NMR imaging of saturation during immiscible displacements

Abstract

The use of magnetic resonance imaging (MRI) for monitoring multiphase displacement experiments for quantitative characterization of fluid saturations is demonstrated. Displacements are conducted with one fluid phase in a porous medium being immiscibly displaced by another. Our objective is to accurately measure porosity and saturation distributions corresponding to one spatial dimension. Measures for the accuracy and resolution, with which the properties are identified, are developed. IntroductionNoninvasive imaging methods, such as X-ray CT scanning and magnetic resonance imaging (MRI), are providing exciting new opportunities for determining accurate rock characterizations and fluid-phase distributions corresponding to local regions within porous media. While much of the work reported for imaging methods have centered around the use of X-ray C T scanning (Potter and Groves, 1989;Vinegar, 1986), the versatility of MRI provides some unique opportunities for even more accurate and complete characterization of many properties associated with fluid-solid systems. Most of the work to date has dealt with static or nonflowing situations. When the porous media contain a single fluid phase, the void volume of the media can be characterized by imaging (Edelstein et al., 1988). MRI can also be used to characterize the distribution of one or more fluid phases in multiphase situations (Hall and Rajanayagam, 1987;Chen et al., 1988;Baldwin and Yamanashi, 1986;. In previous studies, fluid distributions have typically been expressed by a quantity proportional to the intensity of the signal. While these experiments demonstrate the use of imaging, they provide at best only a semiquantitative measure of rock properties and fluid-phase distributions. Porosity and fluid saturations, which are desired for quantitative analysis of fluid transport and storage in porous media, have been reported only as averages for whole samples (Edelstein et al., 1988), rather than resolved spatially in one or more dimensions.Several studies that deal with experiments involving transient multiphase flow in porous media have been reported (Baldwin and Yamanashi, 1986;Chen et al., 1988). Standard twodimensional (slice) imaging procedures were typically used. In some cases, such as the absorption of water by nylon (Blackband and Mansfield, 1986), the flow process is sufficiently slow so that the time required for imaging is not a critical issue. In other cases, the flow has been disrupted to perform the imaging under static conditions (Chen et al., 1988;Baldwin and Yamanashi, 1986). This generally is not desired for quantitative work since some redistribution of fluid phases is to be expected.In this work, we investigate the use of MRI for monitoring multiphase displacement experiments. In our experiments, one fluid phase is immiscibly displaced from a porous medium by a second fluid phase. Our objective is to accurately measure quantities that may be used to characterize the porous media and transport processes-specifically, the porosity and saturation p...

Saturation gradients in drainage of porous media: NMR imaging measurements

“…We also find that in our water-wet material, transverse relaxation in the nonwetting phase depends on local hydrocarbon saturation So, an observation difficult to make without an imaging experiment. Neglect of this variation cannot in general be correct, as recently pointed out by others (Chen et al, 1992b), although the assumption has been made in some previous work (Mandava et al, 1990).…”

“…This provides profiles that are already integrated over the specimen cross section, and overall data acquisition times are short. A similar approach has been taken by Mandava et al (1990) and Chen et al (1992a). Although conceptually straightforward, using this method for quantitative phase saturation profiles requires: (i) calibration of signal levels against a known standard; (ii) attention to instrumental factors; and (iii) quantification of signal loss by transverse relaxation.…”

NMR Characterizations of Properties of Heterogeneous Media

“…While the receiver gain may change only slightly with time during an experiment, the quality factor may change significantly (Abragam 1961;Mandava et al 1990). The specific magnetization intensity can be determined by placing a reference standard containing a known quantity of fluid into the imaged volume next to the sample .…”

Determining fluid saturations during multiphase flow experiments by NMR imaging techniques

“…It has been found that the volume-averaged porosities calculated from NMR image data match satisfactorily with the average porosities obtained using gravimetric methods. Although it is straightforward to extend the same approach to dynamic experiments (Mandava et al, 1990;Chen et al, 1992Chen et al, , 1993 for estimation of saturation distributions, the accuracy of such estimates has not been verified. Independent measurements are desired since there are several experimental conditions associated with dynamic experiments and low saturations which could compromise the accuracy of saturation determination.…”

“…Spin density imaging methods can be used to determine information concerning porosity and saturation distributions (Edelstein et al, 1988;Baldwin and Yamanashi, 1986;Mandava et al, 1990;Chen et al, 1992Chen et al, , 1993. Accurate quantitation of these data is difficult because of the characteristically short relaxation times of fluids in porous media and because of variation of relaxation properties with position and saturation.…”