Pumps and Systems, February 2007
Natural gas is a clean-burning energy source well suited as a boiler fuel, vehicle fuel, and heating residences as well as large structures. Coalbed methane (CBM) is a natural gas containing virtually 100 percent methane (CH4) produced from coal seam reservoirs.
CBM is a non-conventional hydrocarbon that is fundamentally different in its accumulation processes and production technology. It is often produced at shallow depths with large volumes of water of variable quality, through a borehole that allows gas and water to be produced to the surface. The typical extraction process involves these steps:
1) Tunneling along a seam of coal to a point below a water table and at a depth below the water table sufficient for dissolved methane to be present in the water.
2) Separating a portion of the water containing dissolved methane while below the water table.
3) Reducing pressure on the separated portion for extracting dissolved methane.
4) Removing the extracted methane.
5) Discharging the separated portion of water after extracting dissolved methane.
CBM resources represent valuable volumes of natural gas within and outside of areas of conventional oil and gas production. CBM currently provides 10 percent of all domestic gas in the U.S. and, as an industry, has grown rapidly as other sources have depleted. Many coal mining areas support current CBM production; other areas containing coal resources are expected to produce significant volumes in the future.
Close-up of a 30-cm section of rock core containing part of a coal seam. The coal incorporates many thousands of hairline fractures, known as cleats, in which methane gas accumulates.
This same opportunity is now being explored in the U.K. in response to concerns about rapidly declining U.K. offshore gas reserves. CBM is the same natural gas that is found in U.K. offshore gas fields and is piped to households, but it is stored within the coals where it was formed.
Composite Energy Ltd. is a new U.K. independent energy company that is undertaking a program of work to evaluate the potential for natural gas production from deep coal seams. The company's main focus is CBM extraction, with various sites across the central belt of Scotland. Composite's preliminary investigations in Scotland could lead to the development of a gas field that would supply clean natural gas directly to the local and national gas network, providing energy for homes and industry.
The company wants to recover the natural gas found within deeper coal deposits that have not been disturbed by mining. This involves drilling slim 'horizontal' holes along the coal seams and reducing the pressure within the coals by pumping water. This releases the gas, which rises through the borehole to the surface where it can be collected and delivered to the gas network or local users.
The initial test-drilling is being carried out by new rigs, purchased by Composite Energy, to ensure quiet, quick, safe and clean drilling of the wells. Each drilling location requires a small site which is occupied for operations for only short periods of time. The initial test work involves site preparation, drilling of the wells, and initiation of water and gas flows. All sites, when no longer required or effective for gas production, will be fully restored and returned to their original purpose.
Composite's G-55 rig at the Airth 6 well was its inaugural CBM project, meaning a successful operation here was especially important. This well was also the first horizontal CBM well to be drilled in the U.K., and deployment of a progressive cavity pump (PCP) system represented another first for the sector. The operation had to be completed on budget and within days of the pump's delivery to the U.K.
The Composite G-55 rig at the Airth 6 well, close to the M9 motorway. The low height of the rig is seen in comparison with the adjacent mobile phone mast (behind the motorway gantry).
Weatherford International Ltd. was contracted to design, supply, and install a PCP system for methane
extraction in the horizontal openhole section within a five-week time frame. Weatherford designed and installed a BMW Model 56-1200 PCP, complete with tag bar, 1-gal (4.3-L) gas engine with hydraulic skid package, and rod guide centralizers.
The G-55 rig is a modern design incorporating the latest drilling technology. Its automated systems require only a small crew and minimal maintenance during drilling operations.
The PCP system was delivered to the rig site, where the client-owned drive head was reworked as the tubing was being pulled. The PCP system was then picked up and successfully run into the openhole section to the planned setting depth of 3,450-ft (1,052-m) and brought on line. The total time for this project, from design to on line, was five days ahead of the five-week target time frame.
If this initial well and others produce successful results, a second phase of development activity will be undertaken by Composite to produce natural gas over a period likely to extend over many years. This second stage will involve the drilling of additional wells and the installation of gas-handling equipment and underground pipelines connecting the wells to the local and national gas network.