Page 30 - Handbook of Natural Gas Transmission and Processing Principles and Practices

FIGURE 1.1 Schematic geology of natural gas resources.

1.3.1. Conventional Gas
Conventional gas is typically “free gas” trapped in multiple porous zones in naturally occurring
rock formations such as carbonates, sandstones, and siltstones. Conventional natural gas generally
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occurs in deep reservoirs, either associated with crude oil (associated gas ) or in reservoirs that
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contain little or no crude oil (nonassociated gas ).
Conventional gas is typically found in medium to highly porous reservoirs with a permeability
greater than 1 millidarcy (mD). Pressure moves the gas towards the production wells (i.e., natural
flow). As such, it can be extracted via traditional techniques that are relatively easy and inexpensive
(Speight, 2013).

1.3.2. Unconventional Gas

Unconventional gas is the collective term for natural gas held in formations that are different from
conventional reservoirs. Natural gas resources such as shales, tight gas sands, coal (also known as
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coalbed methane), geopressurized aquifers, and gas hydrates are often referred to as
unconventional gas resources. However, the three main types of unconventional natural gas
resources are: shale gas, tight gas, and coalbed methane.

Shale gas is extracted from shales (source rock, usually at depths greater than about 1500 m),
which have naturally low permeability. The gas that shale contains (mainly methane) is either
adsorbed onto the organic matter or left in a free state in the tiny void spaces (pores) of the rock.
The only way to produce commercially meaningful quantities of gas from shale is by generating
large hydraulic fractures in the formation through a process commonly known as “fracking.” Large
volumes of water and sand (proppant) are pumped downhole to open fractures in the shale
formation. The sand fills the fractures so that the soft shale does not “heal” when the well is put on
production. Initially, the well produces a substantial portion of the injected water. After that, gas
will be produced at initially high rates, but those rates generally fall off over time.
Tight gas is trapped in ultra-compacted reservoirs characterized by relatively high porosity but
with very low permeability (due to their laminated structures). Interconnections between the rock
pores in the sandstone that contains the gas are very small, so the gas migrates through them with
great difficulty.
Coalbed methane (also known as coal seam gas) is trapped in coal deposits, often near the surface.

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