Reservoir engineering - Fluid Saturation

8/13/2019 Reservoir engineering - Fluid Saturation

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Dr. Prashant Jadhawar

Sr Lecturer - Petroleum Engineering

School of Engineering

University of Portsmouth

023 9284 2373, [email protected]

ENG591 (P21980)

15 Nov 2013


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Saturation definition

Concepts, types and effects during oil and gas flow

in porous media

Lab measurement of saturations

21/11/2013 Dr Prashant Jadhawar 3

Initial Fluid Distribution in Petroleum Reservoirs:

Water zone (or bottom water) at the bottom, oil-water transitionzone, oil zone, oil-gas transition zone, and gas zone (or gas cap) atthe top.

Fluid Saturation

21/11/2013 Dr Prashant Jadhawar


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Oil-water reservoir:

A reservoir that contains oil and water.It is often called oil reservoir.

Oil-gas reservoir: A reservoir thatcontains oil and gas in addition toirreducible water.

Oil-water-gas reservoir: A reservoir thatcontains oil, gas, and water. It is alsocalled associated oil-gas reservoir.

Gas reservoir: A reservoir that containsgas and water that may be irreducible.It is also called non-associated gasreservoir.

Fluid Saturation


The term saturation represents the fraction, or percent, of the porevolume occupied by a particular fluid in a multiphase system (eg. oil,gas, or water).

total volume a fluid i

pore volume.

It depends on volume of each phase in the pore space

Relative permeability is a function of saturation.

Does not include fluids interaction at the interface

For a reservoir above bubble point pressure (Pb), Sw+ So= 1.

For a reservoir below b, Sw+ So+ Sg= 1.

Fluid Saturation

Saturation (Si) =



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Saturation of oil, So = Vo/ VpSaturation of gas, Sg = Vg/ VpSaturation of water, Sw = Vw/ Vp

All saturation values are based on porevolume, not the bulk or gross volume.

Saturation of each individual phase rangesbetween 0 to 1 (or 0 to 100 %), and

Sw+So+Sg = 1.0

Above equationscan be written as

Fluid Saturation


In equilibrium statefluids are separatedaccording to theirdensity i.e., Oil overlainby gas and underlain bywater

Determination:

Saturation can be determined from

Core analysis (based on drilling cores)

petro-physical log analysis

with the help of capillary pressure curve,

various other methods

Why important Effective hydrocarbon porosity: The porosity portion that is

related to oil and gas.

Hydrocarbon pore volume: The pore volume that is occupiedby both oil and gas.

Fluid Saturation



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Rock wettability:The capacity of a fluid to spread on the surface of the rock.Therefore, we may have water-wet rock, oil-wet rock, orintermediate-wet rock.

Wetting phase:

The phase that spreads on the surface of the rock.

Non-wetting phase:

The phase that does not spread on the surface of the rock.

Fluid Saturation


Water-wet rock:A rock that permits water tospread on the surface of itsgrains.

Oil-wet rock:A rock that permits oil to

spread on the surface of itsgrains.

Intermediate-wet rock:A rock that is equally wettedby oil and water.

Fluid Saturation



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Displacing phase:The phase that pushes or displaces

fluids ahead of it in a displacement

process.

Displaced phase:

The phase that is pushed or

displaced ahead of a displacing

phase in a displacement process.

Mobile phase:

Phase that has the capacity to flow.

Immobile phase:

The phase that seizes to flow.

Fluid Saturation


Imbibitions:

A process during which the

saturation of the wetting phase

increases. Is gas injection in

an oil reservoir an imbibition

process or a drainage

process? Why?

Drainage:

A process during which the

saturation of the wetting phase

decreases.

Fluid Saturation



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Critical oil Saturation Soc when oil phase remains immobile untilit exceeds this certain value above which the oil phase startsmoving

Oil at Soc remains in the pores and will not flow until itssaturations exceed this value.

