Abstract
A micro-annulus (MA) is defined as a high permeability zone or gap initiating/occurring at the casing-cement and cement-formation interfaces during the wellbore life span. An MA can significantly compromise wellbore integrity by establishing enhanced fluid flow pathways. This study uses a staged finite element approach to simulate wellbore integrity during various loading steps of wellbore operations under downhole conditions. Particular emphasis is placed on the processes of cement poro-elastic property evolution, volume variation, and pore pressure variation as part of the cement hardening step. The resulting state of stress during the life cycle of a typical injection well (i.e. hardening, completion, and injection) is analyzed to assess the onset and evolution of micro-annuli at various interfaces of the composite wellbore system under downhole conditions. The results show that cement shear failure is observed at the casing-cement interface during pressure testing (excessive wellbore pressure); and tensile debonding failure initiates at the cement-formation interface due to cement shrinkage during hardening and injection-related cooling (thermal cycling). Sensitivity analyses considering several parameters show that: (1) the degree of poro-elastic bulk shrinkage has significant implications for both shear and tensile failure initiation - the less the cement shrinks, the less likely the failure initiation is; (2) cement integrity increases with increasing depth; (3) cement pore pressure evolution has significant implications for tensile failure - if cement pore pressure decreases more, higher temperature differences can be sustained before an MA occurs; and (4) cement temperature fluctuations during hardening promote initiation of debonding failure. In summary, the results presented indicate that establishing downhole conditions to quantitatively analyze MA generation is necessary. The results are different compared to laboratory studies without considering/simulating downhole conditions. The knowledge from this study can raise the awareness of predicting and evaluating MA under downhole conditions and can be used to supplement and improve future laboratory experiments.
Recommended Citation
W. Zhang and A. Eckert, "Micro-Annulus Generation under Downhole Conditions: Insights from Three-Dimensional Staged Finite Element Analysis of Cement Hardening and Wellbore Operations," Journal of Rock Mechanics and Geotechnical Engineering, vol. 12, no. 6, pp. 1185 - 1200, Elsevier, Dec 2020.
The definitive version is available at https://doi.org/10.1016/j.jrmge.2020.03.003
Department(s)
Geosciences and Geological and Petroleum Engineering
Research Center/Lab(s)
Center for High Performance Computing Research
Keywords and Phrases
Cement hardening; Debonding; Micro-annulus; Pore pressure; Poro-elastic bulk shrinkage; Staged finite element analysis; Temperature fluctuation
International Standard Serial Number (ISSN)
1674-7755
Document Type
Article - Journal
Document Version
Final Version
File Type
text
Language(s)
English
Rights
© 2020 The Authors, All rights reserved.
Creative Commons Licensing
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.
Publication Date
01 Dec 2020
