Abstract

This paper details three field applications of combined low dose hydrate (LDHI) and corrosion inhibitor (CI) chemicals in different assets in the Southern North Sea (UKCS) gas producing area. The design and application philosophy is discussed as well as the criteria necessary to manage a safe and efficient chemical transition. The three fields all displayed mid-level degrees of sub-cooling (4 - 8 °C) with operating pressures up to 70 bar with variable water breakthrough. Corrosion was severe in some cases with over 700 ppm of H2S production combined with 1.1 mol % CO2 in the produced gas.

The paper goes onto describe the cost benefits of such applications including increased equipment efficiency, logistical savings of single chemical deployment and lower maintenance costs. In addition improved hydrate and corrosion control was achieved over the incumbent chemical. This approach achieved cost savings, including a saving of $3 million in the first year of application on one of the fields. Environmental benefits have also been realised with reduced chemical usage and discharge and improved environmental profile of the combined products when compared to the originally selected single application chemicals.

Introduction
Hydrates

Hydrates were first described in 1810 by Sir Humphrey Davy[1] and were reported as forming when gas (predominantly methane) and water combine under suitable temperatures and pressures. Although snow like in appearance, they can form at temperatures much higher than the freezing point of water. Hydrates are actually cage-like structures called clathrates and have two common forms - Type I and II.[2] Hydrates typically form with low molecular gas compounds such as methane, ethane and propane. Other species such as nitrogen, carbon dioxide and hydrogen sulphide can also promote hydrate formation. Hammerschmidt reported the formation of hydrates in gas pipelines in 1934.[3] Hydrates are known to plug flowlines, pipelines, valves and other equipment whilst removing hydrate blockages is a dangerous task potentially resulting in a hydrate missile. This can be caused by the rapid dissociation of a hydrate at the outer edges combined with the pressure build up caused by the blockage.

A number of methods exist for controlling hydrates but the most common method is the use of thermodynamic inhibitors, such as methanol or glycol (monoethylene glycol, MEG). These treatments are effective by lowering the freezing point of an aqueous solution, similar to anti-freeze in a car engine. Methanol is a low cost chemical which can be recovered from the process steam but increases risk to the environment and in handling. MEG is also recoverable but requires higher injection rates than methanol and viscosity can be a limiting factor for injection via long sub sea tie-backs. MEG regeneration can also be subject to salt fouling. Triethylene glycol (TEG) can be used to de-water wet gas and as such remove a required component of hydrates.

In order to overcome the issue of supply of large volume of chemical, regeneration, handling and environmental issues, a new generation of low dose hydrate inhibitors (LDHI) were developed. Two types of LDHI presently exist; kinetic hydrate inhibitors (KHI) and anti-agglomerates (AA). The particular type selected for hydrate control is dependent on a number of factors including the severity of the hydrate problem, the water cut in the pipeline, the amount of hydrocarbon present and the period of time hydrate control is required.[4] KHI's work by delaying initial hydrate nucleation. The mechanism of inhibition does not alter the thermodynamics of hydrate formation, but is a surface adsorption phenomena of the inhibitor such that growth is retarded, thus delaying hydrate formation for a given time, known as induction time. AA's differ from kinetic inhibitors in that they allow a certain amount of growth of gas hydrate but then act to suppress the continued propagation and agglomeration by dispersing the hydrates in the oil or condensate phase. With AA's the brine:hydrocarbon fluids ratio and composition of these fluids are more influential on performance. Both KHI's and AA's are referred to as LDHI's since much lower treatment rates are required than compared to thermodynamic inhibitors.

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