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Rock Mechanics and Rock Engineering

, Volume 50, Issue 12, pp 3121–3140 | Cite as

Creep of Posidonia Shale at Elevated Pressure and Temperature

  • E. RybackiEmail author
  • J. Herrmann
  • R. Wirth
  • G. Dresen
Original Paper

Abstract

The economic production of gas and oil from shales requires repeated hydraulic fracturing operations to stimulate these tight reservoir rocks. Besides simple depletion, the often observed decay of production rate with time may arise from creep-induced fracture closure. We examined experimentally the creep behavior of an immature carbonate-rich Posidonia shale, subjected to constant stress conditions at temperatures between 50 and 200 °C and confining pressures of 50–200 MPa, simulating elevated in situ depth conditions. Samples showed transient creep in the semibrittle regime with high deformation rates at high differential stress, high temperature and low confinement. Strain was mainly accommodated by deformation of the weak organic matter and phyllosilicates and by pore space reduction. The primary decelerating creep phase observed at relatively low stress can be described by an empirical power law relation between strain and time, where the fitted parameters vary with temperature, pressure and stress. Our results suggest that healing of hydraulic fractures at low stresses by creep-induced proppant embedment is unlikely within a creep period of several years. At higher differential stress, as may be expected in situ at contact areas due to stress concentrations, the shale showed secondary creep, followed by tertiary creep until failure. In this regime, microcrack propagation and coalescence may be assisted by stress corrosion. Secondary creep rates were also described by a power law, predicting faster fracture closure rates than for primary creep, likely contributing to production rate decline. Comparison of our data with published primary creep data on other shales suggests that the long-term creep behavior of shales can be correlated with their brittleness estimated from composition. Low creep strain is supported by a high fraction of strong minerals that can build up a load-bearing framework.

Keywords

Shale Creep Fracture closure Hydraulic fracturing stimulation Brittleness 

List of symbols

A, B, c1,2, m, t0, s, k

Creep constants

Br

Brittleness

d

Indenter diameter

F

Volume fraction

\(h,\dot{h}\)

Indentation depth, rate

n

Stress exponent

P

Confining pressure

Q

Activation energy

T

Temperature

t

Time

V

Activation volume

\(\varepsilon ,\dot{\varepsilon }\)

Axial strain, strain rate

ϕ

Porosity

σ

Differential stress

σnet

Indenter stress

Notes

Acknowledgements

This work was supported by the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 640979 (SXT). We thank Masline Makasi for sample handling, Stefan Gehrmann for sample and thin section preparation, Anja Schreiber for TEM foil preparation, Tobias Meier for providing porosity data and Michael Naumann for assistance with deformation experiments. Insightful comments of two anonymous reviewers improved the manuscript.

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Copyright information

© Springer-Verlag GmbH Austria 2017

Authors and Affiliations

  1. 1.GFZ German Research Centre for GeosciencesPotsdamGermany

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