Natural Gas Engineering

(CHE-484)
Lecture 2-3:

Introduction

1
What is Natural Gas?
• Petroleum: A naturally occurring, complex
mixture of hydrocarbons (minor amount of
inorganic compounds)
• Natural gas is a subcategory of petroleum
‒ Gaseous fossil fuel
‒ Oil fields and natural gas field
• Grouped with other fossil fuels; however, unique
characteristic
Abbreviated timeline for the use of natural gas
• Known for many centuries, but initially used for religious
purposes rather than as a fuel
• Chinese drilled the first known natural gas well in 211 BC
• In Europe, natural gas was unknown until it was discovered
in Great Britain in 1659
Conversion of parent organic material into petroleum
• Not understood
• Petroleum originates from plants and animal
remains that accumulate on the sea/lake floor along
with the sediments that form sedimentary rocks
Factors that may contribute:
a. shearing pressure during compaction, heat, and
natural distillation at depth
b. presence of catalysts and time
c. bacterial action
d. possible addition of hydrogen from deep-seated
sources
Formation of natural gas
• Organic matter is the remains of ancient flora and
fauna that was deposited over the past 550 million
years
• Organic debris mixed with mud, silt, and sand on the
sea floor; buried over time.
• Anaerobic environment
• Exposed to increasing amounts of pressure and heat,
the organic matter decomposed to hydrocarbons
• “Product of decomposed organic matter”
An anticlinal reservoir containing oil and associated gas.
Typical Composition of a Natural Gas
• Methane, a major
component of the gas
mixture
• Inorganic compounds
‒ not combustible
‒ cause corrosion
• Heating value of
natural gas usually
varies from 700 to
1,600 Btu/scf.
Classification of natural gas accumulations in
geological traps
• Reservoir
‒ Porous underground formation
‒ Individual bank of hydrocarbons confined by
impermeable rock or water barriers
• Field
‒ consists of one or more reservoirs all related to
the same structural feature
• Pool
‒ contains one or more reservoirs in isolated
structures
Natural Gas Industry
• Natural gas was once a by-product of crude oil
production
• More efficient use of natural gas is of paramount
importance
• Any gas sold to an industrial or domestic
consumer must meet designated specification
• Composition of natural gas can vary significantly
as the product flowing out of the well can change
with variability of the production conditions
In the early years of the natural gas industry:
‒ the gas accompanied crude oil
‒ find a market or be flared
‒ flared in huge quantities
‒ gas production often short-lived

• The rapidly growing energy demands of Western

Europe, Japan, and the United States could not be
satisfied without importing gas from far fields.
Liquefied natural gas:
‒ liquefied by a refrigeration cycle
‒ reduce its volume to the point where it becomes
economically attractive to transport
‒ reduced to about one six-hundredth of its original
volume
‒ Non-methane components are largely eliminated
‒ transported efficiently and rapidly by insulated
tankers
‒ At the receiving terminals, regasification
• Current production from conventional sources is
not sufficient to satisfy all demands for natural gas.
Classification of Gas Wells
• Gas wells
‒ gas-oil-ratio (GOR) being greater than 100,000
scf/stb

• Condensate wells
‒ gas-oil-ratio (GOR) being less than 100,000
scf/stb, but greater than 5,000 scf/stb

• Oil wells
‒ gas-oil-ratio (GOR) being less than 5,000 scf/stb
Types of Natural Gas
• Non-associated gas
‒ reservoirs with minimal oil
• Associated gas
‒ The gas dissolved in oil under natural conditions
in the oil reservoir
• Gas condensate
‒ Mixture of low-boiling hydrocarbon liquids
obtained by condensation
‒ Predominately C5H12, varying amounts of
higher hydrocarbon up to C8H18, little C1-C4
 • Fuels derived from natural gas:
‒ one quarter of the total world energy supply

Natural gas is used as a source of energy in all sectors of the U.S.

economy (Louisiana Department of Natural Resources 2004).
Example Problem
Natural gas from the Schleicher County has a heating
value of 1,598 Btu/scf. If this gas is combusted to
generate power of 1,000 kW, what is the required gas
flow rate in Mscf/day? Assume that the overall
efficiency is 50 percent.
 Natural Gas Engineering
(CHE-484)
Lecture 4-5:

Natural Gas Industry

18
• Greenhouse gases (GHGs) warm the Earth
- absorbing energy
- slowing the rate at which the energy escapes to space
• GHGs act like a blanket insulating the Earth
• Different GHGs can have different effects on the Earth's
warming

