Effective stress and minimum velocity trends

Ebrom, Dan; Heppard, Phil; Thomsen, Leon; Mueller, Mike; Harrold, T.; Phillip, Lindy; Watson, Peter

doi:10.1190/1.1851148

Cited by 4 publications

(4 citation statements)

References 8 publications

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“…Since overpressure is usually observed in shale or mudstone, it is not warranted to extrapolate our measurements on sandstone directly to such pore-pressure predictions. If, however, similar phenomena of effective-stress coefficient exist in mudstone (Hornby 1996;Ebrom et al 2004), the accuracy of porepressure prediction can be improved by taking the effectivestress coefficient into account (Carcione et al 2003).…”

Section: Discussionmentioning

confidence: 99%

Influence of pore pressure on velocity in low‐porosity sandstone: Implications for time‐lapse feasibility and pore‐pressure study

Hofmann

Batzle

et al. 2006

Geophysical Prospecting

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A B S T R A C TAs seismic data quality improves, time-lapse seismic data is increasingly being called upon to interpret and predict changes during reservoir development and production. Since pressure change is a major component of reservoir change during production, a thorough understanding of the influence of pore pressure on seismic velocity is critical. Laboratory measurements show that differential pressure (overburden minus fluid pressure) does not adequately determine the actual reservoir conditions. Changes in fluid pressure are found to have an additional effect on the physical properties of rocks. The effective-stress coefficient n is used to quantify the effect of pore pressure compared to confining pressure on rock properties. However, the current practice in time-lapse feasibility studies, reservoir-pressure inversion and pore-pressure prediction is to assume that n = 1. Laboratory measurements, reported in both this and previous research show that n can be significantly less than unity for low-porosity rocks and that it varies with porosity, rock texture and wave type.We report the results of ultrasonic experiments to estimate n for low-porosity sandstones with and without microcracks. Our results show that, for P-waves, n is as low as 0.4 at a differential pressure of 20 MPa (about 3000 psi) for a low-porosity sandstone. Thus, in pore-pressure inversion, an assumption of n = 1 would lead to a 150% underestimation of the pore pressure. Comparison of the effective-stress coefficient for fractured and unfractured samples suggests that the presence of microfractures increases the sensitivity of P-wave velocity to pore pressure, and therefore the effective-stress coefficient. Our results show that the effective-stress coefficient decreases with the differential pressure, with a higher differential pressure resulting in a lower effective-stress coefficient. While the effective-stress coefficient for P-wave velocity can be significantly less than unity, it is close to one for S-waves. I N T R O D U C T I O NPressure strongly influences the mechanical and transport properties of rocks, such as porosity, velocity, permeability and resistivity. In a fluid-saturated rock, both pore pressure and confining pressure control the rock properties. Seismic and borehole techniques measure these rock properties in order to infer subsurface information. For example, time-lapse seis- * mic is often employed to find bypassed reserves and reservoirpressure distribution. Differences in seismic attributes (a combination of compressional-and shear-wave velocities and density) between monitor and base surveys are used to infer pore pressure and saturation changes caused by depletion and injection.To relate changes in seismic attributes to reservoir conditions, a thorough understanding of pressure and saturation effects on rock properties is essential. Gassmann's theory (1951) is generally accepted as a prediction tool for velocity change

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Section: Discussionmentioning

confidence: 99%

Influence of pore pressure on velocity in low‐porosity sandstone: Implications for time‐lapse feasibility and pore‐pressure study

Hofmann

Batzle

et al. 2006

Geophysical Prospecting

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“…Eaton (1975) developed a number of exponential empirical formulae linking various log parameters to pore pressure gradients. Using Terzaghi's approximation Eaton's equation for P‐wave velocity can be written as (Ebrom et al 2004): where σ eff, o is the predicted effective stress, σ eff, n is the expected normal stress assuming hydrostatic pressure, V p , o is the observed P‐wave velocity and V p , n is the expected P‐wave velocity assuming hydrostatic pore pressure. Bowers (1999) and Ebrom et al (2003) use information of the burial history and depth dependent nature of compaction to modify the exponent and increase the accuracy of the pore pressure prediction.…”

Section: Pore Pressure Predictionmentioning

confidence: 99%

Section: P O R E P R E S S U R E P R E D I C T I O Nmentioning

confidence: 99%

“…Terzaghi (1936) made the assumption that the effective stress (σ eff ), is equal to the differential stress: where, σ mean is the mean stress (average of the three principle stresses) and Pp is the pore pressure. While this is a good approximation for low stresses, improvements can be made for high stress areas (Furre 2002; Hornby 1996; Ebrom et al 2004). Eaton (1975) developed a number of exponential empirical formulae linking various log parameters to pore pressure gradients.…”

Section: Pore Pressure Predictionmentioning

confidence: 99%

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Investigation into the use of 2D elastic waveform inversion from look‐ahead walk‐away VSP surveys

Roberts

Singh

Hornby

2008

Geophysical Prospecting

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A B S T R A C TPore pressure in sediments beneath salt in the Gulf of Mexico varies widely creating a potentially dangerous and difficult drilling challenge. Estimating elastic parameters of sediments beneath salt is key to the prediction of pore pressure and reducing the drilling risk in exiting the base of the salt. In this paper we investigate the ability of 2D waveform inversion to recover the elastic parameters in the sedimentary layer beneath the salt from a walk-away VSP (vertical seismic profile) carried out with the receivers in the salt, with the objective of estimating pore pressure at the base of the salt (to be estimated using traditional methods). We propose an effective method for performing the inversion and apply this method to a blind test of a large and realistic synthetic dataset. To facilitate the design of a VSP survey suitable to this type of inversion we also present an analysis into the effects of the receiver array receiver spacing. It is shown that the resulting velocity estimates are sufficiently accurate to predict the pore pressure within the limits required for drilling.

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Exploration in the Indian Offshore Basins—Some Challenging Issues Related to Imaging and Drilling

Bastia¹,

Radhakrishna²

2012

Developments in Petroleum Science

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Effective stress and minimum velocity trends

Cited by 4 publications

References 8 publications

Influence of pore pressure on velocity in low‐porosity sandstone: Implications for time‐lapse feasibility and pore‐pressure study

Influence of pore pressure on velocity in low‐porosity sandstone: Implications for time‐lapse feasibility and pore‐pressure study

Investigation into the use of 2D elastic waveform inversion from look‐ahead walk‐away VSP surveys

Exploration in the Indian Offshore Basins—Some Challenging Issues Related to Imaging and Drilling

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