WHAT IS NATURAL GAS

• Natural gas is a natural occurring mixture of light hydrocarbons consisting mainly of methane but also contains varying amounts of heavier alkanes and other impurities such as carbon dioxide, hydrogen sulphide,helium, nitrogen, water.
• Natural gas is a fossil fuel formed when layers of decomposing plant animal matter are exposed to intense heat and pressure over thousands of years.
• Natural gas is a non-renewable resource because it cannot be eplenished on a human time frame.

SOURCES OF NATURAL GAS

• Associated Gas: Occurs in the reservoir in conjunction with oil
• Gas Cap Gas: gas overlying the oil layer
• Solution Gas: dissolved in reservoir oil
• Non-associated Gas: occurs in the reservoir with little or no Oil as free gas
• Gas Condensate

PROPERTIES OF NATURAL GAS

• Specific gravity: ratio of apparent molecular weight of gas to that of air itself, a mixture of gases.
• Pseudo-critical properties: critical properties of the gas determined using the mixing rule which takes consideration of the critical properties of the different components of the gas.
• Viscosity: this is a measure of the resistance to flow exerted by the gas.
• Compressibility factor: also called deviation factor, reflects how much the real gas deviates from the ideal gas at different pressure and temperature.
• Density: mass per unit volume of the gas
• Formation volume factor: ratio of the gas volume at reservoir condition to the volume at standard condition.

CATEGORIES OF NATURAL GAS IN ORDER OF HEAVINESS

Natural Gas : 80-90% Methane, Ethane

Liquefied Petroleum Gas (LPG)

• Propane or butane or a mixture of both
• Exists in two forms
• Removed as a bye product of gas
• Processing & in oil refining

• Motor Fuel or Gasoline
• Kerosene
• Heating Oil or Diesel
• Heavy Fuel Oil
• Asphalt for road paving

NATURAL GAS AND GAS LIQUIDS

COMPONENTS OF TYPICAL NATURAL GASES

HYDROCARBON	COMPOSITION
METHANE	70%-98%
ETHANE	1- 10%
PROPANE	Trace-5%
BUTANE	Trace-2%
PENTANE	Trace-1%
HEXANE	Trace-1/2%
HEPTANE+	Trace (Usually none)
NITROGEN	Trace-15%
CARBONDIOXIDE	Trace-1%
HYDROGEN SULPHIDE	Trace (Occasionally)
HELIUM	Up to 5%, usually trace or none

LIQUIFIED NATURAL GAS

• • Liquefied Natural Gas (LNG)

LNG is Natural Gas refrigerated to a point where it changes to a liquid

• Cooling temperature of -162°C or -260°F
  Natural Gas Liquefaction is a temporary change for purposes of transportation & storage

• They are heavier Hydrocarbons removed from Natural Gas during processing

   COMPRISES

• LPG – propanes, butanes
• Natural Gasoline – pentanes and heavier liquids used for lending motor gasoline

GAS LAWS

Our starting point in the study of the properties of real gases, we sider a hypothetical fluid known as an ideal gas.

An ideal gas is fluid in which the volume occupied by the molecules is significant with respect to the volume occupied by the total fluid,

There are no attractive or repulsive forces between the molecules between the molecules and the walls of the container, and all collisions of the molecules are perfectly elastic

This means that, there is no loss in internal energy upon collision.

At low pressures, most gases behave like the ideal gas at under normal distribution pressures.

• Boyles law: This states that if the temperature of a given quantity as is held constant, the volume of gas varies inversely with the absolute pressure.
• Charles law: This states that the volume occupied by a fixed amount of gas is directly proportional to its absolute temperature at a constant pressure
• Simple Gas law: This is a combination of the separate relations of the Boyles and Charles law
• Gay- Lussac’s Law: At constant volume, as pressure increases, temperature increases
• Ideal Gas law: This is an equation of state for an ideal Gas

PHASE BEHAVIOUR OF PURE SUBSTANCE

Phase behavior describes the state (Liquid or Gas ) of a fluid under any known pressure and temperature conditions.

