Benefits of Low-Dosage Hydrate Inhibitors

Abstract

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 The paper has not been peer reviewed.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. 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.