A multiscale FEM to model CO₂ sequestration in saline aquifers

Abstract

A major cause of the attenuation levels observed in seismic data from sedimentary regions is the mesoscopic loss mechanism, caused by heterogeneities in the rock and fluid properties greater than the pore size but much smaller than the wavelengths of the fast compressional and shear waves. The main objective of this paper is to apply a numerical upscaling method to determine the plane wave complex modulus of a viscoelastic solid long-wave equivalent to a fluid saturated poroelastic (Biot’s medium) with mesoscopic-scale heterogeneities in the form of brine-CO2 patches. This is achieved by applying time-harmonic compressibility tests at a selected set of frequencies to a representative sample of bulk material. These tests are modeled as boundary value problems stated in the space-frequency domain. This numerical upscaling approach was applied using data from the CO2 sequestration Sleipner-field case. A numerical flow simulator to represent the CO2 injection and storage was combined with a wave propagation model in order to obtain synthetic seismograms. The flow and petrophysical parameters were determined to obtain synthetic seismograms resembling actual field data. The simulations yield CO2 accumulations below the mudstone layers and synthetic seismograms which successfully match the typical pushdown effect, observed in actual field data.