Influence of temperature on dynamic stiffness properties of high porosity North Sea chalk
By Tobias Orlander
Abstract:
A geomechanical testing program was designed to investigate the influence of temperature on the dynamic stiffness properties of high-porosity North Sea chalk.
The North Sea Basin is geologically undergoing subsidence, and the studied chalk has presumably never experienced higher temperatures than in situ.
Accordingly, the dynamic stiffness moduli were derived from elastic wave velocities and bulk density at ambient (≈25°C) and in situ temperature (75°C to 75°C).
For heating, strain reduction from thermal expansion is effectively a frame stiffening mechanism. A counteracting effect is temperature-induced decrease in mineral stiffness.
Hence, the ratio between temperature-induced decrease in mineral stiffness and thermal expansion coefficient governs temperature-induced in- or decrease in frame stiffness.
This study characterised and mechanically tested a series of chalk core plugs with a carbonate content of around 90%. The core plugs had high similarity in initial petrophysical properties, with porosity and liquid permeability around 43% and ≈1 mD, respectively.
The core plugs were assigned to ambient or in situ temperature and hydrostatic or uniaxial compaction. Comparing dynamic moduli across testing temperatures shows decreased frame stiffness for increased testing temperature during both uniaxial and hydrostatic compaction.
The respective magnitude decrease for compressional, shear, and bulk modulus correlate with the temperature-induced stiffness decrease of calcite. The most significant magnitude decrease is seen on Mdyn and the lowest on Gdyn, suggesting a direct link between temperature effects on calcite and the chalk frame stiffness.