Deformation and Acoustic Behavior of Oil Well Cements




Guo, William Yi

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The experiments described in this thesis were designed to assess the evolution of deformation and acoustic behavior of cement slurry as it hardens. Laboratory facilities and test procedures developed over four decades in the University of Texas Rock Mechanics Laboratory were used to characterize static and dynamic deformation, failure, and inelastic behavior of two cement mixtures as they cured from freshly-mixed slurry to intact material. The experiments provide documentation of material behavior during curing in a carefully controlled environment, to develop a better understanding of physical and chemical processes that are active during curing. The database of mechanical properties will provide useful inputs into numerical modeling of well-bore casing behavior. It is anticipated that the shakedown measurements in this project will form the basis for the design of more elaborate studies of cement-casing behavior in cemented casings.Two cement recipes were used. Slurry mixtures for each of the specimens were placed immediately after mixing into the testing system. Length changes and compressional wave velocities were measured during an initial 24-hour curing and during several tri-axial deformational cycles carried out at 24-48 hour intervals, over a one-week-long-term curing period. Eight specimens of Mix #1 cement and seven specimens of Mix #2 cement were tested. Specimens were cured under ambient, 200 psi, and 1 kpsi confining pressure. Several of the tests were repeated to assess measurement reproducibility. Excess water layer was removed from the top of some of the poured specimens as it was produced, for approximately 30 minutes prior to the application of confining pressure, to assess the effects of initial water content on mechanical behavior. Experiments were carried out to determine the degree of cement stiffness and Non-Recoverable Strain (NRS) using static deformation and ultrasonic pulses through the cement slurry during hardening. Data were used to determine P-wave velocity, NRS, Young’s modulus, and static and dynamic modulus. Axial stress vs. axial strain and total axial stress – axial strains vs. elapsed time were plotted for each test sample. P-wave velocity, Young’s modulus, and static and dynamic constrained modulus were plotted versus elapsed time for both mix #1 and #2 (H2O wicked and not wicked). Results show that P-wave velocity increased non-linearly with elapsed time; Young’s modulus increased with each succeeding stress-strain unloading cycle, and static and dynamic constrained modulus both increased non-linearly with elapsed time.



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