Growing demand for Liquefied Natural Gas (LNG) is leading to increased production of natural gas from less attractive reservoirs. These are generally contaminated with sulphur components and located in inhospitable environments such as deep offshore waters. Conventional upstream processing in such locations is either infeasible or excessively expensive, so more and more production is by full wellstream transfer, thereby reducing the upstream facilities to the bare minimum. The consequences are that the midstream facilities need to be much more complex and require very special design attention.

This paper describes how the contaminants (e.g. hydrogen sulphide, mercaptans, carbon dioxide, water, salts, organic acids) and the necessary additives (e.g. corrosion inhibitor, hydrate inhibitor) are dealt with in the upstream and midstream designs whilst complying with ever more stringent environmental requirements (e.g. sea water quality, sulphur dioxide emissions, hydrocarbon losses). It also describes how the various contaminants and additives can interact, (e.g. forming emulsions, solids) and can cause additional problems such as foaming, fouling or corrosion.

The paper concludes that a functioning and cost-effective line-up can only be achieved by using an integrated approach, in which all the processing steps such as pipelining, separation, stabilisation, treating and NGL extraction are considered together.

Conventional & Historical Developments

Given a choice of which fields to develop, companies have always chosen the easiest and hence cheapest and most profitable ones. The consequence is that historically LNG plants have been supplied from sweet reservoirs in accessible locations.

When the undesirable constituents of the LNG plants feed gas are only carbon dioxide (CO2) and traces of sulphur components, the acid gas is called sweet. If sulphur components are present in higher concentrations, specifically hydrogen sulphide (H2S), carbonyl sulfide (COS) and organic sulphur components such as mercaptans, the gas is called sour.

When the upstream gas supply to an LNG plant is sweet and comes from an easily accessible field, it is convenient and cheap to dry the gas in the upstream facilities before pipelining the gas to an LNG plant. Consequently, the receiving facilities for such uncontaminated streams are simple. This is illustrated in figure 1, which shows a typical offshore development with a sweet reservoir in shallow water.

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