Seventy percent of all natural gas going to market from, Alberta is produced from sour gas fields. Without the recent advances in design, construction and operation of gathering systems to collect this sour gas for treating at central points prior to transmission in large diameterlines, this potential would not have been realized. The Canadian Standards Association draft gas pipe line code contains requirements for material and equipment, design, installation, inspection, operation and control of these systems.
In 1952 Alberta's first central gas treating plant, capable of producing elemental sulphur from hydrogen sulphide in the raw natural gas commenced operation with a capacity of 30 long tons of sulphur per day. During the past 15 years, because of the demand for -clean dry gasfor domestic fuel and the world demand for elemental sulphur, nineteen major treating plants capable of producing elemental sulphur have been built in Alberta with total production in excess of 5,000 long tons ofsulphur per day. The total maximum day delivery to main transmission lines is now in excess of 2 billion cubic feet with some 70% of this coming from the sour gas fields.
In general there are two main methods by which sour gas is gathered in the fields; one method is by removal of water at the well and the other is by transporting the whole well stream. The major problems are hydrate control and corrosion control which are much more critical in a sour gassystem than in a sweet gas system.
DRY SOUR GAS GATHERING – In a dry system the sour gas is dehydrated at the wellhead to remove sufficient water to lower the hydrate formation temperature to less than the lowest flowing temperature in the system. The most common methods are as follows:
Glycol Dehydration – Usually used where operating pressures are in the order of'1200 psig or less. If the gas being collected is not too sour, the standard glycol dehydration unit may be used and the regeneration fuel is taken from the well stream. Where the well stream is very sour, the use of well stream gas for fuel creates problems in pollution control and therefore sweet fuel gas is returned from the central treating plant through a separate system usually in the same ditch as the sour gas gathering system. In some cases the rich glycol is piped to the central plant for treating and the lean glycol returned in a separate line.
Dry Desiccant Dehydration – This type of system has been used for operating pressures in the 2000 psi range where it has been considered necessary to effect almost complete removal of water fromthe system.
WET SOUR GAS GATHERING – In a wet system it is necessary to provide sufficient heat to maintain the gathering system temperature at all points above the hydrate point of the sour gas being transmitted. The most common methods are as follows:
Heater Systems – This is the most common installation in Alberta for wet systems.