PPE 2, Overview of Reservoir Eng (Lecture 2)

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

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Aims:


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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



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Works with others

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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?

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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.

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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

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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

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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)

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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

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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 ?

PPE 2, Overview of Reservoir Eng (Lecture 2)

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Transcript of PPE 2, Overview of Reservoir Eng (Lecture 2)