Abstract
Debonding of well cement towards casing or rock, i.e. microannuli formation, is a typical failure mode of cement barriers. Consequently, such microannuli may act as leak paths during leakages of downhole fluids such as hydrocarbons or CO2. Current leakage models usually assume that microannuli have uniform geometries with homogeneous apertures, which results in linear Darcy flow. However, several recent studies have shown that microannuli do not have uniform geometries and subsequently, fluid flow through real microannuli is complex and non-linear.This paper presents results of CFD simulations of fluid flow through real microannuli geometries, where the microannuli were experimentally created and visualized in 3D by X-ray computed tomography (CT). Simulations were performed with three different fluids of varying viscosity and density – methane gas, liquid water, and liquid oil. The results visualize how the non-uniform flow path geometry and surface roughness of real microannuli create tortuous flow patterns, especially locally around bottlenecks and other heterogeneities. It is also seen that these complex flow patterns result in non-linear dependencies between flow rate and pressure difference for the less viscous fluids, i.e. gas and water, whereas the more viscous oil display linear flow.
Highlights
Throughout the life cycle of a well, the integrity of well barriers must be maintained to prevent uncontrolled flow of downhole fluids to the surface, freshwater aquifers, or surrounding formations
Cement is an important well barrier material, and during well con struction, cement slurry is placed in the annulus between the formations and the casing strings to provide zonal isolation (Nelson and Guillot, 2006)
The flow at the neighboring area shifted into transverse streamlines to ward the front face with velocity reduction
Summary
Throughout the life cycle of a well, the integrity of well barriers must be maintained to prevent uncontrolled flow of downhole fluids to the surface, freshwater aquifers, or surrounding formations. Defects could form in the cement sheath from several mechanisms, such as shrinkage and exposure to temperature and pressure variations during normal production opera tions (Bois et al, 2011; Nygaard et al, 2014; Therond et al, 2017). Typical such defects are microannuli, i.e. when there is no or poor bonding between the cement and the surrounding casing or formation
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