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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

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    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

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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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    Residual oil: water drive (waterflood)

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    Normal displacement of oil by water in a single pore channel

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    Residual oil: water drive (waterflood)

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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)

    Residual oil: water drive (waterflood)

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    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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    Residual oil: water drive (waterflood)

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

    Residual oil: Gas drive (Gasflood)

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    Normal displacement of oil by gas in a single pore channel

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    Residual oil: Gas drive (Gasflood)

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    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

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    Average saturation:

    The average saturation of Phase l of nrock samples.

    Fluid Saturation

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    Fluid Saturation

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    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)

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    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

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    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.

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    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.

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    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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