Field-data analysis and hydromechanical modeling of CO2 storage at In Salah, Algeria

The permeability of a reservoir is a key parameter to determine the pressure response due to production and injection and its concomitant geomechanical response. A fractured reservoir affects the fluid flow by increased permeability, which can be further enhanced by elevated pore pressure. The CO2 storage project at Krechba, In Salah, Algeria, was concluded in 2011, but it provided unique and valuable geomechanical data and still remains an important site to study geomechanical processes. It has previously been shown that fracture injection, i.e. injection of CO2 above fracture pressure, is an important transport mechanism. Here we analyze the pressure response from the many temporary shut-ins during injection to justify a proposed correlation between pressure and permeability (power-law expressions) in the reservoir. The correlation is validated using field-data: pressure response in the reservoir and surface heave from InSAR data. Although the shut-in curves from pressure data at In Salah cannot provide the permeability directly, due to its complex injection history (rate and duration), they can show how the permeability varies with pore pressure and provide evidence of the fracture pressure. A recently developed efficient up-scaled numerical model of fully coupled poroelasticity and two-phase flow that effectively captures the main processes in the high-aspect ratio reservoir of In Salah, allows for the current analysis. This model illustrates that a static geomodel cannot explain the observed pressure response and surface heave, and is subsequently used to fit the parameters in the proposed correlation for the pressure-dependent reservoir permeability. Although there are three injection wells at In Salah, KB501, KB502 and KB503, this study is supported primarily by the data from KB501 and KB503. The correlation for the reservoir permeability provides a good match with both the pressure response in the reservoir and the surface heave above the injection wells, thus illustrating that irreversible and non-elastic processes can be approximated with non-linear material properties. For KB502 the geological setting is much more complicated and the response and behavior around the injection well is strongly depending on the behavior of a large and intersecting fracture zone and it still remains crucial to characterize properties of fault/fracture zones, such as thickness, transmissivity, stiffness and porosity.