Component Global Warming Potential (GWP)

Carbon dioxide (CO2) 1

Methane (CH4) 28-36

Nitrous oxide (N2O) 265-298

Chlorofluorocarbons The GWPs for these gases can be in the thousands or tens
(CFC) of thousands
Rapid growth of NG consumption
• The consumption of natural gas in all end-use
classifications increased rapidly since World War
II.
• Factors contributing to this rapid growth:
1. development of new markets
2. replacement of coal as fuel
3. making petrochemicals and fertilizers
4. strong demand for low-sulfur fuels
Natural Gas Reserves
• Two terms are frequently used to express natural
gas reserves
i. Proved reserves
ii. Potential resources
Natural Gas Reserves (Continued)
• Proved reserves: those quantities of gas that have
been found by the drill
- Proved by known reservoir characteristics such
as production data, pressure relationships
- Volumes of gas can be determined with
reasonable accuracy
• Potential reserves: those quantities of natural gas
that have not yet been found by the drill
- Believed to exist in various rocks of the Earth’s
crust
- Future supplies beyond the proved reserves
Methodologies to estimate the future potent of
Natural Gas
• Based on:
i. growth curves
ii. discovery rates
iii. extrapolations of past production
iv. exploratory footage drilled
• Empirical models of gas discoveries and
production have also been developed
• Converted to mathematical models
Methodologies to estimate the future potent of
Natural Gas (Cont’d)
• Volumetric appraisal of the potential undrilled
area
• Different limiting assumptions have been made,
such as
i. drilling depths
ii. economics
iii. technological factors
iv. water depths in offshore areas
Methodologies to estimate the future potent of
Natural Gas (Cont’d)
• Huge disparity between “proven” reserves and
potential reserves
• In the case of the highly mature and exploited
United States, the potential remaining gas reserve
estimates vary from 650 Tcf to 5,000 Tcf
• Proved natural gas reserves (in 2000) were about
1,050 Tcf in the United States
• On the global scale, it is more difficult to give a
good estimate of natural gas reserves
• Sources: US EIA, OPEC, BP,…
Types of Natural Gas Resources
• The natural gases can be classified as
i. conventional natural gas
ii. gas in tight sands/tight shales
iii. coal-bed methane
iv. gas in geopressured reservoirs
v. gas in gas hydrates
Types of Natural Gas Resources (Cont’d)
i. Conventional natural gas is either associated or
non-associated gas.
a) Associated or dissolved gas is found with crude oil
‒ Dissolved gas is that portion of the gas
dissolved in the crude oil
‒ Associated gas (sometimes called gas-cap gas)
is free gas in contact with the crude oil.
‒ All crude oil reservoirs contain dissolved gas
and may or may not contain associated gas
Types of Natural Gas Resources (Cont’d)
b) Non-associated gas is found in a reservoir that
contains a minimal quantity of crude oil.
‒ Some gases are called gas condensates or
simply condensates.
‒ Although they occur as gases in underground
reservoirs, they have a high content of
hydrocarbon liquids.
‒ On production, they may yield considerable
quantities of hydrocarbon liquids.
Tight gas
• Refers to natural gas reservoirs produced from
reservoir rocks with very low permeability
• Considerable hydraulic fracturing is required to
harvest the well at economic rates
• These resources are sealed in extremely
impermeable hard rock
• Making the underground formation extremely
“tight”.
Tight gas (Cont’d)
• Tight gas reservoirs are generally defined as
- Matrix permeability less than 0.1 millidarcy (mD)
- Matrix porosity less than 10%
• Tight gas can also be trapped in
- Sandstone formations
- limestone formations that are impermeable or
nonporous, also known as tight sand.
• The majority of production is controlled by naturally
occurring fractures and is further influenced by
bedding planes and jointing
Coalbed methane
• Methane gas in minable coal beds with depths less
than 3,000 ft.
• Although the estimated size of the resource base
seems significant, the recovery of this type of gas
may be limited owing to practical constraints

Geo-pressured reservoirs
• In a rapidly subsiding basin area, clays often seal
underlying formations and trap their contained fluids
• After further subsidence, the pressure and
temperature of the trapped fluids exceed those
normally anticipated at reservoir depth.
Gas hydrates
• Gas hydrates are snow-like solids
• When each water molecule forms hydrogen bonds
with the four nearest water molecules to build a
crystalline lattice structure that traps gas molecules
in its cavities.
• Gas hydrates contain about 170 times the natural
gas by volume under standard conditions.
• Highly concentrated form of natural gas
• Extensive deposits of naturally occurring gas
hydrates have been found in various regions of the
world
• Considered as a future, unconventional resource of
natural gas.
 Natural Gas Engineering
(CHE-484)
Lecture 6:

