Advanced cement integrity evaluation of an old well in the Rousse field

摘要

The Rousse field, located in the Lacq basin in the southwest of France, is a site for a pilot carbon dioxide (CO2) storage project operated by Total. Since 2010, CO2 has been injected into a depleted gas field at a depth of 4.5 km through RSE-1, a 45-year old production well converted to an injection well. Older wells are commonly recognized as the most likely pathway for CO2 to migrate from the injection zone to other zones or to the surface. A number of factors could increase the potential risk for these wells: completions and production history may have created defects in the sealing elements (cement, steel and elastomers); the history itself may not be known in sufficient detail to estimate a reliable risk figure and finally the injected or produced fluids may have corroded the structure Old wells, whether they are converted to injectors or are in the path of the injection pressure field, could require renewed evaluation and preventive repairs as part of the field-level containment management plan, if their integrity assessment request so. As part of the due diligence process, advanced cement and steel evaluation logging tools were run in the RSE-1 well to investigate the bottom kilometer of the two kilometer-thick caprock. The 3D integrity map produced by the acoustic logging tools was analyzed for the presence of connected defects (pathways) that could lead to unwanted CO2 migration. The logs were also compared to the original 40 year old low-resolution sonic log to assess the extent of degradation, if any. A particular issue for wells drilled before the 1990's, when technological advances almost eliminated the problem, is that of mud channels left behind during the cement placement process. These defects present the highest risk because of their relatively large flow area. Since the well was completed in 1967, special attention was paid to the occurrence, connectivity and extent of channels. Even though imaging of the cemented annulus revealed an eccentered casing, almost lying on the rock face, and an oval borehole, the few mud pockets trapped between casing and rock were confirmed to be intermittent, with good cement providing hydraulic isolation between them. The comparison of the through-casing caliper to the formation sonic log enabled identification of borehole breakouts, caused by stress anisotropy, as the reason for the ovalization of the borehole section. Casing-cement bonding was confirmed to be excellent (it actually improved compared to the original log), and model-based inversion of imaging data suggests that cement-formation bond is also good. Defect-oriented analysis of the logs thus confirmed that RSE-1 provided containment across the caprock and was suitable for conversion to a CO2 injector without any repair or improvement work.