Chapter One-O&G-1

Shell Special Intensive Training Program Phase IV Oil and Gas Processing

Univation Page 1 of 46 12/01/00

Chapter One

PETROLEUM FLUIDS COMPOSITION

TABLE OF CONTENTS Reservoir Fluid Composition 2

Methods of Crude Oil Analysis 2 Types Of Crude Oil Bases: 5

HYDROCARBONS 9 Physical Properties of Hydrocarbon 9 Paraffin Hydrocarbons 14 A. Physical Properties of Paraffins 14 Chemical Properties and Uses of Paraffin 19 Naphthenes (Cycloalkanes) 23 Physical and Chemical Properties of Naphthenes 25

Aromatics 25 Properties of Aromatics 26 Physical and Chemical Properties of Benzene. 26 Other Hydrocarbon Compounds 28 Non Hydrocarbon Compounds of Natural Gas 28

TYPE OF RESERVOIR FLUIDS 31 A. Oil 31 B. Gases 33

Gas and Oil Relationship 35 A. Gas Oil Ratio (GOR). 35 B. Reservoir Gas and Oil Relationship 37 C. Basic Sediment and Water (BS&W) 38 D. API and API Correlation 38

Sampling and Analysis of Reservoir Fluids 44 Oil and Gas Processing 45 Crude Assays 45 Reservoir Simulation 46



Chapter One

PETROLEUM FLUIDS COMPOSITION Reservoir Fluid Composition Reservoir fluid is a complex mixture of several compounds, most of which, are hydrocarbon compounds. The rest are water, sulphur compounds (eg H2S), carbon dioxide (CO2), nitrogen, Oxygen (O2), helium and heavy metals such as mercury. Reservoir fluids commonly occur in the reservoir as liquid (oil) or gas, or as combination of gas and liquid, where the gas is dissolved in the liquid. It must be mentioned however that, petroleum, which is the reservoir fluid, does occasionally occur in the solid or semi-solid form. Apart from the solid state, petroleum would be found in a thick tar-like jelly form and as a light and clear oil. The state, in which petroleum is found, depends on the amount of the lighter or heavy hydrocarbons present in it. Petroleum with higher proportion of methane, will invariably be gas, while that with higher proportion of heavy high-number carbon atoms will either be liquid, semi-solid or solid. Generally, when the petroleum consists of larger molecules of hydrocarbons it is liquid and it is called petroleum oil or crude oil. The colour of petroleum ranges from light clear to dark black colour. This also is a function of the type of hydrocarbons that it contains predominantly, and the type and amount of the non-hydrocarbon compounds. Methods of Crude Oil Analysis

Crude oil consists of thousands of different compounds ranging from methane with one carbon atom to hydrocarbon molecules having 100 carbon atoms. The most commonly found hydrocarbon groups in crude oil are paraffins, naphthenes and aromatics. Two other hydrocarbon groups, olefins and diolefins, are sometimes found in refined products of crude oil. The exact analysis or separation of crude oil into its multitude of compounds is impossible due to the number. As such, analysis or separation of the crude oil is normally done into fractions consisting of several compounds having individual boiling points falling into a particular boiling point range. There are three methods of reporting this analysis. 1. Ultimate Analysis:

This lists the composition of the crude oil in terms of percentage of the elements of the various compounds; such as percentage of elements of carbon, hydrogen, oxygen, sulphur, nitrogen, etc. This method of analysis says very little about the amount and type of compounds in the crude, and about the characteristics of the crude oil. It is however very useful in determining the quantity of sulphur in the crude which helps to



determine the quantity to be removed. Table 1-1 illustrates elemental analysis of several crude oil from some parts of the world.

Table 1-1 Ultimate Chemical Analysis of Petroleum

Component (%) Petroleum Specific

Gravity

Temp. (C)

C H N O S Base

Pennsylvania pipeline

0.862 15 85.5 14.2 Paraffin

Mecook, WV 0.897 0 83.6 12.9 3.6 Paraffin Humbolt KS 0.912 85.6 12.4 0.37 Mixed Healdton, OK 85.0 12.9 0.76 Mixed Coalinga, CA 0.951 15 86.4 11.7 1.14 0.60 Naphthene Beaumont, TX 0.91 85.7 11.0 2.61* 0.70 Naphthene Mexico 0.97 15 83.0 11.0 1.7* 4.30 naphthene Baku, USSR 0.897 86.5 12.0 1.5 Colombia, S.Am. 0.948 20 85.62 11.91 0.54 2. Chemical Analysis:

The chemical analysis gives the composition in terms of percentage of paraffin, napthenes and aromatic type compounds present in the crude oil. Paraffins are the straight or branched chain, single bond, saturated hydrocarbons. Naphthenes are hydrocarbon compounds that are saturated having single bonds but with the carbon atoms arranged in ring. They are also called cycloalkanes, cycloparaffin or alicyclic hydrocarbons. Aromatics hydrocarbon are made up of molecules of benzene or their multiples. The benzene molecule consists of six carbon atoms bonded together in a ring with six-hydrogen atoms see fig. 1-1. Any compound of this nature that behaves chemically like benzene is also considered to be aromatic. The chemical analysis of crude can be determined with a lot of accuracy by means of chemical reaction or solvent test. This analysis gives an idea of the usefulness of the refined products, since the proportion of the paraffin, naphthenes or aromatics should be a pointer. Nevertheless, the actual quantity or amount of such refined products cannot be ascertained from the analysis. Table 1-2 illustrates the result of chemical analysis of several fractions of four crude oils.



Fig. 1-1 Structures of Paraffins, Naphthenes and Aromatics

Table 1.2 Chemical Analysis of Petroleum

3 Evaluation Analysis

This consists primarily of a fractional distillation of the crude oil followed by physical property tests. These property tests are carried out on the distillation products to determine their API gravity, viscosity, pour point etc. This method makes it possible for the yield of the crude oil and its properties at the refinery to be predicted, since the refinery itself uses fractionation process.