• Pure substances include : Methane, Heptane. Bubble point and dew point pressures of pure substances are the same
• Mixture of two pure substances: Methane plus Heptane. Bubble point and dew point pressures are not the same
• Critical Point: Pressure and temperature conditions at which all properties of liquid and Gas becomes identical and the two cannot be distinguished from each other

Phase Diagram for a pure substance

PHASE BEHAVIOUR OF RESERVOIR FLUIDS

RESERVOIR FLUIDS

• MixturChanges in pressure ( in reservoir & wellbore) and temperature (in the wellbore) have a pronounced effect on hydrocarbon behavior
• P-T Diagram are useful for showing the effect of P&T on the physical state of a hydrocarbon system

FEATURES OF PHASE OF RESERVOIR FLUID

• Critical Point: P&T where the liquid and gas phase are not distinguishable
• Bubble Point: P&T point where the first drop of liquid comes out of the vapor phase
• Saturation Pressure: Pressure where a second phase starts to evolve from a single phase
• Two Phase Region: P&T domain where Gas and liquid co-exist in equilibrium
• Retrograde Region: The spotted area in a P&T domain, where fluid behavior is reverse of the conventional behavior
• Quality Lines: Loci of points of equal liquid volumes within the two-phase region

FLUID CLASSIFICATION BY PHASE BEHAVIOUR

• For a mixture having more of light component, the phase envelope is relatively small and located at low temperatures, the critical point is far down the LHS.
• As heavy component composition increases, the phase envelope covers wider ranges of P and T.
• Higher the amount of n-heptane greater the slant towards right; two phase region exists at even higher temperatures, critical point shifts to right.
• Qualitatively, phase behavior of petroleum reservoir fluids is similar, i.e., reservoir gases have relatively small phase envelopes, whereas reservoir oils have relatively large phase envelopes.

 

PHASE BEHAVIOUR OF THE THREE CLASSES OF NATURAL GAS

GAS CONDENSATE

• Also called Retrograde Gases.
• The phase envelope covers a wider pressure range.
• Gas condensates contain fewer of the heavy hydrocarbons that black oils or volatile oils.
• The critical point on the phase envelope is represented by low pressure and temperature.
• Initially the reservoir fluid is a single phase vapor in the reservoir at point 1
• As the pressure decreases the fluid exhibits retrograde dew-point at point 2
• Further reduction in the pressure cause liquid to precipitate in the reservoir ( usually unrecoverable)
• Revaporization of the liquid begins at low pressures
• However, the above can only be observed in the laboratory, as such low pressures are never encountered in the reservoir

WET GAS

• Predominantly smaller molecules.
• The phase envelope covers a wide pressure and narrow temperature range.
• The word ‘wet’ does not mean that the gas is wet with water but refers to the hydrocarbon liquid or the condensate.
• The critical point on the phase envelope is represented by very low pressure and temperatures.
• A wet gas exists solely as a single phase vapor in the reservoir
• The pressure path 1-2 in the reservoir does not enter the phase envelope
• No liquid is formed in the reservoir However, separator conditions lie within the phase envelope, resulting in some liquid formation at the surfaceDRY GAS
• Primarily methane with some intermediates.
• The phase envelope is narrow and small in comparison to wet gases.
• The word ‘dry’ indicates that the gas does not contain any heavy molecules to form liquid or condensate.
• A Dry gas exist solely as a single phase vapor in the reservoir as well as the separator or surface conditions
• The pressure path 1-2 in the reservoir does not enter the phase envelope and is well outside the phase envelope
• The reservoir temperature is much higher than the cricondentherm

SOURCES OF NATURAL GAS

• Associated petroleum Gas (APG): Or associated Gas, is a form of natural gas which is found with deposits of petroleum, either dissolved in the oil or as a free “gas cap” above the oil in the reservoir.
• Associated gas is/was generally regarded as an undesirable by-product, which is either re-injected , flared or vented
• Non-Associated gas: Are gases that are not found in connection with any crude oil in the reservoir. The non- associated gas reserves are developed to produce natural gas.
• Condensate Gas: Is agas that contains low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields. It condenses out of the raw gas if the temperature is reduced to below the hydrocarbon dew point temperature of the raw gas
• Shale Gas: is a natural gas that is found trapped within shale formations, shale is fine-grained, clastic sedimentary rock composed of mud that is a mix flakes of clay minerals and many fragments of other minerals, especially quartz and calcite

• Biogas: This refers typically refers to a mixture of gases produced by the breakdown of organic matter in the absence of oxygen.

