Studies on New Gas Hydrate Inhibitors

Abstract

Abstract Traditional hydrate inhibitors such as methanol and glycols have been in use for many years, but at today's oil prices the demand for cheaper methods of inhibition will be required. The development of alternative, cost-effective and environmentally acceptable hydrate inhibitors is a technological challenge for the oil and gas production industry. These new hydrate inhibitors can lead to very substantial cost savings, not only for the reduced cost of the new inhibitor but also in the size of the injection, pumping and storage facilities. Thus it is possible to redesign production facilities on a smaller scale. A few chemicals have been tested in field trials already with mixed success. Certainly, there is room for more active and cheaper inhibitors. We will review the relative merits of the two main types of hydrate inhibitor, "kinetic inhibitors" and "anti-agglomerators" regarding their applicability, and discuss how test results obtained in the laboratory compare to real field trials. We will also describe new techniques for comparing the effectiveness and activity of new hydrate inhibitors and present laboratory results on various chemicals. Patented examples will also be mentioned. Finally, a comparison of model systems with real fluids with regards to hydrate formation and inhibition will be given. Introduction The oil industry is more than ever on the look-out for cheaper methods for producing oil and gas. In some mature basins such as the North Sea, to maintain the same level of production as today the oil industry must find cheap solutions to building out the many small and medium fields that have already been discovered. Many of these smaller fields could be built out with underwater installations and coupled to existing platforms. One cheap solution for such fields could be sub-sea multiphase production. This requires knowledge of multiphase flow as well as cheap inhibitor technology for hydrates, wax, corrosion, scale etc. In this case, the cost of inhibiting hydrate formation can be an important factor, even more so in deep water production. Besides chemical treatment and the use of pipeline insulation, a novel method is being developed for a field in the UK sector of the North Sea. This involves maintaining the produced fluids outside the hydrate region by having an outside jacket around the production line into which hot water is injected from the platform. Gas Hydrate Structures Gas hydrates can form several structures. If one is trying to prevent hydrate formation in production, it can be important to know which structure you are dealing with. The most well-known and most researched hydrate structures are called Structure I and Structure II. Structure H is a third form which was more recently discovered and has attracted attention as it contains a large cavity that can hold larger molecules than either structure I or II, e.g. some saturated cyclic hydrocarbons such as cyclopentane. Evidence for Structure H was found in a natural gas hydrate collected from the sea floor in the Gulf of Mexico. However, no proof yet exists that Structure H forms in oil or gas production lines. Which hydrate structure is formed depends mainly on the gas molecules, their size relative to the cavity, and the relative stabilities of the structures. The chemical nature and shape of the guest molecule may also influence the type of structure, Both methane and ethane form structure I hydrates. Methane can occupy both the small and large cavities of this structure but the filling of the small cavities that provides the major stabilising effects. With pure ethane gas the molecules only fit the larger of the two cavities in structure I. The larger hydrocarbon gas molecules, propane, n-butane (with a smaller hydrate former) and iso-butane, form hydrates of structure II entering only the large cavities (which are larger than the large cavities in structure I). A typical natural gas production mixture containing Cl-C4 components will preferentially form structure II hydrates. P. 531