Practicalities of thermodynamic hydrate inhibitor distribution within hydrocarbon systems under steady state and dynamic operations

摘要

Hydrate formation in pipelines and processing facilities is a frequent problem within the oil and gas industry. The conditions that tend to initiate the hydrate formation include: low temperature, high pressure, and gas in contact with "free" water, which are inherent in subsea multiphase transportation systems. Hydrate formation during transportation and / or processing can cause shutdowns and even destruction of valuable equipment. Due to the catastrophic and costly consequences of hydrate formation, various mitigations are available to prevent hydrate development within multiphase transport. One of the most common mitigations is impeding hydrate formation in the free water phase by injection of a Thermodynamic Hydrate Inhibitor (THI).The most used inhibitors are Methanol, Mono-Ethylene Glycol (MEG) and Di-Ethylene Glycol (DEG). Typically, Methanol is used when regeneration of the hydrate inhibitor is not necessary, whilst MEG and DEG are used when the regeneration processes are cost effective. Determining the required THI concentration in the aqueous phase is needed to quantity THI volumes for injection and regeneration. However, the amount of THI required is affected by dynamic operations that the multiphase pipeline may experience due to the hydraulic instability, subsea installed equipment and imposed operational constraints. The other area which affects the THI rates is evaluating the amount of water present; this is a very uncertain area due to the error in reservoir predictions through to the significant error in water metering. All of these aspects are required in developing the hydrate management philosophy and subsequently the operating procedures. The injected THI may distribute into three possible phases: (a) the vapour hydrocarbon phase, (b) the liquid hydrocarbon phase and (c) the aqueous phase. It is crucial to thermodynamically analyse the behaviour of the THIs within the multiphase system during all potential operations. This paper examines the practicalities of injecting THIs into a multiphase pipeline system and the areas of uncertainty which should be considered in design. These include the effect of THI injection location, how the THIs travel along the multiphase pipeline, the impact of changes in operating state, i.e. initial start-up, shutdown and restart, and finally, the impact of metering uncertainty and controllability. It has been concluded that a full investigation of all areas of uncertainty coupled with a rigorous transient analysis of the multiphase system is needed to determine the THI volumes for injection and regeneration as well as for developing the hydrate prevention strategy and operating procedures.