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8/10/2019 PPE 2, Overview of Reservoir Eng (Lecture 2)
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An Overview of Reservoir Engineering
Dr Nejat Rahmanian
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An overview of Reservoir Engineering
Part IScope of Reservoir engineering
Basic concerns of Reservoir Engineering Define SPE (Society of Petroleum Eng. ) definition of
reserves and reserves categories
Part II
Concept of Stack Tank Oil In Place, Recovery Factor
Part III
Hydrostatic pressure, Reservoir Drive Mechanisms
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Part I
Scope of Reservoir engineering
Basic concerns of Reservoir
Engineering
Define SPE definition of reserves andreserves categories
Describe general terms for reservesestimation.
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What is Reservoir Engineering?
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Reservoir Engineering
5
Aims:
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Reservoir Engineering
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Aims (Contd):
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The phase of engineering which deals with the transfer of fluids
to, from or through the reservoirs
Goal of reservoir engineering
Discover productive reservoir
Set up development project
Attempts to optimize hydrocarbon recovery
Derive information required to optimal production from the
reservoir
Reservoir Engineering
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Works with others
8
RESERVOIR
ENGINEERING
Petropyhsics
Production
ProcessEngineering
Exploration/
Geophysics/Geology
Economics
General Engineering/
Platform topsideDesign
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Where is Reservoir Engineering?
Geology
Geophysics
Exploration Well
Discovery Well
Reservoir EngineeringWell LocationWell Assessment
Reservoir Assessment
Exploration
Development
Development PlanProduction
Transport
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Basic Concerns of Reservoir Engineers
How to calculate the volume of the initial
hydrocarbon present in the reservoir ?
How much of the initial fluids have been
recovered ?
How much is left ?How to increase recovery economically?
What data are needed?
10
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Good Reservoir Engineer
Understand geology & geological process that
give rise to the reservoir.
Understand the nature of reservoir fluid and
rocks
Be familiar with fluid mechanics &
mathematical methods to solve problem.
11
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Activities of Reservoir Engineering
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Productionand
Optimization
Reserve
Estimation
Development
Planning
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PART II
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Describe basic concept of STOIIP
Explain what is FVF (Formation Volume Factor)
Explain what is RF (recovery Factor)
Highlights different between reserves & STOIIP
Calculate STOIIP
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PART II
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Stock Tank Oil Initial In Place (STOIIP)
Same as STOOIP
Refer to oil in place before the commencement of
production
Depends on
Volume of rock containing oil
percentage porosity of the rock in the reservoir
percentage water content of that porosity
amount of shrinkage that the oil undergoes when
brought to the Earth's surface
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The volume of oil and gas in-place
Depends on area coverage of the reservoir ~A
The thickness of the reservoir ~hn
The porevolume ~ Pore space occupied by Oil ~S0 or (1-Sw)
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h
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reservoir
Stock
Tank
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What is Formation Volume Factor (FVF)?
Dimensionless factor for the change in volume between
reservoirandstandard conditionsat surface
What happen?
High reservoir temperature and pressuredecreases
Gas bubbles out of the oil
The volume of the oildecreases
Stabilized oil undersurfaceconditions =stock tank oil
Oil reserves are calculated in terms of stock tank oil volumes
rather than reservoir oil volumes
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Recovery Factor (RF)
RF ~indicates theproportion of the in-place HCexpected to
be recovered.
Varygreatly among oil fields. Depends on drive mechanism.
Change over time based on operating history and in
response to changes in technology and economics. May rise over time if additional investment is made in
enhanced oil recovery techniques such as gas injection,
water-flooding or microbial enhanced oil recovery.
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What is the different between reserves
and oil in place?
To convert in place volumes to reserves we need to
multiply the STOIIP by the recovery factor
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Reserves = (STOIIP) (RF)
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Example
(a) Calculate the original-oil-in-place for a reservoir with
4000 acres and thickness of 115 ft. The following
conditions are reported for this reservoir, pressure of
3000 psia, the formation volume factor is 1.43 bbl/STB,
water saturation (Sw) is 30% and porosity is 21%.
(b) Determine reserve in place if Recovery Factor is 28%
(a) N=Ah(1-Sw)/Bo=(4000*7758)(115)(0.21)(1-0.3)/1.43
N=3.7 108 STB
(stock tank barrels refers to the volume of oil after production, at surface pressure and
temperature (as opposed to reservoir conditions).
NB: Conversion factor (acre-ft * 7758 = barrels)
(b) Reserve in place=N*RF=(3.7 108 )(0.28)=1.034 108 Barrel
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Evolution of the Reserve Estimate
22
Oil
Water
Suggest
OWC
cross section view of a reservoir structure
Where do we locate the first exploration well and get
involved in large exploration expenditure costs ?
F1
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Oil
Water
Alternative locations of Exploration Wells
F2
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Oil
Water
Interpretation after Exploration Well drilled and cored.
F3
Confirm
OWC
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What can you see?
