About Unconventional Gas
Unconventional Gas vs Conventional Gas
Natural gas from either conventional wells or unconventional wells is the same substance – a mixture of hydrocarbons, comprised mainly of methane. The named designation of ‘conventional’ or ‘unconventional’ is determined by the unique characteristics of the individual reservoirs.
The term unconventional gas refers to the natural gas contained in rock formations or coal seams. These resources are also known as natural gas from coal (NGC), coal bed methane (CBM), shale gas, tight gas sands and gas hydrates.
Unconventional gases are primarily stored through adsorption (adhesion of the molecules of gases, liquids, or dissolved substances to a solid surface) to the coal itself. Not initially in a gaseous state, production of these reserves is technically complex. Horizontal drilling techniques and formation fracturing methods allow for reduced pressure in the coal seams, resulting in the detaching of methane molecules and the flow of gas through existing fractures. Typically, unconventional wells contain large quantities of natural gas and have a longer production life than traditional natural gas deposits, implying longer term economic and reserve life advantages for the developer.
Conversely, conventional gas is found in the pore space of the rock versus the rock formation. Production, therefore, involves the relatively simple process of drilling and extracting gas from the reservoir pores with the output and economic return generally strongest at the start of the well.
Shale Gas
Shale gas is natural gas that is produced from reservoirs predominantly composed of shale with lesser amounts of other fine grained rocks rather than from more conventional sandstone or limestone reservoirs. The gas shales are often both the source and the reservoir for the natural gas which is stored in three ways:
- adsorbed onto insoluble organic matter called kerogen
- trapped in the pore spaces of the fine-grained sediments interbedded with the shale
- confined in fractures within the shale itself
Gas shales are considered continuous type natural gas plays in that they are pervasive across large geographic areas and the reservoirs generally have a long life (Hill and Nelson, 2000). Most gas shales have very low permeability (they are a 'tight' gas), and production rates are usually quite low with low recovery factors that are a fraction of conventional reservoirs. The area extent of the deposits and the longevity of shale gas wells compensate for low flow rates. Commercial production is very sensitive to drilling and completion costs. In the US, fracture stimulation is almost always needed to achieve economic gas recovery.
Shale gas is produced in much the same way as conventional reservoirs. Due to the low permeability inherent in gas shales, stimulation is almost always required. Fracturing the shale is required in most situations to achieve economic production. In the US, gas shale reservoirs have proven to have fairly low production rates with a long well life and fairly low recovery rates. This means that drilling and completion costs have to be kept to a minimum. Additionally, technologies continue to be developed to increase the recovery percentage and effectively stimulate the shale at minimal cost.
Lakeview and Island Shale Gas Potential
