Overburden pore-pressure changes and their influence on 4D seismic

Holt, R.M.; Bauer, Andreas; Bakk, Audun

doi:10.1190/segam2017-17727948.1

Cited by 5 publications

(5 citation statements)

References 7 publications

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“…For the sandy facies, the Skempton A parameter varies between about 0.15 for 0° orientation and up to 0.45 for 90° orientation. Anisotropy effects of the Skempton A parameter have previously been observed for other shales as well (Holt, Bauer and Bakk ).…”

Section: Resultssupporting

confidence: 66%

Static and dynamic stiffness measurements with Opalinus Clay

Lozovyi

Bauer

2018

Geophysical Prospecting

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In this work, an experimental study was carried out with the aim of reconciling static and dynamic stiffness of Opalinus Clay. The static and dynamic stiffness of core plugs from a shaly and a sandy facies of Opalinus Clay were characterized at two different stress states. The measurements included undrained quasi‐static loading–unloading cycles from which the static stiffness was derived, dynamic stiffness measurement at seismic frequencies (0.5–150 Hz) and ultrasonic velocity measurements (500 kHz) probing the dynamic stiffness at ultrasonic frequencies. The experiments were carried out in a special triaxial low‐frequency cell. The obtained results demonstrate that the difference between static and dynamic stiffness is due to both dispersion and non‐elastic effects: Both sandy and shaly facies of Opalinus Clay exhibit large dispersion, that is, a large frequency dependence of dynamic stiffness and acoustic velocities. Especially dynamic Young's moduli exhibit very high dispersion; between seismic and ultrasonic frequencies they may change by more than a factor 2. P‐wave velocities perpendicular to bedding are by more than 200 m/s higher at ultrasonic frequencies than at seismic frequencies. The static undrained stiffness of both sandy and shaly facies is strongly influenced by non‐elastic effects, resulting in significant softening during both loading and unloading with increasing stress amplitude. The zero‐stress extrapolated static undrained stiffness, however, reflects the purely elastic response and agrees well with the dynamic stiffness at seismic frequency.

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Section: Resultssupporting

confidence: 66%

Static and dynamic stiffness measurements with Opalinus Clay

Lozovyi

Bauer

2018

Geophysical Prospecting

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“…According to Cheng (1997), the significance of Equation 6 is that pore pressure can be generated by incremental normal as well as shear stress, the latter being an effect similar to the concept of Skempton's A s parameter. The undrained pore pressure response to an isotropic stress change (∆𝜎 = ∆𝜎 = ∆𝜎 ) is independent of material orientation relative to the symmetry axis, since B ij = B s is expressed as a volume-weighted average of the two invariant components (Holt et al, 2017). However, the undrained pore pressure generated from an anisotropic stress change depends on stress orientation.…”

Section: Poroelastic Theory and Undrained Pore Pressure Generationmentioning

confidence: 99%

Experimental evaluation of predicted undrained pore pressure generation as function of stress path and orientation in the Draupne shale

Soldal

Choi

Skurtveit

2022

Preprint

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Injection or production of fluids from subsurface reservoirs lead to stress changes affecting both reservoir and surrounding rocks. For low-permeable caprocks overlying such reservoirs, the movement of pore fluids to or from the formation is restricted and the immediate and short-term response to changes in the stress field will be undrained. Consequently, stress changes transfer partly into pore pressure changes. The aim of the current study is to investigate theoretical means of forecasting the undrained pore pressure generation in the Draupne Formation shale and to compare predictions with experimental results. Predictions are based on measurements from a single undrained triaxial test on a sample with known orientation, and a combination of Skempton’s classical formulation and anisotropic poroelastic theory. The predicted pore pressures are compared to measured pore pressures from a series of triaxial tests on samples with various orientations exposed to different total stress paths. First, it is confirmed that the normalized undrained pore pressure measured is linearly connected to the total stress path. Then it is demonstrated that a tensorial pore pressure parameter can be used to accurately predict the influence of stress orientation on generated pore pressure. Lastly, it is experimentally confirmed that the two predictions can be combined to predict the pore pressure arising from stress changes along any compressional stress path and orientation. The observations herein may contribute significantly to the understanding of induced pore pressure in low-permeable materials and provide valuable input to geomechanical modeling of various field operations.

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“…For other overburden shales, less pronounced dominance of the poroelastic pore pressure parameters B 33 over its in-plane counterpart B 11 may also play a role in decreasing the quality of approximation. However, the comparison of Lista shale with other overburden shales described by Holt et al (2017Holt et al ( , 2018a, Lozovyi and Bauer (2019), Soldal et al (2021) and Duda et al (2022) suggests that Lista's pore pressure parameters lie within a range typical for relatively soft caprock shales (in terms of static moduli and strength). In the case of Lista shale, parameter B 11 = 0.53, parameter B 33 = 1.51 and the ratio between them B 11 /B 33 = 0.35.…”

Section: Discussionmentioning

confidence: 89%

“…Cheng (1997) demonstrated that Skempton's parameters can also be derived from anisotropic poroelasticity. Holt et al (2017) applied this approach for vertically transversely isotropic (VTI) shales and proved experimentally that the value of A S depends on the angle between the direction of axial stress (1)…”

Section: Introductionmentioning

confidence: 99%

Anisotropic Poroelastic Modelling of Depletion-Induced Pore Pressure Changes in Valhall Overburden

et al. 2023

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Stress and pore pressure changes due to depletion of or injection into a reservoir are key elements in stability analysis of overburden shales. However, the undrained pore pressure response in shales is often neglected but needs to be considered because of their low permeability. Due to the anisotropic nature of shales, the orientation of both rock and stresses should be considered. To account for misalignment of the medium and the stress tensor, we used anisotropic poroelasticity theory to derive an angle-dependent expression for the pore pressure changes in transversely isotropic media under true-triaxial stress conditions. We experimentally estimated poroelastic pore pressure parameters of a shale from the Lista formation at the Valhall field. We combined the experimental results with finite element modelling to estimate the pore pressure development in the Valhall overburden over a period of nearly 40 years. The results indicate non-negligible pore pressure changes several hundred meters above the reservoir, as well as significant differences between pore pressure and effective stress estimates obtained using isotropic and anisotropic pore pressure parameters. We formulate a simple model approximating the undrained pore pressure response in low permeable overburden. Our results suggest that in the proximity of the reservoir the amplitude of the undrained pore pressure changes may be comparable to effective stresses. Combined with the findings of joint analysis of locations of casing deformation and total and effective stresses, the results suggest that pore pressure modelling may become an important element of casing collapse and caprock failure risk analysis and mitigation.

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Overburden pore-pressure changes and their influence on 4D seismic

Cited by 5 publications

References 7 publications

Static and dynamic stiffness measurements with Opalinus Clay

Static and dynamic stiffness measurements with Opalinus Clay

Experimental evaluation of predicted undrained pore pressure generation as function of stress path and orientation in the Draupne shale

Anisotropic Poroelastic Modelling of Depletion-Induced Pore Pressure Changes in Valhall Overburden

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