Evaluation analysis curves produced by this method or analysis are illustrated in fig.1-12 and these curves make it possible to predict the refined products physical properties. Such predictions from the evaluation curves are used to determine the product yield from the crude oil. The predictions can be for the refinery fractionation column to be set to operate for maximum gasoline yield as in Table 1-3, or for it to be set for maximum lube oil and diesel yield as in Table 1 4. A simulated distillation called ASTM Test Method D2887 which uses gas chromatograph is being researched upon to produce simulated distillation that will produce crude oil evaluation curves with very small samples in very short period of about one hour. The present fractional



distillation column uses a gallon of crude oil and takes two days for the analysis.

Table 1-3 Evaluation Operated for Maximum Gasoline Yield

Table 1-4 Evaluation Operated for Maximum Lube Yield

Types Of Crude Oil Bases:

There are three methods of classifying the bases of crude oil. 1. General Classification: This has three traditional classifications used to determine crude oil bases. These are Paraffin base, Intermediate base and Naphthene base. This general classification is still sometimes used and it is quite useful except that it leads to ambiguity when trying to classify crude oil that may exhibit one set of characteristics for its lighter ends and another set for its heavy ends (fractions). 2. USBM Hempel Distillation Classification This method classifies all crude oils into two bases in accordance to two key fractions resulting from the Hempel distillation of the crude oil. These are:

a) Key fraction 1, is a fraction which consists of compounds that will boil between 482 0F and 527 0F at atmospheric pressure.



b) Key fraction 2, is fraction with compounds that will boil between

527 0F and 572 0F at 40mm absolute pressure. The two fractions well separated are then tested for API gravity. Key fraction 2 is in addition tested for cloud point. In naming the crude oil after the distillation and testing, the base of the light ends i.e. key fraction 1 is named first before the base of the heavy end i.e. key fraction 2. Furthermore, if the cloud point of the key fraction 2 is above 5 0F, the term Wax Bearing is added and the term Wax Free is added if the pour point is below 5 0F. In general, the key fraction 1 and key fraction 2 refer to the gasoline and lube oil fractions respectively. For example, paraffinintermediatewax free crude would mean a crude oil that has paraffin characteristics in the gasoline portion and intermediate characteristics in the lube portion and it has very little wax. Table15 illustrates bases of crude oil by the USBM methods.

Table 1-5 Bases of Crude by USBM Hempel Method

3. Numerical Correlation Indices This is an index that gives a numerical correlation for the crude oil base. There are two correlation indices used in the oil and gas industry. a) Characteristics Factor

This factor, developed by Watson, Nelson and Murphy, is the most useful in the industry. It is commonly referred to as Watson K-factor, UOP Characterisation factor or UOP-K factor.

The higher the K of any crude, the more paraffin it is, the higher the boiling point, the higher the API, the higher the tendency towards gasoline state. Conversely the smaller the K, the more aromatic, the lower the boiling point, the less the API, the thicker and higher tendency to liquid/lube state. Table 15 and 1-6 illustrate K-factors for certain



crude oil with other physical properties. Apart from API, boiling point and specific gravity a number of other crude oil and crude products properties have been found to relate pretty well with the K-factor. Table16 illustrates this further with different types of crude with their K-factors, hydrocarbon contents, etc. Table 1-6 also shows that the more complex the Hydrocarbon, the lower the K-factor.

It is determined by the following equation:

K =

3BT Eqn. 1-1

or

K =

3 460+BT

T B = Molar Average Boiling Point temperature in 0R TB = Normal Boiling Point 0F = Specific Gravity at 60 0F K = Crude Characteristics Factor.



Table 1-6

Characterisation Factors for Some USA Crudes

b) Correlation Index

This method has been used extensively at the United States Department of Energy (USDOE) at Bartlesville Energy Technology centre (BETC), which has been making distillation analysis since 1920s and has done so for virtually all crudes and for all oil fields in the U.S. The correlation index is defined as follows:

IC = 473.7 - 465.8 + 87552 Eqn. 1-2 BT = Crude specific gravity at 60 0F BT = Crude average normal boiling point temperature in 0R

Generally, Low IC indicates paraffin and IC = 0 indicates purely paraffinic crude. High IC indicates high degree of aromatically. Actually benzene has IC = 100.



HYDROCARBONS From the evaluation analysis of the several and various crude from all over the world, it is clear that hydrocarbons constitute between 80 99 % of most crude oils. Most of the reservoir compounds whether occurring as gas, liquid or solid, are made up of hydrocarbons. Hydrocarbons are compounds made up of just carbon and hydrogen elements. These two elements combine to form various and different chemical compounds. Fig 1-2 consists of major hydrocarbons found in one typical crude oil. Physical Properties of Hydrocarbon

Type of Physical Properties a) Intensive Properties:

These are properties, which are independent of the quantity of the material present e.g. Density, specific volume and compressibility.

b) Extensive Properties:

Properties whose value depends on the quantity of the material present. Examples are volume, mass etc.

1. Molar Mass or Mole

This is the weight of any compound equal in number to its molecular weight. It is the number of weight units equal to its molecular weight. It is expressed in

Pound mole The compound weight in pounds equal in number

to its molecular weight. Gram Mole The compound weight in grams equal in number to

its molecular weight. Ton mole The compound weight in tons equal in number to

its molecular weight.

Pound Mole = 453.59 Gram mole. A pound mole of all compounds occupies the same volume (379.4 FT3) at standard conditions (14.7 psia & 600F) and contains the same number of molecules.

2. Molecular weight

Molecular weight of any compound is expressed in pounds per pound mole or gram per gram mole or tons per ton mole. In Table 1-9 are some commonly found compounds in petroleum fluids with their respective molecular weights and pound moles.



Figure 1.2 Major Hydrocarbons in Crude Oil



3. Dew Point Pressure When gas is saturated with liquid phase, any reduction in pressure will result in liquid formation. This is the case in condensate reservoir. The pressure at which liquid starts to form is referred to as dew point pressure.

4. Hydrocarbon Structure.

The hydrocarbon Structure is made up of carbon atoms bonded (attached) to one another forming the backbone. The hydrogen atoms are then bonded (attached) to the carbon atoms (see fig. 1-1). The bond is a covalent bond which is a result of sharing of electrons from both elements. The carbon and carbon bond can be single, double or triple, while the carbon and hydrocarbon bond is always single. Fig. 1-2 illustrates structures of methane, ethylene and acetylene indicating single, double and triple bonds respectively.

