MAGNETIC RESONANCE METHODS FOR THE CHARACTERIZATION OF THE PORE SPACE IN VUGGY CARBONATES

Understanding the petrophysical properties of vuggy carbonate rocks is rendered particularly challenging by the extreme complexity of their pore space. Significant variability of carbonate depositional environments and susceptibility of carbonate sedimends to diagenesis results in pore spaces comprising length scales ranging from nanometers to millimetres (and beyond). Characterizing the size distribution and connectivity of pores spanning several orders of magnitude of the length scale is a key issue in carbonate petrophysics that has not been completely resolved. Nuclear spin diffusion in the susceptibility-contrast induced internal field (DDIF-NMR) has been recently proposed for probing multiple length scales in sedimentary rocks. This method has hitherto been applied to only a few samples and its ability to discern the entire spectrum of pore length scales actually present has not been fully evaluated. Motivated by the need to measure the pore size distribution in vuggy carbonate rocks exhibiting structure over disparate length scales, ranging from less than a micron (matrix pores) to millimetres (vugs), we carry out a DDIF-NMR study of real and synthetic vuggy porous media. Synthetic media having controlled amounts of matrix and vuggy porosity serve as standards. They are made by first sintering known amounts of glass beads and calcium carbonate particles of known sizes, and then dissolving the carbonate particles in an acid. DDIF-NMR results are presented for three synthetic and three real vuggy samples. These results are complemented by 3D-MRI data obtained at a resolution sufficient to resolve millimetre-size vugs and independently determine their amount and connectivity. Additionally, mercury porosimetry and statistical image analysis (SIA) of large (3-cm wide), high-resolution images of thin sections are employed to independently determine the complete pore size distribution. Several assumptions behind the interpretation of DDIF-NMR data (fast diffusion, weak encoding and weak pore coupling conditions uniformly fulfilled for pores of all sizes) and the obvious upper and lower detection limits of this technique are likely reasons for the fact that pore size distributions determined by DDIF-NMR and SIA exhibit only qualitative agreement. Nevertheless, DDIF-NMR provides quantitative information about the fraction of total porosity due to presence of vugs which agrees with the independent results obtained from 3D-MRI. A deeper analysis of the precise origin of the deviations between DDIF-NMR and SIA results must await the development of a more rigorous method for the interpretation of DDIF-NMR data.