Evaluating Old Wells for Conversion to CO₂ Injectors: Experience From the Rousse Field Available to Purchase

The RSE-1 well operated by Total is located within the Rousse field in the southwest of France. The 43 year-old well, a gas producer from a now depleted reservoir has undergone extensive research in order to determine its suitability for CO₂ storage. The main objective, in terms of containment, is to avoid that the injected CO₂ migrates back to the atmosphere or to potable aquifers. Existing wells are often identified as a potential weak point in the containment system, because cement, casing and elastomers can be degraded by CO₂ over time. Since any defects within the cement sheath may accelerate this aging mechanism, the material behind the casing should be fully characterized especially across the caprock.

Cement bond log data was available from the 43 year old RSE-1 well, which only gave a qualitative indication of the casing-cement bond, but did not allow a detailed characterization of the material behind the casing. To gain more detailed information, new cement evaluation logs were acquired over the lower 905 m of a 2000 m thick caprock using a new generation 3D ultrasonic cement imager coupled with an array sonic imager. The use of different, independent measurements revealed good casing-cement bond that could be compared to the original logs from 1967 to confirm no degradation over the production life of the well. However, annulus imaging revealed breakouts and a very eccentered casing (almost touching the borehole wall), both of which led to the presence of mud pockets. Detailed evaluation of these pockets indicated that they are not connected and that zonal isolation over the 836 m cemented section of the caprock is indeed achieved. Modeling of cement imaging data also suggested good cement-formation bond, without any external microannulus presence.

Detailed characterization of the material behind casing, casing eccentralization and intermittent mud pockets provided an improved understanding in terms of well integrity. The results showed that the RSE-1 well nevertheless provides adequate hydraulic isolation for the injection of CO₂ and that the cement sheath can therefore be eliminated as a potential weak spot in terms of containment.