Abstract
Cement slurry is pumped downhole to create cement sheaths that induce zonal isolation and maintain well integrity throughout the well’s lifetime. This task is difficult to achieve due to adverse wellbore conditions and rig operations. In particular, hydraulic fracturing stimulation generates cyclic stresses detrimental to the cement sheath. The primary research objective was to conduct cyclic fatigue tests on cement sheaths reinforced with polypropylene fibers, then compare results with cement sheaths that were not fiber reinforced. The secondary research objective was to observe crack initiation and propagation in computed tomography (CT) scans of the cement sheath before, during, and after cyclic fatigue testing.
An experiment was built to mimic wellbore conditions during hydraulic fracturing. Cement sheath samples were cured at wellbore conditions of 6,000 psi and 250°F, then exposed to pressure and temperature cycles to evaluate fatigue performance. Internal casing pressure was applied to a restraining cement sheath, then cycled from the wellbore confining pressure to a positive pressure differential and back to the confining pressure. Intermittent temperature cycles were also applied from wellbore temperature to room temperature. Samples endured the applied stresses initially, but failed after reaching their respective fatigue limits due to cyclic fatigue.
Two baseline slurries without fibers were tested: one contained Class H cement plus 35% silica flour by weight of cement (bwoc) while the other contained Class H cement plus 35% silica flour bwoc plus 8% bentonite bwoc. Polypropylene fibers were added at a concentration of 0.75% by volume of batch (bvob) with the intention to improve performance of the baseline slurries. Results showed that fiber reinforcement increased the fatigue life of cement sheaths tested with 0 psi confining pressure, however it lowered the fatigue life of cement sheaths at 6,000 psi confining pressure. Three types of failure were observed including radial cracks, disk cracks, and cement-casing debonding. Cross-sectional CT scans displayed mature radial and disk cracks, but not the gaps formed from debonding. In conclusion, further testing is needed to assess fiber reinforcement’s ability to improve cement sheath durability in oil and gas wells.
