Abstract

A wellbore cement sheath is expected to provide zonal isolation and borehole integrity during well construction and well life. Cement sheaths mechanically interact with other elements in the wellbore region to stresses from geological processes and operational activities. Quantifying these interacting physical components and processes has technical, economic, and environmental implications of great, and growing, significance.

Staged finite element procedures during well construction sequentially consider the stress states and displacements at and near the wellbore. The model replicates complicated stress states arising from simultaneous action of far-field stresses, overburden pressure, cement hardening and shrinkage, debonding at the interfaces, and plastic flow of cement sheath and rock formation. Presently, temperature and flow are not included. The technique tracks the time-dependent behavior of cement slurry, curing (with or without shrinkage), and hardened cement during the critical period after slurry placement.

Material models for casing, cement and rock formation, failure criteria for cement, formation and interface bonds were calibrated using published information and experimental data. Calculations were conducted for various loading and unloading scenarios, geometric configurations, properties of rock formations, and cement slurry formulations. Results are discussed in terms of field implications, for example:

  1. interface micro-channels may or may not develop, depending upon shrinkage magnitudes;

  2. simplifying modeling assumptions that are often used, such as 2-D stresses and/or deformations, may obscure critical casing, cement, and formation behavior in the wellbore region, and in the producing horizon.

This paper, part of a series quantifying the interacting physical components and processes at and near the wellbore region, initiates useful comparisons of analytical results and field realities. The series illustrates and compares results and practical implications for simple to increasingly complex, but more realistic assumptions, such as isotropic/directional stress states, and isotropic/anisotropic casing, cement, and formation material parameters.

Introduction

During well construction and subsequent wellbore, near-wellbore, and reservoir operations, several physical elements at and in the vicinity of the wellbore interact in response to downhole processes and events. The life of any well - both its longevity and its productivity - reflects an integration of the various processes and events which have occurred during drilling, completion, and production operations.

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