Pulsed Nuclear Magnetic Resonance Studies of Porosity, Movable Fluid, and Permeability of Sandstones

Abstract

The NMR methods described here provide a rapid, nondestructive determination of porosity, movable fluid, and permeability of sandstone. Introduction Fluid flow properties of porous media have long been of interest in such varied disciplines as geology, geophysics, soil mechanics, and chemical, civil, and mechanical engineering. This interest has resulted in numerous models of porous media that have been proposed, tested, and found to be useful under, at proposed, tested, and found to be useful under, at best, only special circumstances. A critical review of most of these models is given by Scheidegger. The application of each new tool to the study of porous media has helped to test some of the existing porous media has helped to test some of the existing theories and to form neat ideas based on the new parameters being measured. In the application of parameters being measured. In the application of nuclear magnetic resonance (NMR) techniques to the study of the properties of fluids in porous media, the theoretical studies of Korringa et al. resulted in a model for the relaxation of spin polarization of protons in a hydrogenous fluid in a pore of a solid. protons in a hydrogenous fluid in a pore of a solid. Seevers verified this model, established a method for measuring the surface-to-volume-ratio distribution in a porous medium, and proposed a technique for determining specific permeability in sandstones. In the present investigation, the uses of NMR methods are extended and a simple model of porous media is developed from an analysis of pulsed NMR measurements. In tills model, the pore spaces of a porous medium are divided into three groups on the porous medium are divided into three groups on the basis of their surface-to-volume ratios. To evaluate predictions of the NMR model, laboratory predictions of the NMR model, laboratory measurements of spin-lattice relaxation time, porosity, permeability, and residual (irreducible) water saturation permeability, and residual (irreducible) water saturation were conducted on more than 150 sandstone samples obtained from four oil fields in North America. Applications are discussed for estimating the volume of movable fluid, and the specific permability, k, of sandstone samples. The former permability, k, of sandstone samples. The former parameter, which can be considered as producible porosity, parameter, which can be considered as producible porosity, is expressed by ..........................................(1) where is porosity in percent of bulk volume and Swr is the residual (irreducible) water saturation in percent of pore volume at a capillary pressure of 50 percent of pore volume at a capillary pressure of 50 psi. psi. Fast nondestructive laboratory methods are described for determining porosity, movable fluid, and permeability of sandstone samples from permeability of sandstone samples from measurements with a pulsed NNM (spin-echo) apparatus. Procedures are given for adapting these methods to Procedures are given for adapting these methods to samples of irregular shape and small size. Theory General In the Korringa, Seevers, and Torrey (KST) model the observed decrease in the spin-lattice relaxation time, T1, of protons of a hydrogenous liquid contained in the pore spaces of a porous solid is attributed to an increase in the correlation time for the random motion of the water molecules and to the presence of paramagnetic centers at the liquid-solid presence of paramagnetic centers at the liquid-solid interface. JPT P. 775

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