Properties of Natural Gas

33
Properties of natural gas include:
1. specific gravity
2. pseudocritical pressure and temperature
3. viscosity
4. gas density
5. compressibility factor

• Essential for designing and analyzing natural gas

production and processing systems.
Specific Gravity
• Ratio of the apparent molecular weight of a
natural gas to that of air
• The molecular weight of air is usually taken as
equal to 28.97
𝑀𝑊𝑎
𝛾𝑔 =
28.97
• where the apparent molecular weight of gas can
be calculated on the basis of gas composition.
• Gas composition measured in a laboratory
Specific Gravity (Cont’d)
• The apparent molecular weight of the gas can be
formulated using mixing rule as
𝑁𝑐

𝑀𝑊𝑎 = 𝑦𝑖 𝑀𝑊𝑖
𝑖=1

• where
‒ MWi is the molecular weight of component I
‒ 𝑦i be the mole fraction of component i
‒ Nc is the number of components.
Specific Gravity (Cont’d)
• A light gas reservoir is one that contains primarily
methane with some ethane.
• Specific gravity of pure methane?
• Rich or heavy gas reservoir may have a gravity equal
to 0.75 or, in some rare cases, higher than 0.9.
Pseudocritical Properties
• Critical properties of a gas can be determined using
the mixing rule
‒ the critical properties of compounds in the gas
• The gas critical properties determined in such a way
are called pseudocritical properties
𝑁𝑐 𝑁𝑐

𝑝𝑝𝑐 = 𝑦𝑖 𝑝𝑐𝑖 𝑇𝑝𝑐 = 𝑦𝑖 𝑇𝑐𝑖

𝑖=1 𝑖=1

• Where 𝑝𝑐𝑖 and 𝑇𝑐𝑖 are critical pressure and critical

temperature of component i, respectively
Example Problem 2.1
For the gas composition given in the following text,
determine apparent molecular weight, pseudocritical
pressure, and pseudocritical temperature of the gas.
Components Mole fraction (- MWi Pci (Psia) Tci (⁰R)
)
C1 0.775 16.04 673 344
C2 0.083 30.07 709 550
C3 0.021 44.10 618 666
i-C4 0.006 58.12 530 733
n-C4 0.002 58.12 551 766
i-C5 0.003 72.15 482 830
n-C5 0.008 72.15 485 847
C6 0.001 86.18 434 915
C7+ 0.001 114.23 361 1024
N2 0.050 28.02 227 492
CO2 0.030 44.01 1073 548
H2S 0.020 34.08 672 1306
Pseudocritical Properties (Cont’d)
• If the gas composition is not known but gas-specific
gravity is given, the pseudocritical pressure and
temperature can be determined from various charts
or correlations developed based on the:
𝑝𝑝𝑐 = 709.604 − 58.718𝛾𝑔
𝑇𝑝𝑐 = 170.491 + 307.344𝛾𝑔

• which are valid for H2S < 3%, N2 < 5%, and total
content of inorganic compounds less than 7%.
Pseudocritical Properties (Cont’d)
• Corrections for impurities in sour gases are always
necessary.
• The corrections can be made using either charts or
correlations such as the Wichert-Aziz (1972)
correction expressed as follows:
Pseudocritical Properties (Cont’d)
• Correlations with impurity corrections for mixture
pseudocriticals are
Pseudocritical Properties (Cont’d)
• Applications of the pseudocritical pressure and
temperature are normally found in natural gas
engineering through pseudoreduced pressure and
temperature defined as:
𝑝
𝑝𝑝𝑟 =
𝑝𝑝𝑐
𝑇
𝑇𝑝𝑟 =
𝑇𝑝𝑐
 Natural Gas Engineering
(CHE-484)
Lecture 7:

Properties of Natural Gas

45
Properties of natural gas include:
1. specific gravity
2. pseudocritical pressure and temperature
3. viscosity
4. gas density
5. compressibility factor

• Essential for designing and analyzing natural gas

production and processing systems.
Viscosity
• Gas viscosity is a measure of the resistance to flow
exerted by the gas.
• Dynamic viscosity (μg) in centipoises (cp) is usually
used in the natural engineering
• If gas composition and viscosities of gas components
are known, the mixing rule can be used for
determining the viscosity of the gas mixture

