. The approach is based on injecting into rock specimens a ferrofluid, a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier, and measuring the magnetic susceptibility in different orientations. The standard anisotropy of magnetic susceptibility (AMS) measurements on dry samples reveal that clay particles in the studied red sandstones are oriented parallel to bedding. Because the magnetic susceptibility after impregnation increases several times, the rock susceptibility can be neglected and therefore the anisotropy of magnetic ferrofluid susceptibility (AMFFS) provides an image of the 3D fabric and porosity in the rock. After injecting the samples with the ferrofluid solution, the AMFFS reveals that the pores have preferred orientation parallel to bedding, as the principal axes of susceptibility mimic the initial anisotropy of the samples before injection. Our results on Triassic red sandstones suggest that both interparticle and intraparticle pores, which are measurable via AMFFS, are present and are largely determined by micas and clays. Interparticle pores are most likely found between clay platelets, whereas intraparticle pores correspond to cleavage-plane pores within clay aggregates. Overall this study highlights that magnetic methods can be readily applied to siliciclastic rocks to characterize pore networks and more important, to determine the preferred orientation of the pores assemblage.