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Benefits of Low-Dosage Hydrate Inhibitors

Topics: Flow assurance

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This paper summarizes historical advancements in low-dosage hydrate inhibitors (LDHIs) over the past 2 decades, discusses their advantages and limitations, and their selection criteria. Historically, hydrate risk has been managed by keeping fluids warm, removing water, or injecting thermodynamic hydrate inhibitors (THIs), commonly methanol (MeOH) or monoethylene glycol (MEG). THIs require high dosage rates; therefore, that technique can pose limitations to production systems in the form of supply, storage, and umbilical-injection constraints. Additionally, high dosages of MeOH can cause crude contamination for downstream refineries. LDHIs continue to offer significant efficiency and cost benefits over other techniques.

Introduction

THIs have long been used by the industry because of their ability to shift the hydrate-equilibrium curve toward higher pressures and lower temperatures by changing the activity of water molecules. MeOH and MEG have proved the most popular types of THI because of their low cost and widespread availability. Though THIs are low-cost, the volume requirement per barrel offsets the cost benefit.

A group of chemicals developed later, LDHIs, differ from THIs because they do not shift the hydrate curve; rather, they interfere with the process of hydrate formation through different mechanisms. The dosage requirement of these inhibitors is much lower than that required for THIs. LDHIs include two categories, kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs).

KHIs increase the induction time for hydrate formation by interfering with the nucleation process or the crystal-growth process. Special surfactants disperse hydrate particles as they form, reducing the tendency of hydrates to stick to the pipe’s inner surface, reducing the chance of plug formation.

AAs modify the agglomeration of hydrate particles, decreasing hydrate-crystal size by affecting hydrate morphology. This leads to a hydrate slurry flow, which decreases the chance of pipeline blockage. Inhibition mechanisms of both KHIs and AAs are discussed in detail in the complete paper, as is the historical development of these LDHIs.

LDHI Applications

Performance Parameters. The two primary parameters for LDHI performance measurement include the following:

Hold Time. This is defined as the time between when a rapidly cooled fluid is at a constant temperature below the ­hydrate-equilibrium temperature and when hydrates first appear. For KHIs, the hold time decreases steeply with the increase in subcooling.

Dosage Rates. To establish actual dosage rates for a LDHI in a specific fluid, technical-feasibility testing would be required with the actual LDHI and fluid under actual pressure and temperature conditions. An approximate range of expected LDHI dosage rates can be given as examples as follows.

Gas System. For a gas system operating with condensed water, the dosage-rate guideline is 5 gal LDHI/bbl of condensed water. Breakthrough of formation water with a saline content of at least 3 wt% would reduce the rate to between 0.5 and 1.0 gal LDHI/bbl of formation water, although it should be recognized that this would be a different LDHI from the one used with condensed water.

Oil System. For an oil system, the dosage rate guideline is 1–3 gal LDHI/bbl of water, a guideline based upon a limited number of oil tests in the Gulf of Mexico. In a number of these tests, LDHIs had to be combined with other chemicals to ensure satisfactory performance, which should be taken into consideration when dealing with waxy, asphaltic crudes.

Temperature Limitations. Some LDHIs will degrade at elevated temperatures over time and will cease to be effective. If temperature-performance data are not available on the actual LDHI to be used, it is recommended to consider 99°C as an upper temperature limit. Any production exposed to these temperatures for an extended period of time should be considered to be degraded. Therefore, these sections will need to be purged of degraded LDHIs before use in planned shutdowns. Depending on the size and length of downhole tubing, this could amount to an extra 30 gal of LDHI for each planned shutdown.

Viscosity. Because production systems operate in wide pressure and temperature ranges, LDHI viscosity is a key performance parameter. In general, the viscosity reduces with increasing temperature and increases with increasing pressure, as shown in Figs. 1 and 2.

Fig. 1—LDHI viscosity change with temperature.

 

Fig. 2—LDHI viscosity change with pressure.

