Simulation of CO_2 distribution at the In Salah storage site using high-resolution field-scale models

摘要

The In Salah CO_2 storage site, Algeria, is an industrial-scale capture and storage project. CO_2 from several natural gas fields within the development is removed from the production stream and injected into a deep saline formation 1.9 km below the surface and several kilometers away from Krechba, one of the gas fields in production. The three horizontal injection wells have been actively monitored since the start-up in 2004. In particular, satellite surveys (InSAR), showing subtle surface deformation, and well data analysis (gas geochemistry and tracers) have been used to indicate the pressure and gas distribution. The 20 meter thick storage formation is pervasively fractured with the predominant joint set (NW-SE) in close alignment with the present-day stress field. The storage formation is also segmented by a number of strike-slip faults related to a regional mid-to-late Carboniferous basin inversion. The heterogeneous nature of the storage formation is a key influence on the distribution of stored CO_2. We use an invasion percolation modeling approach, assuming capillary limit conditions, to simulate the CO_2 migration process. The field-scale model involves 56 million cells with dimensions of 10x10x2 meters. This high-resolution model captures the reservoir heterogeneity with respect to both the fault and fracture distributions. The simulation results are reasonably consistent with the inferred CO_2 distribution after 5 years of injection, and indicate that the current distribution of CO_2 is principally related to the fracture network. Initial results for predictive simulations of the post-injection period are sensitive to, and principally constrained by, the fault distribution and the multiphase flow behavior. The simulation results highlight the role that high-resolution heterogeneous field-scale models can play in developing a comprehensive storage monitoring program