Investigations of porous media using nuclear magnetic resonance secular relaxation measurements and micro-CT image analysis
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Nuclear magnetic resonance (NMR) has been used as a common and powerful tool for petrophysical investigation of fluid-bearing porous media. A common application in this field is the extraction of pore size distributions, which are important descriptors of pore system morphologies. The common technique is to correlate mercury-injection porosimetry (MICP) measurements with NMR T₁ or T₂ distributions to obtain NMR-derived pore sizes. The limitations of MICP include pore-throat sensitivity and percolation effects, which compromise interpretation of results. Micro-CT image analysis has no such limitations, and measurements of pore size are characterized by pore body voxel counts. Presented are image analysis and NMR-correlated results for samples of Berea sandstone and Silurian dolomite. These results are compared to MICP-correlated results and the discrepancies interpreted as pore throat-to-body aspect ratios. Nuclear magnetic resonance pore size distributions are valid for single-phase fluids in the fast-diffusion NMR relaxation regime. When the effects of proton diffusion through internal magnetic field gradients become prominent, however, this relationship becomes entangled. Simultaneous measurement of longitudinal (T₁) and transverse (T₂) relaxation times using combined inversion recovery-CPMG pulse sequences allows for interpretation of a computed NMR attribute called secular relaxation (T₂sec). This quantity is defined as the difference in transverse and longitudinal relaxation rates (1/T₂-1/T₁) and can reveal important pore system properties. Presented are results that extract internal magnetic field gradient strengths based on changes in T₂sec as a function of the NMR experimental parameter τ. Further results consider a two-dimensional χ² analysis to attempt to invert for mean pore size and the difference in transverse and longitudinal surface relaxivities. The benefit of these types of analyses is to provide a simple methodology for inferring the average strengths of internal magnetic field gradients and pore sizes from NMR measurements without the need for independent measurements of pore size distributions, such as from mercury injection porosimetry. In addition, secular relaxation analysis removes the effects of bulk fluid relaxation.