Transient Injection Performance in First UAE CCS Well – Lessons Learned

Abstract Understanding CO2 injectivity is a key element in the scale-up of a CO2 storage project. A one-year injection period in the first UAE CCS well has yielded valuable insights into the impact of transient, thermal flow dynamics on CO2 injectivity. The objectives of this study were to collate and analyze the high-frequency pressure, rate, and temperature data to calibrate a coupled wellbore-reservoir model at a later stage. In addition to monitoring injection rate, surface pressure and surface temperature on continuous basis, the well is equipped with a permanent downhole gauge, as well as fiber optics on both tubing and casing. Combination of this data enabled calculation of fluid properties at downhole conditions using the GERG-2008 equation of state, which subsequently enabled the estimation of the time-dependent k*H product as a proxy for well injectivity. The high-frequency data have revealed several interesting observations since CO2 is sub-critical at wellhead conditions but becomes super-critical in the upper part of the wellbore. First, during injection periods, the flowing bottom-hole pressure (FBHP) increases, as expected, while the wellhead pressure (WHP) decreases; this is a result of strong dependency of density on temperature. Second, after a certain shut-in period, the gauge pressure begins to increase towards a stable plateau; this behavior is also attributed to the impact of increased fluid temperature on density. Third, since the injection rates were fairly low, in the range 2-5 MMscf/d, the CO2 was unable to fully displace water from the vertical wellbore. Also, rather than displacing water away from the wellbore, some of the CO2 would simply dissolve into the brine, potentially causing water ingress into the well during shut-in periods. Finally, the CO2-water interface level in the wellbore depends on the downhole pressure and hence the injection rate. As a result, the effective k*H is found to correlate linearly with injection rate.