Feasibility of Reproduction of Stored CO2 from the Utsira Formation at the Sleipner Gas Field

AbstractThe feasibility to recover stored CO2 from the Utsira formation at Sleipner has been studied by reservoir simulation of a 15 years injection (until 2011) and a subsequent scenario where the CO2 is produced from a horizontal well under the cap rock. There are three important aspects related to the recovery of the stored CO2: -Performance of temporary storage of CO2 for EOR purposes.-Reproduction of mobile CO2 from formations that turn out to be less suitable for storage than anticipated.-The reproduction simulation can resemble an open leaking well. The simulations are based on experimental drainage and imbibition curves for the corresponding injection and production phase. The results show that until 47.7% of injected CO2 can be reproduced during a seven year non-stop production period. The results are based on a 15 year injection period, an irreducible water saturation of 5% and a residual CO2 saturation after imbibition of 25%. Also the amount CO2 reproduced was found to be increasing with decreasing values of irreducible water saturation.Data from pre-injection seismic, well logs and petrophysical data are used to build a reservoir model. The Utsira formation is intersected by thin horizontal discontinuous shale layers that impede the vertical migration and cause entrapment of the CO2 in flat plumes. The transport properties of these shales are obtained by history matching the size of the plumes observed from seismic monitoring. To accommodate the necessary amount of CO2 the lateral size of the model had to be 5.2 km×7.9 km. To minimize numerical dispersion a relatively fine grid was required giving 2.1 million grid blocks in the model.The CO2 plume may be reached by a new well from the Sleipner A platform or through a sidetrack of the existing CO2 injection well and convert it to a producer. Placement of the producer is optimized for maximum reproduction by utilizing the local topography of the top seal. A producer could be drilled to avoid penetration of the cap-rock close to or above the CO2 plume and thereby avoid a potential leakage point after abandonment. The formation is high-permeable and relatively flat. The injected CO2 will therefore accumulate in thin plumes with large lateral extensions. Accordingly, recovery efficiency is low and the formation is less suitable for reproduction of CO2 compared to formations with a significant dome seal or dipping structures sealed by faults. The results give an important confirmation that it is possible to reproduce a significant amount of the mobile CO2 as a remediation option if a serious lack of integrity of the cap rock is detected