Equations have been developed that relate induced polarization (IP) in shaly sands to measurable petrophysical parameters. The induced‐polarization process has been modeled in terms of two mechanisms: clay counterion displacement and membrane blockage. The resulting equations can be used to determine shaliness, brine conductivity, and oil saturation from in‐phase and out‐of‐phase conductivities. Laboratory measurements have confirmed the IP dependence on these variables, as well as on temperature.
Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Summary. Computerized tomography (CT) is a new radiological imagingtechnique that measures density and atomic composition inside opaque objects. Arevolutionary advance in medical radiology since 1972, CT has only recentlybeen applied in petrophysics and reservoir engineering. This paper discussesseveral petrophysical applications, including three-dimensional (3D)measurement of density and porosity; rock mechanics studies; correlation ofcore logs with well logs; characterization of mud invasion, fractures, anddisturbed core; and quantification of complex mineralogies and sand/shaleratios. Reservoir engineering applications presented include fundamentalstudies of CO2 displacement in cores, focusing on viscous fingering, gravitysegregation, miscibility, and mobility control. Introduction X-ray CT is a radiological imaging technique first developed in GreatBritain in 1972 by Hounsfield. CT revolutionized medical radiology by producinganatomical images of extraordinary accuracy and clinical detail. Hounsfield wasawarded the Nobel prize in medicine in 1979 for his contributions. Tounderstand the advantages of CT, first consider conventional X-ray radiography, e.g., chest X-rays. Conventional radiographs view an object from only one angleso that shadows from all irradiated matter along a ray path are superimposed onone another (Fig. 1). The attenuation information along the ray path isaveraged together so that localized regions with small attenuation contrast areobscured. By comparison, CT scanners generate cross-sectional image slicesthrough the object by revolving an X-ray tube around the object and obtainingprojections at many different angles (Fig. 2). From a set of these projections, a cross-sectional image is reconstructed by a back-projection algorithm in thescanner's computer. The cross-sectional image of attenuation coefficients isdisplayed on a cathode-ray-tube (CRT) monitor. The beauty of CT is thatattenuation differences as small as 0.1% can be measured accurately within aninterior region of 2 mm(2) or less. With a fourth-generation medical scanner, the entire imaging process is completed in seconds. CT images (3D) can bereconstructed from sequential cross-sectional slices taken as the sample ismoved through the scanner (Fig. 3). Once this 3D data set has been acquired, any plane through the object can be viewed. For example, Figs. 4A and 4B showhow vertical and horizontal slices can be used to separate gravity and viscouseffects during tertiary gas injection. Instrumentation CT scanners have undergone considerable development since 1972. First-generation scanners used a single pencil-beam source and detectorarrangement. Second-generation scanners improved image quality by use ofmultiple detectors in a translate/rotate configuration. A large improvement inspeed occurred in the third-generation scanners, which used a rotate-onlyfan-beam geometry with source and detectors rotated together around the object. Finally, the fourth-generation scanners use a fan-beam geometry with sourcerotating within a fixed ring of high-efficiency detectors. The second- throughfourth-generation medical CT scanners are satisfactory for petroleumengineering applications because they have adequate X-ray energy and dose forscanning core material. Used medical CT scanners are readily available at asmall fraction of the original cost. JPT P. 885^
The medical x-ray computerized tomography (CT) scanner has proved to be a useful tool for studies of fluid flow in porous media, with particular applications in reservoir engineering and enhanced oil recovery. This paper explains how CT is used to measure the volume fraction of pore space occupied by up to three discrete phases, such as oil, water, and gas. The image processing system, x-ray transparent high-pressure flow equipment, choice of fluid dopants, and x-ray energies are described for scanning of core flood experiments. Examples are given of tertiary miscible carbon dioxide displacements in Berea sandstone.
Introduction In little more than a decade, X-ray computerized tomography (CT) and nuclear magnetic resonance (NMR) imaging have become the premier modalities of medical radiology. Both of these imaging techniques also promise to be useful tools in petrophysics and reservoir engineering, because CT and NMR can nondestructively image a host of physical and chemical properties of porous rocks and multiple fluid phases contained within their pores. The images are taken within seconds to minutes, at reservoir temperatures and pressures, with spatial resolution on the millimeter and submillimeter level. The physical properties imaged by the two techniques are complementary. CT images bulk density and effective atomic number. NMR images the nuclide concentration, Mo, of a variety of nuclei ( H, 19-F, 23-Na, 31-P, etc.), their longitudinal and transverse relaxation-time curves (t1 and t2 ), and their chemical shift spectra. In rocks, CT images both rock matrix and pore fluids, while NMR images only mobile fluids and the interactions of these mobile fluids with the confining surfaces of the pores. Principles CT uses an X-ray source that rotates around the sample to obtain one-dimensional projections of X-ray attenuation at different angles. From these projections, a crosssectional slice through the sample is reconstructed by computer. Finally. a three-dimensional image is reconstructed from sequential cross-sectional slices taken as the sample is moved through the scanner. To obtain both density and effective atomic number, CT images are taken at two X-ray energies. One energy is high enough for the X-rays to be predominantly Compton-scattered and one is low enough for them to be mostly photoelectrically absorbed. A combination of the two images in the computer. on a pixel-by-pixel basis, generates separate images of bulk density and effective atomic number. In NMR imaging, the sample is placed inside an intense, homogeneous, magnetic field, one preferably generated by a superconducting magnet. Radio frequency (RF) coils apply RF magnetic fields at the precise Larmor frequency to cause the particular nuclear species to precess. The RF coils then detect the signals generated by the precessing nuclei. Spatial localization within the sample is obtained by x. y, and z magnetic-field gradient coils that cause nuclei in different parts of the sample to precess at slightly different frequencies. The resulting frequency-modulated signal is Fourier-transformed to obtain the spatial distribution of nuclear-spin density. Unlike CT, an image is obtained of the entire sample volume within the RF coil. JPT P. 257^
The massive size of the oil shale resource in the Western USA, particularly in the Green River Basin, has attracted numerous commercialization attempts from industry over the last 100 years. Although great sums of money have been invested and many professional careers have been devoted to the challenge, efforts thus far have not resulted in a commercial oil shale industry. For more than 40 years, Shell has been active in the Green River oil shale seeking a process that could develop the 800 billion bbls of oil resource (RAND Corporation). Since 1980 Shell has focused on the In-situ Conversion Process (ICP) in which oil shale is heated by thermal conduction from a closely spaced array of electric resistance heaters. At approximately 650oF, the kerogen present in the oil shale is converted to oil, gas and water that can be produced by conventional means. Although the process is subject to mining statutes and regulations in the State of Colorado, no traditional mining is required. This paper provides a brief history of attempts to commercialize oil shale in the United States and an overview of Shell's ICP technology, including its 1940s Swedish oil shale roots. It focuses on Shell's seven field pilots conducted in Colorado that address ICP recovery, heater testing, and freeze wall construction and performance.
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