Steady-State and Transient Studies of Gas Hydrates Formation in Non-emulsifying Oil Systems

Abstract

The dynamics of gas hydrate formation have been investigated for surfactant-free oil mixing systems, containing mineral oil or kerosene as the continuous phase. This subject is particularly relevant in the hydrate management strategies for deep-subsea petroleum production. Experiments were performed in a high pressure cell coupled to a rheometer to investigate the characteristics of hydrates formed under different mixing rates (rotational speeds of 200, 300, and 450 rpm) and water contents (5, 10, 20, and 30 vol %) mixed with the oil. The experimental protocol consisted of six stages: (i) saturation, (ii) cooling, (iii) hydrate formation, (iv) steady state, (v) shut-in, and (vi) restart. Three different types of results were observed. In the first type, all six stages were present and well-defined. In the second type, the experiment stopped during the third stage due to the jamming of the impeller caused by the morphology/amount of hydrate formed. For the last type, the experiment was executed until the shut-in stage but the impeller could not be restarted afterward. The shear (rotational speed) imposed in the system determined whether hydrates formed were dispersed or not: for a given water content with mineral oil, mixing at 450 rpm improved the dispersion state of the dynamic slurry compared to the mixing rate. The higher rotation speed provides mechanical energy for dispersing the water/small dispersed hydrates/breaking the hydrates into small units, which when combined with the viscous nature of mineral oil, results in a system with dispersed hydrates particles. Results using kerosene as the oil phase were shown to be more likely to reach plugging conditions due to the low viscosity of the oil and the poor dynamic stability of the slurry.

A Rheological Study of Parameters That Influence the Formation of Cyclopentane Hydrates

“…On the other hand, for hydrate slurries the increment in viscosity becomes dramatic for certain water cuts and is dependent on the shear rate employed. The increase in the hydrate slurry viscosity caused by the water volume fraction is generally associated with the appearance of more capillary bridges in the system enhancing the attraction among hydrate crystals and, therefore, increasing the agglomerate size. ,, Another mechanism to explain CP hydrate formation was proposed by Karanjkar et al , for systems composed of CP and Span 80. The authors claimed that the viscosity increase is due both to the volume increment of the hairy and porous hydrate structures created by the presence of Span 80 in the CP oil-continuous emulsions and to the agglomeration produced by the liquid capillary bridges increasing the hydrate volume fraction.…”

“…These conditions may be reached, for example, in restart experiments when low shear rates are applied during the hydrate formation process ,, and when emulsions with high water cuts are used . Usually, most of the works reported have been conducted with stable emulsions. , Nonetheless, hydrates may also form in nonemulsified systems, as demonstrated by Majid et al and Charin and Sum . Nonemulsified systems have shown more fluctuations in viscosity curves, probably caused by the nonhomogeneous distribution of hydrate crystals.…”

“…21,35 Nonetheless, hydrates may also form in nonemulsified systems, as demonstrated by Majid et al 36 and Charin and Sum. 37 Nonemulsified systems have shown more fluctuations in viscosity curves, probably caused by the nonhomogeneous distribution of hydrate crystals. It also seems that these systems induce more severe hydrate formation than emulsified ones.…”

Functionalized Nanoparticles for the Dispersion of Gas Hydrates in Slurry Flow

“…This method assumes that the actual hydrate forming system would follow the same correlation. 43 Second, when a high-pressure cell is used, it is often challenging to directly visualize the system inside and determine the distribution of different phases. For example, the apparent viscosity increase may be partially caused by the hydrate agglomeration on the impeller instead of the viscosification of hydrates themselves.…”

“…The procedure for this calibration was previously reported. 43 Briefly, 25 mL of standard silicone oil with different viscosities was measured in the pressure cell for different settings of the rotational speed of the impeller. The correlation obtained between the torque T (μN m) and viscosity μ (cP) isfor a rotational speed of 200 rpm.…”