what is natural Gas,classification of natural gas

Natural Gas

Natural gas consists of Hydrocarbons not condensed at STP.

First four member of Paraffin are in gaseous form

Classification of Natural Gas (Composition)

       I.            Dry gas: entirely composed of methane

    II.            Wet gas: a gas contains ethane with other molecule between 4-5%

 III.            Classification of Natural Gas due to Origin

 IV.            Petroleum Gas: Formed as a by product of Petroleum, if present with oil in reservior called associated gas, if not called non- associated gas

    V.            Coal Gas: formed by modification of coal thermo chemically, world most commercial gas.

 VI.            Bacterial Gas: formed in low temperature near surface, no relation with petroleum e.g., marsh gas.

VII.            Other type of natural gas is:

Mineral Gas: totally unrelated with petroleum, formed during volcanic activity

GOR (Gas Oil Ratio)

Volume of gas within reservoir in relation to the volume of oil

Gas dissolved within oil in reservoir under great pressure and temperature, and gas increases with increase in pressure and effect the property of oil within reservoir.

The pressure is also responsible oil to flow to the surface without any effort.

 If the gas content is sufficient (saturate the oil) within reservoir it caps the oil as “free gas cap”.

Distillation Process/Fractionization: About 70-80% crude oil contains heavy Hydrocarbon higher than C10, The gasoline, kerosene, diesel and heavy oil are middle fractionate, the remaining are residuum.

In the Distillation process, petroleum is separated into different molecular groups:

C1-C4        Gas                                          

C4-C10      Gasoline

C11-C13    kerosene

C14-C18    Diesel fuel (light gas oil)

C19-C25    Heavy gas oil

C26-C40    lubricating oil

    >C40:     Residuum

Natural-Gas Liquid (NGL):

Hydrocarbon that occurs naturally in gaseous or solution within oil in reservoir and are recoverable through fractionation.

Forms of NGL

Condensate

Liquefied Petroleum Gases (LPG):

Condensate

A liquid hydrocarbon of wet gas at STP

Includes pentanes and higher, called as pentane-plus, 10-40% butane may include.

Colour pale yellow, API gravity above 60o.

Liquefied Petroleum Gas (LPG)

Hydrocarbon which are gaseous at STP, but compressed through distillation process.

Used as motor car fuel and mostly (propane and butane)

Non-Hydrocarbon Constituents of Crude oil and Natural Gas:

Sulphur, nitrogen and oxygen and its compounds along with some heavy metals like vanadium and nickel are non-hydrocarbon constituents and are as;

·         Sulphur

·         Nitrogen

·         Oxygen

·         Other element

·         Metals

1. Sulphur and its compounds

Range from 0-3%, found in heavy oil

Some found in elemental form or as compound (H2S) the rest

Attached in petroleum compounds e.g., Mercaptons or Thioalcohol (C2H5SH), disulphides (as diethyl disulphide: C2H5SSC2H5) and cyclic aromatic sulphides (as Thiophene: CnH2            S).

Classification of Crude oil on basis of Sulphur

Sour Crude Oil: contains detectable amount H2S

Sweet Crude Oil: S (wt.) = 0.1-0.2%

Low Sulphur Crude Oil: S (wt.) = 0.3-0.5%

Intermediate Sulphur Crude Oil: S (wt.) = 0.6-1.7%

High Sulphur Crude Oil: S (wt.) = > 1.7%

2. Nitrogen and its Compounds

Amount ranges from 0.1-1.5%, generally 0.2% and present all type of oil

Its compounds are Pyridine (C5H5N) and Quinoline (C9H7N).

Classification of Crude oil on basis of Nitrogen

Low Content “N” Crude oil: N (wt.) = 0.2%

High Content “N” Crude oil: N (wt.) = 0.5%

Oxygen and its Compounds

Ranges from 0.1-4.5% by weight.

It may be present as Free State, very beginning of organic decay (anaerobically) or Phenol as OH in aromatics. It is also present at some amount in CO2

Other Element in Crude oil and gas

Are in trace amounts and are;

In Gas: H (wt.) = 0.1 -0.5%, He= 5ppm (in dry gas or high in nitrogen base gas and Ar (wt.) = 0.1%.

In crude oil: Radon = 0.1-0.4% PCi g-1, Thoron etc.

Metals in crude oil (like Nickel and Vanadium)

Can be found in residuum, with heigh molecular weight 50 carbon or greater atom.

Compounds include: Asphaltene and Porphyrins

Asphaltene: composition variable, in polymertic sheets having aliphatic side chain and S-N-O, and metals which binds molecule of porphyrins.

Porphyrins: hydrocarbon ring complexes containing N and metallic nucleus, metal in invariable either, Vanadium = 30-300 ppm or Nickel= 20-85 ppm (in oily part of bitumen)

Summary

Petroleum has an average composition of 85% C, 13% H, and 2% N, S, and O.

In natural gas, C is slightly lower and H higher.

Distillation separates petroleum into fractions of increasingly higher complexity.

HC types include paraffins, naphthenes, olefins, and aromatics.

Non-HC contain N,S, and 0 and are enriched in the heavier petroleum fractions.

Most HC are used as fuels; only 3% are used to produce over 10,000 types of organic chemicals.

Gas use is increasing strongly despite transportation problems.

Oilfield Water

The water associated with oil and gas pools are known as oilfield waters.

Classification of Oilfield waters

Genetic Classification

Classification on the basis of occurrences

1. Genetic Classification

Classified into three groups;

A: Meteoric Water

The water, as a rain, which percolate from surface to subsurface.

Water of this type contains combined oxygen, carbon dioxide in form of carbonates, bicarbonates and sulphates.

B: Connate Water

The water in which the sediments were deposited, mainly sea water.

There chemistry is the same as sea water, may some time little high in concetration w.r.t various minerals

C: Mixed Water

The water which has both the affinity of meteoric or connate.

Water and has chlorite, sulphate, carbonate and bicarbonates, suggesting multiple origin.

2. Classification on the basis of occurrences

·         Free Water

·         Interstitial (attached) Water

A. Free Water

A water which is ready to flow at any point of pressure release.

They found in the aquifer in their inter connected pore spaces as a continuous body.

B. Interstitial Water

A water which is present at pore spaces of rock.

May coexists with the oil and gas of every pool, and believed to be present at time of rock is being formed and did not escape from their because of capillary action.

They found in oil and gas is less than 10% to 50% or more in pore spaces.