An Invention Promising Less Carbon and Cost Could Allow More Stranded-Gas Development
Authors: Rassenfoss Stephen 
Published: May 2022

A promising innovation with a ponderous name, pseudo dry gas (PDG), opens the possibility of developing stranded-gas fields in remote locations by making it possible to build far-longer tiebacks and ultimately produce more gas.

But it comes with a major challenge: convincing offshore producers that they can rely on a subsea system that separates natural gas from the condensate and water in the production stream.

The benefits are obvious. Being able to move gas and liquids separately is a lot more efficient and avoids problems associated with multiphase flow, particularly slug formation, which requires topsides equipped to catch the slugs. Separation also reduces backpressure, which is expected to increase the ultimate recovery from those fields.

In the 4 years since the idea debuted at the Offshore Technology Conference (OTC), a small team in Worley’s Aberdeen office has built and successfully tested a small-scale prototype of a flow loop with the support of an industry partnership including three unnamed energy companies.

Even with all of that, they needed to establish an added benefit to win a critical government grant to build and test a full-scale prototype for high-pressure testing. This energy-efficient alternative promises to significantly reduce carbon emissions compared to offshore compression and platforms.

“As the energy industry strives toward sustainability, it’s important to support solutions that enable decarbonization,” said Andy McDonald, head of Low Carbon Transition at Scottish Enterprise, which put up £765,000 ($1 million) to support testing, according to a release from Worley.

They are currently building a full-scale unit to be tested later this year at the National Engineering Laboratory in Glasgow, pushing the device closer to the technical readiness level that will allow an offshore field test.

This new approach is competing against two well-established alternatives—platform-based development and compression. Their counterargument is based on the energy efficiency of separation, which limits the compression required and the steel that goes into platforms.

When comparing the energy required, Thomas Lee, lead development engineer at Worley who co-invented PDG, pointed out that the power required for separation is measured in thousands of watts, while compression requires millions of watts.

“It effectively decarbonizes upstream gas production. It gets rid of compression from the gas field. The amount of CO2 from this operation is really low,” Lee said.

The amount of carbon and cost also depends on the location of the compression equipment. Normally it has been on a platform, which provides power and easy access for maintenance but adds costs and carbon. Worley’s inline separator is built to fit into the space filled by a standard joint of pipe (Fig. 1). Onshore compression is also an option, and Equinor and Shell have shown that compression can be done subsea.

Compression has been used in projects around the world, the majority of which are in fields that hold reserves of 10 Tcf or more (Fig. 2).