Effects of RF attenuation in magnetic resonance imaging on quantitative measurements in oil cores

JOURNAL

OF MAGNETIC

RESONANCE

95, 574-580

( 199 1)

Effects of RF Attenuation in Magnetic ResonanceImaging on Quantitative Measurements in Oil Cores M. A.

ROBINSON

Chemical Engineering Department, University of Wyoming, Laramie, Wyoming 82071 Received

February

26, 199 1; revised

June

3, 199 1

Although several papers have been written about the use of magnetic resonance imaging to study porosity and saturation distributions in oil cores (l-6), there has never been a detailed investigation into the effects of RF attenuation (RF inhomogeneity) on the numerical results. Instead, the authors assume that the effects of RF attenuation are negligible because such effects are not apparent in the images. However, in order to estimate the error of MRI measurements of porosity and saturation, a quantitative determination of RF attenuation effects is desired. In addition, if the error due to RF attenuation is large, corrections are required in order to obtain accurate saturations and porosities. This paper presents the results of studies investigating the effect of RF attenuation on a sample of pure fluid such as water or mineral oil. Such results are useful in understanding what factors can contribute to RF attenuation inside oil cores. In addition to these results, an approach is presented which quantitatively shows the effect of RF attenuation inside two brine-saturated oil cores: Indiana limestone and Berea sandstone. RF attenuation can be attributed to two properties of the imaged fluid: its dielectric constant and its electrical conductivity ( 7). An artifact pattern of RF attenuation often appears in images of bottles of water, and Clover et al. have shown that the artifacts become more pronounced as the conductivity increases or the dielectric constant decreases. In this paper, dielectric effects are ignored, while the effect of varying the conductivity of the sample is investigated. Since RF attenuation increases when the sample inside the imager has high conductivity, RF attenuation should be very large in an ionic fluid, but much smaller for a nonpolar fluid. However, the effect of RF attenuation is less clear for fluid inside a porous sample. If the bulk fluid has high conductivity, but a core saturated with the same fluid has high resistivity (low conductivity), what are the effects of RF attenuation? This paper attempts to answer this question. Several experiments investigating conductivity effects were performed. For the bulk fluids, the experiments involved making an image of the fluid and then calculating the average signal and its standard deviation across the image. A percentage error was then calculated by dividing the standard deviation by the average signal. All experiments were performed on a General Electric CSI Omega 2 T imager, operating at 85 MHz proton frequency. Acustar shielded gradients and a bird-cage 0022-2364191

$3.00

Copyright 0 1991 by Academic Press, Inc. All n&s of reproduction in any form reserved.

574

575

NOTES TABLE 1 Imaging Parameters Sample

Experiment number Number of averages Recycle time (s) Number of slices Axial field of view (mm) Image field of view (mm) Image resolution Echo time (ms)

Pure fluid

Indiana limestone

Berea sandstone

1-8 2 2 8 150 90 128 32

9 2 1.5 32 100 15 128 1.5

10 2 4 64 128 64 64 1.2

RF coil were used. The use of a bird-cage coil is recommended, because it has an RF field uniformity which is significantly greater than that of a saddle coil (8). A 3D FT pulse sequence was used to acquire the data. The parameters for the experiments are shown in Table 1. Eight experiments were performed to test the effects of fluid conductivity, container composition, and positioning material on the magnitude of RF attenuation. The fluids were CuS04-doped water for high conductivity (resistivity = 6.0 Q m) and mineral oil for low conductivity (resistivity = cc ) . Two containers, a 250 ml glass bottle and a 250 ml polyethylene bottle, were used. Both had the: same inner diameter of 59.3 mm. The sample was positioned inside the magnet using either a piece of foam underneath the sample or a cylindrical nylon sample positioner. Table 2 shows the various combinations which were tested, as well as the calculated error due to RF attenuation. Figure 1 shows the images for CuS04-water, while Fig. 2 shows the images for mineral oil. The gray-scale variations present in the images are indicative of RF attenuation. Clearly, these results would require correction if any quantitative information were desired from these images. The errors range from 7.4% for experiment 5 to 17.7% for

TABLE 2 Bulk Fluid RF Attenuation Measurements Exp.

