Deformation and failure of rock samples probed by T1 and T2 relaxation.
The objective of the study was to pinpoint the effect of stress induced rock matrix alterations on NMR-wireline-log measurements by means of laboratory T1 and T2 relaxation time measurements. The research activities were subdivided into two major parts: NMR relaxation measurements on a brine saturated outcrop sandstone (Red Wildmoor Sandstone) during uniaxial compressional tests and NMR relaxation measurements on artificial sandstone samples prepared with defined crack patterns. T1-measurements performed on Red Wildmoor samples during compaction showed a decrease in the mean relaxation rate 1/exp<ln (T1) up to a point assumed to characterize the yield strength of the rock. The rate decreased most probably as a result of the compaction of micropores. This trend was reversed when the rock failed. The relaxation rate increased again, most probably through the generation of fresh mineral surfaces at broken cementations or grain contacts. T2-relaxation was measured on artificial sandstone samples using CPMG pulse sequences with echo times between 175 micros and 1400 micros. Samples with larger crack volumes and crack orientation orthogonal to the B0-field showed consistently smaller exp<ln (T2) than the samples without any cracks. Further calculations indicate that internal magnetic field gradients were largest for samples having a larger number of cracks and crack orientations orthogonal to the B0-field.
van der Zwaag
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SINTEF, N-7465 Trondheim, Norway. Claas.v.d.Zwaag@npd.no
Veliyulin
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Skjetne
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Lothe
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Holt
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Nes
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D016880:Anisotropy; D019015:Geologic Sediments / Q000032:analysis*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D013314:Stress, Mechanical