Thixotropic yield stress materials show a shear-induced solid-liquid transition at the yielding point, characterized by yield stress and yield strain. It is well known in the literature that the elastic modulus and the yield stress of thixotropic materials increase with aging time. In the current work, we propose a discussion on the brittleness of a suspension of swollen bentonite in water, focusing mainly on the role of aging times on the yield strain and on the critical strain at the linear to nonlinear viscoelastic transition of the material. The yield strain was measured in creep and constant shear rate start-up experiments, whereas the linear to nonlinear viscoelastic transition was evaluated from Fourier transforms on transient data in oscillatory shear stress amplitude sweeps. We show that aging increases material brittleness since the yield strain decreases with the resting time. On the other hand, the linear to nonlinear viscoelastic transition strain is surprisingly unaffected by the aging process. Other thixotropic systems were also investigated: 8 and 10 wt. % suspensions of bentonite in water and a 2 wt. % suspension of Laponite® in tap water. These lead to similar observations, showing constant linear to nonlinear viscoelastic strains and decreasing yield strains over increasing aging times. These findings bring relevant information to the intricate open-discussion issue on how to describe the behavior of thixotropic materials below the yield stress.
In deep-water drilling operations,
gas hydrates may be formed when
light compounds flow from the reservoir into the wellbore as a result
of a kick. The crystalline structure must be broken so as to restart
the drilling fluid circulation and consequently to remove the hydrate
slurry from the wellbore. Although hydrate formation is an important
industrial issue, there is still a lack of information about the rheological
properties of hydrate slurries formed in drilling fluids. In the current
work, hydrate formation was induced by adding tetrahydrofuran (THF)
to the water-based drilling fluid, allowing hydrate crystallization
at atmospheric pressure. The structure formed is the sII that is the
structure of natural gas hydrates usually found in oil and gas fields.
Rheometric tests that were conducted reveal that the hydrate slurry
formed in the drilling fluid is a time-dependent elastoviscoplastic
material in which the microstructure is irreversibly affected by shear.
We investigated the influence of shear during the hydrate formation,
and the results show that the elastic modulus and the yield stress
of the statically (without shear) formed hydrate slurry are more than
1 order of magnitude larger than those dynamically formed (imposing
shear). Finally, the hydrate slurry at the solid-like regime has a
brittle structure as the linear-to-nonlinear transition limits are
observed at very low oscillation strains (in the order of 10–3%). These findings can bring new perspectives for improving the techniques
and procedures for flow startup of hydrate slurries in drilling fluid
operations.
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