Geology and drilling in the Bowen Basin
Queensland’s Bowen sedimentary basin has Australia’s largest coal reserves. Rich in both coal and gas deposits, it contains almost all the state’s hard coking coal.
Bowen Basin coal seams are generally laterally continuous. They are moderate to high in gas content, however low permeability means that the gas does not flow easily through the seam.
Horizontal, in-seam drilling has been used successfully to extract gas from Bowen Basin coals since the 1990s. Wells of this type are more suitable to the target Bowen coal seams than other well types, such as vertical wells or wells stimulated with hydraulic fracturing.
In seam drilling and multi-lateral wells
With in-seam drilling, a bore is drilled from the surface and rotated so it is horizontal by the time it intersects the target coal seam. In some cases, it may intersect a second, vertical well bore through which gas and water flow to the surface.
One of the well types designed by Arrow are the 'multi-lateral' wells. In multi-lateral wells, branches run off the sides of the main horizontal bore to expose more of the seam and produce more gas. The multi-lateral design allows greater exposure of the coal seam while minimising surface impact.
Where suitable, wells in the Bowen Basin will typically be drilled on a multi-well pad, meaning that up to six wells may be drilled on each pad. This approach means a significant reduction in Arrow's surface footprint.
A well pad will typically consist of both vertical and lateral wells, and will be mirrored by another well pad with the same number of wells approximately 400m apart. This is known as 'paired' well pads.
Fraccing in the Bowen Basin
Arrow is not fraccing in any of its current production fields and anticipates little, if any fraccing in the initial stages of the Bowen Gas Project. Arrow's mainstay technique will likely remain non-fracced, co-joined horizontal and vertical wells – "surface to in-seam" wells – including multi-lateral wells.
Fraccing – hydraulic fracture stimulation – has been used worldwide for decades to enhance the flow of gas. In CSG, a fluid (99.5% water and sand) is pumped at high pressure down a cased well to perforations in the casing at the target coal seam. The pressure causes radiating cracks along the brittle coal seam and water carries the sand into the cracks to hold them open after the pressure is released.
Australian regulations ban potentially dangerous chemicals in frac fluids, including the BTEX group. Governments closely regulate fraccing, imposing risk assessment; risk management; monitoring, testing and reporting processes.