Fig. 1-3.

Single, Double and Triple Bonds of Hydrocarbons

5. Saturated Hydrocarbons: These are hydrocarbons with very strong

single bonds. The alkanes or paraffins are saturated hydrocarbons. They are considered saturated, because their chemical desire for reaction with most reactants is satisfied or saturated. They are very inactive chemically.

6. Un-Saturated Hydrocarbons: These are made up of weak double or

triple bonds which give room for further reactions chemically. Alkenes, and Alkynes are un-saturated hydrocarbons.



7. Isomerism: This is the occurrence of a hydrocarbon of the same chemical formula in two or more chemical structures. In most cases one of the structures is a straight chain while the other is branched. For example, Butane C5 4 H5 10 has n- butane and Iso-butane, and pentane C5 H 12 has n-pentane, Iso-pentane and neo- pentane. Table 1-7 consists of possible number of isomer per number of carbon atoms in alkane isomers.

Table 1-7 Number of Possible Alkane Isomers

Number of Atoms Number of Isomers 1 or 2 or 3 1 4 2 5 3 6 5 7 9 8 18 9 35 12 355 15 4,347 18 60,523 25 36,797,588 40 62 x 1012

Figure 1.4 Isomers of Butane and Pentane



Table 1-8 Properties of Isomers of Hexane

Fig 1-4 illustrates isomers of butane and pentane .The number of isomers for each molecule increases with the number of carbon atoms the paraffin has. Table 1-8 illustrates the physical properties of isomers of hexane. As branching increases, there is a decrease in intermolecular attraction resulting boiling point reduction. Branching also causes changes in the way the molecules fit into the crystal lattice of the compound solid and this causes the melting point of the isomer to be different.

8. Homologous Series

In the study of hydrocarbons, they are usually classified into families or series. This family is known as a homologous series. Each series is made up of compounds or members with similar chemical structures and have graded physical/chemical properties that form a predictable pattern. These graded physical properties differ from one member to another according to the number of carbon atoms in the structure. If the homologous series to which a compound belongs is known, its chemical and physical properties can be inferred from the corresponding properties of the other compound in the homologous series. We shall therefore study the physical and chemical characteristics of hydrocarbons in accordance to the various hydrocarbon families (series) present in reservoir fluids.

Reservoir fluids consist mainly hydrocarbon homologous series which include of Alkanes, Alkenes, Cyclic Aliphatic Hydrocarbons (which include naphthenes, cycloalkenes and cycloalkadienes), and Aromatics.



The series that are most important to the oil industry include the Paraffin, Cycloparaffin, Aromatics and Refined Hydrocarbon Products of Crude. Some of these hydrocarbon compounds do not occur naturally in the reservoir, but can be found in the refinery (such as those in d above), while others occur naturally but the proportion is so small that they are usually ignored. Nevertheless, there are others, whose occurrence is small, but they cannot be ignored. These must be considered in design process of the process facilities. Paraffin Hydrocarbons

The paraffin hydrocarbons are saturated openchain (straight chain) hydrocarbons having a general formula of Cn H2n+2. The straight chain hydrocarbons are referred to with the term normal ( n- ), while the ones with branched chains go with the term iso- ( i- ), for example; normal butane (n-butane) and iso- butane (i-butane). See fig. 1-5. Typical members of the paraffin series include methane, ethane, propane, butane, pentane etc. They are characteristically non-reactive, hence the name paraffin which means little affinity for reaction with a lot of compounds (e.g. water). In some countries, the name paraffin is synonymous with petroleum products to laymen and they refer to kerosene as paraffin oil, medicinal oil as liquid paraffin and solid paraffinic petroleum as paraffin wax. Paraffins constitute the main composition of natural gas. A. Physical Properties of Paraffins

1. Number of carbon atoms of member compounds is such that each successive member has additional one carbon atom. The lowest member of the series is methane with one carbon atom. Table 1-9 illustrates other paraffins and some of their physical properties.

2. The molecular structure is such that the carbon atoms are attached to

one another in either straight chain or branched chain pattern, with the hydrogen atoms attached to the carbon atoms to satisfied or saturate the four valency of each carbon. (see fig 1-4)

3. They are generally colourless, odourless and, as far as can be

ascertained, tasteless when pure, whether in solid, liquid or gaseous state.

4. They burn with bluish flame. Leaving little carbon residue. 5. They are all saturated hydrocarbons, which means the carbon valency

is satisfied as mentioned earlier. 6. Each compound of the series is heavier than the preceding one (with

lower carbon atom number) by 14 in molecular weight i.e.- CH4.



Table 1-9 Some Properties of Normal Paraffin

7. The specific gravity increases with increase in carbon atom number.

This is usually measured relative to air for the series gases and relative to water for the series liquids and solids. See table 1- 11.

8. The freezing point increases as the carbon number increase in the paraffin series. Methane the lowest in the series has freezing point of 1840C (3000F) and this increases up till paraffins of C20 and higher, whose freezing points are above room temperature, and are therefore solids in their pure state.

9. The boiling points of the paraffins indicate the difference between those that are gases, and those that are liquids or solids, under normal conditions. As indicated in table 1-9, it will require very low temperature cooling for methane and ethane to be liquefied. Their boiling points are - 259F and -128F respectively. The boiling points of propane and butane are 44F and 31F respectively and these compounds can be liquefied easily. See table 1-11 for physical constants of hydrocarbons. These four compounds are gases at normal temperature and pressure. Pentane with boiling of 36C (97F) is a liquid under normal conditions though a very volatile liquid. The rest members of the paraffin senses are liquid until C20 and higher, which are solids. Except for the smaller paraffins, the boiling point increases between 20C to 30C for each carbon atom increase in the paraffin carbon chain. This increase also applies to all the homologous series of hydrocarbons.



10. The heating value or calorific value is a measure of the heating properties of fuel, and can be expressed as a quantity of heat generated per unit of liquid volume or per unit of vapor volume, per unit of weight. Table 1-11 illustrates these values in both per ft3 of vapor volume and per unit of weight. It will be seen that the heating values related to vapor volume, increases considerably with increasing number of carbon atoms. Butane, for example, gives about 3 times the values of methane.

