Pressure Transient Analysis in CO2 Storage Projects - From Reservoir Characterization to Injection Performance Forecast

CO2 injection into saline aquifers causes fast pressure diffusion in large reservoir domains due to low water-rock compressibility. This gives a possibility to enlarge explored reservoir domain using pressure transient analysis (PTA).

Installation of permanent downhole gauge (PDG) in the well injecting CO2 into the Tubåen formation of the Snøhvit field provided pressure recording during three years of injection history with many shut-in periods. Analysis of time-lapse pressure transients from both injection and fall-off periods revealed unique pressure derivative trend indicating flow barriers. This inspired the use of analytical reservoir model to match both pressure transients and the whole injection history.

Usually PTA provides input data for reservoir simulation reducing uncertainty in history matching. Another advantage of PTA models is highlighted in this study: capability of long-term injection performance forecast. The field case confirmed that a reliable model may be assembled based on interpretation of a single injection or fall-off pressure response (e.g. well test prior to injection), which provides accurate forecast. Such a well test, interpretation and forecast do not require in-depth reservoir characterization and simulation. The key point is accounting for major features of the reservoir governing injection performance such as aquifer size and flow barriers.