Gas treatment

Harald Tønnesen og Finn Harald Sandberg, Norwegian Petroleum Museum

Natural gas removed from the wellstream through pressure reduction is a mix of several different light hydrocarbons, with methane as the main component. Ethane, propane and butane are also present. Statfjord’s gas production was initially injected back into the reservoir.

— Gas pipe at Statfjord A. Photo: Jan A. Tjemsland/Norwegian Petroleum Museum

Once Statpipe became operational in 1984, however, the Norwegian share of that output was transported to Kårstø. This processing plant north of Stavanger separated out the various components (a process known as fractionation) for onward sale. From 2007, all the natural gas from Statfjord has been piped to the UK via the Tampen Link pipeline and Britain’s gas gathering system.

The diagram above illustrates the way the gas flows through the treatment process. As the wellstream moves from one separator to another, the pressure is reduced and the gas removed. Once separated out, the gas must be compressed and dewatered before being exported by pipeline to land or injected back into the reservoir. Four centrifugal compressors are connected in series on a single shaft driven by a gas turbine. Gas leaving the fourth-stage separator is cooled and led to a scrubber where condensate (light oil) gets removed and returned to the oil, while extracted water is pumped to the treatment plant.

The gas then flows to a compressor where its pressure is boosted. Gas from the other separator stages is fed in at various stages in the compression process. Pressure is raised in the first compression stage from 0.7 to 5.9 bar, then 5.9 to 21.7 bar in stage two, 21.7 to 67.9 in stage three, and 67.9 to 216 bar in the final stage.

The gas must be cooled between each pressure rise before continuing to the next stage. Between the third and fourth stages, it passes through a glycol-based dewatering unit. Prior to the start of pipeline exports, all the gas was injected back into the reservoir and had its pressure raised to 380 bar in a fifth stage using a piston compressor.

The Statfjord late life project is recovering the residual gas in the reservoir for sale to the UK. When this began in 2007, some of the compressor equipment was modified to handle larger volumes of gas at lower pressure.

Gas and water injection

Injecting gas back into Statfjord had a positive impact on field economics by helping to maintain reservoir pressure over a longer period. The gas was also “banked” for later sale. The injection compressor is a turbine-driven double-acting piston unit with six cylinders, which raises the gas pressure from 216 bar at 45°C to 380 bar at 86°C.

Also used to maintain reservoir pressure, the water injection system on the field delivers treated seawater to the injection wellheads. Chlorinated seawater is taken from the joint outlet of the crude oil coolers and pumped into a deoxygenator before flowing to five injection pumps. Each of the latter can deliver 68 000 barrels per day at a pressure of 190 bar to the injection manifold.