Numerical Investigation of Fractured Cement Degradation by Carbonated Brine Injection in A Tortuous Rough-Walled Fracture

Open Access
Author:
Zhang, Tianji
Graduate Program:
Petroleum and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 16, 2016
Committee Members:
  • Zuleima T Karpyn, Thesis Advisor
  • Li Li, Thesis Advisor
  • Hamid Emami Meybodi, Thesis Advisor
Keywords:
  • cement
  • CO2
  • degradation
  • chemical
  • flow
  • fracture
Abstract:
The presence of fractures within the cemented column of a well could potentially aggravate the chemical degradation of cement during geological carbon sequestration, especially under dynamic flowing conditions of carbonated brine. The integration of reactive transport and the representation of fracture geometry and flow field complexity is critical for an accurate evaluation on wellbore cement integrity, while this integration is still missing in many numerical simulators. The present work consists of a numerical study to investigate the influence of fracture geometry and flow field representation on the numerical prediction of chemical degradation of wellbore cement in the presence of a single, highly conductive, rough-walled fracture. The fracture structure and flowing conditions used are based on laboratory data of an eight-day core-flood experiment of carbonated brine through a fractured composite cement-rock core. The three-dimensional fracture structure was obtained from microtomography imaging and used in the calculation of the fracture flow field using computational fluid dynamics (CFD) to honor the complex fracture geometry. Later, a series of reactive transport models of cement degradation were built with different simplifications of the fracture geometry and flow field to investigate the impact of the flow field complexity on the predicted cement degradation and property evolution. Results from this work demonstrate that fracture geometry and flow field complexity have a dominant impact on cement property evolution. In the model with the best representation of the fracture geometry and flow field, the total average porosity within cement zones increases from 15% to 20%, while the simplest model reaches a maximum average porosity of 16% with shallower acid penetration. Changes in CO2–brine injection flow rate were found to shift the chemical degradation of cement from pore closure (fracture healing) to self-limiting gapping of apertures. Comparison of cement evolution predictions also suggests that two-dimensional reactive flow models of fractured cement with a smooth-wall, cubic law assumption for the fracture flow field are likely to underestimate cement degradation.