Experiments with Additive – AA-LDHI

The additive used to avoid hydrate agglomeration is a surface active agent (surfactant). It will act at the droplets interface. As consequence, it also increases homogeneity and stability of the emulsions under flow formed before hydrate formation. Throughout the emulsion formation and

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rheological study, this influence should be seen. The additive dosage in each experiment is very low.

The different percentages of additive used in this study are a feedback from the hydrate crystallization experiments and will be explained in § III.2.2.

Figure III.5 shows the rheological study for shear stabilized emulsions regarding the three water cut groups (high, intermediate and low) with anti-agglomerant additive. As the blanks (Figure III.1), these experiments (Figure III.5) present the same plateau of pressure drop for the same flow rate during the rheological study. However, it is observed that the pressure drop presents more stable behavior, which means that experiments with additive have higher degree of homogeneity and stability.

With the purpose of understanding what happens in the case where additive is used, it is necessary to evaluate the chord length distribution during the rheological study (Figure III.6).

Comparing experiments, it can be observed that the initial chord length distribution shape sharply changes with varying water cut. The sharpest peak of chord length number is of 900 chords at high water cut, 1800 at intermediate water cut and 200 at low water cut, while the corresponding quantity for the blanks does not vary too much when water cut is varied (400, 700 and 500, respectively). The quantity of chords found is related to the dispersed phase. Relating it with the number of chords found in presence and absence of additive, this shows that experiments with additive have more dispersed droplets than experiments without additive. However, the evolution of the number of chords detected in experiments with additive varying the water cut is not similar to blank experiments. Thus, it is necessary to consider that the use of surfactant additive changes the expected shear stabilized emulsion morphology.

The final range (Figure III.6) of measured chord lengths after 400L.h^-1 is the same for high, intermediate and low water cuts and the attained shear stabilized emulsions remain similar until the end of the rheological study. The small droplets are mainly dependent on the surfactant action: if the dosage of additive is very low, the morphology cannot be completely maintained when the flow rate is decreased and less chord lengths are detected.

In the experiment with low water cut (Figure III.6 (c)), the rheological study already starts with a peak corresponding to small droplets (around 9 µm). This probably happens because the additive dosage used in this water cut is more important than the one used in high and intermediate water cuts. The higher dosage influences the shear stabilized emulsion since the lowest flow rate, inducing

119 the formation of more and smaller droplets than in experiments at high and intermediate water cuts in same conditions. When the flow rate is increased to 200 L.h^-1 and 300 L.h^-1, the distribution is maintained. At the highest flow rate (400 L.h^-1), it is expected a more important additive effect, in other words, an increase of the peak corresponding to small droplets. However, it is observed the formation of large droplets ranging from 20µm to 40µm. From this, it is possible to interpret that with increasing flow rate droplets coalescence occurs. As the additive is water soluble, its action can be also related to the quantity of water in the system.

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(a) 90% water cut and 0.005% AA-LDHI

(b) 60% water cut and 0.01% AA-LDHI

(c) 30% water cut and 0.05% AA-LDHI

Figure III.5 – Rheological Study for experiments at (a) 90%, (b) 60% and (c) 30% water cut with additive (AA-LDHI).

121 (a) 90% water cut and 0.005% AA-LDHI

(b) 60% water cut and 0.01% AA-LDHI

(c) 30% water cut and 0.05% AA-LDHI

Figure III.6 – Chord Length Distribution in number during Rheological Study for experiments at (a) 90%, (b) 60% and (c) 30% water cut with additive (AA-LDHI).

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A final analysis refers to the density evolution throughout the rheological study (Figure III.7). This measurement also evidences the additive effect comparing to the blanks (Figure III.4). The density evolution is more regular and the disturbed behavior presents a narrow range when additive is used.

For all water cuts (Figure III.7), the additive effect is evidenced since the beginning of the experiment by detecting a less noisy density measurement comparing to the blanks (Figure III.4).

For intermediate water cut (Figure III.7, green line) the density is still a little noisy due to the presence of the free phase. Once again, a better dispersion is provided as the flow rate is increased.

In agreement with the chord length distribution (Figure III.6 (b)), the density behavior shows that the shear stabilized emulsion partially maintains its new distribution when the flow rate is decreased.

Consequently, the noisy behavior does not reappear.

Figure III.7 – Density evolution during Rheological Study for experiments at (a) 90%, (b) 60% and (c) 30% water cut with additive (AA-LDHI).

An overview of the emulsion behavior throughout the rheological study using the surfactant additive gives the following conclusions:

(1) All shear stabilized emulsions, from high to low water cut (independent of the additive dosage), have a better homogeneity and stability under flow when comparing to the blanks.

This is confirmed by the pressure drop evolution during the study and also evidenced through the density measurements.

123 (2) The peaks observed in the chord length distributions change with the flow rate variation, but this change is less important when the flow rate is decreased after 400 L.h^-1. The presence of additive induces the appearance of smaller droplets with chord length around 10 µm.

(3) A flow rate increase forms a better dispersion with formation of small droplets.

During the rheological study, the flow regime and viscosity is also determined. These topics will be discussed in later section (§ III.3), presenting a comparison between the shear stabilized emulsion and the suspension of hydrates.

Even if these preliminary results were useful to calculate the viscosity of the shear stabilized emulsion and validate its stability and homogeneity under flow, some further work must be done to better understand the behavior of the emulsion and the emulsification process.

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