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 What Is Tight Gas, and How Is It Produced?

While conventional natural gas streams from the earth relatively easily, unconventional gas findsare more difficult to develop and more costly to produce. As technologies and skills improve,unconventional gas is a variable concept because some finds may become more easily or

economically produced over time, no longer making them unconventional. Right now, there aresix main types of unconventional gas, including deep gas, gas-containing shales, coalbedmethane, geopressurized zones, Arctic and subsea hydrates, and tight gas.

Major Tight Gas Reserves in the USSource: EIA, www.eia.doe.gov

nconventional natural gas deposits are likely to account for much of the world!s remainingreserves. According to the "#A, there is more than $%& 'cf of recoverable tight natural gasdeposits in the (, which represents some )*+ of the total natural gas reserves in the country.

elping to boost interest in developing technologies that can overcome the challenges ofproducing unconventional gas resources in the nited (tates, the atural as /olicy Act offersincentives to companies exploring for and producing unconventional gas plays.

 What Is Tight Gas?

'ight gas refers to natural gas reservoirs locked in extraordinarily impermeable, hard rock,making the underground formation extremely 0tight.0 'ight gas can also be trapped in sandstone

or limestone formations that are atypically impermeable or nonporous, also known as tight sand.

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Impermea!e "ores in Tight Gas #ormationSource: USGS, www.energ$.usgs.gov 

While a conventional gas formation can be relatively easily drilled and extracted from the ground

unassisted, tight gas re1uires more effort to pull it from the ground because of the extremelytight formation in which it is located. #n other words, the pores in the rock formation in which thegas is trapped are either irregularly distributed or badly connected with overly narrow capillaries,lessening permeability -- or the ability of the gas to travel through the rock. Without secondaryproduction methods, gas from a tight formation would flow at very slow rates, making productionuneconomical.

While conventional gas formations tend to be found in the younger 'ertiary basins, tight gasformations are much older. 2eposited some 345 million years ago, tight gas formations aretypically found in /alaeozoic formations. 6ver time, the rock formations have been compactedand have undergone cementation and recrystallisation, which all reduce the level of permeabilityin the rock.

'ypical conventional natural gas deposits boast a permeability level of .%) to .7 darcy, but theformations trapping tight gas reserves portray permeability levels of merely a fraction of that,measuring in the millidarcy or microdarcy range.

#n order to overcome the challenges that the tight formation presents, there are a number ofadditional procedures that can be enacted to help produce tight gas. 2eviating drilling practicesand more specific seismic data can help in tapping tight gas, as well as artificial stimulation,such as fracturing and acidizing.

Developing Tight Gas

6ne of the most important aspects of drilling for any petroleum is predetermining the successrate of the operation. 6perators do not 8ust drill anywhere. "xtensive seismic data is gatheredand analyzed to determine where to drill and 8ust what might be located below the earth!ssurface.

'hese seismic surveys can help to pinpoint the best areas to tap tight gas reserves. A surveymight be able to locate an area that portrays an improved porosity or permeability in the rock inwhich the gas is located. (hould wells directly hit the best area to develop the reserve, costs ofdevelopment can be minimized.

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9ost tight gas formations are found onshore, and land seismic techni1ues are undergoingtransformations to better map out where drilling and development of these unconventionalplays. 'ypical land seismic techni1ues include exploding dynamite and vibroseis, or measuringvibrations produced by purpose-built trucks. While these techni1ues can produce informationalsurveys, advancements in marine seismic technologies are now being applied to land seismicsurveys, enhancing the information available about the world below.

ot only providing operators with the best locations for drilling wells into tight gas formations,extensive seismic surveys can help drilling engineers determine where and to what extentdrilling directions should be deviated.

%irectiona! %ri!!ing Source: Mac&en'ie Gas "roject, www.mac(en'iegasproject.com

While vertical wells may be easier and less expensive to drill, they are not the most conduciveto developing tight gas. #n a tight gas formation, it is important to expose as much of the

reservoir as possible, making horizontal and directional drilling a must. ere, the well can runalong the formation, opening up more opportunities for the natural gas to enter the wellbore.

 A common techni1ue for developing tight gas reserves includes drilling more wells. 'he morethe formation is tapped, the more the gas will be able to escape the formation. 'his can beachieved through drilling myriad directional wells from one location, lessening the operator!sfootprint and lowering costs.

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Production Stimulation

 After seismic data has illuminated the best well locations, and the wells have been drilled,production stimulation is employed on tight gas reservoirs to promote a greater rate of flow./roduction stimulation can be achieved on tight gas reservoirs through both fracturing andacidizing the wells.

:racturing, also known as 0fracing,0 a well involves breaking the rocks in the formation apart./erformed after the well has been drilled and completed, hydraulic fracturing is achieved bypumping the well full of frac fluids under high pressure to break the rocks in the reservoir apartand improve permeability, or the ability of the gas to flow through the formation.

 Additionally, acidizing the well is employed to improve permeability and production rates of tightgas formations. Acidation involves pumping the well with acids that dissolve the limestone,dolomite and calcite cement between the sediment grains of the reservoir rocks. 'his form ofproduction stimulation helps to reinvigorate permeability by reestablishing the natural fissuresthat were present in the formation before compaction and cementation.

:urthermore, deli1uification of the tight gas wells can help to overcome some productionchallenges. #n many tight gas formations, the reservoirs also contain small amounts of water.'his water can collect and undermine production processes. 2eli1uification is achieved in thisinstance through artificial lift techni1ues, such as using a beam pumping system to remove thewater from the reservoir, although this has not proven the most effective way to overcome thischallenge.

"ngineers continue to develop new techni1ues and technologies to better produce tight gas.'hrough their efforts, maybe one day, tight gas will no longer be considered an unconventionalplay.