A Corrigan, S N Duncum, A R Edwards, C G Osborne
A series of field trials to test Threshold Hydrate Inhibitors (THI) have been carried out in the wet gas line connecting the Ravenspurn and Cleeton fields. These BP operated assets are located in the UK sector of the Southern North Sea, approximately 50 miles offshore of the Dimlington gas terminal. The flow line is 13 miles long, 16″ in diameter with a normal production rate of 195 mmscf/d (million standard cubic feet per day). During the trial the line pressure was maintained at 75 barg for a throughput of 90 mmscf/d. When methanol injection into the line was stopped, hydrates readily formed. Conditions in the line were 8-9 0C inside the equilibrium hydrate curve. This resulted in an estimated 60- 70 tonnes of hydrate accumulating in the line over a four day period. At these same conditions THI injected at a dose rate of 3000-5000 ppm (based on total water production) prevented hydrate formation. After six days the chemical supply was exhausted and after a delay during which chemical was flushed from the system, hydrates formed as before. The robustness of the chemical to line shut-in condition was also confirmed. These trials conclusively show that a low dosage chemical can be used for hydrate suppression in a wet gas pipeline system.
Gas hydrates are snow like crystalline solids. They are formed by certain low molecular weight hydrocarbons combining with water under conditions of temperature and pressure commonly found in flow lines carrying hydrocarbon fluids during normal production. These compounds contain gas molecules trapped in a metastable “host” crystal lattice made up of water molecules forming a three-dimensional structure. Gases that form hydrates are light, non-polar, and generally have low solubility in water. They are usually C1 to C4 inclusive and may be paraffins or olefins. Other gases found in oil field fluids such as CO2 and H2S will also form hydrates under favourable conditions. Conditions favouring hydrate formation are high pressures (typically >30 bar) and low temperatures (typically <20 C).
Hydrates can form in wet gas, condensate or black oil lines as well as being a natural occurrence in many parts of the world on or below the sea bed in the proximity of a natural gas accumulation. Gas hydrate formation can cause problems during hydrocarbon production by blocking pipelines, valves or other process equipment. It can occur relatively quickly, be difficult to remove and potentially cause serious damage if not removed with care. The problem is becoming more important as natural gas and gas condensate resources are discovered where operating conditions (deep, cold water and on-shore colder climates) surpass the conditions needed for hydrate formation. Often hydrates will form from gas streams (which are produced saturated with water) in downstream transportation networks once the stream has cooled from reservoir conditions. This can cause large pressure drops throughout the system and reduce or stop the flow of natural gas.
Expensive hydrate prevention measures are currently used to maintain gas production. Most commonly, these include the use of methanol as inhibitor, glycol to dry the gas and sometimes heating the gas to stay outside of the hydrate forming region. Systems are also conservatively designed to remain outside of the hydrate region during normal operations. All of the above solutions, which are also true of oil and condensate fields, have large associated economic penalties of one form or another.
BP has been working on low dose hydrate inhibitors for several years and for the past two in collaboration with Shell Research B.V. The term threshold is used to reflect the low dose rates of these inhibitors compared to conventional treatments and is used in preference to the more commonly used term “kinetic” as it does not imply a mechanism of action. THI’s are chemicals that prevent or considerably delay the onset of hydrate formation even though equilibrium chemical thermodynamics would indicate that conditions may be up to 10 C inside the hydrate phase envelope. At more severe conditions a different type of inhibitor can be used (Hydrate Dispersants HD’s) which function in a slightly different manner (this type of inhibitor will not be discussed in this paper). To prove the technical feasibility of this novel form of hydrate inhibition and after extensive laboratory evaluation, a series of field trials was proposed in a large wet gas facility that had a real potential to form hydrates if not treated successfully. The aim was to demonstrate beyond reasonable doubt that THI’s were ready for commercial deployment.
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