ABSTRACT Due to budget constraints, CCS in deep saline aquifers is often carried out using only one injector well and one control well, which seriously limits infering the dynamics of the CO_2 plume. In such case, monitoring of the plume of CO_2 only rely on geological assumptions or indirect data. In this paper, we present a new two-step stochastic P- and S-wave, density and porosity inversion approach that allows reliable monitoring of CO_2 plume using time-lapse VSP. In the first step, we compute several sets of stochastic models of the elastic properties using conventional sequential Gaussian cosimulations. Each realization within a set of static models are then iteratively combined together using a modified gradual deformation optimization technique with the difference between computed and observed raw traces as objective function. In the second step, this statics models serves as input for a CO_2 injection history matching using the same modified gradual deformation scheme. At each gradual deformation step the CO_2 injection is simulated and the corresponding full-wave traces are computed and compared to the observed data. The method has been tested on a synthetic heterogeneous saline aquifer model mimicking the environment of the CO_2 CCS pilot in Becancour area, Quebec. The results show that the set of optimized models of P- and S-wave, density and porosity showed an improved structural similarity with the reference models compared to conventional simulations.

We have performed a series of rock physics measurements under various simulate confining and pore pressure and temperature states to test the seismic response of two tight reservoir samples of the sedimentary basin of the St. Lawrence Lowlands to different CO₂ phases. Results show that the seismic velocity and amplitude can be used to detect the CO₂ phase transition. Laboratory measurements were used to calibrate a stochastic geological model that was used to generate synthetic seimograms reproducing the response to CO₂ injection in the reservoir formations. The modeled CO₂ injection scenario included 15 years of injection followed by 35 years of CO₂ migration. Synthetic time-lapse seismograms were produced after 5, 15 and 50 years form the start of injection. Results show that substitution of brine by CO₂ is responsible of a time-delay in the seismic traces despite very low reservoir permeabilities and porosities. A comparison between a classical blocky model and our stochastic model shows that the blocky model leads to a misinterpretation of the CO₂ effect on the seismic response. Keywords: CO₂, ultrasonic measurements, seismic modeling, time-lapse VSP, low porosity sandstones, CO₂ injection modeling