W8- Gas Treatment & Specifications

Question 1

Well Types

Answer 1

Oil wells: NG called associated gas
Gas wells: NG called non-associated gas
Condensate wells: NG called non-associated gas

Question 2

What is wet gas?

Answer 2

Methane composition less than 85%, might be out 2-phase region at reservoir, but crosses dew point curve and enters 2-phase region at separator

Question 3

What is dry (lean) gas?

Answer 3

Contains few/no liquefiable HCs (outside 2-phase region), and minimal hydrate formation risk.

Question 4

What is Sour Gas?

Answer 4

Contains more than 4ppm H2S (sweet gas for lower concentrations)

Question 5

What is acid gas?

Answer 5

Contains high composition of acidic gases (CO2/H2S) can be sour/sweet

Question 6

What are condensates?

Answer 6

Contains HC mixture which leaves well as gas but condensates during production

Question 7

Natural Gas (NG)

Answer 7

Methane content at 90%, processed to specific composition/energy content

Question 8

Natural Gas Liquids (NGLs)

Answer 8

mixture of ethane, propane, butane and pentane

Question 9

Liquefied Petroleum Gas (LPG)

Answer 9

propane-butane mixture compressed to liquid at room temperature

Question 10

Liquefied Natural Gas (LNG)

Answer 10

NG liquefied by refrigeration (T=162, P=1atm)

Question 11

Compressed Natural Gas (CNG)

Answer 11

NG compressed at 200bar.
Cheaper vs LNG

Question 12

NG Quality Specifications

Answer 12

Volume: measured in standard cubic meters (scm) or feed (scf)
Calorific Value: total energy per unit gas produced during combustion
Wobbe Index: measure of fuel gases interchangeability of fuel gases
Methane Number: indicates knock tendency
H2S+S: max 5+10mg respectively
Mercury: extremely corrosive- must be below 0.001ppb
Dew Point: temp at which each gas condenses at pipeline pressure
Particles: No particulate solids/liquids causing erosion/corrosion
Additives: tetrahydrothiophene to give smell for domestic use (leak detection)

Question 13

NG Treatment Methods

Answer 13

Gas Cooling
Compression
Acid Gas (CO2/H2S) removal
Dehydration
Mercury, Nitrogen or NGLs removal

Question 14

NG Compression Goals

Answer 14

Gas for export;
Reinjection for well;
Gas lift to improve production;
Provide fuel for gas turbines

Question 15

NG Compression

Answer 15

Must comply with dry gas specification (no H2O/gas condensed)

Multistage used as gas compression produces heat so gas coolers are needed after each compressor.
Intercooling reduces compressor load

Question 16

NG Compression Types

Answer 16

Positive Displacement (increase pressure by operating gas in confined space)
Dynamic (high velocity to induce pressure increase)

Question 17

Hydrates

Answer 17

Crystalline structure stabilised by guest (often HC) molecules.

Form in high P, low T environments

More frequent in start-up/shutdown due to pressure changes.

Question 18

Controlling Hydrate Formation

Answer 18

High Flowrate (less time for formation)
Pipe Scanning (hydrate plug detection_
Pressure monitoring (high P = hydrate presence)
Use of inhibitors/dehydrators (desiccants)

Question 19

Advantages/Disadvantages of solid+liquid desiccants (Hydrate)

Answer 19

Solid desiccants: +less affected by P,T, flowrate; -higher capital cost & P drop, high space & weight req.

Liquid desiccants: +lower installation costs and P drop; -glycol susceptible to contamination

Question 20

Advantages/Disadvantages of Hydrate Inhibitors

Answer 20

Thermodynamic: +more efficient; -safety considerations

Kinetic: +low cost, non-toxic; -limited experience

Antiagglomerants: +low volume req., non-toxic; -limited experience

Question 21

Glycol-based NG Dehydration

Answer 21

Remove moisture in NG as water can cause problems such as corrosion, freezing, and reduced heating value, which can lead to operational and safety issues

Question 22

Water Absorption Column Design

Answer 22

Kremser-Brown Equation:
Number of theoretical column stages
N=ln[((yN+1-y0)/(y1-y0))(1-(1/A))+1/A](1/lnA)

Question 23

Acid Gas Removal

Answer 23

CO2/H2S react with water forming strong acids, which corrode equipment + pipes.

Sweetening: removal H2S or CO2 (reduction of sour character).

Methods: absorption, adsorption, membranes, cryogenic processing

Question 24

Natural Gas Liquids (NGLs) Removal

Answer 24

Separate CH4 from NGLs (propane, butane, pentane); these heavier HCs (NGLs) are absorbed into an oil using an absorption tower.

Cryogenic processing: gas blown into expansion chamber, so vapour pressure and temp reduced.
Cooling forces NGLs to liquefy while CH4 still gas

Question 25

Liquefied Natural Gas (LNG) for NG Storage

Answer 25

NG is stored above ground in spherical tanks as LNG is safer and easier due to volume reduction.

NG/CO2 can be reinjected to raise pressure and increase flows

Question 26

Natural Gas (NG) Storage Below Ground

Answer 26

Depleted gas reservoirs
Aquifers
Salt caverns

Question 27

Advantages/Disadvantages of Depleted gas reservoirs

Question 28

Advantages/Disadvantages of Aquifers

Answer 28

Adv: wide availability

Dis: expensive to develop; geological characteristics not well known; dehydration of gas required after extraction; base gas can be 80% total gas storage

Question 29

Advantages/Disadvantages of Salt Caverns

Answer 29

Adv: high wall integrity- structural strength; high deliverability rate

Dis: requires cavern leaching- salt dissolved and extracted using water to create cavern ->expensive;
small size (1% of depleted gas reservoir)

Question 30

Type of Salt Cavern

Answer 30

Salt Dome & Bedded salt