Petrophysical implications of laboratory NMR and petrographical investigation on a shaly sand core

摘要

Nuclear magnetic resonance (NMR) measurements on rocks directly respond to the hydrogen content of the fluids in the pore space. Thus, besides the routine application of NMR in measuring the producible water- and hydrocarbon-filled porosity in the rocks, NMR technology has the potential to characterize the surficial clay-bound water. The exact nature of NMR transverse-relaxation T sub 2 spectra obtainable from the clay-bound water has been discussed in recent studies conducted on both pure clays and reservoir rocks. The paper presents the results of a laboratory investigation on systematically sampled shaly sand rocks. We explore the relationship between the T sub 2 distribution as a function of the rock texture, pore size distribution, clay mineralogy, and clay morphology. A 10-ft conventional core, cut in water-bearing, clay-rich sandstone, was sampled for this study. Mineralogy, texture, pore size distributions, and clay morphology were analyzed by X-ray diffraction (XRD), this section petrography, scanning-electron microscopy 9SEM), and petrographic image analysis (PIA). In addition, fully polarized NMR signals were acquired spacings (T sub e) of 0.3 and 0.6 millisecond. NMR porosity, computed by summing the signal in the T sub 2 spectra, match favorably with the helium porosity, indicating that the full spectrum of pore sizes is included in the NMR porosity. The NMR T sub 2 spectra obtained from brinesaturated samples show three distinct T sub 2 classes: 1 to 3, 5 to 20, and 30 to 200 milliseconds. These appear from core properties to be controlled by pore-size changes, which are primarily influenced by the changes in clay type and morphology. The <5-millisecond components likely correspond with <5-mum sized pores typically trapped within the flakes of detrital illite. Features in the 5-to 20-millisecond range are also likely associated with the authigenic aggregates of kaolinite and chlorite that enclose 5-to 30-mum sized pores. T sub 2 spectra obtained from desaturated rocks support the match of the specific features in the T sub 2 spectra to the clays present in the rocks. Finally, the water in the relatively larger pores (30 to 150 mum) is associated with the signals at about 30 milliseconds and greater.