[CCoE Notice] PETR announcement - PhD Dissertation Defense

Knudsen, Rachel W riward at Central.UH.EDU
Thu Nov 7 15:31:44 CST 2019


The Petroleum Engineering Department

Invites the Cullen College of Engineering
To the
PhD Dissertation Defense
Of
Mohab Dessouki

Failure Mechanisms in Thick-Walled Cylinder Tests: Numerical and Laboratory Analyses on the Role of Plasticity and Borehole Size on Near Cavity Deformations

Date: Tuesday, November 26, 2019

Location: Technology Bridge (Formerly ERP) Building 9, Room 123
Time: 10:00 am – 12:00 pm

Committee Chair: Dr. Michael Myers

Committee Members:
Dr. Lori Hathon, Dr. George Wong, Dr. Victor Dunayevsky, Dr. Maria Nikolinakou, Dr. Sherif Akl
Abstract
Understanding the factors affecting wellbore stability is crucial for safe and cost effective drilling and completion operations.  Thick-wall cylinder (TWC) laboratory tests have been widely used to calibrate wellbore stability models. Although the TWC test is meant to simulate the wellbore geometry, the influence of the finite specimen wall thickness makes the results difficult to upscale. To account for these problems, elastoplastic models should be calibrated to laboratory tests using the MCC material model. These calibrated models were then combined with finite element analysis to investigate failure mechanisms.  Three mechanisms have been postulated: Runaway Instability (RAI), when the strains at the borehole grow uncontrollably; Negative Rate of Work (NRW) of plastic strains which is interpreted to cause borehole spalling; and Considere instability, a catastrophic failure which occurs when global equilibrium conditions cannot be maintained.
To investigate the effect of wall thickness and rock properties on TWC failure mechanisms, a suite of core samples was tested using both multistage triaxial and TWC tests. The Modified Cam-Clay model was calibrated to test results for the multistage triaxial and TWC tests for the three rock types (incipiently cemented Miocene Sandstone, Bentheimer Sandstone, and Austin Chalk). The MCC calibration parameters are stress dependent, meaning the parameters change depending on the experiment run.  A more complicated double yield surface was also required in order to describe the most ductile rock type.
A dependence of TWC test results on wall thickness was observed. The ultimate strength at which the TWC collapses increases with increasing wall thickness. Failure mechanisms around the borehole were investigated using high resolution micro CT scanning. The three different failure mechanisms (spalling, borehole closing, and catastrophic) were all observed.  The MCC model was used to further investigate stresses and deformation present inside the TWC specimen. Near wellbore failure is caused by an extensional deformation gradient that has an approximately constant magnitude for each rock type regardless of the TWC wall thickness.



Anne Sturm.
University of Houston . Cullen College of Engineering
Department of Petroleum Engineering. Academic Advisor
masturm at uh.edu<mailto:masturm at uh.edu>. 832.842.4848. petro.egr.uh.edu

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