Table 1-10 Commonly Found Paraffins in Natural Gas

Table 1-10 consists of commonly found paraffins in natural gas and their features



Table 1.11 Physical Constraints of Hydrocarbons



Table 1.11 Physical Constants of Hydrocarbons (cont.)



Chemical Properties and Uses of Paraffin

Chemically, Alkanes or paraffins are generally non-reactive because of the strong single bond which can only be attacked by very strong reactants at ordinary temperatures. At elevated temperatures, paraffin are attacked by oxygen and burn to CO2 and water, hence their use as fuel. Below are chemical properties and uses of paraffins of natural gas. The main hydrocarbon components of natural gas are relatively small number of the lower paraffin group, such as methane, ethane, propane, butane and pentane. Fig 1-5 illustrates the uses of the various components of natural gas. 1. Methane CH4:

Natural gas consists of mostly methane and little of other compounds. The methane proportion of most natural gas is between 80- 99 %. (see table 1-12). Methane is chemically stable because it is compact and saturated. This chemically stable nature is responsible for it not being used as a feed stock in reaction processes in the industry. There is some progress however, in the development of processes that convert methane to methanol (methyle alcohol) and protein. There are also a number of plants that convert methane to urea and ammonia based fertilizers. In these processes however, it is actually the combustion products of the methane that are being used. The main commercial use of methane is as fuel. It is a good and clean fuel and it is sold and used as such.

Table 1.12 Typical Compositions of Natural Gas

(1) Bachaquero Crude (2) Lake Maracaibo (Lagomar Crude (3) Ex-Separator



2. Ethane C2 H6 The proportion of ethane in the natural gas is in the neighbourhood of 10 15% (see table 1-12). Ethane is a less stable chemical than methane and this property lends ethane to involvement in a lot of reactions, giving rise to a lot of uses resulting in various products in the industry. (see fig 1-5). The uses of ethane include;

a) As a good fuel like other paraffin. b) Can be cracked to form ethylene an olefin hydrocarbon. c) Ethylene is a starting feed for the petrochemical industry to

manufacture PVC and styrene Poly styrene Styrene rubbers Solvents Ethane oxide which is used to manufacture:

Detergents Glycol Fibres of terylene type

3. Propane C3H8

Propane is found in the proportion of about 1 8% of associate natural gas and about 4 3.6% of non- associated natural gas. It is by far less stable chemically than methane and this lends it to greater degree of reaction and a lot of uses in the processing industry. Some of the uses of propane include: a) As fuel. b) Being cracked to from ethylene and thus all the uses of ethylene

enumerated above. c) Being cracked to form propylene which gives rise to wider range of

plastics solvents and fibres. (See fig 1-5). d) Propane (just as butane) can be liquefied either by pressurizing it

under ambient temperature or by reducing its temperature at atmospheric pressure. It is then bottled and transported as liquid in either smaller pressurized containers or in bulk refrigerated tanks and vessels. In this form, it is known as liquefied petroleum gas.(LPG). LPG is used mainly as fuel in areas lacking or inaccessible by fuel pipelines. LPG is also used for table lighter, pocket lighters heaters, cookers and other flame devices. Research and experiment are under-way to use LPG as automobile fuel in various parts of the world.



Figure 1.5 Petrochemical Derivatives of Natural Gas



4. Butane C4 H10 This is chemically more reactive, and its uses are more diversified in the chemical and processing industries.

a) First it is cooled, pressurized to form LPG as propane above, to offer

the already enumerated uses. b) nButane and iso-Butane can also be used as high octane

components of gasoline in the refinery. The proportion incorporated is however limited by vapor pressure and volatility restrictions of the gasoline.

c) Production of butadiene from which sulfolane, nylon and synthetic rubber are made.

d) Production of butylene which is used in the manufacture of various solvents, detergents and additives for lubricating oils and greases.

5. Pentane C5 H12

Pentane and heavier paraffin which are found in natural gas, are often used as mixtures without separating them further into individual components. Many of their uses include: a) In corporation into naphta feed stocks that are used in the

i. Manufacture of fertilizer ii. Cracking to ethylene, propylene, butylene, etc. iii. Reforming to synthesis gas or town gas (substitute natural gas).

b) In corporation into blending stock in the refinery for gasoline or other fuels.

c) Mixed with distiller feed to be split into narrow cuts. d) Separation of iso-putane which is a valuable component of gasoline. e) Manufacture of solvents. f) Manufacture of under-boiler fuel.

Naphthenes (Cycloalkanes)

Naphthene is the common name for cycloalkane which is a group of hydrocarbon compounds, having the carbon atoms in the back-bone structure, arranged in the form of a ring. Cycloalkanes are also known as Cycloparaffins or Alicyclic hydrocarbons. They belong to cyclic aliphatic hydrocarbons whose basic structure is formed on carbon atom ring frame. The naphthene ring is saturated. Their general formula without substituents is Cn H2n which is the same general formula for alkenes but the structural configuration is completely different. The structural configuration difference also accounts for differences in physical and chemical properties between the two homologous series. Typical examples of naphthenes are cyclohexane, and cyclopenthane ( see fig 1-6).



Fig.1-6 Typical Naphthenes



Physical and Chemical Properties of Naphthenes

Physically, naphthenes exhibit properties in accordance to other typical homologous series with regular changes in boiling points, melting points and density. Chemically, different members of the naphthene exhibit different level of chemical reactivity. The level of reactivity is determined by the bond angle and the shape of the ring structure. The strain in the carbon-carbon bond is generally responsible for level of activity and with the normal bond angle of 109.50 in paraffins, the bond is stable and reactivity is low. In cylcohexane, the hexagon frame is somewhat puckered in structure with 109.50 bond angle instead of being flat with 1200 (see fig 1-7.) This therefore accounts for its non-reactivity. The cyclopentane is generally flat with 1080 bond angle and this accounts for its non-reactivity. But the cyclopropane and cyclobutane have bond angle less than 1090 and they are the most reactive of this series. They generally do not occur naturally. Their reactions involve the cleavage of the carboncarbon bond. Cycloalkanes of high carbon atoms achieve configurations with bond angle that ensure stability of the molecular structure and their reactivity level is low. These are also not very common.

