Abstract

Gas dehydration is a very important and integral part of gas production in the upstream and downstream petroleum industry and yet its criticality in term of safe gas operation and design costs are often underestimated or misunderstood.

The produced natural gas nearly always contains water vapour. Depending on the intended use of the gas, this vapour will change into a liquid form upon temperature drop making the process environment corrosive especially if there are accompanying, as they often do, carbon dioxide and or hydrogen sulphide especially in the Middle East area. Upon further temperature drop, the liquid will transform into layers of hydrates that may cause line blockage leading to severe safety concerns and loss of production and revenues. To avoid the potential for these eventualities, the gas must be suitably dehydrated. The dew point of the wet gas decreases with the degree of lowering of the water content of the gas and the method or complexity of drying also change with it. At the same time, the costs for drying shoot up drastically with the degree of drying. For example, the normal specification for a gas being transported through a commercial pipeline is 7 lb/mmscf requiring only about 99.1% TEG purity (if drying by TEG is chosen) and this is easily achieved by a glycol reboiler at the saturation temperature with some gas being sparged through it. Whereas for deep ocean gas injection in a remote subsea location, a TEG concentration of close to 100% may be required and this calls for a gas drying plant with azeotropic distillation or gas drying by adsorption rather than absorption or other licensed methods. On the costs side, the gas drying plant for TEG concentrations greater than 99.99 wt% costs several folds more than 99.90% TEG. It is therefore essential that the degree of the required gas drying is correctly identified upfront in the life cycle to meet the environmental and technical needs specific to the project involved and it must be borne in mind that the complexity of the drying plant or method of drying also changes with the required dry gas specification.

In order to set correctly the specification for the dry gas, we have to make a journey along the gas path and travel with it and analyze dynamically the coldest condition that the gas may be subjected. For instance, for a gas re-injection back into the reservoir, the coldest condition that the process gas may be exposed to, is the very cold seawater temperature for a subsea pipeline in a long and pressurized shutdown situation or it may be the magnitude of the pressure drop across the choke of the injection well. In the latter case it is important to recognize that pressure drop across the choke is a variable itself and therefore the required injection pressure profile for the reservoir is required right from the start which itself is very challenging to correctly determine as the overall gas composition for a recycle gas changes with time and during the whole project life cycle. For a gas exposed to cryogenic conditions, extreme dryness is required lowering the dew point to temperatures less than -60 °C or beyond.

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