Studies on some alkylamide surfactant gas hydrate anti-agglomerants

https://doi.org/10.1016/j.ces.2006.02.016Get rights and content

Abstract

Low dosage hydrate inhibitors (LDHIs) are a recently developed hydrate control technology, which can be more cost-effective than traditional practices such as the use of thermodynamic inhibitors e.g., methanol and glycols. Two classes of LDHI called kinetic inhibitors (KHIs) and anti-agglomerants (AAs) are already being successfully used in the field. This paper describes efforts to develop new classes of AA surfactant with one or two alkylamide groups in the polar head. The goal was to find an AA that was as good as commercial quaternary AAs, which would be economically competitive and more environmentally friendly. The chemistry and environmental properties of the new surfactants are described along with experiments to determine their performance carried out in high-pressure sapphire cells and a wheel loop. The results indicate positive performance for some products but not as good as a commercial quaternary ammonium-based surfactant AA. The best surfactants had one or two carbonylpyrrolidine or isopropylamide groups in the head. The performance of the best AAs was found to be dependent on the hydrocarbon phase and salinity of the water phase.

Introduction

Gas hydrates are clathrates in which water molecules form a hydrogen-bonded network enclosing roughly spherical cavities that are filled with gas molecules (Sloan, 1998). Gas hydrate can form several structures, but a typical natural gas mixture containing C1C4 components will preferentially form Structure II hydrates due to the presence of C3-4 components. The equilibrium conditions for gas hydrate formation are determined by the fluid pressure and temperature. At a pressure of 75–100 bar it is not uncommon that natural gases, light condensates and crude oils containing C1C4 components will form hydrates when the temperature falls below 18–20C. Thus at a seabed temperature of 4–6C, which is quite common in the North Sea, hydrate plugs can form in a sub-sea pipeline either during multiphase flow or shutdown.

Plugs caused by gas hydrate formation are a menace to the oil and gas industry in production lines, during drilling (especially in deep water), and in work-over operations. Methods to avoid hydrate plugs include raising the temperature/heating (e.g., insulation, bundles, electric or hot water heating), lowering the pressure, removing the water and shifting the equilibrium for hydrate formation by adding anti-freeze chemicals. These techniques are often very expensive (such as heated pipelines or the need for methanol regeneration facilities), or do not provide a complete solution (e.g., sub-sea water separation). Hence, there is a clear need for cheaper technologies.

In the last 10–15 years low dosage hydrate inhibitors (LDHIs) have been developed which in many cases can be significantly cheaper to deploy than other methods just described (Kelland, 2006). In particular, the design of a new field development with LDHI technology can give large CAPEX savings (Frostman and Crosby, 2003).

There are two main classes of LDHI:

  • kinetic hydrate inhibitors (KHIs),

  • anti-agglomerants (AA).

KHIs delay hydrate nucleation (and usually also crystal growth) so that one has enough time to transport the fluids to the process facilities before hydrates build up in the line. The higher the subcooling (driving force for hydrate formation), the shorter the induction time to hydrate nucleation. Commercial KHIs are based on specific classes of water-soluble polymer. Unlike AAs, KHIs can also be used in gas lines where there is no liquid hydrocarbon phase.

AAs prevent the agglomeration and deposition of hydrate crystals such that a transportable hydrate slurry is formed in a liquid hydrocarbon phase (Kelland et al., 1994). There are two mechanisms known for this process. The first mechanism discovered by workers at IFP uses a special type of emulsifier (usually polymeric). The emulsifier not only forms water-in-oil emulsions but confines hydrate formation to the water droplets, preventing their agglomeration (Behar et al., 1991). The second method discovered by researchers at Shell involves the use of surfactants designed to attach to hydrate crystal surfaces (Klomp et al., 1995). The head of the surfactant is “hydrate-philic”, the tail (or tails) are hydrophobic. Attachment of the polar head group to the hydrate crystal surface disrupts the hydrate growth process slowing down crystal growth. The tails make the crystals oil-wet making them easily dispersed in the liquid hydrocarbon phase. AAs useful for gas wells have also been developed (Lovell and Pakulski, 2002).