Fig 1-7 Configuration of Cyclohexane with 109.50

Aromatics These are hydrocarbons that have benzene as the building block of their basic structure. Consequently they include benzene and other compounds that have similar chemical behavior as benzene. These chemical and physical behavior or characteristics which distinguish the aromatics from aliphatic hydrocarbons (Alkane, Alkynes and Cyclic Aliphatic) are called aromatic properties. Aromatics are also referred to as Aliphatic Aromatic Hydrocarbons. It should be noted though, that some compounds which display aromatic properties do not have the benzene structure.



Properties of Aromatics

1. Stable unreactive structure as against similar normal cycloalkatriene

2. (i.e. hexagonal hydrocarbon ring structure with 3-double bonds) 3. Having double bonds that behave as 1.5 bonds making it very stable. 4. Double bond does not undergo addition or cleavage reaction. The

benzene molecule is known to be flat hexagonal ring structure with six carbon atoms at each corner of the ring. Each carbon atom is attached (bonded) to a hydrogen atom. All bonds are 1200 apart since the molecule is flat and symmetrical. There are three double and three single bonds; a situation which suggests it to be as reactive as regular cyclohexatriene. Nevertheless, studies and benzene as if they are 1.5 bonds in combination. This gives the benzene structure a more stable structure than the regular cycloalkadiene. It is postulated that benzene in reaction actually behaves as if it has a resonance structure between the two structures below (see fig 1-8). The bond is called a hybrid bond ,1 bond or benzene bond .

Physical and Chemical Properties of Benzene.

1. Compound containing benzene rings, generally, have very pleasant odors. Hence, they are called aromatic hydrocarbon, They are also called arenes.

2. They are generally quite toxic. 3. Some aromatics are carcinogenic i.e. causes cancer. So the inhalation

should be avoided. 4. The usual size and properties, such as boiling point, density, etc are as in

other homologous series (see table. 1-13). 5. Because of the strong benzene bond, aromatics can enter into reactions

with out the benzene structure being altered. 6. The volatile organic aromatics are highly flammable and burn with

luminous, sooty flame in contrast to alkanes and alkenes which burn with bluish flame leaving little carbon residue. They have low smoke point.

7. They have high octane number, although they are mostly pollutants



Table 1-13 Some Physical Properties of Aliphatic-Aromatic Hydrocarbons



Fig. 1-8 Reonance Hybrid of Two Benzene Structures

Other Hydrocarbon Compounds

1. Asphalt: Asphalt can occur naturally as solid or viscous bitumen in natural formation beds. It is also obtained as a residue from petroleum in the refinery. Asphalt blends containing asphalt and altered asphaltic materials which result from process of chemical modification, air-blown, etc, are added to some drilling fluids. They serve diverse purposes such as component in oil-base mud, loss circulation material, emulsifier, fluid loss control agent, wallplastering agent, etc.

2. Resins :

These are solids or semi-solids complex amorphous mixtures of organic compounds having no definite melting point nor tendency to crystallize. They are sometimes found in petroleum. Their uses include being component of compound material being added to mud to impart special properties such as wall-cake, etc.

Non Hydrocarbon Compounds of Natural Gas

1. Water This is usually present as vapor in reservoir fluids in which state it presents no problems. In the liquid form however, it becomes problematic by freezing to form ice and hydrates. It also forms slugs which pose corrosion problems.



2. Sulphur Compounds ` Sulphur compounds found in reservoir fluids include:

a. Hydrogen Sulphide (Sulphuratted Hydrogen H2S) It is colourless with extreme bad odour. It has boiling point of 560 C. It is also harmful to metallic catalyst used in refining process. It is toxic and poisonous. H2S has to be removed from natural gas due to disagreeable effects above. Where its content is so high, it is used for sulphur production. b. Mercaptans Hydrocarbon in which one of the hydrogen atoms has been replaced by a SH radicle. The general formula for mercaptans is RSH where R represents any organic group. They are also known as thiols. They have more disagreeable odor than H2S. Butanethiol and proppanethiol are from skunk secretion and freshly chopped onions respectively. Different types of mercaptan have been found in crude oil which make them to be sour and they must be removed before refining. Fuel containing sulphur is a major pollutant. Below are simple molecular and structural formulas of typical thiols.

c. Carbonyle Sulphide (COS)

d. Alkyl Sulfides This has general formula of RSR. Their odor is distinct but quite agreeable. They have to be removed from crude oil because of their adverse effects such as those of H2S and mercaptans. All of these sulphur compounds produce the pollutant sulphur dioxide (S02) on combustion, and some have very disagreeable odors. Others are corrosive.

Fig 1-9

Typical Thiols Molecular and Structural Formulas



3. Carbon Dioxide (CO2) It is highly corrosive in the presence of water. It is non-combustible, therefore reduces the heating value of gas.

4. Nitrogen

It is non-combustible and reduces heating value of natural gas. If it is present in high amount, it presents problems of in compactibility with other natural gas stream.

5. Helium

It is rarely present in proportion that requires mention, but in some cases its proportion requires that it should be removed. Helium is hardly ever present in liquid fluids.

6. Mercury

It is rarely present but even if it is, it is in traces in which case, it has to be removed.

Table 1-14

Properties of Some Nigerian Crude

LOCATION

FORCADOS TERMINAL

U.Q.C.C.

Origin of Sample Export Blend Outgoing Crude to Forcados

Gas / Oil Ratio (Ft3/BBL)

Date of Sampling 13.09.98 05.10.98 Date of Analysis 23.09.98 06.10.98 Crude Properties

Specific Gravity (at 60/60F)

0.8749 0.8933

Crude API Gravity (at 60F) API

30.23 26.9

Water Content (BW&S) %Vol

0.20 0.80

Kinematic Viscosity at 100F cs

5.57 8.90

Crude Pour Point (max) (ASTM D97-57)F

+10F 0F

Total Acid Number (Mg KOH/g)

0.33 0.32

Sulphur Content (%Wt)

0.12 0.11



TYPE OF RESERVOIR FLUIDS Petroleum fluids have no distinct classification or types because pressure, temperature and composition mainly determine their states. Therefore, there exists an overlapping between fluid type, irrespective of the criteria used for classification. Nevertheless, petroleum fluid type can be graded as follows: A. Oil

Reservoir oil is further classified in terms of level of shrinkage. 1. Low-Shrinkage Oil

This oil has its separator pressure and temperature very close to its bubble point, which means very little amount of gas will bubble out during separation. Invariably, the surface GOR is low and less than 500 SCF/STB. These oils are quite viscous with 0API gravity of 30 or less or heavier. Their colours range from very dark, often black to other greenish or shades of deep colour. About 80mole % of the produced oil remain as liquid at separator conditions.

