Numerical investigations of ocean and earth tidal-induced pressure fluctuations affected by CO2 injection

Abstract

Pore pressure in aquifers shows small fluctuations induced by earth and ocean tides, which decrease when CO₂ is injected due to its higher compressibility than water. This reduction is significant in a CO₂ plume and propagates to outer water-saturated regions if the reservoir has high permeability. The degree of reduction mainly depends on the distance from the CO₂ plume. Therefore, it possibly provides useful information for inversion of CO₂ plume shape and position. Traditionally, the fluctuation induced by tides are understood and formulated using poroelasticity, and there have been studies examining the impact of CO₂ on pressure fluctuation based on one-dimensional analytical solutions for infinite reservoir. In this study, we used fluid geomechanical coupled analysis with a poroelastic continuum model and two-phase flow to investigate the effects of CO₂ injection on pressure fluctuation inside and outside the CO₂ plume. Simulations were conducted for hypothetical reservoirs under ocean tide effects similar to Tomakomai CCS project site and earth tide effects at Otway CCS site. Our numerical simulations showed a similar behavior of an amplitude reduction and a phase-shift observed at Tomakomai CCS site and reproduced amplitude reductions at Otway CCS site. These results indicate the usefulness of numerical simulations for effects of CO₂ on tidal induced pressure fluctuation during CO₂ injection. This work should be regarded as a preliminary step toward future applications that incorporate more realistic CO₂ plume geometries and complex hydro-mechanical conditions beyond the reach of one-dimensional analytical models.