LAUSR

Seismic monitoring of reservoir and overburden performance during subsurface CO2 storage plays a key role in ensuring efficiency and safety. Proper interpretation of monitoring data requires knowledge about the rock physical phenomena occurring in the subsurface formations. This work focuses on rock-stiffness and elastic-velocity changes of a shale overburden formation caused by both reservoir-inflation induced stress changes, and leakage of CO2 into the overburden. In laboratory experiments, Pierre shale I core plugs were loaded along the stress path representative for the in-situ stress changes experienced by caprock during reservoir inflation. Tests were carried out in a triaxial compaction cell combining three measurement techniques and permitting for determination of: (i) ultrasonic velocities; (ii) quasi-static rock deformations, (iii) dynamic elastic stiffnesses at seismic frequencies within single test; which allowed to quantify effects of seismic dispersion. In addition fluid-substitution effects connected with possible CO2 leakage into the caprock formation were modelled by the modified anisotropic Gassmann model. Results of this work indicate that: (i) stress sensitivity of Pierre shale I is frequency dependent; (ii) reservoir inflation leads to the increase of the overburden Young's modulus and Poisson's ratio; (iii) in-situ stress changes mostly affects the P-wave velocities; (iv) small leakage of the CO2 into the overburden may lead to the velocity changes which are comparable with one associated with geomechanical influence; (v) non-elastic effects increase stress sensitivity of an acoustic waves; (iv) both geomechanical and fluid substitution effects would create significant time shifts which should be detectable by time-lapse seismic. This article is protected by copyright. All rights reserved