Coal Seam Gas is primarily methane, which is colorless and odorless found in coal deposits. Natural gas collects in underground coal seams at depths of 300m to 1000m underground. It is sometimes referred to as coal bed methane (CBM) and would be classified under the umbrella of Unconventional Resources. The difference between conventional and unconventional gas is the geology of the reservoirs from which they are extracted and which therefore require different extraction techniques. With coal seam gas comprising mostly of methane, it has a much more simplified approach in drilling, production, storage and separation techniques when compared to producing shale gas in the US. Natural gas can get collected underground in coal seam beds by bonding to the surface of coal particles. These coal seams are generally filled with water. Interestingly, it is the pressure of the water that keeps the gas attached as a thin film to the surface of coal particles.
Like any other formation, a well is drilled to the required depth to access the coal seam gas, Origin, which is the upstream operator in Australia, uses hybrid drill rigs to obtain efficiency. The completion of the well enables some water to be pumped to the surface. Since the water is pumped to the surface the pressure within the formation is reduced which helps the gas to flow to the surface.
What’s intriguing is that hydraulic fracturing is not always required. Undoubtedly, this depends a lot on the formation itself but most of the current coal seam gas production serving Australia Pacific LNG is coming from high flowing wells, which are not hydraulically fractured. In comparison to US shale, to produce from shale plays such as Texas or North Dakota, hydraulic fracturing is a must because the formation is so tight. Without hydraulic fracturing of these shale plays production is not economical. Another key characteristic of hydraulically fractured wells in the US is that production is high for the first 6 months to a year and then drops rapidly. Many scholars have linked this decline to the closing of fractures and the natural decrease in the reservoir pressure of the formation due to production