Summary. This paper presents the use of X-ray tomography to inspect a series of laboratory core-displacement tests. Saturation profiles in thin longitudinal planes of view (slices) in the core are followed as a function of time. Immiscible and miscible displacements in single core plugs and butted composite plugs are presented. Effects of heterogeneity of the core, dispersion of flood fronts, and end effects are observed and discussed. The technique has a large potential to improve the understanding of fluid displacement processes in laboratory core experiments. Introduction Data from laboratory displacement tests on reservoir rock samples are widely used as input parameters in reservoir simulators. Questions on the validity of these data are frequently asked concerning sample heterogeneity, dimensions, end effects, analysis procedures, and conditions. One way to study these effects would be to follow the movement of the fluids inside the reservoir rock sample. Several methods are used to visualize or to quantify saturations during laboratory displacement experiments. Some of these are based on radioactive tracers or an external source, nuclear magnetic resonance, microwaves, and electrical properties. All these methods give only a one-dimensional presentation of saturation. Computerized tomography of X-rays for medical purposes was introduced in the 1970's. Because this tool gives a two-dimensional (2D) image of the interior of an object, it should also provide information on reservoir rock. In our evaluation of the technique and commercial scanners, we performed several displacement tests using different displacement methods. Some of these tests are presented in this paper. However, no systematic study was undertaken to analyze the different parameters' influence on displacement behavior. Principle of Computerized Tomography of X-Rays Radiation of high energy can penetrate material of moderate density, such as reservoir rock. The problem is to reconstruct a detailed image contained in the attenuated high-energy rays transmitted. In 1917, Radon showed mathematically how to unscramble the information in the attenuated energy rays. A geometric interpretation of Radon's theorem emerged in 1972 with a technique called computerized tomography (CT). The device is called a CT scanner. Any object of moderate density is placed between an X-ray source and an array of collimated detectors. The source and detectors are mounted on a "yoke" that rotates. Power is pulsed to the X-ray tube, creating a fan of beams that traverses a thin slice (I to 8 mm [0.04 to 0.3 in.]) of the object (Fig. 1). Between each power pulse the yoke moves, and the next X-ray fan beam traverses the same slice from a slightly different angle. As the yoke is rotated 6.3 rad [360 degrees], as many as 700,000 different X-ray projections are available for mathematical processing. A presentation of the mathematical treatment is found in Ref. 10. The mathematical processing, performed by a computer, gives a synthesized image. The basic synthetic unit is the volume element. The CT slice is composed of many volume elements, each with its own characteristic attenuation, which are displayed as a 2D image matrix of picture elements (pixels) (Fig. 2). Because X-ray attenuations are related to density, the CT image gives the density distribution within every point of the object scanned. Because it is impractical to deal with the X-ray attenuation coefficient, A, in CT scanning, a new relative scale was defined as (1) where mutc = calculated X-ray attenuation coefficientandmuH2O = X-ray attenuation coefficient for water. SPERE P. 148^

This paper was prepared for presentation at the 1999 SPE European Formation Damage Conference held in The Hague, The Netherlands, 31 May–1 June 1999.

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Summary This paper presents examples of xanthan corefloods visualized by X-ray tomography. Water and aqueous xanthan solutions are distinguished from each other by addition of sodium iodide (NaI) at different concentrations. One reservoir sandstone and one outcrop (Rosbrae) sandstone core samples were used. The reservoir sample was naturally divided into two longitudinal zones differing in permeability by about 20-fold. The Rosbrae sample was homogeneous, with a permeability of 450 md. Miscible xanthan/water displacement tests were performed on both plugs. Immiscible displacement of light refined oil by xanthan was performed on the homogeneous sample. Introduction Unrecoverable oil reserves for discovered fields in the North Sea are estimated to be about 3 × 10(9) m3. Improved oil recovery (IOR) offers some advantages over exploration activities. IOR studies are cheaper than drilling, implementation of IOR methods can use existing infrastructure, and the studies involve lower risk. Many North Sea reservoirs are stratified and have permeability contrasts. Injection of polymers can improve the areal sweep efficiency of the waterflood and thus increase the amount of oil recovered. This study was motivated by a field case evaluating the use of xanthan biopolymer. The aim of this work was to study biopolymer flood behavior in porous media by means of computer tomography (CT). CT has proved to be an excellent technique to visualize flow phenomena in porous media, especially in heterogeneous (layered) systems. Methods and Materials The CT scanner used was a Siemens Somatom DRH. The main scanning parameters were tube voltage, 125 kV; measuring time, 5 seconds; dose, 280 mAs; projections, 960; and slice thickness, 8 mm. The principles of CT and its applications in the oil industry have been described by several authors. The scanner is connected on line to a separate image-processing system, Context Vision GOP-300, to enable postprocessing on a higher level; for instance, filtering, color manipulation, contrast enhancement, and calculating fluid saturations and porosities. The coreholder was a Hassler type made of carbon fiber tube with polyamide end pieces pressure tested to 6 MPa. The two rock samples used were a North Sea stratified sandstone core and a homogeneous Scottish outcrop (Rosbrae) sandstone core. The reservoir core consisted of two longitudinal layers with a permeability contrast of 1:20. Table 1 summarizes the rock properties. The fluids were synthetic seawater, solutions of the biopolymer xanthan, glycerol, and light refined oil. The density difference between the xanthan solution and the seawater was less than 0.005 g/mL. In some floods, the fluids were doped with 40 or 200 g/kg NaI to increase the X-ray absorption to gain a better contrast between the fluids in the images. Note that whenever the polymer phase was doped, the dopant was added to the polymer solvent. Thus, the polymer itself was not traced. Table 2 gives the composition and fluid properties of the seawater. The xanthan was formulated in experimental batches at concentrations of 400 and 1,000 ppm with 500 glutaraldehyde added as a bactericide. Neither cell debris removal nor microgel filtration was performed. The viscosity was measured by a Contraves viscometer. Fig. 1 shows the rheological properties of the 400-and 1,000-ppm solutions. The samples were mounted in the coreholder with a net confining pressure of 2 MPa. All CT scans were made in a longitudinal plane of view from inlet to outlet (Fig. 2). All experiments were performed at room conditions. The injection flow rates were maintained at 4 mL/h unless otherwise stated. This flow rate corresponds to an overall apparent velocity of q/A =0.084 m/d.

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