NMR Imaging of Pore Structures in Limestones

Abstract

Summary Nuclear-magnetic-resonance (NMR) imaging was used to examine pore structures and liquid flow paths in rocks nondestructively. Various pore types were recognized in a spectrum of limestones, including growth-framework, moldic, vuggy, between- particle, and within-particle porosities. Pore connectivity was particle, and within-particle porosities. Pore connectivity was examined by fluid displacement and flow-weighted and 3D imaging. Introduction NMR signals from hydrogen in liquid molecules can be used t obtain information about the location, displacement, and flow o liquids in poresystems of rocks. The direct proportionality of the NMR pore systems of rocks. The direct proportionality of the NMR signal frequency to the magnetic fieldstrength is the basis of NMR imaging. The application of a magnetic fieldgradient across a liquid-saturated rock results in a range of NMR frequencies along the direction of the gradient. A series of gradients properly oriented in3D space gives rise to a spatially dependent distribution of NMR frequencies that can be processed by computer to yield an image of the pore system of the rock. Liquid flow during the measurement can modify the NMR signal, allowing flow-rate imaging, even during steady-state flow. Several characteristics make NMR imaging useful for examining geologic core samples. Because it is nondestructive, valuable material need not be destroyed and such multiple experiments as repetitive fluid flow through the core may be conducted with a single sample. In addition, the solid rock matrix does not interfere with measurement of fluids in the pores. Such fluid properties as now rate, diffusion coefficient, and chemical properties as now rate, diffusion coefficient, and chemical composition can be measured (e.g., oil can be distinguished from water). Finally, 3D NMR imaging 1.2 can measure 3D pore structures accurately. In contrast, methods that build a 3D structure from 2Dslices are subject to errors from nonuniform, overlapping, or missing slices, The only other technique available that directly provides 3D pore structures of a rock without these problems is 3D provides 3D pore structures of a rock without these problems is 3D X-ray microtomography. A potential limitation of NMR imaging is the difficulty in obtaining signals proportional to porosity when iron or clay is present in samples, causing the spin-spin relaxation time(T2) of present in samples, causing the spin-spin relaxation time (T2) of the saturating fluid to be short. This commonly is worse for sandstones than for limestones. Using fast switching and strong gradients in the spin-warp imaging methods or using the projection reconstruction method should overcome this limitation. The earliest geologic applications of NMR imaging were profiling water distribution in rocks and imaging bedding planes profiling water distribution in rocks and imaging bedding planes and clay seams in sandstones. Fluid displacements, flow rate, and relaxation time of geologic rock cores have been imaged. Of particular interest is the ability to distinguish oil from water in rocks. Recently, NMR imaging has been used for pore structures in rocks. This enables investigation of processes at the pore level that ultimately control the processes at the pore level that ultimately control the macroscopically observed phenomena. In this report, we first show several examples in which various types of pores in limestones are recognized in NMR images of water-saturated samples. Pore types are classified as the reservoir potential of a rock is described and this classification can help indicate the difficulty of producing oil from the rock. Next, three methods of using NMR imaging to examine pore connectivity are demonstrated. Connectivity is an aspect of pore structure that strongly influences fluid flow through reservoir rock; it is difficult to study by other means. Experimental Details The instrument used is a Bruker MSL-200 NMR spectrometer with mini-imaging and micro-imaging accessories. The capabilities of the mini-imaging equipment and the imaging parameters are a 4.7-T (200-MHZ) magnetic field, 150-mm magnet bore, 60-mT/m magnetic field gradient strength, 6.5- to 17.0-ms echo time, 80-s hard 180' pulse length. 0.1-mm resolution in the image plane and 0.3-mmresolution normal to that plane (slice thickness), and pixel dimensions of 128× 128 to 512 × 512. The microimages were obtained at the same field, but with200-mT/m gradient strength, 2. - to 3.4- ms echo time, 7- s hard 180 pulse length, and 0.05-mm isotropic 3D resolution. The resolution, data acquisition time, and other experimental parameters can be varied to fit the requirements of the investigation. Most rocks examined in this study are limestones cut into25- mm-diameter cylinders 25 to 38 mm long. The microimages were of 4.5 x4.5-mm samples. All the 2D images presented here are slices orthogonal to the cylinder axis and therefore are round in cross section. The terms 2D and 3Dimaging refer to generating and collecting NMR signals. not to the signal display. The 2D spin-warp imaging method, usually with a hard 180' pulse, was used to obtain all the 2D images. The 3D spin-warp imaging method was used to obtain the 3D images. Two methods are available to display 3D arrays of imagedata (sometimes called volume image data), regardless of the generation method(from 3D imaging or from a 3D reconstruction of stacked 2D images). In one method, computed slices through a 3D array are displayed the same way as 2Ddata. It is possible to "step through" a sequence of slices generated from a 3D array of image data in movie like fashion. In addition, 3D arrays of data can be displayed in a rendering that resembles a view of a pore cast with the added flexibility of deleting selecting pores. The method used here to display 3D NMR image data is similar to that described by Herman et al. A cube is drawn at the coordinates of each volume element that contains more than 50%water. Cube faces that would be hidden from the viewer if the cube were opaque are not drawn. Visible faces are displayed with an intensity as if they reflected light diffusely from a point source oriented about 45 from the direction of the viewer and as if they emitted light about 25 % as intense as the point source. Views for selected rotation angles of the sample are point source. Views for selected rotation angles of the sample are generated. Various water saturations, lighting and viewing angles, and light intensities are possible. A computer program was written to define a porous network by identifying adjacent voxels containing more than 50% water. This method allows further examination of pore structures by enabling selecting parts of a poresystem to be displayed. Advanced display programs can show pore system to be displayed. Advanced display programs can show part of the pore structure in a translucent, opaque, or transparent part of the pore structure in a translucent, opaque, or transparent appearance. NMR Imaging of Various Pore Types In Limestones Pores in rocks are classified according to their sizes, shapes, and Pores in rocks are classified according to their sizes, shapes, and modes of formation. They often are classified while the geologic description of the area is generated and can help indicate the difficulty of producing oil from the rock. Choquette and Pray's classification of pore types and Dunham's classification of limestones as modified by Jordan were used. Several examples of NMR imaging of pore types found in coarse-grained limestones from subsurface reservoirs and surface outcrops is discussed below. The quality of the NMR images allows various pore types to be recognized in the images. SPEFE P. 123