Lecture 2- Overview of

the Natural Gas Industry

BASED ON CHAPTER 6 ( PG 125-135), FUNDAMENTALS OF NATURAL
GAS PROCESSING, EDITION 2, BY KIDNAY, PARRISH, AND
MCCARTNEY
Origin of Natural Gas
• Gas and oil are the products of the decomposition of organic matter trapped in
sediments at high pressure and temperature in the past millions of years
◦ Microbial decomposition into kerogen
◦ Kerogen further decomposes to oil and gas
◦ Thermal cracking of oil produces gas
• After being formed, the hydrocarbons are expelled and migrate through porous Kerogen
rock until they are trapped by an impermeable rock layer (cap rock)

Asociated layer -> when oil and gas

presentd together
Reservoir characteristics
• Porosity: void space in the reservoir rock. Typically 0-
20%.
• Permeability: measure of the size of openings
between voids.
• Shape: determines economic viability of the reservoir.
• Some reservoir shapes (e.g. bands) require special
drilling techniques (e.g. horizontal or directional drilling)

• Porosity and permeability can be artificially increased

by hydraulic fracturing
• Requires large amount of water
 non associated gas- when only gas no oil

very difficult to extract

traped on pore

remeber all the names

 why conventional ? find out -> easier to drill

Sources of Natural Gas

• Conventional natural gas: from gas and oil wells
◦ Associated or dissolved gas (coexisting with oil in an oil
reservoir)
◦ Nonassociated or dry gas (produced from gas reservoir and does
not contain liquid hydrocarbons). Can contain NGL and water.
◦ Wet gas or condensate gas gas at reservoir condition
◦ Occurs at high reservoir T and P as a very dense, high pressure fluid
◦ As pressure is reduced in a condensate gas reservoir, the fluid will pass
through the dew point and large volumes of liquid will condense in the
reservoir

• Unconventional natural gas: from nontraditional sources

that became economically feasible with advances in
technology
◦ Tight gas sand
◦ Coal Bed Methane (CBM)
◦ Shale gas
◦ Gas hydrates (not economically viable)
 HHR3#3HGXXKR3#3狣3[Y3MGRRUTY

Associated Natural Gas

• Also known as casing-head gas when stored - gas form upper part

• Dissolved methane and light HCs are released as gas when

pressure on the oil is reduced

Natural
Gas
• Shrinkage
◦ The volume (bbl) of the stock-tank oil per volume (bbl) of reservoir fluid

Reservoir
Fluid
• Oil formation volume factor (FVF)
◦ The reciprocal of shrinkage

Tank Oil
Stock-
• The gas-oil ratio (GOR)
◦ The total scf of gas evolved (60 °F) per barrel of atmospheric-pressure
stock-tank oil
 Classification of Well Fluid
• According to GOR, reservoir fluid may be classified as:

Black Oil Volatile Oil Wet Gas Dry Gas

GOR 2000 3000 50000

Fluid Type GOR
g/oil ratio
Black oil (Low GOR) < 2000
High Shrinkage crude 2000-3300
(High GOR), volatile oil

Gas condensate 3300-50000

Wet Gas > 50000
Dry Gas - GOR
Unconventional Sources: Tight Gas Sands
• Produced from low permeability gas sandstone reservoirs requiring
artificial stimulation to be commercially productive
◦ Permeability is lower than 0.1 millidarcy, compared to > 2 millidarcy for
conventional reservoirs
◦ Porosity is less than 10%
◦ Median pore diameter 0.03-1 µm ( > 2 µm for conventional reservoirs)
◦ Much higher production and drilling cost compared to conventional gas wells
(directional drilling and hydraulic fracturing are common)
 Unconventional Sources: Coal Bed
Methane
• Coal beds contain large amounts of natural gas
adsorbed on coal surface, dissolved in water and as free
gas
• Kept from desorption by overburden pressure and
hydrostatic pressure of water
• Significant amount of water is produced initially and
declines with lifetime
• Poses problems because it contains large amounts of
dissolved solids
• In conventional wells, water production increases over
field life
• Unlike conventional gas wells, reservoir pressure must
be reduced to release (desorb) methane
• Typically shallow wells <1200 m
◦ Low drilling costs
Unconventional Sources: Shale Gas
• Natural gas produced from sedimentary fine grain
rocks (shale)
• Production of shale gas is increasing, especially in
North America
• Requires horizontal drilling and hydraulic fracturing
 studying this because -> while gas traveliling flowing in pipeline this form and cause problem ,
and also there is no any method to extract gas from it so doesn't need to study but problem is there so study.

