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Tight Gas
Tight gas refers to natural gas reservoirs trapped in highly impermeable rock, usually non- porous sandstone and sometimes limestone. It is found in different geological formations from shale gas (although according to some definitions shale gas is a form of tight gas). Over time, rocks are compacted and undergo cementation and recrystallisation, reducing the permeability of the rock. As with shale gas, directional drilling is used and fracking is necessary to break up the rock and allow the gas to flow. In addition to fracking, acidisation is also sometimes used. This is where the well is pumped with acid to dissolve the rock that is obstructing the flow of gas.
shale gas ( Tight Gas)
SHALE GAS IS NATURAL GAS THAT IS TRAPPED UNDERGROUND IN SHALE ROCK WHICH MUST BE FRACTURED TO EXTRACT THE GAS. EXTRACTION CAUSES WATER POLLUTION AND METHANE LEAKAGE WITH SERIOUS CONSEQUENCES FOR CLIMATE CHANGE.
While many of the problems posed by tight gas, such as water pollution and contributing to climate change, are similar to those of shale gas, there are some differences. For example the differing natural carbon content in tight gas means that it stores different kinds of contaminants and therefore produces different pollutants. Shale gas is also generally harder to extract, being even less permeable and requiring more fracking.
how is it extracted?
Shale gas has been known about for a long time. The first commercial gas well in the USA, drilled in New York State in 1821, was in fact a shale gas well. However, it is only since around 2005 that it has been exploited on a large-scale. This has been driven by the huge rise in energy prices resulting from declining fossil fuel reserves and the development of two new technologies, horizontal drilling and advanced hydraulic fracturing, which have opened up reserves previously inaccessible by conventional drilling.
Natural gas is mainly methane and is usually extracted from oil or gas fields and coal beds (see coal bed methane), but it can also be found in shale formations.
Hydraulic fracturing, often just referred to as fracking, is used to free gas trapped in rock by drilling into it and injecting pressurised fluid which creates cracks which release the gas. The fracking fluid consists of water, sand and a variety of chemicals which are added to aid the extraction process such as by dissolving minerals, killing bacteria that might plug up the well, or reducing friction.
Shale is a form of sedimentary rock formed from deposits of mud, silt and clay. Normally natural gas is extracted from sandstone or carbonate reserves, where the gas flows fairly easily once the rock is drilled into. However shale is relatively impermeable, meaning that it is harder for the gas to escape. It is only with the development of horizontal drilling and advanced hydraulic fracturing (see below) that shale gas extraction has become possible.
The fracking process also produces a large volume of waste water, containing a variety of contaminants both from the fracking fluid, and toxic/radioactive substances which are leached out of the rocks (see below).
what is it?
Production from shale gas wells declines very quickly and so new wells must be drilled constantly. This process of continual drilling and fracking means that huge areas of land are covered with well pads where thousands of wells are drilled, with each well requiring millions of litres of water.