Some Applications of Scanning Electron Microscopy to the Study of Reservoir Rock

Abstract

The scanning electron microscope (SEM) provides qualitative information about pore geometry through direct observation of a rock or a pore cast of the rock. This aids in understanding reservoir productivity capabilities. The SEM is useful for locating and identifying minerals, particularly clay minerals -an aid when designing drilling and completion programs. Introduction This paper discusses some applications of the scanning electron microscope (SEM) to the study of reservoir rocks. The SEM is used extensively to provide qualitative information about the pore geometry of rocks, either directly or indirectly through the examination of pore casts. Many of these studies were concerned with relating pore geometry to various tests or rock properties; specific applications also are discussed.The basic principles of the SEM were known in 1935. Experimental instruments were built in Germany and the U.S. before World War II. After the war, the British started a research program that eventually developed a commercial instrument in the early 1960's. Since then, the SEM has become an indispensible scientific tool in all fields of science and industry. Presently, there are approximately 10 SEM manufacturers, with prices ranging from about $17,000 to well over $ 100,000.The SEM provides several unique advantages for the geologist or engineer when compared with other types of microscopes:magnification range of 15 x to a practical upper limit of about 40,000 x for rocks,great depth of field,ease of sample preparation - samples made conductive by a coating of metal applied under vacuum,nondestructive sample study, andthree-dimensional image analogous to reflected light requiring no special techniques for interpretation. These features make the SEM ideal for studying irregular rock surfaces.An energy-dispersive X-ray attachment for the SEM provides qualitative information on chemical elements and aids in identifying minerals. Evaluation and Importance of Microporosity Micropores occur in sandstone and carbonate reservoirs and affect fluid flow properties. These micropores are seen and evaluated best with an SEM. Microporosity, for convenience, has been defined as pore apertures with a radius of less than 0.5 mum.In sandstone reservoirs, clay with associated micropores may occur as detrital laminae, pellets (grains) of silt-sized or coarser material, or as authigenic (that is, newly formed or regenerated) material (Fig. 1). In a water-wet sandstone, the micropores and high surface area associated with the clay minerals are conducive to holding significant amounts of bound water that can affect log calculations for water saturation. Because of improved log-calculation techniques, which take into consideration cation-exchange capacities of clay minerals, this is less of a problem than in the past.Micropores in carbonate reservoirs occur among calcite or dolomite crystals that typically are less than 4 mu m in diameter. JPT P. 1375＾