Abnormal shear wave polarizations as indicators of high pressures and over pressures

Crampin, Stuart; Zatsepin, S. V.; Slater, Carl; Brodov, L. Y.

doi:10.3997/2214-4609.201408674

Cited by 20 publications

(14 citation statements)

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“…APE shows that such polarization changes are characteristic of overpressured reservoirs. Similar large time‐delays and polarization changes have been observed previously in an overpressured reservoir (Crampin et al 1996; Slater 1997), and in the fault gouge of the San Andreas Fault where high pore‐fluid pressures are expected (Liu et al 1997).…”

Section: Outline Of Phase VIsupporting

confidence: 81%

“…This produces a ‘negative anisotropy’ where the shear‐wave polarized parallel to the maximum horizontal stress direction is slower than the perpendicularly polarized wave. A similar observation of a negative anisotropy was made in an overpressurized reservoir by Crampin et al (1996) based on the analysis of Slater (1997). The shear‐wave difference plot, Fig.…”

Section: Discussionsupporting

confidence: 68%

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Processing, modelling and predicting time-lapse effects of overpressured fluid-injection in a fractured reservoir

Angerer

Crampin

et al. 2002

Geophysical Journal International

116

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Summary Time‐lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs. We analyse two 4‐D, 3C onshore surveys from Vacuum Field, New Mexico, USA, where the reservoir of interest is a fractured dolomite. In Phase VI, a time‐lapse survey was acquired before and after a pilot tertiary‐recovery programme of overpressured CO2 injection, which altered the fluid composition and the pore‐fluid pressure. Phase VII was a similar time‐lapse survey in the same location but with a different lower‐pressure injection regime. Applying a processing sequence to the Phase VI data preserving normal‐incidence shear‐wave anisotropy (time‐delays and polarization) and maximizing repeatability, interval‐time analysis of the reservoir interval shows a significant 10 per cent change in shear‐wave velocity anisotropy and 3 per cent decrease in the P‐wave interval velocities. A 1‐D model incorporating both saturation and pressure changes is matched to the data. The saturation changes have little effect on the seismic velocities. There are two main causes of the time‐lapse changes. Any change in pore‐fluid pressures modifies crack aspect ratios. Additionally, when there are overpressures, as there are in Phase VI, there is a 90° change in maximum impedance directions, and the leading faster split shear wave, instead of being parallel to the crack face as it is for low pore‐fluid pressures, becomes orthogonal to the crack face. The anisotropic poro‐elasticity (APE) model of the evolution of microcracked rock, calculates the evolution of cracked rock to changing conditions. APE modelling shows that at high overburden pressures only nearly vertical cracks, to which normal incidence P waves are less sensitive than S waves, remain open as the pore‐fluid pressure increases. APE modelling matches the observed time‐lapse effects almost exactly demonstrating that shear‐wave anisotropy is a highly sensitive diagnostic of pore‐fluid pressure changes in fractured reservoirs. In this comparatively limited analysis, APE modelling of fluid‐injection at known pressure correctly predicted the changes in seismic response, particularly the shear‐wave splitting, induced by the high‐pressure CO2 injection. In the Phase VII survey, APE modelling also successfully predicted the response to the lower‐pressure injection using the same Phase VI model of the cracked reservoir. The underlying reason for this remarkable predictability of fluid‐saturated reservoir rocks is the critical nature and high crack density of the fluid‐saturated cracks and microcracks in the reservoir rock, which makes cracked reservoirs critical systems.

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Section: Outline Of Phase VIsupporting

confidence: 81%

Section: Discussionsupporting

confidence: 68%

Processing, modelling and predicting time-lapse effects of overpressured fluid-injection in a fractured reservoir

Angerer

Crampin

et al. 2002

Geophysical Journal International

116

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“…Stress-perpendicular polarisations caused by high pore-fluid pressures were first recognised in a vertical-seismicprofile (VSP) in a reservoir in the Caucasus Oil Field [55]. The most definitive observation of 90 • -flips has been in fluid-injection experiments in Vacuum Field, New Mexico by the Reservoir Characterisation Project of the Colorado School of Mines [56].…”

Section: • -Flips In Exploration Seismologymentioning

confidence: 99%

A review of shear-wave splitting in the compliant crack-critical anisotropic Earth

Crampin¹,

Peacock²

2005

Wave Motion

164

111

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“…Such 90j flips have previously been observed in vertical seismic profiles in a critically pressurised reservoir in the Caucasus Oil Field (Crampin et al, 1996;Slater, 1997) and in reflection surveys (Angerer et al, 2000(Angerer et al, , 2002 of a critically pressurised CO 2 injection. These 90j flips have also been observed above small earthquakes immediately above the San Andreas Fault in California by Liu et al (1997), who recognised the significance of 90j flips, and also by Peacock et al (1988) and Crampin et al (1990Crampin et al ( , 1991, although at that time, the significance of the 90j flips had not been established.…”

Section: J Flips In Shear-wave Polarisationsmentioning

confidence: 84%

Shear-wave splitting in a critical crust

Crampin

Chastin

Gao

2003

Journal of Applied Geophysics

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Abnormal shear wave polarizations as indicators of high pressures and over pressures

Cited by 20 publications

References 0 publications

Processing, modelling and predicting time-lapse effects of overpressured fluid-injection in a fractured reservoir

Processing, modelling and predicting time-lapse effects of overpressured fluid-injection in a fractured reservoir

A review of shear-wave splitting in the compliant crack-critical anisotropic Earth

Shear-wave splitting in a critical crust

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