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Effect of sub-core scale heterogeneities on acoustic and electrical properties of a reservoir rock: a CO2 flooding experiment of brine saturated sandstone in a computed tomography scannerNormal access

Authors: B.L. Alemu, E. Aker, M. Soldal and Ø. Johnsen
Journal name: Geophysical Prospecting
Issue: Vol 61, No 1, January 2013 pp. 235 - 250
DOI: 10.1111/j.1365-2478.2012.01061.x
Organisations: Wiley
Language: English
Info: Article, PDF ( 2Mb )

Summary:
The effect of sub-core scale heterogeneity on fluid distribution pattern, and the electrical and acoustic properties of a typical reservoir rock was studied by performing drainage and imbibition flooding tests with CO2 and brine in a laboratory. Moderately layered Rothbach sandstone was used as a test specimen. Two core samples were drilled; one perpendicular and the other parallel to the layering to allow injection of fluids along and normal to the bedding plane. During the test 3D images of fluid distribution and saturation levels were mapped by an industrial X-ray CT-scanner together with simultaneous measurement of electrical resistivity, ultrasonic velocities as well as amplitudes. The results showed how the layering and the flooding direction influenced the fluid distribution pattern and the saturation level of the fluids. For a given fluid saturation level, the measured changes in the acoustic and electrical parameters were affected by both the fluid distribution pattern and the layering orientation relative to the measurement direction. The P-wave amplitude and the electrical resistivity were more sensitive to small changes in the fluid distribution patterns than the Pwave velocity. The change in amplitude was the most affected by the orientation of the layering and the resulting fluid distribution patterns. In some instances the change due to the fluid distribution pattern was higher than the variation caused by the change in CO2 saturation. As a result the Gassmann relation based on ‘uniform’ or ‘patchy’ saturation pattern was not suitable to predict the P-wave velocity variation. Overall, the results demonstrate the importance of core-imaging to improve our understanding of fluid distribution patterns and the associated effects on measured rock-physics properties.

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