While conventional natural gas is relatively easy to extract to the surface of the earth in a cost-effective way, unconventional gas is difficult to locate and extract and hence it is cost prohibitive. With fast evolving technological development, the line between conventional and unconventional gas is gradually but surely diminishing. Hence we should understand that the term unconventional gas is a dynamic concept and is subject to reclassification as technologies and better extraction methods evolve.
Currently there are six broad types of unconventional gases: deep gas, shale gas, coal bed methane (coal seam gas), geo-pressurized zones, Arctic and subsea hydrates and tight gas. (For more on coal bed methane, see the article The Unique Challenges of Coal Bed Methane Production.)
Unconventional gas resources generally require more wells, greater energy and water consumption, and sophisticated technology and skills. They require more extensive production operations per unit of gas recovered as compared to conventional gas resources.
What is Tight Gas?
Tight gas refers to natural gas reservoirs embedded in extremely impermeable hard rock, making the underground formation extremely "tight.” Tight gas is also trapped in sandstone or limestone formations that are unusually nonporous, and so are known as tight sand. The tight sand produces dry natural gas.
The reservoir rocks have such tight impermeability that they require sophisticated hydraulic fracturing and horizontal wellbore to stimulate and extract the gas in a cost-effective way. The tight sand definition also applies to coal bed methane, shale gas and tight carbonate reservoirs.
Let us first understand porosity and permeability. Porosity is the proportion of void space to the total volume of rock. Tight gas is held in pores up to 20,000 times narrower than a human hair.
Permeability is the ability of the fluid to move through the pores, and is measured in millidarcy units (named after Henry Darcy). They are not SI units but they are widely used in the petroleum and gas industry. Tight gas reservoirs are generally defined as having less than 0.1 millidarcy (mD) matrix permeability and less than ten percent matrix porosity.
As technology has developed, the permeability guidelines for tight gas have changed onshore from less than 0.1 mD in the 1970s to less than 0.01 mD today, and less than 0.001 mD in the United States.
Tight gas sands (TGS) represent approximately 70% of the unconventional production, with significant reserves yet to be developed. A tight gas reservoir is one that cannot be produced at economically viable flow rates or recover economic volumes of gas unless the well is stimulated by a large hydraulic fracture treatment and/or produced using horizontal wellbores. Although shale rocks also have low permeability and porosity, shale gas is usually considered separate from tight gas, which is commonly contained in sandstone, and at times in limestone too. Hence, the nature of the rock makes the difference.
In a typical tight gas reservoir, the original pore space is reduced by lithification (a process of porosity destruction through compaction and cementation) and diagenesis (a change of sediments or existing sedimentary rocks into different sedimentary rocks during and after their formation).
Tight gas reservoirs are found in several varieties, and they may be:
- Deep or shallow
- High pressure or low pressure
- High temperature or low temperature
- Blanket or lenticular
- Homogeneous or naturally fractured
- A single or multiple layers
World Reserves and Production of Tight Gas
Companies in the United States have been exploring and fracturing low permeability rocks since the 1950s. However, the steep increase in natural gas prices in the 1970s prompted exploration of low permeability gas reservoirs. Along with it came advances in exploration and stimulation technology, which gave an impetus to explore low quality gas reservoirs.
The natural gas shortages were addressed by gas price deregulation, government-funded R&D programs and tax incentives to create the technology required for tight gas development. It is estimated that more than 15% of the consumption and 30% of the production in the US comes from tight gas reservoirs. Shale gas, coal bed methane and tight gas account for about 50% of the natural gas production in the US. (Learn more about shale gas in Is Shale Gas Losing Its Sheen?) This percentage will change in the years to come when conventional reserves are further depleted.
Information about reserves outside the US have not been officially released, but this is constantly changing as new sources are being identified. Development and extraction activities of tight gas have occurred in many countries including Canada, Australia, Argentina, Venezuela, Saudi Arabia, Mexico, China, Indonesia, Egypt and Russia. In Europe, the presence of huge natural gas reserves has resulted in tight gas not receiving the same attention as in the United States. However, this is changing due to technology developments and the discovery of shale gas in the US, which is creating a similar interest in Europe.
Tight Gas – A Hidden Treasure
Tight gas fields can be made economically viable with a combination of horizontal wells and fracture stimulation. Production and permeability guidelines are relative to technology development, current gas price, well cost, fracturing cost, etc. Before tight gas is extracted, studies are conducted on important geological parameters including regional pressure and thermal gradients, the structural and tectonic formations of the rock basin, etc. All these can highly influence the drilling, evaluation, completion and stimulation technique to be adopted to extract this hidden treasure.
The world market for unconventional gases including tight gas is driven by complex factors including geopolitics, world gas prices, technological developments, the demand supply situation resulting in switching the supply sources, cost barriers, etc.
The US is a major driver and leads the development of cost-effective technology. The country has taken a gigantic leap in technology and resources (e.g., manpower, business environment and sophisticated equipment). Drilling is extremely cost-effective compared to the overall world drilling efficiency. This is mainly due to high expertise developed over a long period of time, reduced data collection costs and reducing inefficiencies in operations. In due course Europe and rest of the world is expected to catch up, but it may take a while before that happens.