Exploration well has been drilled and a core recovered
The structure of the field with respect to formations and
contacts redefined
Now we can see an estimate of reserves according to,
proven, probable and possible
25
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Water ConfirmOWC
Confirm
Oil
Probable
Possible
Probable
Possible
After the Exploration well was drilled.
F4
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Top Structure Map (typical)
28
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PART III
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LEARNING OBJECTIVES
Define lithostaticpressure, hydrostatic
pressure and hydrodynamicpressure.
Describebriefly and sketch the pressure
gradients associated with overpressured
and underpressured reservoirs.
Understanddrive mechanism in a reservoir.
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Li h i
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Lithostatic pressures
Pressure / stress imposed on a layer of rock
Caused by the pressure of rock which is transmitted
through the sub-surface by grain-to-grain contacts
Sometimes calledgeostatic oroverburden pressure
Unit ispsi.
Pressure gradient varies according todepth, thedensityof
the overburden, and the extent to which the rocks are
supported by water pressure
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(z) is the density of the overlying rock at depth z and gis the
acceleration due to gravity.p0is the datum pressure, like thepressure at the surface.
H d t ti
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Hydrostatic pressure
Pressure exerted by afluidat equilibrium due to the force
of gravity
Imposed by a column of fluid at rest
Depends on the density of the water(w)which is affected
bysalinity
Column of fresh water ~0.433 psi/ft
With55,000 ppmof dissolvedsalts~0.45 psi/ft
32
His the total height of the liquid column above the
test area the surface, andpatm
is the atmospheric
pressure.
Pressure gauge
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S l ti
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1 foot = 0.3048m
Pressure in formation= 0.47 * 7400 = 3478 psi
Converting to kPa = 3478 / 0.145 = 23986 kPa
Converting to MPa = 23986 / 1000 = 23.99 MPa
Pressure gradient = 0.47 psi/ft = (0.47 / 0.145) kPa/ft =
3.2414 kPa/ft= (3.2414 /0.3048) kPa/m
= 10.63 kPa/m
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Solution
Fluid Pressures in Hydrocarbon Systems
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Fluid Pressures in Hydrocarbon Systems
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Typical
The composition of the respective fluids gives rise to
different pressure gradients.
Gradientswill be determined by thedensityof the fluids
which result from the specificcompositionof the fluids.
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Drive Mechanisms
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Drive Mechanisms
Definition
A reservoir drive mechanism is a source of energy fordriving
thefluidsout to thewellbore.
Types of Reservoir Drive Mechanisms
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Types of Reservoir Drive Mechanisms
Depletion drive
Solution gas drive
Gas Cap drive
Liquid expansion
Water drive
Compaction drive
Gravity drainage drive
Combined drive
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Depletion Drive - Solution Gas Drive
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Above bubble point:
Evacuation of fluids during production causes oil, connate
water and pore space to expand. The expansion is small due
to the low compressibility. Consequently, there is a rapiddecline in pressure.
Lowrecovery
Depletion Drive - Solution Gas Drive
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When pressure drops below thebubble point
Dissolved gas is liberated and expands
Liquids and rocks continue toexpandWhen there is a good verticalpermeability, the freed gas
moves up the structure to form a secondary gas cap.
Solution gas drive:Petroleum commonly has gas
dissolved in it. One of the major
constituents is carbon dioxide (CO2).
As the pressure in the reservoirdecreases, this gas exsolves from
the oil and expands. The expansion
of this solution gas helps maintain
reservoir pressure.
Depletion Drive - Gas Cap Drive
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Depletion Drive Gas Cap Drive
Where there is already
free gasin the reservoir.
Compared to the initial
undersaturated condition
for solution gas drive,
energy is from very highcompressibility of gas cap.
Some energy from
solution gas drive which isalso present.
Oil expansion is
insignificant
Depletion Drive - Gas Cap Drive
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Gas cap drive: Often, thereis a layer of natural gas above theoil. It may be in either liquid or
gaseous form. As the oil is
extracted, the gas expands
because of the extracted oil. This
expansion helps to maintain
natural drive.
The oil rate will not decline as steeply but will depend also on
the placement of the well with respect to the gas cap.
Water Drive Reservoirs
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A water (aquifer ) drive reservoir is a reservoir in contact with a
supporting aquifer.
The aquifer may be a water reservoir with a finite size.
It can also be replenished by some source, e.g. rainwater and so
behaves as an infinite reservoir.
Theoil ratewill remainfairly stableuntil the water reaches the well.Thisimagecannotcurrently bedisplayed.
Water drive: As oil is
extracted from the reservoir,pressure in the layer of
water below the oil forces
the oil up into the reservoir
and hence into the wellbore.
This is the most efficient of
the primary recovery drives.
Thisimagecannotcurrently bedisplayed.
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Gravity Drainage Drive
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Due to the relative density of the fluids and high verticalpermeabilities.
Commonlyoccurs inheavy oilreservoirs.
Combined Drive
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Most reservoirs have a combination of drives during production
period.
Reservoir Performance
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The production characteristics of reservoirs.
Oil-gas ratio Pressure decline
Water production
Can give indications of drive mechanism
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Q & A ?