Constitutive Modeling of Soft Sandstone Degradation under Cyclic Conditions

  • Author / Creator
    Rahimi, Mojtaba
  • The near wellbore region is subjected to cyclic loads due to repeated changes of the mean effective stress. These repeated loads result in plastic deformation and sanding problems which can occur in injection or production wells. The difficulty is more pronounced in weakly consolidated reservoirs since they are more prone to sanding. Therefore it is required to investigate the material response to cyclic loading. In this research, the mechanical behaviour of uncemented and cemented sands under monotonic and cyclic loading are studied. Emphasis is placed on the constitutive modeling. A critical state constitutive model which was developed at the University of Alberta for monotonic behavior of cohesionless sands was chosen as the base model. First with modification of the hardening law, plastic volumetric strain increment and unloading plastic modulus, the original model was modified to describe sand behavior under cyclic loading. The modified model was calibrated and validated against triaxial cyclic loading tests for Fuji River sand, Toyoura sand and Niigata sand. Comparison between the measured and predicted results suggests that the model can capture the main features of sands under cyclic loading. Second the original model was modified for cemented sands. Formulation of the yield function, elastic moduli, plastic modulus, flow rule and other components of the original model were modified. Having incorporated these changes, the radial mapping formulation of bounding surface plasticity was incorporated in the model. The modified model was assessed against monotonic triaxial tests. Third to simulate the mechanical behaviour of cemented sand/soft sandstone under cyclic loading, some further modifications were incorporated into the model. Destruction of the cementation bonds by plastic deformation was considered as the reason for the mechanical degradation of cemented sands. To model cyclic response, the unloading plastic and elastic moduli were formulated based on those of loading. The proposed model was evaluated against laboratory triaxial tests, and the model agreed with experimental observations. Fourth the application of the proposed constitutive model was ultimately extended to cases that are not under conventional triaxial conditions. This was performed by incorporating the inherent anisotropy and b-parameter into formulation of the proposed model.

  • Subjects / Keywords
  • Graduation date
    Fall 2014
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.