In this Case Study, AFS compares the performance of a high-pressure rheometer with an autoclave for hydrate risk evaluation and inhibitor assessment. This overview summary is based on the paper “Utilizing the High-Pressure Rheometer for Comparison of LDHI Effectiveness” originally presented at the Offshore Technology Conference (OTC-OTC-28717-MS-MS).
The industry currently uses three primary testing approaches for hydrate evaluation studies, from most to least conservative:
– Rocking cells
Advantage: lower sample volume requirement, simultaneous testing possible
Disadvantage: high requirements for an LDHI “pass”
Advantage: closer to field conditions
Disadvantage: higher volume requirement
– Flow loops
Advantage: most “field like”
The high-pressure rheometer has been used extensively to study hydrate slurries and provides the possibility for highly accurate hydrate evaluation studies using small sample volumes.
Using a range of fluids and operating conditions, multiple field operation scenarios were simulated (eg. steady state, shut-in, restart) in both an autoclave and a high-pressure rheometer. The test parameters for both the autoclave and high-pressure rheometer have been developed based on experience and rigorous research in the AFS laboratory to best replicate field conditions. Test evaluation criteria was also established which determines whether a test is “pass” or “fail” from a hydrate risk perspective.
Once the pass/fail criteria was established, direct comparisons of the results such as hydrate formation, torque response and overall hydrate risk assessment were used to determine the ability of the rheometer to replicate autoclave results. Results showed that the rheometer not only replicated the results but demonstrated a higher sensitivity to the fluid changes after hydrate formation.
Additionally, and somewhat unexpectedly, the rheometer also replicated several unique occurrences observed in the autoclave in very similar fashion, such as two distinct hydrate formation events during a test. The examples below (Figures 1 and 2) show two distinct hydrate formation events in the autoclave and the high-pressure rheometer. In each case, the first hydrate formation event occurs during the cooldown period while the second hydrate formation event occurs a few hours after reaching the steady state phase of the test.
Figure 1: Unique Hydrate Formation within the Autoclave
Figure 2: Unique Hydrate Formation within the High-Pressure Rheometer
In summary, of the 38 tests studied, only two test results were different with respect to pass/fail between the equipment. In each of those experiments, the higher sensitivity of the rheometer provided more detailed insight into the behavior of the fluids, leading to untreated samples to be considered failures.
The results of the study showed that the high-pressure rheometer had good agreement with the autoclave while providing increased sensitivity and allowing for the utilization of smaller sample volumes. While more research is necessary to further validate the performance of the rheometer in evaluating hydrate risk, it has potential to provide an effective method of performing hydrate evaluation studies in the future.