X-Ray Microtomography: A New Tool for the Characterization of Porous Media

In this paper we describe the application of a new high-resolution X-ray tomographic microscope to the study of porous media. The microscope was designed to exploit the properties of a synchrotron X-ray source to perform three dimensional tomography on millimeter sized objects with micron resolution and has been used in materials science studies with both synchrotron and conventional x-ray sources. An overview of the microscope design, operating modes, the reconstruction algorithm and the data display will be presented. The performance of the microscope using conventional and synchrotron sources will be compared. In this work we have applied the microscope to measure the three dimensional structure of fused bead packs and berea sandstones with micron resolution and have performed preliminary studies of flow in these media with the microscope operated in a digital subtraction radiography mode. Computer graphics techniques have been applied to the data to visually display the structure of the pore body system. Tomographic imaging after flow experiments should detect the structure of the oil-water interface in the pore network and this work is ongoing.Introduction. Since 1984 there has been an continuing development program in our laboratory for digital microradiography and program in our laboratory for digital microradiography and microtomography. The program has had as its principal objective the exploitation of synchrotron x-ray radiation to obtain high resolution full volume tomographic reconstruction of millimeter sized objects with micron resolution. A new high-resolution digital imaging x-ray detector was developed that produces the high quality radiographic images necessary for tomographic reconstruction. During the course of the development of this detector a number of other applications such as digital subtraction microradiography and digital subtraction microtomography have been implemented that utilize the detector's unique capabilities. The design and construction of a new synchrotron beamline has made possible two and three dimensional chemical microscopy using dual energy methods across absorption edges to compute maps of elemental distribution. The ability to rapidly collect resource that is unique in the petroleum industry and represents the most advanced of the few other research instruments of this type available today.DIGITAL MICRORADIOGRAPHY OVERVIEWX-ray source requirements. Differences in the x-ray absorption of materials form the basis for radiography experiments. For example, the difference between the absorption of bone and soft tissue generates the contrast seen in medical radiographs. Microradiography is the extension of this technique to small objects. As the test specimens become smaller radiography becomes more difficult for a number of reasons. In any radiograph, the specimen must absorb a sufficient portion of the incident beam to give a measurable signal either with film or an electronic detector. Optimum results are obtained when the specimen absorbs about 90% of the incident x-rays. For a small specimen to do this, the attenuation per unit length must be high. This condition is satisfied at low x-ray energies (5-30 keV). Complications arise because the attenuation per unit specimen length varies rapidly with energy in this region per unit specimen length varies rapidly with energy in this region as shown in Figure 1. Small changes in the specimen size cause large changes in the measured signal. Conversely, small changes in energy cause large changes in opacity. Monoenergetic x-ray sources are necessary to obtain an accurate measurement of small specimen attenuation. In addition, the spatial resolution in the radiograph is determined by the geometry of the x-ray source, sample and detector. Penumbral broadening due to the finite source size blurrs fine details in the microradiograph. Monochromatic, highly collimated, low energy x-rays are required to obtain quantitative high resolution results. Synchrotron x-ray sources provide such an x-ray source with very high brightness which allows collection of high quality radiographs with short exposure times. Conventional fixed-tube and rotating anode x-ray sources are also used but are far less intense at a few fixed energies which are difficult to make monochromatic.P. 423