Fluid

Sample container

Positioning material

Error

I 2 3 4 5 6 I 8

CuS04-water CuSO1-water CuS04-water CuS04-water Mineral oil Mineral oil Mineral oil Mineral oil

Glass Glass Polyethylene Polyethylene Glass Glass Polyethylene Polyethylene

Foam Nylon Foam Nylon Foam Nylon Foam Nylon

10.0 11.9 12.2 17.7 7.4 8.6 9.9 11.7

NOTES

576

a)

m

FIG. 1. Images of CuS04doped water. (a) Experiment 1: glasscontainer, foam positioner; (b) experiment 2: glasscontainer, nylon positioner; (c) experiment 3: polyethylene container, foam positioner; (d) experiment 4: polyethylene container, nylon positioner.

experiment 4. The variation of the errors indicates that visual inspection is inadequate for estimating the magnitude of RF attenuation. From these results, several conclusions can be drawn. First, the experiments clearly show that mineral oil is affected much less by RF attenuation than the CuS04-water solution (7.4% vs lO.O%, 8.6% vs 11.9%, 9.9% vs 12.2%, and 11.7% vs 17.7%). These results support the theory presented in the paper by Glover et al. ( 7). Second, the sample container has an effect on the magnitude of RF attenuation. The results show that a glass container exhibits less RF attenuation than a polyethylene

FIG. 2. Images of mineral oil. (a) Experiment 5: glass container, foam positioner; (b ) experiment 6: glass container, nylon positioner; (c) experiment 7: polyethylene container, foam positioner; (d) experiment 8: polyethylene container, nylon positioner.

NOTES

577

bottle of the same dimensions. This effect is probably due to the insulating capacity of glass and the fact that the wall thickness for the glass (2.68 mm) was more than that for the polyethylene bottle ( 1.12 mm). Third, the material used to position the sample also aflects the image results. Our data show that using foam underneath the sample to place it near the center of the RF coil is better than using the nylon sample positioner. This effect may be due to the insulating properties of air. Surrounding the sample by nylon may provide a path for RF attenuation which is negligible through air. Indiana limestone and fired Berea sandstone were chosen as the oil core samples, because both were known to be highly homogeneous and thus more likely to yield images free of porosity variations. The cores were prepared by vacuum-saturating them with brine and then dipping them in hot wax to seal them. They were positioned in the magnet using a piece of foam, which was the best material indicated in the experiments on bulk fluids. The parameters that were used in the pulse sequence are shown in Table 1 for experiments 9 (Indiana limestone) and 10 (Berea sandstone). Figure 3 shows an image of the Indiana limestone sample. The dark region at the bottom is an area where the wax broke away from the core, leaving a thin layer of water. RF attenuation appeared insignificant in the image (the resistivity of the saturated core was 110 Q m), but visual inspection was not believed adequate for estimating the error. In addition, an effect due to porosity variation was present and needed to be eliminated or controlled in order to determine the error value. Since Indiana limestone is homogeneous on a large scale, it was concluded that averaging several points together should eliminate porosity variation in the sample. Random subsets of the data were utilized to determine the error from the variation of the average signal of each of these subsets. Thirty random subregions were chosen within the image, and an average signal for that subregion was determined. The thirty averaged signals were then used to obtain the standard deviation and error for the image. Four different-sized subregions ( 6 X 6, 10 X 10, 16 X 16, and 20 X 20) were compared and the results are shown in Fig. 4. As revealed in Fig. 4, the errors decreased monotonically to a minimum of 3.1%, for box sizes larger than 16 X 16. Thus, it was

FIG.

3. Image of Indiana limestone.

NOTES

578

0

100

Subregion

200

300

Size (pixels)

FIG. 4. RF penetration in Indiana limestone.

concluded that subregions of this size effectively eliminated porosity variations, and this error was the true error due to RF attenuation. Therefore, for this image of Indiana limestone, the error is only 3.1%, which is an acceptable value for measurements of porosity and saturation inside an oil core. For an oil core with a porosity of 20%, this corresponds to an error of 0.62% in the measured porosity, and for oil cores with lower porosities, the error is proportionately less. The Berea sandstone had a lower resistivity ( =5.68 Q m) than the Indiana limestone, about the same as the resistivity of the CuS04-doped water. Thus, a higher level of RF attenuation was expected. An image of the Berea sandstone is shown in Fig. 5. The effect of RF attenuation is apparent, but again, no quantitative information is available directly from the image. However, using the method described above, with

FIG.