2. High Shrinkage

This has its separator pressure and temperature quite below its bubble point. Consequently a lot of gas in solution is given off at the separator. The percentage of fluid remaining in separator as liquid is in the neighbourhood of 65%. It contains relatively lower % mole of heavier hydrocarbons. The stock tank oil is usually medium orange to brown colour with 0API greater than 45 and GOR less than 8000 SCF/STB. This is intermediate oil.

3. Retrograde Condensate Gas :

This is gas in the reservoir which gives rise to liquid on the surface due to reduction of pressure and temperature. Reduction also gives rise to liquid formation in the reservoir as production progresses. The mixture contains more lighter hydrocarbon, (96% mole), and fewer heavy hydrocarbons (4% mole). Gas oil ratio is between 18,000 to 70,000 SCF/STB; 0API is between 50 and 60. The STO colour is normally light straw to water white. Retrograde condensate gas is sometimes referred to as distillate or simply oil. Liquid saturation of most condensates seldom exceed 10%. Table1-15 illustrates the various properties of low shrinkage, high shrinkage and retrograde crude while table 1-14 consists of properties of some Nigerian crudes.



Table 1-15 Properties of Typical Low, High Shrinkage and Retrograde Condensate

Crudes

Types of Oil

Low Shrinkage

High Shrinkage

Retrograde Condensate

Gas Sparator

Pressure and Temperature

Close to bubble point

Quite below bubble point

Far from bubble point

Amount of Gas out of Oil

Little High Very high

GOR Low 500 SCF/STB

Medium 8000 SCF/STB

High 18 0000 SCF/STB

Viscosity High Medium Low API Low 30 50 60

% Product Remaining in Liquid at ST

80% 65% 10%

Colour From Black to Greenish or shades of deep colours

Medium orange to deep colour

Light straw to water colour

Remark Low or no shrinkage. Oil contains heavy ends. Black Oil

Intermediate oil or shrinkage oil. Volatile oil.

Gas in reservoir but gives rise to liquid on surface due to pressure reduction. Also in reservoir as production continue due to pressure reduction

Petroleum Fluid Spectrum

Typical Initial

Stock Tank Liquid Fluid Type

Shrinkage Bsto/res bbl

GOR Scf/bsto

API

Color

Low shrinkage Crude Oil (Low GOR, black or ordinary

>0.5 50 Water white Dry Gas - - - -



B. Gases

1. Wet Gas:

Wet Gas is also gas under reservoir condition but contains some heavy hydrocarbons which under standard conditions form liquid. Reduction does not give rise to liquid formation any time of the reservoir production life. The gas contains more of the moderate-size hydrocarbons than regular dry gas. Gas oil ratio (GOR) is up to 100,000 SCF/STB and the stock tank oil (STO) is normally of 0API higher than 50. Colour is normally water white. Butane and propane from wet gas is sometimes liquefied. The term wet referred to the fact that the gas contains enough heavy hydrocarbons that readily form liquid upon pressure temperature reduction. The major difference between wet gas and condensate is that the wet gas exists in the reservoir as gas through the reservoir production life, but each wet gas getting to the surface gives rise to some liquid (condensate). Whereas in the case of retrograde condensate, as the reduction of pressure and temperature which accompany the depletion continue, liquid is formed in the reservoir by retrograde condensation.

2. Dry Gas

This is natural gas which contains primarily methane with small amount of ethane and possibly propane or higher hydrocarbons. Natural dry gas is both gas in the reservoir and at separator. The term dry here, refers to the fact that the gas does not contain enough heavier hydrocarbons to form liquid at the surface. Nevertheless in practice, dry gas does contain some liquid hydrocarbon. Generally, any gas with GOR greater than 100,000 SCF/STB is considered dry gas. Methane content is greater than 97% and C7+ content is between 0 and 0.42%. Table 1-16 shows certain properties of Wet and dry gas reservoir fluids Dowd and Kuuskraa also classified reservoir crude oil in the following manner.

i. Extra Heavy Crude: Crude having 0API less than 10 ii. Heavy Crude: Crude having 0API between 10 and 20 with

viscosity less than 1000 cp. iii. Conventional Crude: Crude having API greater than 20 with

viscosity less than 1000 cp.



Table 1-16 Properties of Wet and Dry Gas Reservoir Fluids

TYPES WET GAS DRY GAS

Condition in Reservoir Gas with some heavy ends Gas with light ends Pressure Reduction Effect

Gives rise to liquid formation Always gas

Size of Hydrocarbon Molecules

More moderate sizes > dry gas

Light ends Mostly methane about

90% C7+ 100,000 SCF/STB STO API > 50 >>50 STO Colour Light straw to water white Water white STO Viscosity Low Very low Remark Called wet due to the large

number of heavy ends but no liquid throughout in the

reservoir

Dry due to as reservoir and surface conditions.

This is a natural gas whose methane is about

90%. Separator Condition Far from bubble point or the

STO close to the dew point of gas

Outside dew point



Table 1-17 Properties and Composition of Certain American Reservoir Fluid

Properties and composition of typical North American reservoir fluid are as contained in table 1-17 above. The composition is given in mole per cent Gas and Oil Relationship In the production of petroleum fluids, oil is invariably produced with natural gas and to a large extent vice versa. A. Gas Oil Ratio (GOR).