Residual oil Saturation Sor The saturation larger than critical oil

saturation

Usually associated with non-wetting phase when it is

displaced by wetting phase

During EOR or encroachment process by water or gas

injection, some oil eventually left and they are characterised

by residual oil saturation

Movable oil saturation, Som = 1 Swc- Soc

Fluid Saturation


Critical gas saturation Sgc gas phase remains immobile until it

exceeds this certain value above which the gas phase starts to

move

Fluid Saturation



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Irreducible water saturation (Sirw):Minimum water saturation or least value of

water saturation that is present in porous

medium.

This minimum reduction dependson the space available between theoil and gasand controlled bycapillary force/size

Distribution is not uniform andvaries with permeability, lithologyand height above the free water

Quantitatively the maximumwater saturation at which waterphase remains immobile

Fluid Saturation


Irreducible water in

water-wet rock: forms a thin filmcovering the surface of rock particles.

oil-wet rock: dispersed (not continuous)

droplets away from rock surface.

Connate water saturation (Swc):

The lowest in-situ water saturationfound in an undeveloped oil or gasreservoir, equal to, or a little less thanthe maximum water saturation atwhich the water phase will remainimmobile.

Fluid Saturation



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Irreducible (or connate) water saturation: in an idealized gravitycapillary equilibrated petroleum reservoir

Fluid Saturation


Residual oil saturation (Sor):

Two definitions:

1. The oil saturation remaining at

the end of primary production or

after either water of gas

displacement process (EOR

process)

2. Lab: Final oil saturation in a

reservoir rock core sample at

the end of laboratory gas

displacement or water

displacement process

Fluid Saturation



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Residual oil saturation (Sor):

The saturation of the oil phase

when it becomes immobile at the

end of oil displacement.

Residual oil in water-wet rockexists

in the form of dispersed (not

continuous) droplets away from

rock surface.

Residual oil in oil-wet rockforms a

thin film covering the surface of

rock particles.

Fluid Saturation


The residual oil in a gas-flooded reservoir exists in smallisolated patches of oil in the largest pores.

Fluid Saturation



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The residual oil in a gas-flooded reservoir exists insmall isolated patches of oilin the largest pores.

Fluid Saturation


The residual oil in a gas-floodedreservoir exists in small isolatedpatches of oil in the largest pores.

Fluid Saturation



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The residual oil in a gas-floodedreservoir exists in small isolatedpatches of oil in the largest pores.

Fluid Saturation


Residual oil: water drive (waterflood)


Normal displacement of oil by water in a single pore channel


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Capillary forces causes water to move ahead faster in low permeability porechannel (A) when water is moving slow through high permeabil ity channel (B)



Capillary pressure gradient causes water to move out and water to move intoa dead-end pore channel when sand is water-wet


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Water drive leaves residual oil in sand because surface films breakrestrictions in sand pore channels

Residual oil: Gas drive (Gasflood)


Normal displacement of oil by gas in a single pore channel


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Residual oil: Gas drive (Gasflood)


Gas first displaces oil from high permeability pore channels. Residual oiloccurs in lower permeability pore channels

Critical gas saturation (Sgc): The saturation of the gas phase at which itbecomes mobile as gas saturation builds up, or that when the gas phaseceases to flow as gas saturation decreases.

Fluid Saturation



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Liquid saturation:

The sum of oil and water saturations.

What is the liquid saturation in the oil-

water transition zone? What is liquid

saturation in the oil-gas transition zone?

Fluid Saturation


Average saturation:

The average saturation of Phase l of nrock samples.

Fluid Saturation



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Fluid Saturation


RockGrain

Oil

water

Porosity (): Capacity of porous medium to store/contain the fluid(O+W+G) in the interconnected pore network (static Property)

Permeability (k): Capacity of porous medium to conduct/transmit fluidsthrough the interconnected pore network

Porosity: Definitions

Saturation (S): Defines Volumetric distribution of the reservoir fluids in theinterconnected pore-network

Gas



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21 Nov 2013

CORES/CORE PLUGS

Lab measurement of: fluid saturation



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Core: A Sample of reservoir rock from a well sectionCoring: A process by which the reservoir rock sample, CORE, isobtained by drilling through oil bearing formation.

