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Study Evaluates Hydrate Antiagglomerants

Topics: Flow assurance

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A common hydrate-management strategy involves the use of large volumes of thermodynamic inhibitors (THIs) to operate outside the hydrate-stability region, However, this strategy represents significant capital expenditure and operating costs. Low-dosage hydrate inhibitors (LDHIs), in the form of kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs), present an economical alternative to THIs. In this study, a quantitative micromechanical force (MMF) has been deployed to study the performance of seven industry AAs. The results illustrate that an effective AA is one that lowers the cohesive forces between hydrate particles.

Introduction

AAs prevent hydrate agglomeration of a steady-state hydrate slurry. Hydrate-philic AAs are surfactants that adsorb to the hydrate/oil interface in preference to the water/oil interface. Current operating experience suggests that AAs may be unsuited to the high water cuts that are characteristic of late field life. A successful hydrate AA lowers the cohesive force or surface free energy of hydrate particles to prevent agglomeration. Rocking cells and high-pressure visual autoclaves provide a semiquantitative assessment of hydrate blockage. However, a knowledge gap exists with regard to whether current industry AAs actually lower hydrate interparticle cohesive and surface free energy. Techniques to deconvolute the mechanism of the action of AAs are required to better understand how AAs affect hydrate blockage.

Experimental Procedure

Micromechanical Hydrate Cohesive Force. A third-generation MMF apparatus was used to measure the cyclopentane hydrate interparticle cohesive force. The MMF uses cyclopentane to create Structure II cyclopentane hydrates, the same hydrate structure present in the field. The cohesive-force-measurement technique was adapted from previous research on hydrate cohesion and asphaltene cohesion.

Interfacial Tensiometry. An optical interfacial tensiometer was used to measure the interfacial tension (IFT) between water and oil phases at ambient pressure and room temperature. A hooked needle with the AA prepared in paraffin oil was placed in a bulk phase of deionized water. The resolution of this apparatus is typically ±1 mN/m and the threshold sensitivity is 1 mN/m. Typically, surfactant-free systems will rapidly reach equilibrium within 1 minute of creating an oil droplet in water. Conversely, the presence of a surfactant may decrease the IFT as a function of time, asymptotically approaching a steady-state value. Because the AAs studied contain surfactant functionality, each IFT was measured for at least 20 minutes, after which point the IFT was not observed to change with time. Each data point represents an average of at least four independent trials.

High-Pressure Visual Autoclave. A visual high-pressure sapphire autoclave was used to measure the resistance to flow of a hydrate slurry in the presence of AA. The experiment was conducted isochorically, where the autoclave cell temperature was controlled by a glycol bath with constant cooling and intermittent heating. In this experiment, ultrahigh-purity methane was used to form hydrate, which forms Structure I hydrates. While there are mild thermophysical-property differences between Structures I and II, a single-component gas was chosen to minimize uncertainty in calculating hydrate volume fraction (HVF) caused by preferential denuding of the gas phase.

Results

In each MMF experiment, two hydrate particles with radii between 100 and 350 µm were prepared and a hydrate/hydrate baseline measurement was taken (consisting of 40 individual pull-off trials). To control for abnormal roughness or morphological characteristics below the optical resolution of the microscope, the baseline measurement was compared with the baseline reported in the literature. If the experimental value was within ±30% of the baseline in the literature, the experiment proceeded to AA trials. Hydrate/hydrate cohesive-force measurement at different mass fractions of AA-2 were compared with oil/water IFT measurements of AA-2 at varying mass fractions.

The hydrate/hydrate cohesive-force measurements are reduced by more than 80% from baseline at the highest concentration reported. In the case of AA-2, similar behavior is observed for the oil/water IFT measurements where the IFT exists at an order of magnitude of higher concentration. It can be concluded that AA-2 is a powerful surfactant because it reduces the oil/water IFT by over an order of magnitude (to the sensitivity limit of the apparatus, less than 1 mN/m). This is suggestive of a strongly emulsifying chemistry which may produce downstream emulsification problems in the flowline. The reduction in the hydrate/hydrate cohesive force is observed at a lower concentration than the reduction in the oil/water IFT. Strong reductions in the hydrate/hydrate cohesive force and the oil/water IFT likely imply that wettability changes to the hydrate surface and water/oil interface have taken place.

Without any AA in the system, hydrates are water-wet and a water droplet will adhere favorably to the hydrate surface. With the addition of enough AA-2 such that it is saturated in the system, the hydrate surface was observed to become oil-wet and rejected a water droplet with which it had come into contact. This evidence supports the hypothesis that surfactant adsorption reduces the hydrate/oil surface free energy.

