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 getting reduced. Hence we should understand that the term unconventional gas is a dynamic variable concept and is subjected to reclassification, as technologies and better extraction methods evolve. Currently there are six broad types of unconventional gases, including deep gas, shale gas, coal bed methane (coal seam gas), geo-pressurized zones, Arctic and subsea hydrates and tight gas.
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”. It is also trapped in sandstone or limestone formations that are unusually nonporous, also 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. Tight gas reservoirs are generally defined as having less than 0.1 millidarcy (mD) matrix permeability and less than ten percent matrix porosity.
Let us understand what is 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 human hair. Permeability is the ability of the fluid to move through the pores. As technology has developed, the permeability guidelines for tight gas have changed onshore from <0.1mD in the 1970s to <0.01mD today and <0.001mD in the US nowadays. They are measured in millidarcy unit named after Henry Darcy. They are not SI units but they are widely used in petroleum gas measurements.
Tight Gas Sands (TGS) represents approximately 70% of the unconventional production with significant reserves yet to be developed. A tight gas reservoir is one that cannot be produced at economic 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 too 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 rock makes the difference.
In a typical tight gas reservoir, the original pore space is reduced by lithification (process of porosity destruction through compaction and cementation) and diagenesis (change of sediments or existing sedimentary rocks into a different sedimentary rocks during and after its formation).
A tight gas reservoir has many facets such as:
- It can be deep or shallow;
- High pressure or low pressure;
- High temperature or low temperature;
- Blanket or lenticular;
- Homogeneous or naturally fractured; and
- May contain a single or multiple layers.
World reserves and production of tight gas
The United States has been exploring and fracturing low permeability rocks since the 1950s. However, the steep increase in the natural gas price in the 1970s gave a fillip to 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 deregulation of the gas prices, government-funded R&D programs and tax incentives to create the technology required for tight gas development. At that time, a tight gas reservoir was defined as one in which the permeability to gas flow was less than 0.1mD. Now this has changed to <0.001mD in the United States. It is estimated that more than 15% of the consumption and 30% of the production in the United States comes from tight gas reservoirs. Shale gas, coal bed methane and tight gas account for about 50% of United States natural gas production. This percentage will change in the years to come when conventional reserves start depleting further.
Reserves outside the USA 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 outside the USA including Canada, Australia, Argentina, Venezuela, Saudi Arabia, Mexico, China, Indonesia, Egypt, Russia, etc. In Europe, the presence of huge natural gas reserves has resulted in tight gas not receiving the same attention as the case in the United States. However, this is changing due to technology developments and discovery of shale gas in the USA which is creating similar interest in Europe.
Tight gas – a hidden treasure
Tight gas field is one that 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 extraction of tight gas, 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 complexity of factors including geo political situation, world gas prices, technological developments, demand supply situation resulting in switching sources of supply, cost barriers etc. The United States is a major driver and has taken lead in having developed cost-effective technology both in extraction and distribution of service lines. The country has taken a gigantic leap in technology and resources (manpower, business environment and sophisticated equipment). Drilling is extremely cost-effective compared to world drilling efficiency. This is mainly due to high expertise developed over a long period of time, reduced data collection costs and cutting down inefficiencies in operations. In due course Europe and rest of the world is expected to catch up but it may take a while before it happens.