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As oil companies try to drain aging reservoirs more efficiently, engineers and geoscientists may come to regret earlier decisions to forgo coring. Once a well has been drilled through a pay zone, it is too late to go back to obtain whole cores, unless the well is sidetracked. However, mineralogy, grain size, saturation, permeability, porosity and other measures of rock fabric can sometimes be determined without coring. With improvements on the early medical CAT- scan technique developed in 1972, geoscientists can take a series of fine, closely spaced X-ray scans through a rock sample to obtain important information about a reservoir.1


Using a


nondestructive technique called microcomputed tomography, a focused X-ray beam creates


“ virtual slices” that can be resolved to a scale of microns, not just millimeters.2


These refinements


also allow the option of examining smaller samples of rock; instead of depending on whole cores for porosity and permeability measure- ments, geoscientists can now use formation cuttings to estimate these properties.3


Although


many companies do not core their wells, they usually employ the services of a mudlogger to catch formation cuttings as they come over the shale shaker. When they don’t have core, geoscientists are finding that a sliver of rock can be highly revealing.


This article reviews the development of X-ray computed tomography (CT) and the ensuing technology transfer from medical to oilfield applications. We describe how the data can be evaluated using immersive visualization tech- niques and discuss a range of oilfield applications that may benefit from this technology. Finally, we will see how this technology served researchers in their evaluation of casing cement and well stimulation treatments.


CT Scan Technology


Originally developed for medical use by Godfrey Newbold Hounsfield in 1972, computed tomog- raphy uses X-ray scans to investigate internal structures within a body, such as those of soft tissue and bone.4


CT overcomes the problem of


superimposition exhibited in conventional X-ray radiography when three-dimensional features of internal organs are obscured by overlying organs and tissues imaged on two-dimensional X-ray film.


Rather than projecting X-rays through a patient and onto a film plate, as with conventional X-rays, the CT process takes a different approach. The CT scanner uses a rotating gantry


to which an X-ray tube is mounted opposite a detector array. The patient is


1. In the m edical  eld, the com puterized ax ial tom ography ( CAT) scan is som etim es also called com puter-assisted tom ography , and is sy nony m ous w ith com puted tom ography .


2. A m icron, or m icrom eter, is eq ual to one m illionth of a


m eter, or m ore com m only , one thousandth of a m illim eter. It is ab b reviated as µ , µ m or m c. In English m easure, a m icron eq uals 3 . 9 3 7 x 10-5 in.


3 . Siddiq ui S, Grader AS, Touati M, Loerm ans AM and Funk J J : “ Techniq ues for Ex tracting Reliab le Density and Porosity Data from Cuttings, ” paper SPE 9 69 18, presented at the SPE Annual Technical Conference and Ex hib ition, Dallas, Octob er 9 – 12, 2005 .


Bauget F, Arns CH, Saadatfar M, Sheppard AP, Sok RM, Turner ML, Pinczew sk i WV and K nack stedt MA: “ What


is the Characteristic Length Scale for Perm eab ility ? Direct Analy sis from Microtom ographic Data, ” paper SPE 9 5 9 5 0, presented at the SPE Annual Technical Conference and Ex hib ition, Dallas, Octob er 9 – 12, 2005 .


4 . Houns eld GN: “ A Method of and Apparatus for Ex am ination of a Body b y Radiation such as X - or Gam m a Radiation, ” British Patent No. 1, 283 , 9 15 ( August 2, 19 7 2) .


5 . For m ore on X -ray CT: Pub lication Services Departm ent of the ODP Science Operator. http: / / w w w - odp. tam u. edu/ pub lications/ 185 _ SR/ 005 / 005 _ 5 . htm ( accessed J anuary 27 , 2006) .


6. Feldk am p LA, Davis LC and K ress J W: “ Practical Cone-Beam Algorithm , ” J ournal of the Optical Society of Am ericaA1, no. 6 ( J une 19 84 ) : 612– 619 .


placed in the center of the gantry, while the opposing X-ray source and detectors rotate around the patient. With the patient positioned roughly in the middle of the source-receiver plane, the rotating gantry allows a series of closely spaced radiographic scans to be obtained from multiple angles. These scans, or radiographic projections, can then be processed to obtain a 3D representation of the patient (below).


CT radiographic projections are based on the differential attenuation of X-rays caused by density contrasts within a patient’s body. This


patient from this equation, attenuation is a function of the energy of the X-ray as well as the density and atomic number of


the elements


through which the X-ray passes. The correlation is fairly straightforward: lower-energy X-rays, higher densities and higher atomic numbers generally result in greater attenuation.5 Digital projection data are converted into a


computer-generated image using tomographic- reconstruction algorithms to map the distribu- tion of attenuation coefficients.6


This distribution


can be displayed in 2D slices, composed of points that are shaded according to their attenuation


> Thoracic CAT scan. Manipulating color and opacity values of different tissues provides phy sicians w ith an unob structed view of a patient’ s lungs and sk eletal sy stem . ( Im age courtesy of Aj ay Lim ay e, VizLab , The Australian National University . )


attenuation represents a decrease in energy as X-rays pass through various parts of the body. Some tissues scatter or absorb X-rays better than others: thick tissue absorbs more X-rays than thin; bone absorbs more X-rays than soft tissue, while fat, muscle or organs allow more X-rays to pass through to the detectors. Removing the


values (see “ Moving from 2D Points to 3D Volumes,” page 6). Thus, in hospital scans, bone would typically be assigned a light color to correspond with its comparatively high attenuation value, while air-filled lung tissue might be assigned a darker color corresponding to low attenuation values.


Spring 2006 5


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