At In Salah, the CO2 injection took place in three wells (KB-501, KB-502 and KB-503) located downdip of the partially depleted Krechba gas field (dotted white line in the Figure below). Injection in these wells 1) generated an uplift of up to 2.5 cm at the surface (red-colored areas in the Figure below), 2) created fractures that might compromise the top seal, and 3) formed a flow path between the KB-502 injector and the KB-5 observation well.
Surface deformation was used as a means to estimate the extent of the CO2 plume. The plume was expected to migrate south-westward into the updip Krechba gas field structural closure. Instead, the CO2 migrated northward through a saddle and towards another closure, moving beyond the block boundary. This was previously identified as the main project risk.
Multiple revisions to the risk assessment were undertaken and the decision was made to halt injection after 7 years, based on concerns regarding top seal integrity. Between 2004 and 2011, a total of 3.8 Mt (million metric tons of CO2) were injected which was only 22% of the original injection target of 17 Mt. This project is a good example of unexpected events leading to an early termination. It also highlights the importance of an effective monitoring program to ensure early identification of significant events and rapid implementation of mitigation actions.
