A NEW process of dehydrating subsea gas without the use of toxic chemicals could pave the way for opening up previously unviable gas fields.
Curtin University has completed successful pilot plant testing of the process at its Clean Gas Technology Australia Laboratories.
Natural gas always contains water and, to prevent blockages in deep subsea pipelines, companies inject chemicals such as glycol and methanol into the gas.
Typical gas pipelines can develop hydrates at temperatures below 12 degrees Celsius— hydrates are combination of gas and water that form a crystalline, ice-like texture—and are potentially disastrous because they can block pipelines.
To avoid hydrates, the pipeline needs to be kept warm or the water needs to be treated with chemical additives or removed.
Dr David Parks, a postdoctoral fellow in Curtin's Department of Petroleum Engineering, says techniques similar to the new process had previously been trialled as a surface solution.
“The novelty of our studies is the utilisation of the technique in a way that makes it suitable for implementation subsea,” he says.
“Subsea systems must have extremely high reliability and ideally operate with little or no intervention.”
With the new process, natural gas is taken from the subsea well at high pressure and temperature and cooled to just above the hydrate-formation temperature by passing it through a heat exchanger (cooled by surrounding cold seawater).
“It’s then sent to the dehydration vessel where the pressure is reduced, thereby expanding the gas and reducing its temperature—the Joule–Thomson effect,” Dr Parks says.
“Hydrates present in the dehydration vessel, together with the cooling caused by this expansion, reduce the water content of the natural gas to a level that is safe for gas transportation through the subsea pipeline.
“The next step is to take this proof of concept and develop a complete solution that is suitable for a field trial.
“Various developmental issues will need to be resolved before any trial is planned and implemented,” he says.
Current chemical methods used for dehydrating subsea gas are expensive, meaning companies usually exploit large gas fields.
“The outcomes of our experiments suggest the technology has significant merits,” Dr Parks says.
“It saves costs, is small and simple enough for subsea installation, and requires minimal maintenance.
“We are having discussions with a number of companies to fund further research and development to take the concept to a field implementable solution.
“Our subsea dehydration process can open up projects that are currently too expensive, making stranded fields economical,” he says.