Stress and pore‐pressure dependence of sound velocities in shales: Poroelastic effects in time‐lapse seismic

Bauer, Andreas; Lehr, Christian; Korndorffer, Frans; Linden, Arjan van der; Dudley, J. W.; Addis, Tony; Love, Keith; Myers, Michael T.

doi:10.1190/1.3059221

Cited by 10 publications

(6 citation statements)

References 7 publications

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“…The main effect is a slow-down of seismic waves in the overburden, associated with vertical stress decrease ( extension) in the overburden due to the effective stress increase ( compaction) in the reservoir. The impact of pore pressure changes on the 4D response was pointed out by Bauer et al (2008), but appears to be neglected in conventional 4D data analysis.…”

Section: Introductionmentioning

confidence: 99%

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

Holt¹,

Bauer

Bakk

2017

SEG Technical Program Expanded Abstracts 2017

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SummaryChanges in seismic two-way travel time in the overburden can be attributed to pore pressure changes in the underlying reservoir. Since cap rock permeability is very small, these stress changes occur without drainage on short term, which means that, depending on stress path, pore pressure may also change in the overburden. The direct impact of this on 4D seismic has not been thoroughly addressed in the literature. Here, laboratory data with an overburden shale core are presented, showing stress path dependent changes in P-wave velocity and pore pressure. The expected impact on 4D seismic can be significant.

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Section: Introductionmentioning

confidence: 99%

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

Holt¹,

Bauer

Bakk

2017

SEG Technical Program Expanded Abstracts 2017

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“…Shales, abundant in the overburden, are known for their low permeability. It is commonly assumed that shales exhibit undrained static stiffness (Bauer et al, 2008;Delle Piane et al, 2011;Islam & Skalle, 2013;MacBeth & Bachkheti, 2021;Sarout & Guéguen, 2008;Soldal et al, 2021;Thompson et al, 2021). In low-permeability formations, the immediate response to reservoir depletion is a heterogeneous undrained pore-pressure change that occurs within a substantial volume surrounding the reservoir, encompassing the entire overburden (Duda et al, 2023;Yan et al, 2023).…”

Section: Discussionmentioning

confidence: 99%

“…Shales, frequently encountered in the overburden, exhibit inherent anisotropy and are significantly stress‐sensitive. This results in substantial velocity changes that depend on angle and stress path, which is commonly defined as the ratio between horizontal and vertical stress variations (Bakk, Holt, Bauer, et al., 2020; Bauer et al., 2008; Herwanger & Koutsabeloulis, 2011; Holt et al., 2018; Hornby, 1998; Pervukhina et al., 2008; Sarout & Guéguen, 2008; Scott, 2007). The first time‐lapse seismic studies at Ekofisk (Hall et al., 2002) and Valhall (Guilbot & Smith, 2002) demonstrated a significant strain dependence of velocities in the overburden.…”

Section: Introductionmentioning

confidence: 99%

Third‐order elasticity of transversely isotropic field shales

Bakk,

Duda,

Xie

et al. 2023

Geophysical Prospecting

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The formations above a producing reservoir can exhibit large mechanical changes, creating a risk of significant subsidence and loss of rock integrity. These changes can be monitored by time‐lapse seismic acquisition, which measures the corresponding velocity changes via time‐shifts. Third‐order elastic theory can be used to connect subsurface strains and stress changes to these seismic attribute changes. Existing models assume isotropic strain dependence of the dynamic stiffness in shales. It is important to re‐evaluate this isotropic assumption considering the inherent anisotropy of shales and their abundance in the overburden. Thus, we instead propose a third‐order elastic model with a transversely isotropic strain dependence of the dynamic stiffness. When calibrated, this new model satisfactorily predicted P‐wave velocity changes determined in undrained laboratory experiments conducted on overburden field shales, covering a wide range of propagation directions and stress variations. The shales exhibit anisotropic dynamic strain sensitivity, resulting in a significantly higher strain sensitivity predicted for Thomsen's anisotropy parameters epsilon and delta subjected to a uniaxial strain parallel to the horizontal bedding plane compared to the vertical direction. Geomechanical modelling, considering a depleting disk‐shaped reservoir surrounded by shales, was employed to predict the dynamic stiffness changes of the overburden using the laboratory‐calibrated third‐order elastic model. The overburden time‐shifts increased with offset angle, peaking at about 45°, suggesting a strong influence of shear strains on the time‐shifts. In contrast, a corresponding model with an isotropic third‐order elastic tensor, calibrated to the same data, exhibited a significantly lower sensitivity to the shear strains. These results underscore the importance of considering the anisotropic strain dependence of the dynamic stiffness when studying shales. Interpreting offset‐dependent trends in pre‐stack time‐lapse seismic data, along with geomechanical modelling and an appropriate strain‐dependent rock physics model, can assist in quantifying subsurface strains and stress changes.

