Abstract

ABSTRACT: Cement initial effective stress is defined as the state of stress after waiting on cement (WOC). Many factors can affect cement’s initial effective stress distribution, including formation environment, cement formulation, and hydrostatic pressure. Our recent experiments using a plug shape setup show that under high-pressure conditions, cement initial effective stress at the cement-casing interface usually drops below hydrostatic pressure but higher than zero, and cement permeability is in the range of nano-Darcy. In a typical cement annulus geometry, we care about initial stress at both the cement-casing interface and the cement-formation interface. The total stress and pore pressure distribution across the annulus may not be isotropic. Therefore, the initial effective stress in the radial direction may not be the same. This non-isotropic initial effective stress distribution places the cement-casing interface and cement-formation interface at different levels of risks under pressure and temperature oscillations caused by carbon injection into the subsurface through wells. 1. INTRODUCTION Cement’s initial state of stress has confused the petroleum industry for decades. The initial state of stress includes total stress, pore pressure, and effective stress. The total stress determines the carbon leakage threshold at the interface, while the effective stress controls the poromechanical response of cement during carbon injection through wells (Meng et al., 2021a). The total stress and effective stress may or may not show consistent behavior considering the effect of cement pore pressure. Accurately understanding initial stress is essential to well plugging & abandonment and maintaining well integrity in carbon sequestration wells. The initial stress has proven to strongly affect cement failure (Saint-Marc et al., 2008; Bois et al., 2012; Meng et al., 2021b). Working on well integrity simulations, most researchers either assume initial effective stress as zero (Zhang et al., 2017), or equal to hydrostatic pressure when cement is in a liquid state (Gray et al., 2009; Nygaard et al., 2014). Such discrepancy in assumption causes great uncertainty in analyzing safe operational windows for maintaining well integrity during carbon injection.

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