Unconventional Sources: Gas Hydrates

• Gas hydrate is a crystalline water-based solid in
which small gas molecule is trapped inside a cage
of hydrogen bonded water molecules
• Stable at high pressure and low temperature
• Deep ocean floor with low temperature contains
significant amount of gas hydrates
◦ 9.1 quadrillion m3 in US
◦ 1.3 trillion m3 in north Alaska

• Not currently commercially viable

Natural Gas Characteristics
• Colorless, odorless, and tasteless
• Neither corrosive nor toxic
• Lighter than air with specific gravity of about 0.6 with respect to air
• Highly flammable (burns easily & completely)
• High ignition temperature
• Narrow flammability limits (4-14%)
• Doesn’t explode in unconfined environment

• Liquefied NG (LNG) occupies 1/600th the gas

volume with specific gravity of 0.45 because 1% of methane is not able to heat surrounding, air
with respect to water molecule eat that heat.

above 14% is contains less oxygen.

Natural Gas Constituents
• Methane, Ethane, Propane, n-butane, i-butane, n-
Hydrocarbons pentane, i-pentanes, cyclopentanes, hexane and
heavier

• Hydrogen Sulfide (H2S), carbonyl sulfide (COS),

Sulfur Species carbon disulfide (CS2), mercaptans (RSH), sulfides
(RSR), disulfides (RSSR) (if H2S > 2 mol%)

• Carbon Dioxide (CO2)

Gases • Inerts: Nitrogen (N2), Helium (He), and Argon (Ar)
• Hydrogen (H2) and Oxygen (O2)

• Water, Mercury
• NORM: Naturally Occurring Radioactive Materials
Others e.g. radon
• Solids (iron sulfides, rust, reservoir fines)
Natural Gas Constituents: Hydrocarbons
Typical Gas Compositions (mol%)

• Gas compositions vary substantially from

field to field, and from region to region
• Mostly methane

• Hydrocarbons present are primarily

alkanes but aromatic compounds (e.g.
benzene) are found in small amounts
(especially in associated gases)
• Aromatics pose environmental problems
Natural Gas Constituents: Gases
• Can contain up to 30% nitrogen which must be removed because it lowers the
heating value of NG
• Few hundredth of a percent of He is typically present which is usually separated
and sold
• Small amount of argon, oxygen and hydrogen
◦ Usually undetected by chromatographic analysis and lumped with N2

• Hydrogen sulfide and carbon dioxide (acid gases) are found in many NGs and
may be present in very high percentages
Natural Gas Constituents: Acid Gases
• Both H2S and CO2 are corrosive when present with water
H2S
Human Reaction
• H2S is extremely toxic ppmv
0.01-0.1Threshold odor level
• The allowed concentration of H2S in pipeline-quality gas is 4-16 ppmv 0.9 Detection Limit
1 Clear Detection "Rotten-egg" Oder
• NG containing less than 4 ppmv of H2S is regarded as Sweet 10 Permissible exposure limit (PEL)
15 Short-term exposure limit (STEL)
• NG containing more than 4 ppmv H2S is regarded as Sour 10-150 Loss of smell in 2-15 min
Loss of sense of smell in 2-15
• CO2 can be tolerated up to 1-2% but it lowers the heating value of NG 150-200 minutes. hemorrhage and death in
8-48 hrs
Coughing, collapse,
• H2S and CO2 are removed by sweetening process gas absorption process 500-600
unconsciousness with 2 min
• Pure CO2 can be produced, dehydrated and used for CO2 floods used in enhanced oil recovery
co2 is pushed from down that Coughing, collapse,
push oil up 600-
• H2S can be injected into underground wells or converted to elemental Sulfur unconsciousness, and death with
15000
2 min
 water does corrosion and also hyderaties formation

Natural Gas Constituents: Water

• Most gas produced contains water ranging from trace amounts to
saturation
• Water or brine are separated in a knockout drum
• NG is further dehydrated using glycols or solid desiccant
• Water vapor specification for NG is 7 lbm/MMscf which correspond
to dew point of 32 °F knockout drum
• Cryogenic plants require “bone-dry” gas (dew point as low as -150 °F)
• At high pressure and low temperature, hydrates may form which are
capable of plugging the flow lines
Natural Gas Constituents: Others
• Mercury
◦ Can be present in NG in concentrations ranging from 1 ppb to 230 ppm
◦ Causes corrosion of aluminum tubes in heat exchangers
◦ Typically removed by molecular sieves or sulfur impregnated activated carbon

corrosion,