Pore network characterization in travertines based on high resolution Computer Tomography (CT) and Nuclear Magnetic Resonance (NMR)

Pore network characterization in carbonates based on high-resolution Computer Tomography (CT) and Nuclear Magnetic Resonance (NMR). Soete, J.1, Foubert, A.1, Claes, S.1, Claes, H.1, Mohnke, O.2, Klitzsch, N.2, Özkul, M.3, Swennen, R.1 1Dept. Earth and environmental sciences, Celestijnenlaan 200E, B-3001 Heverlee, Belgium (jeroen.soete@ees.kuleuven.be). 2Applied Geophysics, E.ON ERC, RWTH Aachen University, Aachen, Germany. 3Pamukkale university, Dept. of Geological Engineering, Denízlí, Turkey. Porosity-permeability correlations in complex carbonates are poorly understood. The contribution of micro-, meso- and macropores to the connectivity needs to be unravelled. The aim of the study is to develop a multidisciplinary approach that leads to a better assessment of poro-perm relations. The focus lies on the integrated use of petrophysical analysis, CT, NMR and the necessity of the integration of these techniques for understanding the poro-perm relationships. The samples used for this study are travertine (Denizli Basin, Turkey) and are selected from four facies: i.e. pool, reed, cascade and waterfall facies. First, the porosity and permeability are measured on discrete plugs (2 cm Ø), with a helium porosimeter and steady state permeameter. Secondly, pore-size distributions of the samples are determined quantitatively based on CT, NMR and MIP. NMR experiments were performed with a Halbach magnet, generating a 0.22 tesla static field in a cylindrical region, with a proton resonance frequence of 9.6 MHz. Plugs (2 cm Ø) and microplugs (0.7 cm Ø) were scanned with a skyscan 1172 µCT, respectively at voxel resolutions of 28 and 4.8µm, resulting in the 3D visualization and digitalization of the pore network. Helium porosity varies between 3-45%. Permeability between 0.001 and >10000mD. However, no clear poro-perm relation could be observed. The skeletonization illustrates that pore networks are significantly different for different facies. The pool facies is dominated by fenestral porosity. The reed, cascade and waterfall facies contain increasingly more reed-moldic porosity. Interconnected reed-molds result in high permeability values. The omnipresent vuggy porosity explains the petrophysical heterogeneity of the samples. Separate vugs are in particular problematic. These vugs are isolated units, or solely connected to the pore network by interparticle or intercrystalline porosity. As a consequence, separate vugs contribute to the effective porosity, but do not significantly increase permeability, which is mainly influenced by pore throats. NMR porosity values are consistently a few percentages lower than He porosity. This is due to water loss from vuggy pores prior to the measurements. Porosity trends are, however, similar. The pool facies averages 5% NMR and 9% He porosity, the reed facies 7.8 and 13.5%. The influence of sample dimensions was tested by scanning plugs and microplugs. Total porosity increased with 2-4% for the microplugs, indicating that many pores have diameters smaller than 28µm. NMR and MIP indicate, in agreement with CT, that a dominant group of micro-, mesopores and pore throats with radii between 1 and 100 µm is present in all facies. So, micro- and mesopores are probably a controlling factor for connectivity and thus permeability in the studied travertine facies. Tracer and NMR diffusion experiments, but also permeability simulations on the skeletonised pore network will give a better understanding regarding this topic. The study illustrates how the combined use of NMR and CT leads to a better understanding of pore networks in carbonates. NMR has the advantage of being non-destructive and provides information on porosity, pore size distributions and permeability. Petrophysical data, digitized pore networks, pore throat sizes, macro- and microporosity distributions will allow to ameliorate existing pore network models and to verify their reliability when compared to observed pore networks in carbonates.status: publishe