Simply put, this is the ratio of gas to oil. The GOR is a major and important property of petroleum fluids. This relationship can further be looked upon in term of the way gas is present in or produced with oil. This line of thought gives rise to two types of GOR. 1. Solution Gas Oil Ratio

This is the ratio of the amount of gas dissolved in oil i.e. the total amount of gas in solution with oil. This mostly referred to petroleum fluid in the reservoir and it depends on the composition of the fluid, the pressure and temperature of the reservoir. Solution Gas Oil Ratio is measured in standard cubic feet/stand barrel (SFC/STB). The higher the reservoir



pressure, the higher the solution GOR and the higher reservoir temperature the lower solution GOR.

2. Produced Gas Oil Ratio

This is the amount of gas produced from the reservoir fluid per oil produced at the stock tank. It depends on the reservoir fluid composition, and the pressure and temperature at the separator. Low separator operating temperature gives rise to low GOR and high stock tank (STO) and vice versa with high separator operating temperature. At any separator operating temperature, the higher the separator operating pressure, the lower the GOR since more gas will be compressed into the liquid with less gas being released and vice versa. Generally, the more the content of the fluid that is of the lighter end hydrocarbons, the higher the GOR of the solution or produced GOR. GOR is a good indicator of several properties of petroleum fluid, both in the reservoir and produced condition. The composition of the fluids in terms of hydrocarbon type is easily indicated by the GOR. A high GOR indicates a fluid with major lighter ends hydrocarbon, and a low GOR indicates a fluid with heavier ends hydrocarbon. Lighter ends are C1 C5/C6 and heavier ends are G7+. As a matter of fact, very high GOR is an indication of high methane content and it is also an indication of lighter colour of crude, while low GOR indicates more C7+ content and dark color crude. Correlation between GOR and API is generally difficult to achieve and on the whole the GOR and API of produced liquid from reservoir fluid from the same reservoir, may be different depending on the number of stages of separation it goes through, and the operating pressure and temperature of the separator(s).

3. Other terms worth mentioning in the study of GOR are:

a. Average GOR which is the total gas produced for a particular period divided by the total produced oil in that period and then divided by the number of days in that period. This is more or less the average GOR per day.

b. Cumulative GOR is the total gas produced in a period divided by the oil produced in that period.



B. Reservoir Gas and Oil Relationship

This is the relationship between the gas and oil under the reservoir condition. Gas is present with oil in the reservoir in three ways: 1. Solution-Gas:

As mentioned earlier, gas is dissolved in the liquid phase and becomes known as solution gas. Solution gas can be found in under-saturated oil reservoir, where it assists in the depletion process in what is called Solution Drive Mechanism . The reservoir fluid is produced by liquid expansion above bubble point pressure, and by gas expansion below bubble point pressure by gas already out of solution.

2. Gas Cap

A gas cap is a portion of the reservoir above oil (liquid) occupied by gas alone and it is formed as a resulted of the liquid having been saturated with gas. Gas cap assists in reservoir recovery by expanding to push down the oil to the well bore as the reservoir pressure drops due to production. Its expansion also prevents solution gas from coming out of the liquid, thus retarding pressure decline.

3. Liquid in Gas

Gas is also present in gas reservoir with a lot of liquid droplet in it. The liquid condenses with reduction in pressure and temperature either in the reservoir as in the Retrograde condensate or on the surface as in wet gas.

4. Solubility of Gas in Oil

The solubility of natural gas in oil depends on the reservoir pressure, temperature and the fluid composition. When pressure and composition are constant, the higher the temperature the lower the solubility and vice versa. With constant composition and temperature, the higher the pressure the higher the solubility. When pressure and temperature are constant, fluid with composition of both gas and liquid nearing each other will have higher solubility. That means, solubility will be higher for a combination of higher specific gravity gases and higher API crude.

Generally, solubility is defined as the degree to which gas will dissolve in oil and it is limitless because it is only limited by pressure, temperature and composition of lighter molecules present in the fluid.

5. Saturation

Crude oil is said to be saturated with gas, if upon little reduction of pressure, gas will come out, and it is said to be undersaturated with gas,



if no gas comes out upon little pressure reduction. Undersaturation means the oil is gas deficient and it can still take more gas.

6. Bubble Point Pressure The pressure at which the first bubble of gas comes out of the crude oil at the crude temperature.

7. Formation Volume Factor The volume in barrels, that one stock tank barrel of oil occupies in the reservoir at the reservoir pressure and temperature with all the solution gas that can be held in the oil at that pressure and temperature.

C. Basic Sediment and Water (BS&W)

This is the amount of colloidal particles (Sediment and Water) and solids present in the reservoir fluid measured as a percentage of the total fluid. D. API and API Correlation

The standard measurement for stock tank oil (STO) gravity is the API gravity. This is expressed in API degrees.

API = 5.1315.141 o

Eqn. 1-3

o = Oil specific gravity The API degree of stock tank oil is a major parameter for easy indication of the quality and properties of oil, hence its major application as a yardstick for the sale of oil. In the light of the above, it has become necessary to correlate API with other properties of the crude and stock tank oils. 1. API Correlation with Molar Mass

Generally, the higher the API, the lighter the fluid, which indicates low content of heavier hydrocarbons, high content of lighter hydrocarbons and as such, lower molecular weight and molar mass. Fig. 1-10 shows this inverse relationship. Tables 1-17, although, only indicate molecular weight for C7+, also show this inverse relationship numerically for different types of petroleum fluids and representative petroleum fluids from different reservoirs.



Fig. 1-10 Crude Oil and Condensate Properties/API Correlations

2. API Correlation with Specific Heat/Total Heat Content

The specific heat in Btu/OF-Ibm increases as the API increases at a particular temperature. This shows that fluids with high API will exhibit high specific heat (see fig.1-11). It therefor shows that fluids with high API will require more heat to gain a degree per lb. mass. Fig. 1-11 is for a crude with charaterization factor of 11.8, other crudes will therefore have to be corrected by a correction factor. `The specific heat obtained has to be multiplied by the correction obtained. If condensation or vaporization



occurs during heat exchange process, heat requirement is handled by total heat-content. API correlation is shown in fig. 1-11b. Note that specific heat is required to know how much heat, a fluid is taking or required or how fast the temperature is rising

Table 1-18

Mole Composition and Other and some Properties of Single Phase Reservoir Fluid

3. API correlation with Percentage Crude Distilled

The portion or percentage of the total crude that can be distilled can also be determined or inferred by the API of the crude. Since the API relatively indicates the content of the heavier hydrocarbon molecules of the fluid, it is obvious that this content inferred, is the content related to that which cannot be distilled out. The API/percentage distilled curve in fig. 1-13 illustrates this relationship.

