Coda-Wave Based Monitoring of Pore-Pressure Depletion-driven Compaction
of Slochteren Sandstone Samples from the Groningen Gas Field
Abstract
Pore-pressure depletion in sandstone reservoirs is well known to cause
both elastic and inelastic compaction, often resulting in notable
surface subsidence and induced seismicity. Recent studies indicate that
in such cases inelastic strain, which is often neglected in
geomechanical models, represents a significant proportion of the total
strain throughout reservoir production. While there has been
considerable effort to quantify the proportion of continuous inelastic
deformation from the mechanical response of laboratory samples, there
has been little focus to date on the associated acoustic response
throughout compaction. With this in mind, we employ three coda-wave
based processing methods for the active source monitoring of ultrasonic
velocity, scattering power, and intrinsic/scattering attenuation during
the pore-pressure depletion of core samples from the Slochteren
sandstone reservoir in the Groningen gas field (the Netherlands). Our
results corroborate previous studies suggesting that initially,
inelastic deformation occurs primarily along intergranular boundaries,
with intergranular cracking developing towards the end of depletion and
particularly for the highest porosity samples. Furthermore, analysis of
Biot type intrinsic attenuation indicates that this compaction occurs in
several stages of predominately intergranular closure, transitioning
into predominantly intergranular slip/cracking, and eventually
porosity-dependent intragranular cracking. We demonstrate how this
segmentation of pore-pressure driven compaction can be used to
characterise differences in sample properties, and monitor the evolution
of microstructural inelastic deformation throughout depletion. We
further discuss the feasibility of in/cross-borehole monitoring of
reservoir compaction, for both improved geo-mechanical modelling and
early warning detection of induced seismicity.