Low Dosage Hydrate Inhibitors (LDHI): Advances in Flow Assurance Technology for Gas Production Systems

Abstract

Abstract The use of Low Dosage Hydrate Inhibitors (LDHI) is one of the optimum methods to control gas hydrate formation issues and provide flow assurance in oil and gas production systems. The application of this technology has several advantages to operators including providing the opportunity for significant cost savings, extending the life of oil and gas systems, as well as providing the option to use combination products. This paper will update the review of the use of Kinetic Hydrate Inhibitors (KHI) and Anti-Agglomerants (AA) through further recent case histories. It will also illustrate how these products can be used in conjunction with Corrosion Inhibitors (CI) as is typically required in gas condensate production systems and oil systems, either as individual products or as combination products. The recent illustration of the development and application of LDHIs for sour systems at moderate sub-coolings in the Middle East will be provided via case histories. Further review will illustrate the use of LDHIs with extended induction times and LDHIs which may be used in conjunction with thermodynamic inhibitors to extend the useful life and effective capacity of glycol systems. Hydrates may form different structures depending on the gas composition of the produced fluids. The nature of the LDHI used needs to take this into consideration; and recent product developments and applications in this area will be described. Introduction Flow Assurance and Natural Gas Hydrates. In the production of natural gas fields and oil fields; the issues of flow assurance are paramount. Such issues can include scale deposition; wax deposition; asphaltene issues and the formation of gas hydrates. These issues can lead to blockages and restrictions which can lead to the impairment of flow. This paper focuses on the important flow assurance issues of gas hydrate formation; leading to the possible formation of gas hydrate plugs and blockages. It is demonstrated how these issues can be successfully overcome in the field; through technology developments which have been appropriately transferred to applications at both oil and gas producing assets. From the beginning of their life cycle, offshore gas fields can require continuous gas hydrate protection. This is required as the produced hot gas cools, as it flows from the well, typically via an uninsulated subsea flowline, leading to the formation of condensate and water. In contrast, oil wells may only start producing water a couple of years into their life cycle. From the point of water production onwards, however, they may require protection from hydrate formation. Gas hydrate formation occurs when natural gas molecules are surrounded by water molecules to form 'cage'-like structures. Gas hydrates are similar in appearance to ice. Both materials have crystalline structures that exhibit similar characteristics - with the important difference that the natural gas hydrate has a natural gas guest molecule as an integral part of its structure.1–4 Examples of typical hydrate forming gases include Nitrogen, Carbon Dioxide (CO2), Hydrogen Sulfide (H2S) and light hydrocarbons (such as methane through to heptanes). Gas hydrates typically form at lower temperatures and higher pressures. The temperature at which hydrate formation occurs is not necessarily very low. Depending on the gas composition and the pressure, gas hydrates can form at temperatures of up to 86°F where gas co-exists with water.5–6 Hydrate Plug Remediation - Options and Costs; and the Control and Inhibition of Natural Gas Hydrate Formation. Hydrate plug formation and subsequent remediation can be a costly occurrence. Hydrate plugs may take days to months to dissociate depending on the system conditions and the remediation actions taken. This is costly in terms of deferred production. The action of trying to locate a blockage, particularly in an offshore production system, is also difficult. An important part of the design and operation of both onshore and offshore production systems concerns hydrate inhibition and control of hydrate formation, in order to prevent the formation of hydrate blockages.7–8