(𝜇𝑔𝑖 𝑦𝑖 𝑀𝑊𝑖 )
𝜇𝑔 =
(𝑦𝑖 𝑀𝑊𝑖 )
Viscosity
• Gas viscosity is very often estimated with charts or
correlations developed based on the charts
• The gas viscosity correlation of Carr-Kobayashi-
Burrows involves a two-step procedure
1. the gas viscosity at ambient conditions is
estimated from gas-specific gravity and inorganic
compound content
2. the atmospheric value (µl) is then adjusted to
pressure conditions by means of a correction
factor on the basis of reduced temperature (Tpr)
and pressure state of the gas
Gas Density
• Natural gas is compressible, its density depends
upon pressure and temperature.
• Gas density can be calculated from gas law for real
gas with good accuracy:

• where m is mass of gas and ρ is gas density

• Taking air molecular weight 29 and R = 10.73 (psia-
ft3/mole-⁰R)
 Natural Gas Engineering
(CHE-484)
Lectures 8-9:

Properties of Natural Gas

50
Properties of natural gas include:
1. specific gravity
2. pseudocritical pressure and temperature
3. viscosity
4. gas density
5. compressibility factor

• Essential for designing and analyzing natural gas

production and processing systems.
Compressibility Factor
• Gas compressibility factor is also called deviation
factor, or z-factor
• Reflects how much the real gas deviates from the
ideal gas at given pressure and temperature
𝑉𝑎𝑐𝑡𝑢𝑎𝑙
𝑧=
𝑉𝑖𝑑𝑒𝑎𝑙 𝑔𝑎𝑠
• Introducing the z-factor to the gas law for ideal gas
results in the gas law for real gas as: 𝑝𝑉 = 𝑛𝑧𝑅𝑇
• n is the number of moles of gas, pressure p is in psia, volume V in ft3, and
temperature in °R, the gas constant R is equal to 10.73 (psia-ft3)/(mole-⁰R)
Measurement of Compressibility Factor
• The gas compressibility factor can be determined on
the basis of measurements in PVT laboratories.
• For a given amount of gas, if temperature is kept
constant and volume is measured at 14.7 psia and an
elevated pressure p1, z-factor can then be
determined with the following formula:
𝑝1 𝑉1
𝑧=
14.7 𝑉0
• where V0 and V1 are gas volumes measured at 14.7
psia and p1, respectively
Estimation of Compressibility Factor (Brill and Beggs)
• Brill and Beggs yield z-factor values accurate enough
for many engineering calculations
‒ set up for computer solution
Estimation of Compressibility Factor (Cont’d)
Example Problem 2.3
For the natural gas (sp. gr. = 0.65, 10% nitrogen, 8%
carbon dioxide, and 2% hydrogen sulfide), estimate z-
factor at 5,000 psia and 180 °F.
Formation Volume Factor
• Ratio of gas volume at reservoir condition to the
gas volume at standard condition

• unit of formation volume factor is ft3/scf

• used in mathematical modeling of gas well inflow
performance relationship (IPR)
Expansion Factor
• Gas expansion factor is defined, in scf/ft3

• unit of formation volume factor is ft3/scf

• used for estimating gas reserves
Compressibility of Natural Gas
• Natural gas can be pressurized using a compressor in
which the volume of the gas is decreased
• Typically, natural gas is compressed using pressure
on the order of 2900-4300 psi, which gives a 200- to
250-fold reduction in the volume of the gas
• The compressibility factor appears in equations
governing volumetric metering.
Compressibility of Natural Gas (Cont’d)
• The conversion of volume at metering conditions to
volume at defined reference conditions …..
• When gas is compressed, work is done and thus it
gets hotter (necessary to cool gas during or after
compression)
• When it is expanded adiabatically it gets colder (used
to cool gas during treatment to remove liquids)
• The isothermal gas compressibility is extensively
used in determining the compressible properties of
the reservoir.
Compressibility of Natural Gas (Cont’d)
• Gas usually is the most compressible medium in the
reservoir
• Gas is difficult to store in the gaseous state outside
the reservoir to provide flexibility of supply.
• Gas compressibility is defined as

• For a non-ideal gas, the compressibility is?