Biogas can be produced from regionally available raw materials such as recycled waste. It is a renewable energy source and in many gases exerts a very small carbon footprint.

Biogas is produced y anaerobic digestion with anaerobic bacteria or fermentation f biodegradable materials such as manure, sewage, municipal waste, green waste, plant material, and crops.

• Synthetic natural gas: is a fuel gas that can be produced from fossil fuels such as lignite coal, oil shale, or from biofuels. synthetic natural gas is produced from natural gas through fischer-Tropsch process in Gas-to-Liquid technology.
• Coal bed methane: coalbed gas, coal seam gas, or coal-mine methane (CMM) is a form of natural gas extracted from coal eds. The term refers to methane adsorbed into the solid matrix f the coal.

It is called ‘sweet gas’ because of its lack of hydrogen sulfide. The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk.

Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane stored within the coal by a process called adsorption.

PETROLEUM TREATMENT TRAIN

FIELD PROCESSING OF NATURAL GAS

• Natural gases produced from gas wells are normally complex mixture of different compounds. A typical gas well stream is a high-quantity, turbulent, constantly expanding mixture of gases and hydrocarbon liquids, intimately mixed with water vapor, free water and sometimes solids.
• The well stream should be processed after bringing it to the surface.
• Field processing consists of four basic processes; separating the gas from free liquids and solids such as crude oil hydrocarbon condensate, water and entrained solids, processing the gas to remove condensable and recoverable hydrocarbon vapors.
• Processing the gas to remove condensable water vapor, processing the gas to remove other undesirable compounds hydrogen sulphide or carbon dioxide.

ACID REMOVAL

• Natural-gas, or any other gas mixture, containing significant quantities of hydrogen sulfide, carbon dioxide or similar acidic properties, is called acid gas. Sour gas is only for when the gas contains H2S
• CO2 is removed from gas by carbon capture which involves: post-combustion, pre-combustion and oxy-fuel combustion.
• Post-combustion : The CO2 is captured after the fossil fuel is ed and it produces flue gases (CO2, water vapour, sulfur des, nitrogen oxides).
• Pre-combustion: The gas is heated in pure oxygen, resulting a CO and H. the mix is treated with a catalytic converter with which then produces more H along with CO2.
• CO2 can also be removed with amine in CO2 removal unit.

GAS SWEETENING

• It involves the processes used in separating acid gases from natural gas, thus changing the sour gas to sweet gas.
• This is done through either solid bed adsorption or the use of chemical solvents SOLID BEBEDADSORPTION
• In solid bed absorption processes the gas stream flows through a fixed bed of solid particles that remove the acid gases and hold them in the surface of the bed. When the bed is saturated with acid gases, the vessel is removed from service and the bed regenerated or replaced. There are commonly used solid bed adsorption processes: iron oxide bed, zinc o
• xide bed and molecular sieves bed processes.

Chemical Solvents

• Chemical solvent processes use an aqueous solution of a weak base to chemically react with and absorb the acid gases in the natural gas stream.
• The absorption occurs as a result of the driving force of the partial pressure from the gas to the liquid.
• The reactions involved are reversible by changing the system temperature or pressure, or both. Therefore, the aqueous base on can be regenerated and thus recirculated.
• The majority of chemical solvent processes are either an or carbonate solution.

GAS DEHYDRATION

• Dehydration is the removal of water vapor from the gas using regenerable absorption in liquid triethylene-glycol (TEG),commonly referred to as glycol dehydration, deliquescent chloride desiccants, and or a Pressure Swing Adsorption (PSA) unit which is regenerable adsorption using a solid adsorbent.

Water must be removed:

• To Prevent the formation of hydrates that will restrict or block the movement of gas in the line.
• To Prevent corrosion in the line to minimize free water condensing in the pipeline thereby reducing the internal cross section area of the pipe allowable for flow and causing partial blockage, and consequently obstructing gas flow through put

METHOD OF GAS DEHYDRATYION

• Absorption: Glycol dehydration
• Absorption: Molecular sieve, silical gel or activated alumina
• Condensation: Refrigeration with glycol or methanol injection
• Adsorption is a process where water vapor molecules adhere to the surface of solid desiccants like silica gel, activated alumina, or molecular sieves.
• The gas passes through a bed of these desiccants, which capture and hold the water molecules on their surface. This method is effective for removing water vapor from natural gas to very low levels.
• Absorption involves water vapor being taken up by a liquid desiccant, such as triethylene glycol (TEG), in a contactor tower. The natural gas flows through the tower and comes into contact with the glycol, which absorbs the water vapor from the gas. The water-laden glycol is then regenerated by heating to remove the absorbed water, allowing the glycol to be reused.

