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Pumps are primed for coal-bed methane gas extraction

04 May 2011

While many coal seams are no longer worked in this country, these almost forgotten sources of energy are the subject of a revival of interest as entrepreneurs seek to realise the potential of coal-bed methane for power generation. Pumps play a key role in the extraction process

For centuries coal was king in Lancashire and Cheshire, but over the past 50 years the industry has declined to a point where it is almost non-existent. Seams have either been worked out or deemed too expensive to develop. Coupled with this has been the move to clean power, making coal highly undesirable. However, those almost-forgotten coal seams are getting a wake-up call as engineers develop methods to realise the potential of coal-bed methane (CBM), the naturally occurring hydrocarbon gas that is trapped in the carbon structure of coal.

At Doe Green near Warrington on the north bank of the River Mersey, Nexen Exploration UK is now running a pilot production CBM extraction plant which is providing sufficient high quality gas to generate 0.5 megawatts of electricity. Such is the quality of the gas and the potential resources available, further commercial exploitation is a distinct possibility.

The site was first opened some four years ago with the drilling of two well bores into the coal seam laterals in order to actively remove water and gas from the formation. It is the water pressure in the seam that keeps the gas in the coal, so by relieving the water pressure the gas is free to flow under controlled conditions to the surface. The procedure is termed 'dewatering' and it is here that pump technology plays an essential role and to this end Cat Pumps is providing an efficient and effective solution for this application.

Originally Nexen tried electrical submersible pumps for dewatering, which, in terms of functionality, worked well, but getting the correct size for the 3,300ft deep well bore was difficult because there was no indication as to how much water would be produced. Also, in the event of the pumps failing, removing them for repair would be a major exercise.

After a rethink, Nexen looked at using a Jet Pump system for dewatering. In order to make this work, water needs to be injected into a Jet Pump located at the bottom of the well bore. In reality this is basically a venturi which forces water at high pressure through a small orifice from which it expands into a tapered throat section creating an area of low pressure around the jet pump. It is this area of low pressure that pulls water in from the formation to allow the gas to flow to the surface through one bore and the water through another.
 
The water drawn from the well bore is re-circulated back through the jet pump, with any excess water being transferred to storage and then removed off site. Should the dewatering process stop, the coal seam and the well bore will fill with water and prevent the gas from being released.

In order to maintain dewatering, the jet pump needs to be supplied with water at high pressure. Back in 2008, Nexen initially brought in a temporary pumping skid to provide the constant stream of water required. Whilst this did the job it was not considered fit for purpose over the long term so a decision was taken to employ surface pumping skids designed specifically for the duty. Working in conjunction with contractor Biogas Technology Limited based in Sawtry in Cambridgeshire, a solution to the high pressure pumping problem was found in the form of Cat Pumps' high pressure triplex positive displacement pumps.

The role of the Cat Pumps is to pump water from a top tank into the well head and into the jet pump at a nominal rate of 33 litres/min and then back to the surface through an annulus and ultimately back into the top tank. The process extracts about 10% more water from the formation so the excess is pumped to a storage tank and then removed for off-site treatment and disposal. The Cat Pump is pumping water at pressures of up to 140bar at variable speeds to provide the flow and pressure demanded by the jet pump.

Each of the skids supplied by Biogas Technology contains two Cat Pumps Model 3521 triplex plunger pumps and both pumps run on a cycle depending on the water level in the bore well. The pumps will kick in when the water level rises to a certain point and continue operating until the water levels drops back down again and the pump shuts down. This happens on a continuous basis. Because the skids are exposed to the elements, the pumps and pipelines are provided with covers to protect them against excessive cold weather, even though the water temperature can be in the region of 38oC.

The nature of the water being pumped has given rise to concern in respect of its salinity and levels of suspended solids. In order to reduce wear on the pump seals and plungers - not to mention the piping - filter bags have been retrofitted to remove excessive amounts of suspended solids in the re-circulated water.

Initially the Cat Pumps were operated non-stop 24 hours a day, which was not part of the original plan; however, by reconfiguring the pump down hole arrangements they are now run twice daily for three hours at a time to maintain the desired water level in the bore well.
Based on the contribution that Cat Pumps made in achieving the desired performance for the Doe Green prototype production facility, Nexen has now brought them in to develop a portable unit for use at potential gas production sites in other parts of the UK.


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