Abstract

Cement sheath integrity is a critical part of maintaining wellbore integrity during the wellbore lifecycle and needs to be thoroughly investigated under representative downhole conditions. This paper utilizes an integrated experimental and numerical approach to investigate the cement hardening process and evaluate the severity of wellbore damage under downhole conditions for the entire wellbore lifecycle. The integrated approach includes data collection for downhole conditions and a staged Finite Element Analysis (FEA) framework to evaluate cement failure occurrence. The modeling results indicate that the state of stress developed in the cement after hardening fundamentally affects cement failure occurrence under various loads. The integrated approach indicates that the cement strength decline due to degradation affects the onset of failures (i.e., debonding, radial cracks, and disking) significantly; excessive shrinkage during cement hardening promotes onset of debonding failure which largely endanger the wellbore integrity in the long-term. In summary, to obtain a robust evaluation of wellbore integrity, a systematic evaluation of the cement hardening process and the associated parameters considering downhole conditions where the cement is actually used is suggested. A cement system with less shrinkage, lower Young's modulus, and higher strength can effectively improve wellbore integrity.

Introduction

Greenhouse gas (GHG) emissions are the major contributor to global warming and abnormal & severe climate. In 2018, methane leakage associated with the energy industry accounted for ~9.5 percent of all US greenhouse gas emissions (US GHG Inventory, 2018). The US GHG Inventory (2018) estimated that the annual total methane leakage from abandoned gas wells had increased by over 50% from 1990 to 2018. Considering the vast number of active wells drilled during the recent unconventional energy boom and their gradual progression towards becoming inactive or abandoned in the next several decades, the methane emission from inactive and abandoned wells will become increasingly severe. Moreover, feasibility studies evaluating inactive wells for CO2 sequestration (Nygaard 2010), waste material storage (Celia et al., 2004), and geothermal energy exploitation (Sui et al., 2018) also demonstrate the need for long-term wellbore integrity.

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