ABSTRACT

Efforts have been made for many years to observe pore structure of sedimentary rocks on a microscopic scale. A better description of the pore structure in reservoir rock would aid in the development of relations between flow through tube network models and flow in consolidated reservoir rock. The unique qualities of the scanning electron miscroscope (SE3/I) make it the ideal tool to use in a study of pore structure.

Previous methods of studying pore structure included plastic casts of pore space and optical microscopy.1-7

There is now available a new method to observe three-dimensional flow channels in reservoir rock: a scanning electron microscope having variable magnification from X20 to X20,000 and approximately 1000 times the depth of focus of the conventional light microscopes. This microscope also can obtain stereoscopic images8 and can measure and diagram flow channels so that it is possible to construct a network of tubes that will model consolidated reservoir rock.

THEORY OF THE SCANNING ELECTRON MICROSCOPE

The scanning electron microscope (SEM) functions by projecting a beam of electrons through magnetic focusing lenses at a specimen and recording secondary electrons excited by the primary beam. The amount of secondary electron emission portrays the topography of the specimen because more secondary electrons are emitted from high points than from low points. A schematic of the SEM is shown in Fig. 1.

The property of the SEM that is of most importance in this study is its great depth of focus. The depth of focus results from using electrons [Fig. 1--Schematic diagram of scanning electron microscope (Available in full paper)], rather than light. In the SEM, no electron trajectory is inclined more than 0.5 ° from the optic axis, in contrast with the more than 60 ° in optical microscopy (Fig. 2).

The major problem encountered with the SEM is the buildup of a space charge on insulating specimens under bombardment by the 20 key primary beam. To prevent charge buildup, insulating specimens must be coated with a conducting film to drain away the space charge. Fig. 2--Depth. of focus (Available in full paper)

SAMPLE PREPARATION

The technique used to prepare rocks for examination with the SEM is the most critical part of obtaining micrographs. To obtain information about the pores of a rock, a surface must be prepared in such a way that the grains are cut through without being pulled from the rock matrix. This is difficult because the grains are stronger than the matrix. The problem was solved by application of techniques used to make thin sections. Rock samples 5 cm in diam by 5 cm thick were mounted in polyester resin, with care being taken not to allow the resin to enter the pores. The samples were ground on a steel lap until they appeared fiat, then transferred to a Granton-Vanderbilt polishing machine for final polishing with diamond abrasive. The specimens were cleaned in a sonic vibrator to remove all abrasive from the pores. The samples prepared in this manner were scrutinized under a stereo light microscope at X45.

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