Monitoring fluid migration in a CO2 storage reservoir by distributed fiber optic strain sensing: A laboratory study

Abstract

To ensure the safety of carbon dioxide (CO2) geological storage, it is important to effectively monitor the pore pressure, reservoir deformation and fluid plume migration, which poses new challenges to monitoring techniques and tools. In this paper, an innovative monitoring method is presented based on distributed fiber optic strain sensing (DFOSS) to detect the strain response distribution induced by fluid injection in real time. The original strain field distribution in the reservoir rock and the effect of the effective stress on the strain response were obtained for different confining and pore pressure combinations. Then, the rock strain during displacement was measured to dynamically monitor the movement front of the wetting phase. The results showed that the core strain response curves under the influence of the confining and pore pressures exhibit significant edge effects, with lower strain values in the vicinity of the artificial confinement facility than in the reservoir. In addition, the overall strain response exhibited an increase corresponding to the increase in the confining pressure. A higher confining pressure was observed to mitigate the pore pressure expansion-related strain in the rock itself. This proves that the selection of a large-depth subsurface reservoir could help to increase the CO2 injection pressure and rate, and the formation pressure threshold value identified in this study varied between 6 to8 MPa. Furthermore, the full-field strain history showed that fiber optic technology can be employed to accurately monitor the detailed characteristics of front movement and core deformation during wetting fluid displacement, including the history of the strain distribution due to two-phase motion and the change in the two-phase moving speed. The analysis results collectively validated the applicability of this monitoring technique for core-scale testing. The data and conclusions obtained could contribute to a deeper comprehension of the DFOSS reservoir monitoring capability. Furthermore, these findings hold relevance for subsequent experimental analyses, numerical simulations, and field applications at carbon capture and storage sites.