LDHI Operations

KHIs. The physical properties of KHIs are highly variable depending upon the products used. Most KHIs are predominantly water-soluble polyamides that are formulated mostly in a solvent package having a high flash point. Typical viscosities range between approximately 10 and 100 cp at a temperature of 60°F. KHIs are only soluble sparingly in water at temperatures above 104°F and care must be taken that they do not precipitate when injected in a hot production stream in which the temperature is higher than the cloud point.

AAs. Operations with AAs are not distinctly different from operations with THIs as long as the system is designed for using AAs. However, for AAs to perform optimally, a liquid hydrocarbon phase must be present to act as the carrier of the hydrate crystals. AAs can be used in gas/condensate systems when the volume fraction of hydrates is not higher than 40% of the total liquid volume. For oil systems, this number may be much lower (less than 20 vol%) because the viscosity of the suspension increases with increasing viscosity of the carrier oil, whereas particles can also contribute to the total solid loading of the oil, which may impair pipeline-flow capacity. AAs also have minimum ­requirements for water salinity.

Because AAs are designed to keep hydrates dispersed in oil or condensate, the hydrates separate exceedingly slowly from the hydrocarbon liquid. The produced suspension must be heated before the water can be separated from the oil; thus, a topside heater for hydrate slurry is required.

AAs are superior to KHIs with respect to the maximum achievable subcooling. In general, AAs have no limit to the maximum allowable shut-in time, which is another major advantage of AAs over KHIs.

LDHI Screening Considerations

KHIs. Although KHIs are applicable under most producing scenarios, certain conditions must be considered when evaluating a potential application. At water salinity levels greater than approximately 17% sodium chloride, the polymer may come out of the solution, thereby reducing KHI effectiveness. A solution of KHI in water does not provide protection from freezing or icing conditions in the line being treated or in the KHI storage tank. A solution of KHI cannot be used for melting ice or hydrate plugs. It is recommended to have other strategies in place, such as a small quantity of THIs, for remediation purposes in the event of a blockage.

The delivery system for KHIs must be designed to provide sufficient dosage to achieve a hold time greater than the water residence time in the pipeline. If the gas is undersaturated with respect to water, the water in the KHI solution will evaporate and leave a high-viscosity fluid. This can be addressed by using more MEG.  

AAs. As with KHIs, AAs are applicable under most producing conditions, but other factors must be taken into consideration when evaluating a potential application. Some AAs have a maximum salinity criterion that is normally not exceeded with produced water.

Because AAs disperse (i.e., emulsify) polar hydrate crystals in a nonpolar oil or condensate phase (i.e., continuous phase), they may sometimes require a demulsifier for oil and water separation. Furthermore, the addition of a heater upstream or heat coil inside a separator may be required to melt the hydrate crystals.

Another consideration for AAs is that the water cut should be less than 50%. Higher water cuts can invert the emulsion from water-in-oil to oil-in-water, therefore making the AA ineffective.

Conclusions

LDHIs have provided considerable benefits during the past 2 decades in comparison with THIs. Significantly lower inhibitor concentrations (from 0.1 to 1.0 wt% polymer in the free-water phase) result in lower dosage rates than those required for MeOH.

The advantages of LDHIs also include

  • Lowering of inhibitor loss caused by partitioning into the gas phase, particularly in comparison with MeOH
  • Reduction of capital expenses through decreased chemical-storage and injection-rate requirements
  • No requirement for regeneration because the chemicals are not yet currently recoverable. This is especially important for offshore operations where payload and deck space are critical.

LDHIs have been reducing ­operating expenses in many cases through decreased chemical consumption and delivery frequency and thus transportation and handling costs. Additionally, production rates can increase wherever ­inhibitor-injection capacity or flowline capacity is limited. LDHIs also lower the maintenance of pump and delivery systems, eliminate MeOH in topside and downstream operations, and reduce environmental and safety risks because of lower toxicity.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 28905, “Advances in LHDIs and Applications,” by Y.D. Chin and A. Srivastava, Subsea Engineering Technologies, prepared for the 2018 Offshore Technology Conference, Houston, 30 April–3 May. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.

Benefits of Low-Dosage Hydrate Inhibitors

01 September 2019

Volume: 71 | Issue: 9

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