Tight gas is a type of natural gas, which is extracted from reservoirs with low permeability, formed by sandstones or carbonates.
Tight gas reservoirs are formed in the same way as conventional gas reservoirs; however the rock that is saturated with gas from parent rocks has a very low permeability. Thus, the compacted gas rocks are the gas reservoir that cannot be developed through conventional vertical drilling technology because of low production rate. It is necessary to improve the permeability of such reservoirs to extract natural gas out of them. For this purpose, the technology of hydraulic fracturing executed from several vertical wells is applied.
Typically, low permeability reservoirs with natural gas deposits are composed of sandstones.
Sandstones are clastic sedimentary rocks formed as a result of rock formations destruction, transportation of fragmental particles by air or water and further accumulation. Accumulated sand is transformed into sandstone rocks as a result of compaction by pressure of overlying deposits and cementation by the precipitation of minerals into the pore spaces between sand grains. Sandstones deposits with a capacity of sediments from tens to hundreds of meters are the most suitable for the extraction of unconventional gas. The lenticular sandstones formed in the result of the penetration of water through cracks and pores, may not be suitable for the unconventional gas extraction.
Unlike conventional gas resources, the tight gas is not associated with traditional traps, but is concentrated across the lower parts (depression, bolson) of oil and gas basin over the large areas. Therefore, the tight gas resources are greater than the traditional natural gas resources. It should be noted that some conventional gas fields may be located in the near of unconventional gas deposits.
The compacted sandstone is characterized by small ratios between the pore space and the total volume of rock (porosity) as well as the low ability of moving fluid through the pores (permeability). Tight gas is defined as having less than 10 percent porosity and less than 0.1 millidarcy permeability. Due to the low permeability, the tight gas does not have a traditional gas-water contact and preferably is located below the hypsometric saturated section.
The scientific research on the tight gas has begun in North America in 1970s and actively goes on, especially in the US. In the United States the tight gas is mainly extracted from both the Mesozoic basins of the Rocky Mountains and from the Paleozoic basins of the North American platform.
In Ukraine sandstones are widespread in the coal-bearing deposits of Dnieper-Donets basin. The open porosity of the compacted sandstones varies from 2-4 % to 7-9%, and the average gas content is typical from 2-3 to 9 cubic meters of gas per 1 cubic meter of rock. Gas saturation of sandstones feasible for extraction of natural gas is detected at the depths from 900 to 3500 meters and the maximum gas saturation of sandstones is expected at the depths ranging from 1200 m to 2450 m. The main prospects of tight sands gas extraction in Ukraine are associated with the southern part of Yuzivska area.