The four Norwegian gas/condensate field developments Troll, Snøhvit, Ormen Lange and Åsgard are used together with amongst others Britannia Satellites (ConocoPhillips) and Shah Deniz (BP) to illustrate experience and technology status related to injection and recovery of MEG (Mono Ethylene Glycol).
A comparison between MEG and MeOH is presented. Advantages, limitations and cost elements (CAPEX/OPEX) by use of glycol (MEG) vs. alcohols (Methanol/Ethanol) are reviewed and discussed in relation to their applicability.
Based on experience, typical operational problems within closed loop MEG systems with salt- and water removal are carbonate scale deposits in pipeline and recovery systems, accumulation of corrosion products and other small particles and carry-over/foaming. A holistic approach for proper design and prevention of these and other incidents are presented.
Proper hydrate management is vital for all field developments. For short and moderate tie-backs, flowline insulation (maintaining fluid temperature above the hydrate formation temperature) combined with a depressurisation strategy, is normally the basic method. For developments with cold well fluids, systems which are difficult to depressurise or restart (deep water), and for general improvement of the field regularity, flowline heating is an additional element. For deepwater developments, the hydrate management is often based on displacement with stabilized oil, or built around a subsea separation concept /4/. For appropriate fluids, "Cold Flow" may turn out to be the next quantum leap. However, for long distance gas-condensate tie-backs with complex subsea gathering architecture, chemical hydrate inhibitors (glycols and alcohols) are considered the best option.
Chemical hydrate inhibitors can be arranged in two main categories:
Thermodynamic Hydrate Inhibitors (THI)
Low Dosage Hydrate Inhibitors (LDHI)
This means that THIs are the robust choice for long distance gas-condensate tie-backs. The thermodynamic inhibitors of widest use are methanol (MeOH) and monoethylene glycol (MEG). Other alcohols and glycols may be used, but two main factors making methanol and MEG the most common thermodynamic inhibitors are hydrate suppression performance (see Table 2) and cost.
Rough costs are given in Table 1, where it is seen that the common thermodynamic inhibitors are relatively cheap per unit volume. However, since large quantities may be necessary to suppress the equilibrium temperature below the lowest operational temperature expected in the system, the infrastructure cost may add up to significant amounts. Necessary considerations to make when designing for a hydrate strategy with thermodynamic inhibitors are storage volumes and regeneration facilities.
