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Compaction Phases and Pore Collapse in Lower Cretaceous Chalk: Insight from Biot’s Coefficient

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Abstract

Successful management of chalk reservoirs for various subsurface applications as well as construction in chalk/limestone formations rely on descriptions of compaction behaviour commonly predicted from laboratory experiments. This study aims to understand and describe better the compaction behaviour that oil and water-saturated chalk undergo. Seven North Sea Lower Cretaceous chalk samples with initial porosity ranging from 31 to 45% were compacted hydrostatically in the laboratory. The traditionally named elastic, transitional, elastoplastic and strain hardening phases were identified from stress–strain curves. The observed compaction behaviour is described in phenomenological terms based on the interpretation of Biot’s coefficient as a measure of grain-to-grain contact area within the chalk frame. Biot’s coefficient was derived from elastic wave velocities and bulk density via Gassmann fluid substitution by first approximations assuming a simple calcite-bearing rock frame. Biot’s coefficient identifies both elastoplastic and elastic phases in the initial compaction phase traditionally denoted as the elastic phase. The plastic component of the elastoplastic phase presumably originates from closure of micro-crack introduced by unloading and equilibration from core recovery. Biot’s coefficient is a reliable indicator of pore collapse, and a specific constant magnitude of purely elastic strain controls the onset of pore collapse. In situ reservoirs presumably only experience the elastic strain during effective stress changes, not the elastoplastic behaviour seen in experiments. Yet, as laboratory experiments often form a calibration background for large-scale models, quantifying the plastic component of elastoplastic phases and pore collapse from pure elastic strain provides new insight to improve models and avoid the unphysical use of porosity as a controlling physical parameter.

Highlights

  • During initial compaction, both elastoplastic and elastic phases exit and are identified from Biot’s coefficient.

  • During compaction, a local minimum Biot’s coefficient derive from ultrasonic velocities is a reliable indicator of pore collapse.

  • A critical and purely elastic strain derived from applied stress and dynamic moduli is the controlling mechanism of pore collapse.

  • Identification of subphases and a critical pore collapse elastic strain provides new insight into calibration of reservoir model from experiments.

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Data Availability

Experimental data from this study are available from the corresponding author upon reasonable request.

Abbreviations

α :

Biot’s coefficient

δ :

Change and increment

φ :

Porosity

φ N :

Nitrogen porosity

ρ b :

Bulk density

ρ s :

Grain density

k k :

Klinkenberg permeability

k L :

Intrinsic permeability

IR :

Insulable residue

S BET :

Specific surface

S IR ,BET :

Specific surface of IR

σ M :

Total mean stress

σ M ,eff :

Effective mean stress

σ A :

Axial stress

σ R :

Radial stress

P :

Pore pressure

ε V ,E :

Elastic volumetric strain

ε V ,E,col :

Pore collapse elastic strain

ε V ,P :

Plastic volumetric strain

ε V :

Total volumetric strain

ε V ,E,col :

Pore collapse elastic strain

ε A ,LVDT :

Axial strain from LVDT

ε A ,SG :

Axial strain from strain gauges

ε R ,SG :

Radial strain from strain gauges

V P :

Compressional wave velocity

V S :

Shear wave velocity

G dyn :

Dynamic shear modulus

M dyn :

Dynamic compressional modulus

K :

Fluid saturated undrained bulk modulus

K dyn :

Dynamic bulk modulus

K fl :

Fluid bulk modulus

K fr :

Bulk frame modulus

K min :

Mineral bulk modulus

K sta :

Static bulk modulus

References

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Acknowledgements

The authors kindly acknowledge the Danish Underground Consortium (TotalEnergies E&P Denmark, Noreco & Nordsøfonden) for providing access to core material and granting permission to publish this work. This research has received funding from the Danish Offshore Technology Centre (DOTC) under the TRD Lower Cretaceous program. From the Technical University of Denmark, we thank Ditte Jul Valentin for technical support with measurements. Morten Leth Hjuler recorded backscatter electron micrographs. Professor Ida Lykke Fabricius is thanked for critically reading this manuscript. We thank the technical staff from the company Geo for helping with the experimental setup.

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Author notes

  1. Tobias Orlander

    Present address: Geo - Subsurface Expertise, 2800, Kgs. Lyngby, Denmark

Authors and Affiliations

  1. Danish Offshore Technology Centre, Technical University of Denmark, 2800, Kgs. Lyngby, Denmark

    Tobias Orlander

  2. Rambøll DK, 2300, Copenhagen, Denmark

    Helle F. Christensen

Authors
  1. Tobias Orlander
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  2. Helle F. Christensen
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Correspondence to Tobias Orlander.

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Orlander, T., Christensen, H.F. Compaction Phases and Pore Collapse in Lower Cretaceous Chalk: Insight from Biot’s Coefficient. Rock Mech Rock Eng (2024). https://doi.org/10.1007/s00603-024-03963-x

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