Recent Advances in the Characterization of Porous Media Using Computer Assisted Tomography of X-rays

ABSTRACT

Computer Assisted Tomography (CAT) Scanning has been used as a tool for the development and/or testing of methodologies aimed at the non-destructive characterization of porous media. During this work over one hundred pieces of core were tested. The cores represent two heavy oil reservoirs (unconsolidated media), two sandstone reservoirs, one dolomitic sandstone, one limestone reservoir and over six dolomitic reservoirs (consolidated media). Several hundreds of calibration tests on reservoir rock and other samples were run. Some of the information was obtained using dual energy scanning. Experience on the operating procedures of the CAT scanners was obtained.

A computer package, which was developed in house, was used for the analysis of the x-ray images and the prediction of core properties such as bulk density, porosity and effective atomic number. The proposed methods were also used for characterization of reservoir heterogeneities such as vugs, fractures, shale, bedding planes, clay clasts, stromatoporoids, etc. Macroscopic accumulation of minerals such as anhydrite and sulphur was detected by the scanner. The accuracy of the method compared to conventional core analysis is quite satisfactory with the exception of very tight, very dense, full diameter samples. In terms of flow visualization the CAT scanner has been used for the determination of fluid saturations in immiscible and miscible displacement experiments with up to three phases present. Accuracy varies depending on the type of porous medium and fluids used.

Through the exploration that followed this technology in the recent years, a number of critical factors have not been given the appropriate consideration. These factors include proper calibration of the machines, proper determination of the associated errors at the pixel level for both porosity and fluid saturation measurements, drifting of the measurement due to changes in the x-ray source and the detectors, effects of dopants on CT number measurements and the limitation of dual energy scanning. Improvements on the technique could address increasing resolution and speed. A custom made core analysis CAT scanner could become a routine tool for core analysis. Complementary use of x-rays with some other source of energy would result in a better characterization of native core material.

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