News

Britain’s 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 Limited is now running a pilot production CBM extraction plant which is providing sufficient high quality gas to generate 0.5 megawatts of electricity (Fig. 1). 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.

Nexen is using a Jet Pump system for dewatering. 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 at the bottom of the well shaft. 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. The jet pump needs to be supplied with water at high pressure and this is provided by surface pumping skids designed by Biogas Technology Limited, Sawtry in Cambridgeshire, using Cat Pumps’ high pressure triplex positive displacement pumps (Fig. 2).

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 (Fig. 3). 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 water is pumped at pressures up to 140 bar at variable speeds to provide the flow and pressure demanded by the jet pump.

Each of the skids supplied by Biogas Technology contains two of our 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 Cat Pumps are fit for purpose and the manufacturer provided us with an arrangement that complied with a design specification which we handed to them”, comments Dave Kerr, Nexen’s CBM Technical Advisor. “They came to site when we needed their input and, when we had to make adjustments and rework certain aspects of our design; they recommended design changes that would eventually make the system work as it should have done from day one.”

Based on the contribution that our pumps have at Doe Green, Nexen has now brought us in to develop a portable unit for use at potential gas production sites in other parts of the UK. “Initial reports demonstrate that the portable unit has worked perfectly and as a result of this collaboration we are very comfortable in specifying Cat Pumps for future projects,” continues Dave Kerr.