Using the MMF technique, a quantitative ranking of AAs was deployed to measure how effective each AA was at reducing hydrate cohesive force. This ranking is presented in Fig. 1, where two investigations were conducted:

  • Four generations of a vendor’s chemistry (compared in Fig. 1a)
  • Four current chemistries on the market are compared (Fig. 1b). The plots on the right side are generated by taking the highest hydrate-cohesive force reduction measured at the highest concentration for each AA and reporting the reduction as a percentage reduction from baseline.
Fig. 1—Comparison of how hydrate cohesive force changes with surfactant/AA mass fraction (left) and the final hydrate cohesive-force reduction (right) for: (a) four successive generations of AA formulations for a particular vendor and (b) four current AA formulations. Error bars are 95% confidence intervals for the 40 measurements.

 

The figure illustrates that all AAs reduce hydrate cohesive force, but not all AA chemistries reduce hydrate cohesive force equally. Later generations of AA chemistries provide much stronger reductions in the hydrate cohesive force, and the produced effect is more consistent. For all current-generation chemistries, the hydrate cohesive-force reductions exceed 80%. This consistent result can be used as a benchmark for the minimum reduction of hydrate cohesive-force for AA performance; that is, a current commercial AA should be generating a greater than 80% hydrate cohesive-force reduction. The hydrate cohesive force and oil/water IFTs were compared for AA-2 using the MMF and IFT, respectively. These results supported visual observations in the MMF about wettability changes in the system prompted by the AA. It is unclear, however, whether AAs that strongly reduce hydrate cohesion must also change wettability.

While AA-2 is an effective AA, providing a greater than 80% reduction in the hydrate cohesive force, it also has over an order of magnitude reduction in the oil/water IFT, modifying the system wettability. However, other AAs produce a similar reduction in the hydrate cohesive force without the order of magnitude reduction in the oil/water IFT.

To compare the results from the MMF with those of a more-traditional technique for AA qualification, all seven AAs were tested in a high-pressure visual sapphire autoclave under the conditions outlined in the methodology section of the complete paper. In this work, the authors have used a condensate from a field in northwest Australia. An appreciable rise in relative torque after 25% HVF is observed and an erratic torque signal in the autoclave indicates substantial hydrate growth and agglomeration. All AAs produce a lower relative torque than the one seen in the blank system. In comparing systems at the same HVF, it can be inferred that the hydrates must be more dispersed relative to the blank system, implying that all AAs disperse hydrates to a degree. Secondary observations can be made with respect to AA-6, where a much lower rise in the relative torque followed by a sharp spike at approximately 36% HVF is observed. This is indicative of jamming behavior, a conclusion supported by observations of wall deposition in the cell. For current-generation AAs, low relative torques are observed (within a factor of two of the relative torque before hydrate formation). In developing ranking criteria, a metric that a maximum relative torque of 1–2 for an AA can be considered a “pass” is consistent with the current data. Interestingly, AA-2 (purple diamonds, Fig. 1) limited the total conversion of hydrate to approximately 20% HVF, suggesting that the AA likely has secondary crystal-growth-­inhibition characteristics. These might be slowing the growth such that full conversion is not achieved in the duration of the autoclave experiment, which is approximately 24 hours. For all AAs, visual observation is critical because significant deposition or ejection of hydrate outside of the liquid onto the walls in the gas overboard can lead to an underestimation of the relative torque required to drive a certain HVF in the fluid.

Conclusions

In this work, the performance of seven AAs in the MMF were compared and the maximum cohesive-force reduction obtained by each AA was reported. These results have been compared with the maximum oil/water IFT reduction for each AA. Of the seven AAs studied, four were current-generation and three were previous generations of AA chemistries. A general improvement in the cohesive-force reduction exists with successive generations of AA chemistry. Current-generation AAs reduce hydrate cohesive force more than 80% from baseline. An AA that strongly reduces hydrate cohesive force does not necessarily reduce oil/water IFT by the same amount. In comparing these results with those of AA performance in a high-pressure visual autoclave, it is evident that current-­generation AAs maintain the relative torque of the system with hydrate within twice the system torque before hydrate formation, indicating a good correlation between the results in the MMF and the autoclave.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 28578, “Quantitative Ranking and Development of Hydrate Antiagglomerants,” by Shane A. Morrissy, Temiloluwa O. Kuteyi, Mike L. Johns, Eric F. May, SPE, and Zachary M. Aman, SPE, University of Western Australia, and Stuart F. McKay, SPE, Woodside Energy, prepared for the 2018 Offshore Technology Conference Asia, Kuala Lumpur, 20–23 March. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.

Study Evaluates Hydrate Antiagglomerants

01 September 2019

Volume: 71 | Issue: 9

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