• For an ideal gas, the compressibility is?
 Natural Gas Engineering
(CHE-484)
Lectures 10-11:

Characteristics of reservoir

61
Characteristics of reservoir
• Mathematical relationships describe the flow
behavior of reservoir fluids
• These relationships depends on the characteristics
of the reservoir
• Primary reservoir characteristics
i. Types of fluids in reservoir
ii. Types of flow regimes
iii. Number of fluid flowing in the reservoir
iv. Reservoir geometry
Types of reservoir fluids
Reservoir fluids are classified into three groups:
1. Incompressible fluids
2. Slightly compressible fluids
3. Compressible fluids

• Isothermal compressibility coefficient (c)

- the controlling factor in identifying the type of
the reservoir fluid
- Describe mathematically in terms of fluid
volume as:
Types of flow regimes
Three types of flow regimes to describe the fluid flow
behavior and reservoir pressure distribution as a
function of time
1. Steady-state flow
2. Unsteady-state flow
3. Pseudo-steady state flow

Number of fluid flowing in the reservoir

Complexity of mathematical expressions for
volumetric performance and pressure behavior
depends on the number of mobile fluids in reservoir
Reservoir geometry
• Significant effect of the shape of reservoir on its
flow behavior

• Most reservoirs have irregular boundaries

‒ Mathematical description of their geometries
is only possible with numerical simulators

• For many engineering purposes, the actual flow

geometry is represented by one of the following
flow geometries:
‒ Radial flow
‒ Linear flow
‒ Spherical or hemi-spherical flow
 Natural Gas Engineering
(CHE-484)
Lecture 12-13:

Gas production system

66
Gas production system
• Maximize gas production in a cost-effective manner
• A complete gas production system consists of
i. Reservoir (supplies wellbore with gas)
ii. Well (path for the production fluid to flow from
bottom hole to surface and offers a means to
control the fluid production rate)
iii. Flowline (leads the produced fluid to separators)
iv. Separators (remove gas and water from the crude
oil)
v. Pumps/Compressors
vi. Transportation pipelines
Gas production
1. Gas Reservoir Deliverability
2. Wellbore Performance
3. Well Deliverability
1. Gas Reservoir Deliverability
• Potential of gas production rate from the well
• Evaluated using well inflow performance relationship
(IPR)
- Gas production rate as a nonlinear function of
pressure drawdown (reservoir pressure minus
bottom hole pressure)
• Reservoir properties control the inflow performance
of wells
• Gas well IPR also depends on flow conditions
(transient, steady state, or pseudosteady state flow)
Gas Reservoir Deliverability (Cont’d)
• Pressure drawdown is the difference between the
reservoir pressure and the flowing wellbore pressure
i. drives fluids from the reservoir into the wellbore
ii. greatest impact on the production rate of a well

• The drop in reservoir pressure related to the

withdrawal of gas from a producing well
i. Low-permeability formations
ii. pressures near the wellbores can be much lower
than in the main part of the reservoir
iii. pressure decline in the reservoir
Drainage radius (re)
• For a single well in a circular reservoir
• Bigger drainage areas lead to lower productivities!
• Bigger drainage areas also extend reservoir pressure
over a larger area
Drainage radius (Cont’d)
• The radius of the approximate circular shape around
a single wellbore from which the hydrocarbon flows
into the wellbore.
• The drainage radius of a single well will help
determine how many wells will be needed to most
efficiently drain the reservoir.
Pay zone
Skin factor
• Usually during drilling, completion or workover
operations, the materials such as mud, cement slurry
or clay particles enter the formation
• Reduce the permeability around the wellbore
(wellbore damage)
2. Wellbore Performance
• Achievable gas production rate from the well is
determined by
i. wellhead pressure
ii. flow performance of production string (casing,
tubing or both)

• The flow performance of production string depends

on
i. geometries of the production string
ii. properties of fluids being produced
2. Wellbore Performance (Cont’d)
• Wellbore performance analysis involves establishing
a relationship between
i. tubular size
ii. wellhead pressure
iii. bottom hole pressure
iv. gas flow rate
v. fluid properties
• Understanding wellbore flow performance is vitally
important to gas engineers for designing gas well
equipment and optimizing well production conditions
• Gas can be produced through tubing, casing, or both
in a gas well, depending on which flow path has a
better performance
3. Well Deliverability
• determined by the combination of
- well inflow performance (deliverability of the
reservoir)
- wellbore flow performance (resistance to flow of
production string)
4. Choke Performance
• Gas production rates from individual wells are
controlled for preventing water coning and/or sand
production
• Choke is a device installed at wellhead or down hole
to cause a restriction to flow of fluids, and thus
control gas production rate
• Pressure drop across well chokes is usually very
significant
• No universal equation for predicting pressure drop
across the chokes for all types of production fluids
• Different choke flow models are available from
literature