Both KHIs and AAs are added at low concentrations, often around 0.1–1.0 wt% active concentration. This can be contrasted with the 10–50 wt% needed for thermodynamic inhibitors or “anti-freezes” such as methanol, glycols or salts (Kelland et al., 1995). The two new types of additive have different field application ranges related to performance, field conditions, fluid properties, and the properties of the additives, including their environmental impact. The best commercial AAs can perform at higher subcoolings than KHIs. Therefore, for deep water applications AAs are the only class of LDHI that can be deployed.

AAs based on quaternary ammonium surfactants are now commercially available and being deployed in a number of fields (Frostman, 2000, Frostman and Downs, 2000, Frostman and Przybylinski, 2001, Frostman et al., 2003, Mehta et al., 2002, Klomp et al., 2004, Thieu and Frostman, 2005). However, in sectors such as the Norwegian North Sea these products are not considered environmentally friendly. Therefore, it was our goal to find alternate chemistries that would be greener but could also compete on a performance and cost basis with the quaternary AAs. This paper discusses new classes of alkylamide surfactant AAs, results of their performance in cells and wheel loops as well as their environmental properties. The test procedures used are discussed first, then the design and testing of chemicals is given in a chronological order.

Section snippets

Test equipment and experimental procedures

Tests were carried out in two different types of equipment, a high-pressure sapphire cell and a wheel loop simulator. The sapphire cell system, the wheel simulator and experimental procedures are described in details elsewhere (Kelland et al., 2006).

Results and discussion

The basic idea was to find an alternative head group to the quaternary ammonium group in AA surfactants that would interact with hydrate surfaces thus reducing the growth rate of crystals. Since it was known that certain KHI polymers reduced the growth rate of hydrate crystals it seemed appropriate to place the active groups in KHIs into AA surfactants. The most studied polymers are polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap). Both polymers are known to interact well with

Conclusions

The chemistry and environmental properties of new alkylamide surfactant AAs have been described along with experiments to determine their performance carried out in high-pressure sapphire cells and wheel loops. The results indicate positive performance for some products up to 13–16C subcooling but not as good as a commercial quaternary ammonium-based surfactant AA. The best surfactants had one or two carbonylpyrrolidine or isopropylamide groups in the head (e.g., FX-OTMAA/LTMAA 5/1 and

Acknowledgements

The authors wish to thank the members of our joint industry project BP, Total, Statoil, Conoco Phillips and Clariant for financial support of this work.

References (29)

  • Z. Huo et al.

    Chemical Engineering Science

    (2001)
  • B. Kvamme et al.

    Journal of Molecular Graphics and Modelling

    (2005)
  • T.F. Makogon et al.

    Journal of Crystal Growth

    (1997)
  • Behar, P., Kessel, D., Sugier, A., Thomas, A., 1991. Advances in hydrate control. Proceedings of the 70th Gas...
  • T. Carver et al.

    Faraday

    Transactions

    (1996)
  • Frostman, L.M., 2000. Proceedings of the SPE Annual Technical Conference and Exhibition, SPE 63122, 1–4 October,...
  • Frostman, L.M., Crosby, D., 2003. Low dosage hydrate inhibitor experience in deep water. Deep Offshore Technology...
  • Frostman, L.M., Downs, H., 2000. Proceedings of the Second International Conference on Petroleum and Gas Phase...
  • Frostman, L.M., Przybylinski, J.L., 2001. Proceedings of the International Symposium on Oilfield Chemistry, SPE 65007,...
  • Frostman, L.M., Thieu, V., Crosby, D.L., Downs, H.H., 2003. Proceedings of the SPE International Symposium on Oilfield...
  • Kelland, M.A., 2006. Energy and Fuels, in...
  • Kelland, M.A., Klug, P., 1998. World Patent Application WO...
  • Kelland, M.A., Svartaas, T.M., Dybvik L., 1994. Proceedings of the Society of Petroleum Engineers Annual Technical...
  • Kelland, M.A., Svartaas, T.M., Dybvik, L., 1995. Proceedings of the Society of Petroleum Engineers Offshore European...
  • Cited by (0)

    View full text