Differentiation of Cement & Creeping Formation Behind Casing Key to Successful Plug and Abandonment

Abstract

The Gyda field in the North Sea operated by Repsol was proven in 1980 and the platform started producing in 1990. In June 2017, the Norwegian authorities approved the decommissioning plan for the Gyda field. The decommissioning scope included the permanent plugging of 32 wells in the field. Decommissioning is estimated to cost several hundred million dollars and is expected to finish in 2022. As per the NORSOK standards, each well needs to have confirmed barriers to isolate inflow zones, both for preventing from flowing to the surface and hindering crossflow between them. Cement and creeping formation are both considered to be potentially effective barrier elements. However, the criteria and verification methods used to confirm formation creep and cement as barrier elements are different and hence require an innovative interpretation technique which is presented in this paper. As per the regulations and standards, it is critical not only to evaluate the quality of the circumferential bond for cement and formation creep but also to determine their respective bond length. The most important measurement to accurately determine those criteria in each well is through the ultrasonic and flexural attenuation tool. However, interpretation to differentiate formation creep from cement presents challenges, especially when they have similar ultrasonic properties. Quite often, they coexist at the same depths on different sides behind the casing. Barrier evaluation becomes even more challenging with added complexities such as borehole mud settling due to high deviation, high eccentricity, casing damage, or presence of a microannulus. This paper discusses the techniques and interpretation methods used to accurately evaluate barrier elements, differentiate between cement and formation creep, estimate the tops of cemented areas, and eliminate complex challenges posed by mud, deviation, eccentricity, and wet microannulus sections. Successful and accurate determination of the potential presence and location of annulus barrier elements has been fundamentally important for Repsol to meet the regulatory requirements. A special interpretation technique was established using integrated data evaluation to differentiate creeping formation from cement. This technique successfully determined accurate barrier intervals, helping to meet all the regulatory requirements. The processes and methods have been audited and evaluated by the Petroleum Safety Authority Norway.