Abstract

The use of Low Dosage Hydrate Inhibitors (LDHIs) is one of the optimum methods to control gas hydrate formation issues and provide flow assurance in offshore 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 gas systems, as well as providing the option to use combination products. This paper will review the use of Kinetic Hydrate Inhibitors (KHIs) and Anti-Agglomerants (AA) through 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, either as individual products or as combination products. The novel illustration of the use of LDHIs in sour conditions will also be provided via case histories at low to moderate subcoolings. Hydrates can form different structures depending on the gas composition of the produced fluids. The nature of the LDHI used needs to take this into account and new product developments in this area will be covered.

Introduction

Natural Gas Hydrates. When considering the production of hydrocarbon fluids from offshore gas systems, flow assurance is a significant issue which needs to be taken into account. An important element of flow assurance is the investigation into the possibility of gas hydrate formation and its subsequent control. The inhibition of gas hydrate formation needs to be taken into account as part of the production of offshore gas, especially as hydrates typically form at lower temperatures and higher pressures. Production facilities, particularly offshore wells and offshore transmission lines, may be operating under conditions where hydrate formation is favourable.

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. Examples of typical hydrate forming gases include Nitrogen, Carbon Dioxide (CO2), Hydrogen Sulfide (H2S) and light hydrocarbons (such as methane through to heptanes). 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.

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