MECHANICAL, PIEZORESISTIVE AND FRACTURE BEHAVIOR OF VARIOUS TYPES OF SMART CEMENTS
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Cement sheath integrity is an important factor that contributes to the long economic production life of the oil well. Hence there is a need for monitoring the performance of the cement sheath during the entire service life. This study analyzed a well monitoring system based on electrical resistivity and resistance to effectively monitor the behavior of various classes of cement (API classes A, G and H) sheaths by making them bulk sensing materials. Addition of small amount of carbon fiber enhanced the self-sensing property of the oil well cement making the cement a smart material. Piezoresistivity of the smart cements were studied under various loading conditions up to 28 days of curing. Up to 250% change in resistivity was observed during various types of loading conditions. Mechanical properties also were characterized and the 28 day elastic modulus for different cements varied between 2.7x106 to 3.5x106 psi. Also the Poisson’s ratio varied between 0.15 to 0.2. Split tensile strength varied between 230 to 285 psi while flexural strength was in the range of 385 to 430 psi. Fracture properties were characterized for oil well cement. Stress intensity factor KI was in the range of 0.3 to 0.6 MPa.√m and the Crack Tip Opening Displacement (CTOD) varied between 3 to 6 µm. Impedance characterization of the smart cements used in this study identified them as special bulk material which has resistance only and the capacitance effect was negligible. The monitoring methods studied using lab scale models of oil well were successful in monitoring the placement of different types of fluids used in the well including cement slurry. Also a piezoresistive repair material was developed to repair damaged oil well to regain 90% of strength and self-sensing properties.