Cement Sheath Integrity Implications of Scaled-Down Wellbore Casing-Cement-Formation Sections

Abstract

Many offshore petroleum wells have serious cement sheath integrity issues that imply costly repairs, limitations with respect to short- and long-term use of planned or converted production and injection wells, and safety issues. The annular cement sheath is exposed to a wide variation of thermal and pressure loads that potentiality may result in zonal isolation failure throughout its lifecycle. While several experimental and numerical work manage to map highly loaded scenarios that leads to radial cracks and micro-annulus generation, little effort has been taken to investigate and quantify the impact of moderate pressure and thermal loads on cement sheath stresses. This paper presents a 2D finite element assessment of impacts of numerous material properties, geometric parameters, pressure and thermal loading variables contributing to cement sheath stresses. A centralized wellbore section of casing-cement-formation is considered, representing a production casing string. This work is conducted as a preliminary effort in order to develop a down-scaled laboratory set-up, that represents realistically the impact of thermal and pressure loads on cement sheath stresses. This paper introduces a discussion about the capabilities of a down-scaled well section to characterize the stress distribution around the annular cement sheath of a conventional 9 5/8″ production casing. Results indicate that a down-scaled configuration exposed to comparable internal pressure and temperature profile resembles the stress distribution associated with completion and production operations of wells. It is shown that a proper characterization of the cement stress regimes requires the combined effect of pressure and thermal variations. Sensitivity studies conducted on cement sheath stresses, for both wellbore and down-scaled configurations, have assessed the relative influence of mechanical and thermal properties as drivers of cement stresses. For well operations with substantial wellbore temperature variations such as production and frac job, particularly uncertain parameters such as cement-formation-Young’s modulus and thermal expansion coefficient of cement have shown significant impact on annular cement sheath stresses. In terms of combined pressure and thermal loading, cement stresses have proved to be more sensitive to temperature differential than pressure differential variations.