Reservoir engineering

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Dr. Prashant Jadhawar

Sr Lecturer - Petroleum Engineering School of Engineering University of Portsmouth 023 9284 2373, [email protected]

Fluid saturation

By

Dr. Prashant Jadhawar

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 transition zone, oil zone, oil-gas transition zone, and gas zone (or gas cap) at the top.

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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 that contains oil and gas in addition to

irreducible water.

Oil-water-gas reservoir: A reservoir that

contains oil, gas, and water. It is also called ‘associated oil-gas reservoir’.

Gas reservoir: A reservoir that contains

gas and water that may be irreducible. It is also called ‘non-associated gas reservoir’.

The term saturation represents the fraction, or percent, of the pore volume 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), S_w+ S_o= 1. For a reservoir below b, S_w+ S_o+ S_g= 1.

Fluid Saturation

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Saturation of oil, S

_o

= V

_o

/ V

_p

Saturation of gas, S

_g

= V

_g

/ V

_p

Saturation of water, S

_w

= V

_w

/ V

_p

• All saturation values are based on pore volume, not the bulk or gross volume. • Saturation of each individual phase ranges

between 0 to 1 (or 0 to 100 %), and S_w+S_o+S_g= 1.0

Above equations can be written as

In equilibrium state fluids are separated according to their density i.e., Oil overlain by gas and underlain by water

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 occupied by both oil and gas.

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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, or intermediate-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.

Water-wet rock

:

A rock that permits water to spread on the surface of its grains.

Oil-wet rock

:

A rock that permits oil to spread on the surface of its grains.

Intermediate-wet rock

:

A rock that is equally wetted by oil and water.

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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.

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.

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• Critical oil Saturation S_oc– when oil phase remains immobile until it exceeds this certain value above which the oil phase starts moving

– Oil at S_ocremains in the pores and will not flow until its saturations exceed this value.

• Residual oil Saturation S_or– 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, S_om= 1 – S_wc- S_oc

• Critical gas saturation S_gc– gas phase remains immobile until it exceeds this certain value above which the gas phase starts to move

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Irreducible water saturation (S_irw):

Minimum water saturation or least value of water saturation that is present in porous medium.

– This minimum reduction depends on the space available between the

oil and gas and controlled by capillary force/size

– Distribution is not uniform and

varies with permeability, lithology and height above the free water

– Quantitatively the maximum

water saturation at which water phase remains immobile

Irreducible water in

• water-wet rock: forms a thin film

covering the surface of rock particles. • oil-wet rock: dispersed (not continuous)

droplets away from rock surface.

Connate water saturation (S_wc): The lowest in-situ water saturation found in an undeveloped oil or gas reservoir, equal to, or a little less than the maximum water saturation at which the water phase will remain immobile.

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Irreducible (or connate) water saturation: in an idealized gravity

–capillary equilibrated petroleum reservoir

Residual oil saturation (S_or):

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

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Residual oil saturation (S_or):

The saturation of the oil phase when it becomes immobile at the end of oil displacement.

Residual oil in water-wet rock exists

in the form of dispersed (not continuous) droplets away from rock surface.

Residual oil in oil-wet rock forms a

thin film covering the surface of rock particles.

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

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Residual oil: water drive (waterflood)

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

Residual oil: water drive (waterflood)

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

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

Residual oil: Gas drive (Gasflood)

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

Critical gas saturation (S_gc): The saturation of the gas phase at which it becomes mobile as gas saturation builds up, or that when the gas phase ceases to flow as gas saturation decreases.

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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?

Average saturation:

The average saturation of Phase l of n rock samples.

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Rock Grain

Oil water

Porosity (Φ):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 fluids through the interconnected pore network

Porosity: Definitions

Saturation (S): Defines Volumetric distribution of the reservoir fluids in the Gas

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Lab measurement:

Fluid Saturation

By

Dr. Prashant Jadhawar

Outline

∗ Saturation

CORES/CORE PLUGS

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Core: A Sample of reservoir rock from a well section

Coring: A process by which the reservoir rock sample, CORE, is obtained by drilling through oil bearing formation.

A core sample (Fig 1):

Maximum: 10 m in length and 15 cm in diameter (Dandekar, 2006) Typically in the laboratory coreflood: 1.5 inch in diameter and 6 inches in Length

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Core Plugs

Core plugs: 1.5 inch in diameter 3 inch in length

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Types of Core Tests:

To evaluate the various properties of the petroleum reservoirs, core samples taken from the representative section of the oil bearing formation are subjected to a

procedure of thorough laboratory analysis called “CORE ANALYSIS”. The so obtained information through the whole core or core-plug analysis is used in the further evaluation through formation evaluation, reservoir development and the reservoir engineering 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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Fluid Saturation: Lab Measurement

Two difference approaches:

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

In-direct:

1. use of some other measurements on core plug samples such as capillary pressures based on which the fluid saturations are determined

2. traditional well-logging techniques where fluid saturations are measured 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

Fluid Saturation: Lab Measurement

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 600o_{C 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.

Retort Distillation

• The volumes of oil and water are measured directly and plotted against time.

• A horizontal or a plateau in the plot of collected oil and water volume versus the heating time indicates no further

extraction of pore fluids.

The oil and water saturations are subsequently determined by applying Equations

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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 Dean–Stark extraction, cleaning, and drying;

M_gthe 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 Dean–Stark; Vg the volume of the gas (unknown, to be determined);

Vo the volume of the oil (unknown, to be determined); Vw the volume of the water recovered from Dean–Stark; ρg the density of the gas; ρo the density of the oil; and ρw the density of the water.

Retort Distillation

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 laboratory test:

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

boiling solvent, then condensed and collected in a calibrated trap. This gives the volume of water in the sample.

• Graduated tube will have two liquid phases water and a mixed hydrocarbon phase containing toluene and oil from the rock sample.

• 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.

Distillation extraction

Boiling point = 110

Toluene completely miscible with extracted oil