Paper: A METHOD FOR STUDYING PRESSURE CYCLING EFFECTS ON CEMENT INTEGRITY IN AN ANNULUS

In a well bore, the cement sheath is subjected to pressure changes from various well operations such as changes in the densities of displacement fluids, pressure testing, fracturing, and remedial operations. Failure of cement sheath in annulus due to pressure changes can lead to debonding from the formation or the casing causing microannulus formation, and/or produce cracks in the cement matrix thereby providing flow paths for fluids such as oil, water or gas. Such failures lead to expensive remedial operations in a producing well, or continuous monitoring in an abandoned well. Designing cement slurries which can withstand cyclic pressure changes helps allow for extended periods of trouble free operations. However, experimental techniques to measure failure resistance of set cement in an annulus to pressure changes are not available. This paper presents a method of testing cement compositions using well bore models with pipe-in-pipe configuration in which cement was circulated in the annulus, cured and subjected to cyclic pressure loads by pressuring and depressurizing the inner pipe. The failure of the cement was measured by the flow rates during pressurization and depressurization cycles of a fluid containing a dye through the cement column as a function of applied pressure. The method was applied to several low density cement formulations. The failure mode at the end of the tests was investigated by cutting the models into several segments and inspection of the cement under fluorescent light. Effects of experimental parameters such as temperature at the time of failure testing, annulus pressure, and slurry design were tested. The results and their implications in designing slurries for long term cement performance are discussed.