An Evaluation of the Cement Sheath Quality of Casing Sections Recovered During a Well Abandonment Operation

Abstract

Abstract Cemented casing sections were recovered from the upper part of a Norwegian North Sea production well during a permanent abandonment operation. The barrier quality of the cement sheath, which has been sandwiched between two casing strings for more than 30 years since the well was constructed, has been investigated. Measurements recorded using acoustic logs, fluid leakage testing, and core plug analysis are evaluated, and the results are presented. Two sections were selected from those recovered during well abandonment, the deepest from approximately 260 m depth and a second from the interval covering the top of cement (TOC). The leakage properties of these sections were measured using water and nitrogen. Core plugs were recovered from the top and bottom of each section and petrophysical, chemical, and mechanical properties were measured. The casing ends were sealed with pressure-tight bulkheads enabling acoustic logging to be performed under pressure. The sections were logged at their initial condition: "dry", as delivered onshore and thereafter "wet" after attempting to saturate them with water. Initial leakage testing with nitrogen enabled the distribution of the fluid leakage paths through the ends of each section to be visualized. The leakage path properties were observed to vary axially along each section and also for different positions around the azimuth of the outer casing. The acoustic logs were analyzed and provided a detailed map of the axial and azimuthal variations of the cement bonded to the inner casing of each well section. The variation of the log response recorded under dry and wet conditions was correlated with the leakage property variations measured along and around the casing sections. The log response was found to be consistent with the physical observations and leakage test results. The presence of microdebonding between the casing and cement and the transition to mud solids and fluid pockets above the top of the cement were readily observed. A unique and comprehensive data set has been acquired comprising acoustic logs and laboratory measurements of water and gas leakage rates and cement properties, recorded on casing sections retrieved after more than 30 years of operational exposure downhole. The measurements enabled the cement log analysis to be compared directly to physical measurements of the well barrier quality.