5. Image of Berea sandstone.

579

NOTES

lldifferentsubregions(lX1,2X2,4X4,6X6,8X8,10X10,12X12,14X14, 16 X 16, 18 X 18, and 20 X 20), it is possible to obtain an estimate of the RF attenuation effect. The results, shown in Fig. 6, indicate that the error due to RF attenuation in this sample is approximately 4.5%. For a core with an average porosity of 20%, this corresponds to an error of almost one porosity unit. In many cases, this would be unacceptably high, and corrections would be required. Alternatively, the effect of RF attenuation could be reduced by saturating the core with decane or some other nonconducting fluid. RF attenuation can create problems during the imaging of bulk fluids, especially if the fluid has high conductivity. In addition, the material surrounding the fluid (both container and positioner) can enhance or diminish the effects of RF attenuation; it is desirable to use as little material as possible for containing and positioning the sample. If quantitative measurements are desired on bulk fluid images or systems of high conductivity, corrections are necessary in order to account for RF attenuation. In addition, one could perform the experiments using quadrature transmission and reception, as suggested in the paper by Glover et al. ( 7). Such a method, while not completely eliminating RF attenuation, would improve the uniformity of the field over that of the linear polarization method. RF attenuation was not a problem for the Indiana limestone core. The tortuosity through the core (as measured by its high resistivity and low overall conductivity) effectively shields the core from RF attenuation. These results indicate that corrections are not required to account for RF attenuation when quantitative measurements of porosity, saturation, or other values in Indiana limestone are made. In addition, the use of quadrature detection is not indicated for this oil core sample. RF attenuation was noteable in the Berea sandstone sample, causing an error of 4.5%. This effect can partly be attributed to the higher conductivity of the Berea, which

0

100

Subregion FIG.

6. RF penetration

200

Size

300

(pixels)

in Berea sandstone.

400

NOTES

580

is caused by its higher permeability and porosity as compared to those of the Indiana limestone. Corrections to the data are indicated for this sample. The method described above can be used to quantify the effect of RF attenuation inside homogeneous oil cores. Since the magnitude of RF attenuation can increase with object size ( 7), it is suggested that this method be applied to a large-diameter homogeneous oil core to estimate the maximum error which can occur. The core should be chosen to have the maximum conductivity expected in the experiments to be performed. Through such a measurement, two goals are accomplished: an upper limit of the error in porosity and saturation values is obtained, and one can establish whether corrections need to be made in order to account for RF attenuation. ACKNOWLEDGMENTS This material is based upon work supported under a National Science Foundation Graduate- Fellowship. The author thanks Rob Satchwell for valuable discussions during the preparation of the manuscript. REFERENCES P. ROTHWELL AND H. J. VINEGAR, Appl. Opt. %I,3969 ( 1985). BLACKBAND et al., SPE Form. Eval. I,3 1 ( 1986). J. VINEGAR, J. Pet. Technol. 38,257 ( 1986). A. BALDWIN, SPEIDOE 14,884, 39 (1986). D. HALL AND V. RAJANAYAGAM, J. Magn. Reson. 14, 139 ( 1987). A. HORSRELD, C. HALL, AND L. D. HALL, J. Magn. Reson. 87, 319 (1990). H. GLOVER, C. E. HAYES, N. S. PELC, W. A. EDELSTEIN, 0. M. MUELLER, H. R. HART, C. J. HARDY, M. O’DONNELL, AND W. D. BARBER, J. Magn. Reson. 64,255 ( 1985). 8. C. E. HAYES, W. A. EDELSTEIN, J. F. S~HENCK, 0. M. MUELLER, AND M. EASH, J. Magn. Reson. 63, 1. 2. 3. 4. 5. 6. 7.

W. S. H. B. L. M. G.

622 (1985).