Surface deformation-based reservoir monitoring technologies, such as Tilt, GPS and Interferometric Synthetic Aperture Radar (InSAR), have been successfully applied to monitor fluid flow or pressure changes in the reservoir and fluid migration to shallow depths. To obtain the subsurface fluid movement from the surface deformation requires performing a geophysical inversion. To get meaningful results from the inversion process requires diligent selection of the inversion method and reservoir block sizes, as well as the application of physically reasonable constraints. The focus of this paper is to provide a workflow and guidelines for field application by studying the inverse problem, the solution methods and associated error estimates for the unknown model parameters, and the resolving power for each parameter. As a field case demonstration, the methodologies are applied to a CO2 monitoring project with InSAR data. Also, the subsurface movement of CO2 will be presented.
Technologies capable of measuring small-scale surface deformation, such as Tilt, GPS and InSAR, have been successfully applied to monitor fluid flow or pressure changes within the reservoir[1,2,3,4], and fluid migration to shallow depths. Surface-based monitoring techniques have an inherent cost advantage over downhole based methods and can generally monitor a much larger area.
The increasing number of Carbon Capture and Sequestration (CCS) projects worldwide provides great incentive to develop economical techniques to monitor conditions that result from the long-term injection of CO2, such as the extent of the CO2 plume, fracture development that can affect cap rock integrity, and fluid movement out of a zone. Surface deformation measured by these technologies can provide important information on subsurface fluid flow, pressure changes in the reservoir, and fluid migration to shallow depths. The use of surface deformation to monitor fracture growth during hydraulic fracture treatments, fluid migration during steam or waterflood operations, and even monitoring magma movement in volcanic areas is now common.
Obtaining the subsurface movement of CO2 from surface deformation requires performing a geophysical inversion. Application of the available inversion techniques is quite straightforward; however, model resolution and the associated uncertainties of the inverted model parameters have not been discussed or addressed in previous papers[1,2,4]. But, model parameter resolution estimates were discussed briefly in a 2008 paper by Vasco, et al, that appeared in Geophysics[3]. However, it failed to address the uncertainties in the estimated model parameters.
Initially, this paper will describe the general inverse problem involved with surface deformation monitoring of subsurface changes, and then demonstrate the model uncertainty through the covariance matrix and a newly defined variable. To conserve space, yet provide completeness, the model resolution and data resolution matrices, and the inversion techniques are presented in Appendices A and B. Finally, two synthetic examples will be presented for detailed illustration and discussion followed by InSAR monitoring results for a CO2 sequestration project in the Krechba field, Algeria.
Reservoir fluid injection or production, CO2 sequestration, and thermal processes such as steam flooding, Cyclic Steam Stimulation (CSS), and Steam-Assisted Gravity Drainage (SAGD) are all processes that have the potential to deform reservoirs.
