Tracking CO2 Plume in Deep Saline Formations Utilizing a Time-lapse Pressure Tomography Approach

Abstract:

CO₂ storage in deep saline formations is considered as an attractive option to cut down greenhouse gas emissions. Among the major challenges is the development of efficient technologies for controlling and monitoring the evolution of CO₂ plumes during and after injection in the underground. As an alternative to the most commonly used geophysical approaches for subsurface characterization, we propose a pressure-based tomographical approach to track CO₂ plume history. By taking into account the direct relationship between saturation and flow properties, pressure tomography has the potential not only to detect a plume but also to estimate the saturation of CO₂. The experimental set-up of pressure tomography involves injection of brine or CO₂ at variable depths (sources). We use a time-lapse approach, considering first the CO₂-free formation, and then the multi-phase CO₂-brine system. By applying a rapid eikonal-based inversion technique, pressure fluctuations at observation locations (receivers) are utilized to reconstruct the spatial distribution of the apparent single-phase and mixed-phase diffusivity. Evolution of the plume shape is then delineated by comparison of diffusivity tomograms derived from different times. Finally, an integrated value of CO₂ saturation within the plume is obtained by means of a single-phase proxy. Applicability of this novel approach is evaluated in different virtual formations. The time-lapse pressure tomographic investigation revealed that knowledge about the spatial heterogeneity of permeability has a remarkable impact on proper characterization of plume shape.