Characterizing the Behavior of Smart Oil Well Cement for Enhancing the Monitoring of Cement Sheath Integrity
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Oil well cementing is one of the most important and yet complex steps of a drilling project. One of the challenges in maintaining the well integrity is in monitoring the performance of oil well cement (OWC) used in the construction of the well. Recent oil well failures have clearly identified the importance of well cement on economic and environmental aspects of well drilling and completions. Currently, there is no technology available to monitor cementing operations in real time from the placement of cement through the service life of the wells. In this study, oil well cement was modified with carbon fibers to have better sensing properties, smart cement, so that its behavior can be monitored at various stages. A series of experiments evaluated the cement behavior with and without modifications in order to identify the most reliable sensing properties that can also be easily monitored. Tests were performed on the cement from the time of mixing the slurry to hardened state. The results indicated that the initial electrical resistivity varied based on the type and amount of additives used in the cement. Electrical resistivity responses during curing of cement all exhibited the same trend with time. Resistivity dropped initially by about 10% to 12% to reach a minimum, and then gradually increased as the hydration progressed. The change in resistivity within the first 24 hours of curing varied from 50% to more than 300% depending on the composition. A new quantification concept has been developed to characterize the compressive strength development of cement based on electrical resistivity changes in the first 24 hours of curing. Modifying the cement with 0.1% of carbon fibers, improved the piezoresistive behavior of the hardened smart cement without affecting the rheological and setting properties. The resistivity change at peak stress was about 400 times higher than the change in the strain at failure. Effect of water-to-cement (w/c) ratio and curing temperature were investigated on the performance of smart cement. It was observed that resistivity of cement with different w/c ratios followed the same pattern but the characteristic parameters were significantly varied. Experiments revealed that the water-to-cement ratio, temperature and additives affect the rheological properties of OWC slurries, and a new hyperbolic model was proposed to predict the shear stress-strain rate relationship of cement slurries.
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