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Magnetic Resonance Imaging, Vol. 12, No. 2, pp. 215-216, 1994 Coovriaht 0 1994 Elsevier Science Ltd Prkedin the USA. All rights reserved

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l Short Communication

MRI OF A WATERFLOOD ON A RESERVOIR CHALK SAMPLE

DAN OLSEN

Geological Survey of Denmark, Thoravej 8, 2400 Copenhagen NV, Denmark

CSI of a waterflood experiment on a chalk sample from a North Sea hydrocarbon field has been performed. The method works well, giving separate maps of water and oil distributions. Difference images yield detailed informa- tion about fluid movements.

Keywords: Chalk; Chemical shift imaging; Fluid displacement; Difference image.

EXPERIMENTAL PROCEDURE Table 1. Petrophysical and NMR parameters

A 4.7 T Sisco imager with 31 cm bore (Sisco, Fremont, California) was used to analyze corefloods on reservoir chalk samples from North Sea hydrocarbon fields. The imager was equipped with an unshielded standard gra- dient set capable of producing gradients of 2 G/cm.

Sample id: TWC2-32 Sample shape: cylindrical Diameter: 3.8 cm K water = 6.3 mD @ 22”C, 20 ml/h * = 45.9% T, = 0.65 s @ S, = 100% T, = 15.9 ms @ S, = 100% Image slice thickness = 6.0 mm Image size 128 x 128 pixels Image resolution X-direction = 0.78 mm Image resolution Y-direction = 0.47 mm

Field: Tyra

Length: 6.4 cm

Chemical shift imaging (CSI) was performed with a 2D spin echo pulse sequence, utilizing a gauss-shaped 90” pulse, with a bandwidth of 667 Hz for discrimina- tion of the signal to be observed. Separation between oil and water peaks was 806 Hz, and linewidths were between 65 and 147 Hz, enabling imaging with little mutual interference between oil and water.

A sample of soft, friable, Maastrichtian chalk with low clay contents was selected. Before cleaning, a pre- sumably primary lamination, and many healed hairline fractures were visible. After cleaning, the sample looked homogeneous, apart from a few trace fossils. Petro- physical and NMR parameters are given in Table 1. The sample may be considered as representative for the res- ervoir chalk of the Tyra field.

quisitions. Each acquisition period lasted approximately 2 h. Fluid flow was stopped during acquisition periods. Repeat acquisition of images after 110 min of non-flow shows negligible fluid movement, even in regions of strong saturation gradients.

RESULTS

Cleaning was performed by miscible cold flushing. The sample was then embedded in epoxy together with end pieces and fittings, Saturation to an S, of 100% was accomplished by flooding, and finally an S, of 3 1.6% was attained by flooding with Isopar L oil (Ex- xon Chemicals). Saturation was determined by mass balance calculation.

Bulk spectra show good separation of water and oil peaks (not shown). Water saturation determinations from the spectra show good correlation to determina- tions from production data, but are systematically 3- 5%-points lower.

The sample was mounted in the imager, and con- nected to a metering pump. A waterflood was con- ducted at a flowrate of 30 ml/h, giving a mean speed of 138 cm/day for the fluids in the sample.

Bulk spectra and CSI images were acquired during six acquisition periods, including initial and final ac-

The CSI images show a number of lithological fea- tures, including layering, a trace fossil and hairline frac- tures (not shown).

The most informative display of the progression of the waterflood was obtained from difference images (Fig. 1). They were produced by pixelwise subtraction of intensities in adjacent images. Thus, difference im- age l-2 presents the difference in intensity between im-

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216 Magnetic Resonance Imaging 0 Volume 12, Number 2, 1994

Diff. l-2 APV= 0.09

Diff. 2-3 APV= 0.17

Diff. 3-4 APV= 0.18

Diff. 4-5 APV= 0.35

Diff. 5-6 APV= 8.82

Fig. 1. Difference images displaying progression of a water- flood. APV = Pore Volume fluid injected between acquisi- tions. Inlet is at left.

ages from acquisition periods 1 and 2. The presented difference images only display pixels with increasing water signal.

A series of difference images displaying pixels with decreasing water signal displays no signal at all in the sample (now shown). This absence of signal implies, that water saturation was never decreasing in any part of the sample during the experiment. It also implies, that the imager was stable during the experiment.

Important information is gained from the difference images:

A distinct waterfront is seen until acquisition 4. Af- terwards no front is seen. Breakthrough occurred shortly after acquisition 4. Irregularities in the front at acquisition 2 are attributed to layering and trace fossils.

A hairline fracture strongly influenced the displace- ment, see arrow Hl on Fig. 1. The waterfront crossed the lower part of the hairline unimpeded, but was stopped along the upper part, see difference image 3-4. Later in the experiment S, was rising in the shadow zone behind the hairline, but images in a transverse section (not shown) indicate, that this oc- curred by circumvention of the blockade rather than by crossing. The oil saturation in the right part of the sample re- mained high, indicating the operation of strong cap- illary end effects, and/or channelling through open fractures.

CONCLUSIONS

A displacement experiment conducted in an NMR imager resulted in a comprehensive and consistent data set comprising:

1.

2.

3.

4.

Bulk saturation determinations. Lithological characterization of structures inside the sample. Separate images of water and oil distributions, which may be used for semiquantitative saturation determinations in regions of interest. Detailed description of fluid movements during the experiment.

The experimental procedure utilizing intermittent flow resulted in fluid distributions that were stable during acquisition periods.

Acknowledgmenrs-The work was supported by EC grant no. JOUF-

0019-C under the Joule programme. The NMR work was conducted

at Danish Research Center of Magnetic Resonance.