Absorption Dehydration scheme

Adsorption Dehydration scheme

TYPES OF DESSICANT

• Bauxite/-naturally occurring
• Mineral(AI2O3)
• Alumina – a purer bauxite
• Gels- composed largely of SIO2 or alumina gel.
• Molecular Sieves- a calcium-
• Sodium alumino- silicate (zeolite)
• Carbon (charcoal)-a carbon, Product activated to have adsorptive Capacity.

MERCURY AND NITROGEN REMOVAL

• Mercury is removed by using adsorption processes such as; activated carbon or regenerable molecular sieves.
• Though, not common, nitrogen is sometimes removed and rejected by one of the three process
• Cryogenic process (Nitrogen Rejection Unit) using low temperature distillation.
• Absorption process using lean oil or a special solvent as the absorbent.
• Adsorption process, using activated carbon or molecular sieves adsorbent. This process may have limited applicability because, it is said to incur the loss of butanes and heavier hydrocarbons

NGL RECOVERY

• The next step is to recover the natural gas liquids (NGL). This is achieved using cryogenic processes or absorption methods, where the gas is cooled or absorbed to condense and separate out the NGLs, which include ethane, propane, butanes, and natural gasoline.
• A common method for NGL recovery is cryogenic expansion, where natural gas is cooled to extremely low temperatures, causing heavier hydrocarbons to condense into liquids.
• Another method uses absorption processes where a liquid solvent, typically a light oil, absorbs the NGLs from the natural gas.
• The separated NGLs are transported via pipelines, rail, or truck to storage facilities or directly to end-users.
• Specialized infrastructure, such as refrigerated tanks and pressurized containers, is required to handle the different NGL products safely and efficiently

GAS TRANSPORTATION

• Natural gas, as a result of the storage difficulties, needs to be transported immediately to its destination after production from a reservoir.
• Efficient and effective movement of natural gas from producing from producing regions to consumption regions requires an extensive and elaborate transportation system
• Natural gas can be transported through:
• Pipelines: This accounts for most of the gas moved internationally

The usual means of distributing gas to local consumers

• Liquefied Natural Gas(LNG)
• Compressed Natural Gas
• Gas to Solid
• Gas to Power
• Gas to Liquid
• Gas to Commodity

LIQUIFIED NATRAL GAS (LNG)

• LNG is produced by cooling natural gas to approximately -162°C (-260°F), transforming it from a gaseous state to a liquid.
• This process, known as liquefaction, reduces the gas volume by about 600 times, making it much more efficient to store and transport.
• The liquefaction process also involves removing impurities like water, carbon dioxide, hydrogen sulfide, and certain hydrocarbons that can freeze and
• Once liquefied, LNG is stored in specially designed insulated tanks to maintain its low temperature.
• It is transported to their destination in large, cryogenic sea vessels known as LNG carriers.
• At the destination, LNG is unloaded and transferred to regasification terminals. Here, the LNG is heated and converted back into its gaseous state
• This natural gas is then fed into pipelines for distribution to end-users, including residential, commercial, and industrial consumers.

COMPRESSED NATRAL GAS (CNG)

• CNG is produced by compressing natural gas to less than 1% of its volume at standard atmospheric pressure.
• The compression process involves using specialized compressors to pressurize the gas to around 200-250 bar (2900-3600 psi).
• CNG is stored in high-pressure tanks or cylinders, which can be made from steel, aluminum, or composite materials.
• These tanks are mounted on vehicles or used in stationary storage facilities. CNG transportation typically involves moving the gas in specially equipped trucks with high-pressure cylinders, enabling the supply to areas without pipeline infrastructure.
• CNG is primarily used as a fuel for vehicles, including cars, buses, and trucks, due to its clean-burning properties and lower cost compared to gasoline or diesel.