A core sample (Fig 1):

Maximum: 10 m in length and 15cm in diameter (Dandekar, 2006)

Typically in the laboratory

coreflood: 1.5 inch in diameterand 6 inches in Length(www.corelab.com)


Core plugs:1.5 inch in diameter3 inch in length



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Types of Core Tests:To evaluate the various properties of the petroleum reservoirs, core samples takenfrom the representative section of the oil bearing formation are subjected to aprocedure of thorough laboratory analysis called CORE ANALYSIS. The so obtainedinformation through the whole core or core-plug analysis is used in the furtherevaluation through formation evaluation, reservoir development and the reservoirengineering studies.

CORE ANALYSIS is generally categorized into two types:

1. Routine or conventional core analysis (RCAL)

2. Special Core Analysis (SCAL)



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Two difference approaches:

Direct: preserved core plug samples or rather plug-end trims of thecore plug, that is, a small section of the rock sample removed from apetroleum reservoir

In-direct:

1. use of some other measurements on core plug samples such ascapillary pressures based on which the fluid saturations aredetermined

2. traditional well-logging techniques where fluid saturations aremeasured in situ,



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Principle of leaching - process of removal of liquids from solids(rock sample in this case)

Methods:

1. Retort distillation: Uses heat

2. Dean-Stark extraction: Both heat and organic solvent


1) Gas saturation

- mercury injection into fresh sample (50 atm) (closed system)

- gas compresses or dissolves

- volume of injected mercury = volume of gas

2) Oil Saturation

- atmospheric distillation by retort (650 deg C)

3) Water Saturation

- by retort

- include pore water, not water of hydration

- Dean Stark



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STEPS:1. Weigh the rock sample (crushed or core

plugs) its bulk volume measured or

calculated.

2. Place the sample is in a cylindrical metal

holder with a screw cup at the top and a

hollow stem projecting from the bottom.

3. Seal the top and the place the sample

holder in a retort oven.

4. using thermostat controller apply the

temperature as high as 600 oC to the core

sample to vaporise the water.

5. Vaporising water and oil is condensed in

the condensing tube and then collected

in receiving vessel.


The volumes of oil and water are

measured directly and plotted againsttime.

A horizontal or a plateau in the plot ofcollected oil and water volume versus theheating time indicates no furtherextraction of pore fluids.

The oil and water saturations are subsequently determined by applyingEquations



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The oil and water saturations are subsequently determined by applying Eqn

WW be the wet weight of the rock sample (or as-received sample);DW the dry weight of the rock sample after DeanStark extraction, cleaning,and drying;Mg the weight of the gas (unknown, to be determined);Mo the weight of the oil (unknown, to be determined);Mw the weight of the water recovered from DeanStark;Vg the volume of the gas (unknown, to be determined);Vo the volume of the oil (unknown, to be determined); Vw the volume of thewater recovered from DeanStark; g the density of the gas; o the densityof the oil; and w the density of the water.


Samples are usually destroyed in this test due to the high

temperature and for this reason small-diameter samples or"plugs" (small cores from the well core), are normally used.The calculation of the oil and water saturation is straightforward.The following parameter values are derived from the laboratorytest:



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Heat the flask containing the toluene solvent The water in the sample is vaporised by

boiling solvent, then condensed andcollected in a calibrated trap. This gives thevolume of water in the sample.

Graduated tube will have two liquid phaseswater and a mixed hydrocarbon phasecontaining toluene and oil from the rocksample.

The water phase, due to its higher density,settles at the bottom of the graduated tube;the solvent (mixed) overflows and drips back

over the rock sample. The process is continued until no more

water is collected in the receiving tube. Volume of oil = (loss in weight of Sample)

(weight of the water removed from it). Saturations are calculated from the volumes.

Distillationextraction

Boiling point = 110

Toluene completely

miscible with extracted oil


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Reservoir engineering - Fluid Saturation