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“…The depleted zone whilst contracting will result in a lower vertical total stress above it, the original vertical stress being transferred, in part, to the adjacent undepleted regionsstress arching (Kenter et al, 2004, Yassir et al, 2006, Staples et al, 2007, Bauer et al 2008): This vertical stress reduction above the reservoir and the transfer of higher vertical stresses to adjacent undepleted regions has been observed for depleting HPHT fields. The depleted zone whilst contracting will result in a lower vertical total stress above it, the original vertical stress being transferred, in part, to the adjacent undepleted regionsstress arching (Kenter et al, 2004, Yassir et al, 2006, Staples et al, 2007, Bauer et al 2008): This vertical stress reduction above the reservoir and the transfer of higher vertical stresses to adjacent undepleted regions has been observed for depleting HPHT fields.…”

Section: Stress Evolutionmentioning

confidence: 90%

An Overview of Geomechanical Engineering Aspects of Tight Gas Sand Developments

Addis¹,

Yassir²

2010

Proceedings of SPE/DGS Saudi Arabia Section Technical Symposium and Exhibition

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Tight gas sand exploration and development is a key focus as markets around the world exploit local resources for natural gas. Tight gas sands have matrix permeabilities in the micro-Darcy range and low porosities (typically ~5%). Commercial production is possible from these reservoirs when they have a conductive natural fracture system, and when they are stimulated by hydraulic fractures. However, both the formation matrix and the natural fracture systems can be stress-sensitive, i.e. they close during depletion, reducing the gross permeabilities which pose numerous challenges to establishing sustained economic flow rates. Geomechanical engineering is central to the development of these tight gas resources: Economic flow rates and commerciality of the field developments can only be achieved by stimulation, through fracturing and dilation of the reservoir formations; imposing a "man-made" permeability network on the geological system. This paper discusses geomechanical considerations of wellbore orientation, in-situ stress systems, and connectivity between hydraulic fractures with the natural fracture system, for single and multi-staged fracturing.A major consideration during field development of tight gas sands is the stress evolution accompanying drawdown and depletion. It is now well established that reservoir pressure changes have an effect on both the stress magnitudes and the stress directions in the sub-surface. Two major consequences of the stress changes are addressed: 1) Well placement: Understanding the stress evolution accompanying depletion is central to optimally plan the well locations, infill locations, and hydraulic fracture treatments. Stress changes result in fracture re-orientation following the re-completion, and an increase in the productivity and ultimate recovery from these wells. In addition, optimally placed new infill wells may become more productive than the initial wells as a result of improved connectivity with open fracture systems.2) Sweet spot enlargement: Greater production and ultimate recovery is commonly observed where the fracture system is "relaxed" or "critically stressed". As the stresses change accompanying depletion, so can the sweet spots, with potentially negative impact on development. Geomechanical engineering can be used in field development planning to define depletion strategies which produce an optimum stress evolution in the reservoir to open up these tight formations, by enhancing the conductivity of the natural fracture system. Thus, the productive sweet spots provided by nature may be enlarged to increase the available gas resources and higher ultimate recoveries, by increasing the area available for commercial development.The geomechanical engineering aspects of tight gas sands are discussed, highlighting some potential areas where tight gas sand developments can be optimised.

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Stress and pore‐pressure dependence of sound velocities in shales: Poroelastic effects in time‐lapse seismic

Cited by 10 publications

References 7 publications

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

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

Third‐order elasticity of transversely isotropic field shales

An Overview of Geomechanical Engineering Aspects of Tight Gas Sand Developments

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