What is Tight Gas?
Tight gas is natural gas which is difficult to access because of the nature of the rock and sand surrounding the deposit. Because this gas is so much more difficult to extract than natural gas from other sources, companies require a large financial incentive to go after it; as energy prices rise, so does interest in extracting tight gas. Several global oil and gas companies control significant tight gas reserves, including BP, which has also sunk substantial resources into learning more about extracting tight gas. Normally, natural gas is fairly easy to access. When a deposit is identified, a well can be sunk, and the gas naturally flows into the well, making it easy to pump the gas to the surface and to distribute it from there. This is because natural gas is normally surrounded by deposits of porous rock, with lots of small holes for the gas to seep through. Sometimes, the gas literally pumps itself. In the case of tight gas, the surrounding sandstone, shale, or other rock is not so
Tight gas is natural gas found in reservoirs with low porosity and low permeability (generally sandstones with less than 1/10th of a millidarcy permeability). Tight gas requires multiple fracturing to in order for any significant amount of gas to be available. Drilling for tight gas has been compared to drilling a hole into a concrete driveway–the rock layers that hold the gas are very dense, so the gas doesn’t flow easily. Until recently, tight gas was considered non-economic to produce. Recent technological advances in a number of different directions have made it increasingly possible to extract tight gas. The higher recent prices of natural gas have also tended to support unconventional natural gas production. There is a huge amount of tight gas in place. Even with technological advances, the challenge is finding a way to extract it economically. Where can tight gas be found? This is a map from the Advanced Resources International website. Figure 7 Based on Figure 7, one can see t
“Tight gas” refers to natural gas produced from reservoirs that have very low porosities and permeabilities. Reservoirs are usually sandstone, although carbonate rocks can also be tight-gas producers. The standard industry definition for a tight-gas reservoir is a rock with matrix porosity of 10% or less and permeability of 0.1 millidarcy or less, exclusive of fracture permeability.
The term “tight gas” has not been consistently defined but is generally referred to as the natural gas contained in low permeability sandstone, siltstone and carbonate reservoirs where the application of drilling technology or reservoir stimulation techniques is required to establish economic recovery. Tight gas resources are most similar to conventional gas resources in reservoir lithology and gas storage mechanism. They are commonly distinguished from conventional resources by an arbitrary area and “pay-weighted” in situ permeability cutoff. Other forms of unconventional gas are more easily distinguished from conventional and tight gas storage mechanisms (adsorption and hydrates, for example) and their unconventional reservoir lithology (shale and coal).
