Estudo de propriedades reológicas de suspensões de hidratos

Abstract

Hydrates are crystalline structures that resemble ice, formed through the occlusion of small molecules, such as light hydrocarbons, within cage-like structures composed of water molecules, usually under thermodynamic conditions of high pressure and low temperature. The formation of gas hydrates leading to plugging is regarded as the main issue by the oil and gas industry. While thermodynamic inhibitors have been employed throughout the 20th century to prevent the occurrence of these structures, high costs and environmental regulations have prompted a change in approach. Managing risks associated with the presence of hydrates in transportation lines has undergone a paradigm shift in the 21st century. Keeping the hydrates in suspension is an option, but it only feasible if there is no crystals agglomeration or wall deposition. The aim of this thesis is to analyze the rheological characteristics of hydrate suspensions during crystal formation and transportation. To accomplish this, the Haake MARS III rotational rheometer was utilized. For experimental purpose, tetrahydrofuran (THF) was chosen as the hydrate formation agent. Due to its unique ability to form sII hydrate structures, which are similar to those that occur in the majority of natural gas systems, at atmospheric pressure and temperatures above the water freezing point, THF is frequently used by research groups as a model. Four fluids were used as the basis for the suspension: (i) water; (ii) THF; (iii) brine; and (iv) water-based drilling fluid. Shear influence during hydrate formation and its impact on microstructure degradation were investigated in all cases. A more comprehensive analysis was performed for the drilling fluid samples, which included assessment of yield stress and system brittleness in the solid-like regime. In THF-water systems, samples containing water in excess exhibited a shorter induction time. The application of shear rates at 100 s-1 and 300 s-1 did not result in any significant differences in the induction time of the first three samples. However, a shorter induction time, a faster transition to the steady-state, and a higher rate of particle growth and agglomeration were the results of the increased shear rate in drilling fluid samples. The experimental findings indicate that the THF hydrate suspension in drilling fluid is an elasto-viscoplastic material that exhibits time-dependent behavior, in which the microstructure is irreversibly affected by shear. The elastic moduli and yield stress of samples formed at rest (without shear) were one order of magnitude higher than those obtained under shear. Interestingly, it was revealed that hydrates formed under static conditions have lower resistance to deformation compared to those formed under shear. Finally, visual observations indicated that the presence of limestone particles in the drilling fluid composition had a direct influence on the size of hydrate crystals, as their growth and agglomeration were restricted due to particle interactions between barite and hydrate.