4. API Correlation with Mean Boiling Point

Fluids with low API contain heavier hydrocarbons which means high mean average boiling point temperature. The relationship between the two is inverse and this is illustrated in fig. 1-10.

5. API Relationship with Carbon/Hydrogen with Ratio

The carbon to hydrogen weight ratio of typical hydrocarbon indicates whether it is a heavy or light hydrocarbon and this ratio for a fluid indicates whether it is made up heavier ends or lighter ends hydrocarbon. High ratio indicates heavy hydrocarbon. The relationship between this ratio and API is therefore inverse (see fig.1-10).



6. API/Viscosity Relation Viscosity is the measure of the ability of fluid to flow. This is measured generally in Saybolt Universal Seconds (SUS). It is designated as SUt . For Engineering purposes and calculators, the viscosity is measured in Centipoise (cp). Contipoise viscosity measurement is designated as o . The relationship between the two is:

SUSU

otto

7.149219.0 =

Eqn. 1-4

O = Viscosity cp O = Specific gravity of oil measured at that temperature tSU = Universal Saybolt viscosity in seconds

Fig. 1-11a Specific-Heat and Heat Content Vs API Correlation



Fig. 1-11b Specific Heats of Mid-Continent liquid oils with a correction factor for other

bases of oils

Fig. 1-12 Relation Between Viscosity and Temperature for Fluids of Different API

values



Generally, for a liquid of a particular API, the higher the temperature the lower the viscosity. The relationship between viscosity and API is a little bit difficult to come up with, but an estimate can be made from Fig. 1-12 which illustrates an approximate relationship between viscosity, temperature and characterization factor of fluids of different API values. But the trend is that, the higher the API, the lower the viscosity with temperature being constant. Table 1-19 consists of major reservoir fluids properties.

Fig. 1-13 Evaluation Curves of Intermediate-Base Crude of Characterization Factor

11,65



Table 1-19 Physical Measurements of Reservoir Fluids

No. Property Unit 1. Temperature 0F, 0R 2. Pressure psi, psig, psia 3. Vapour Pressure 0F, 0R 4. Ried vapour-pressure 0F, 0R 5. API Gravity (Liquid) 0API 6. Specific Gravity none 7. Density 1b/ft3,1b/gal, b gm/cc kg/m3 8. Viscosity certipoise(cp), sus (secs) 9. Boiling Point 0F, or psia. 10. Dew point 0F, or psia 11. Pour Point 0F 12. Smoke point 0F 13. Freezing point 0F 14. Aniline Point 0F 15. Molecular weight Ib/ lb-mole 16. Refraction Index none 17. Characterisation Factor none 18. Correlation Index none 19. Equilibrium Ratio (K-value) none 20. Base sediment & water (BS&W) % 21. Sulphur Content % 22. CO2 Content % 23. Nitrogen Content % 24. Octane Number none 25. Formation Volume Factor FVF res bbl/STB. 26. GOR SCF/STB 27. Compressibility none 28. Molal Mass lb-mole, bm-mole, ton-mole 29. Specific Heat btu/gm/0C Sampling and Analysis of Reservoir Fluids Two methods are used for the sampling of reservoir fluids. 1. Lowering a substance sampling equipment on wireline into the well. 2. Collecting samples of gas and oil at the surface and later combining them

in proportion to the GOR as measured at the sampling. It is always advisable to collect samples early in the reservoir life preferably just after completion.



Fluid sampling provides the following information. 1. Solution gas oil ratio. 2. Stock tank gas oil ratio. 3. Separated gas oil ratio at different pressure. 4. Phases volume. 5. Stock tank oil API. 6. Formation volume factor. 7. Bubble point pressure of reservoir fluid. 8. Compressibility of saturated reservoir fluid. 9. Viscosity of reservoir oil as a function of pressure. 10. Fractional analysis of casing head gas sample and saturated reservoir

Fluid sample. Sampling for analysis is extremely important and necessary due to the fact that reservoir fluid consists of thousands of components to the extent that its true composition is impossible to ascertain. The fluid composition is therefore Characterized into discrete fractions reflecting a particular range in property value or behavior. Composition characterization is employed to achieve Oil and Gas Processing, Crude Assays and Reservoir Simulation. Oil and Gas Processing

The fluid from the reservoir is processed on the surface to produce a salable oil and gas, either as an end product or feed product for other processes. The fluid analysis provides the required composition for the thermodynamic property calculations. Properties such as density are required to size separators and pipes. Enthalpy and entropy are required to determine heat exchangers and compressors duty. Equilibrium ratio values are required for the determination of the amount of gas and liquid to be separated out of a particular reservoir stream by the separator. Crude Assays

This is the total analysis of the crude sample to determine its API, Pour Point, Reid Vapor Pressure, H2S, and Salt, Sulfur, Water and Sediment contents. It also involves the analytical methods employed to determine or estimate the yield of the various cuts or products that can be obtained from the crude at the refinery. Two basic and popular methods used for this analysis are: 1. Chromatograph.

This determines the individual content of the light end (C1 to C5/C6) of the crude.

2. True Boiling Point Distillation

This defines the heavier hydrocarbons in the crude in terms of a plot of the distillation temperature versus the percentage volume distilled. The heavier end or high boiling point materials of the crude are therefore represented by boiling point ranges or pseudo-component such as

150 - 250 0C BP 20% Crude Volume



250 - 350 0C BP 42% Crude Volume 350 - 450 0C BP 15% Crude Volume and so on.

Generally, to have the best characterized pseudo-component, one of the crude other properties such as specific gravity must be known. Reservoir Simulation

To have a proper understanding of the reservoir fluid and reservoir behavior, an accurate representation of the fluid composition is required for its simulation. Proper characterization of the high boiling point fraction is therefore a necessity. Which means the C7+ must be properly represented in the reservoir phase equilibrium calculations. An accurate crude analysis is the answer.

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Chapter One-O&G-1