Historically, conventional natural gas deposits have been the most practical and easiest deposits to mine. However, as technology and geological knowledge advance, unconventional natural gas deposits are beginning to make up an increasingly large per cent of the supply picture.
So what exactly is unconventional gas? A precise answer to that question is hard to find. What was unconventional yesterday may, through some technological advance or ingenious new process, become conventional tomorrow. In the broadest sense, unconventional natural gas is gas that is more difficult or less economical to extract, usually because the technology to reach it has not been developed fully, or is too expensive.
For example, prior to 1978, natural gas that had been discovered buried deep underground in the Anadarko basin was virtually untouched. It simply wasn’t economical or possible to extract this natural gas. It was unconventional natural gas. However, the passage of market-based rate regulation and the passage of the Natural Gas Policy Act provided incentives towards searching for and extracting unconventional natural gas, and also spurred investment into deep exploration and development drilling, making much of the deep gas in the basin conventionally extractable.
Therefore, what is really considered unconventional natural gas changes over time, and from deposit to deposit. The economics of extraction play a role in determining whether or not a particular deposit may be unconventional, or simply too costly to extract. Essentially, however, there are six main categories of unconventional natural gas. These are: deep gas, tight gas, gas-containing shales, coalbed methane, geopressurized zones, and Arctic and sub-sea hydrates.
Deep Natural Gas
Deep natural gas is exactly what it sounds like – natural gas that exists in deposits very far underground, beyond ‘conventional’ drilling depths. This gas is typically 15,000 feet or deeper underground, quite a bit deeper than conventional gas deposits, which are traditionally only a few thousand feet deep at most. Deep gas has, in recent years, become more conventional. Deep drilling, exploration, and extraction techniques have substantially improved, making drilling for deep gas economical. However, deep gas is still more expensive to produce than conventional natural gas, and therefore economic conditions have to be such that it is profitable for the industry to extract from these sources
Tight Natural Gas
Another form of unconventional natural gas is referred to as tight gas. This is gas that is stuck in a very tight formation underground, trapped in unusually impermeable, hard rock, or in a sandstone or limestone formation that is unusually impermeable and non-porous (tight sand). In a conventional natural gas deposit, once drilled, the gas can usually be extracted quite readily, and easily. A great deal more effort has to be put into extracting gas from a tight formation. Several techniques exist that allow natural gas to be extracted, including fracturing and acidizing. However, these techniques are also very costly. Like all unconventional natural gas, the economic incentive must be there to incite companies to extract this costly gas instead of more easily obtainable, conventional natural gas.
Natural gas can also exist in shale deposits, which formed 350 million of years ago. Shale is a very fine-grained sedimentary rock, which is easily breakable into thin, parallel layers. It is a very soft rock, but it does not disintegrate when it becomes wet. These shales can contain natural gas, usually when two thick, black shale deposits ‘sandwich’ a thinner area of shale. Because of some of the properties of these shales, the extraction of natural gas from shale formations is more difficult and perhaps more expensive than that of conventional natural gas.
Coal bed Methane
Coal, another fossil fuel, is formed underground under similar geologic conditions as natural gas and oil. These coal deposits are commonly found as seams that run underground, and are mined by digging into the seam and removing the coal. Many coal seams also contain natural gas, either within the seam itself or the surrounding rock. This coal bed methane is trapped underground, and is generally not released into the atmosphere until coal mining activities unleash it. Historically, coal bed methane has been considered a nuisance in the coal mining industry. Once a mine is built, and coal is extracted, the methane contained in the seam usually leaks out into the coal mine itself. This poses a safety threat, as too high a concentration of methane in the well creates dangerous conditions for coal miners. In the past, the methane that accumulated in a coal mine was intentionally vented into the atmosphere. Today, however, coal bed methane has become a popular unconventional form of natural gas. This methane can be extracted and injected into natural gas pipelines for resale, used as an industrial feedstock, or used for heating and electricity generation.
What was once a by-product of the coal industry is becoming an increasingly important source of methane and natural gas.
Geo pressurized Zones
Geo pressurized zones are natural underground formations that are under unusually high pressure for their depth. These areas are formed by layers of clay that are deposited and compacted very quickly on top of more porous, absorbent material such as sand or silt. Water and natural gas that are present in this clay are squeezed out by the rapid compression of the clay, and enter the more porous sand or silt deposits. The natural gas, due to the compression of the clay, is deposited in this sand or silt under very high pressure (hence the term ‘geo pressure’). In addition to having these properties, geo pressurized zones are typically located at great depths, usually 10,000-25,000 feet below the surface of the earth. The combination of all these factors makes the extraction of natural gas in geo pressurized zones quite complicated. However, of all of the unconventional sources of natural gas, geo pressurized zones are estimated to hold the greatest amount of gas.
Methane hydrates are the most recent form of unconventional natural gas to be discovered and researched. These interesting formations are made up of a lattice of frozen water, which forms a sort of ‘cage’ around molecules of methane. These hydrates look like melting snow and were first discovered in permafrost regions of the Arctic. However, research into methane hydrates has revealed that they may be much more plentiful than first expected. Estimates range anywhere from 7,000 Tcf to over 73,000 Tcf. In fact, the USGS estimates that methane hydrates may contain more organic carbon than the world’s coal, oil, and conventional natural gas – combined. However,research into methane hydrates is still in its infancy. It is not known what kind of effects the extraction of methane hydrates may have on the natural carbon cycle or on the environment.