Relationship between Hydrate-Free Hold Time and Subcooling for a Natural Gas System Containing Kinetic Hydrate Inhibitors

Transcription

1 Relationship between Hydrate-Free Hold Time and Subcooling for a Natural Gas System Containing Kinetic Hydrate Inhibitors This study observed simple chemical kinetics behavior for the first appearance of hydrates in a natural gas system containing kinetic hydrate inhibitors (KHI s). Four KHI s were tested in a flowing miniloop with fluids comprised of a synthetic gas mixture, a stabilized natural gas condensate and fresh water. The Hold Time or time to first indication of hydrate formation was determined as a function of KHI concentration and subcooling. The results fit an Arrhenius type function of the natural log of the reciprocal of the Hold Time versus subcooling. The experimental work was done at one pressure, 2250 psig (15.5 Mpa). The observed Hold Times ranged from minutes to 900 hours. The range of KHI concentrations tested was from 0.025% to 0.30 % weight of active polymer. The range of subcoolings tested was from 9 o F to 25 o F (5 o C to 13.9 o C). The procedure for measuring Hold Time was to first determine the equilibrium dissociation temperature of the fluids in the loop before adding any KHI. This was done at least twice for every time the loop was charged with fluids. Next, a KHI was injected at the lowest concentration and Onset experiments were performed. An Onset run is where the miniloop is cooled monotonically until hydrate formation is detected. The loop was cooled from above the hydrate temperature at a rate of about 6 o F per hour. The temperature at which hydrate formation occurred rapidly was determined at least twice. Figure 1 is an example of an Onset run. In this case, hydrates were first detected at 5.8 hours by the observation of noise in the differential pressure curve (Pump DP). The differential pressure is measured across the gear pump that circulates the fluids in the miniloop. Slight changes in the apparent viscosity of the fluids result in changes of the differential pressure. Hydrates cause an erratic change in the viscosity and the differential pressure before there is any visible indication of hydrate formation. The Hold Time for an Onset run such as this is typically set equal to 0.1 hours, a sort of default value for a time that is not explicitly determined. After measuring Onset temperatures, the Hold Time was measured for various values of Subcooling. The miniloop was cooled from above the hydrate temperature to a specific temperature above the Onset temperature and below the equilibrium temperature. The loop temperature was held isothermally until hydrate formation was detected. The amount of time the contents of the loop were hydrate free at the isothermal temperature was defined as the Hold Time. In normal determinations of the Hold Time, the miniloop temperature is lowered to a fixed temperature by the same path as the Onset run, but hydrate formation

2 is slow and does not occur for hours or days. Figure 2 is an example of a 251- hour Hold Time observation. The first indication of hydrate formation is the erratic behavior of the differential pressure starting at 256 hours. Some of the Hold Time runs resulted in hydrate formation rates that were so slow that it was necessary to terminate the run before hydrate formation was observed. When this occurred, the data point was included in the figures as a data point at the last observed Hold Time with an arrow pointing to longer Hold Times to indicate that hydrates had not formed. Figure 3 is a semi-log plot of (1/Hold Time) versus subcooling for three concentrations of ekhi polymer. These results can be summarized by three linear functions that have a common slope at all KHI concentrations and subcoolings tested. One possible assumption is that the true functions of (1/Hold Time) versus Subcooling cannot cross each other for different concentrations. Therefore, in this study it was assumed that the true function is best fit by linear functions that are parallel. The best fit to the data was obtained by regressing each set of data for each KHI concentration to determine the slope of the line at each concentration. The slopes were averaged to obtain one common slope. The data for each concentration were then fit to the regression line by adjusting only the value of the intercept. This method is called the Forced Intercept Method. The resulting curves for the functions derived in the procedure described above were calculated in a spreadsheet and plotted over the experimental Hold Time data for three KHI s. The calculated results and the corresponding experimental data are shown in Figures 4, 5 and 6. The plots of Hold Time versus Subcooling show the same trend for all four KHI s. It was observed that the Hold Time increased exponentially as the subcooling value of the isothermal temperature step was decreased. This behavior has been observed in other gas condensate systems. The apparently simple kinetics observed for these experiments are affected by certain species not present in these experiments. The kinetics become complex and do not fit the simple Arrhenius relationship when certain components are present.

3 Figure 1. Earliest hydrate formation detected by differential pressure across gear pump during cooldown to maximum measurable formation rate at 49 o F.

4 Figure 2. Earliest hydrate formation detected by differential pressure across gear pump after 251 hours Hold Time at 53 o F.

5 Figure 3. Plot of Ln (1/Hold Time) versus Subcooling for ekhi polymer

6 Figure 4. ekhi Hold Time versus Subcooling (ekhi is the polymer of isopropylmethacrylamide)

7 Figure 5. TROS 544 Hold Time versus Subcooling

8 Figure 6. Ondeo Nalco EC6481A Hold Time versus Subcooling