(2) a-Reservoir Fluids

8/6/2019 (2) a-Reservoir Fluids

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Reservoir Fluids

Reservoir Fluids

Schlumberger 1999

A


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Reservoir Fluids

Definitions

Fluid Contacts

Oil in Place OIP The volume of oil in the

reservoir in barrels or cubic metres.

Gas/Oil Ratio GOR The gas content of the oil.

API Gravity API Oil gravity.


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Reservoir Fluids

Fluids in a Reservoir

A reservoir normally contains either water or

hydrocarbon or a mixture.

The hydrocarbon may be in the form of oil or gas.

The specific hydrocarbon produced depends on the

reservoir pressure and temperature.

The formation water may be fresh or salty.

The amount and type of fluid produced depends on the

initial reservoir pressure, rock properties and the

drive mechanism.


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Reservoir Fluids

Hydrocarbon Composition

Typical hydrocarbons have the following

composition in Mol Fraction

Hydrocarbon C1 C2 C3 C4 C5 C6+

Dry gas .88 .045 .045 .01 .01 .01

Condensate .72 .08 .04 .04 .04 .08

Volatile oil .6-.65 .08 .05 .04 .03 .15-.2

Black oil .41 .03 .05 .05 .04 .42

Heavy oil .11 .03 .01 .01 .04 .8

Tar/bitumen 1.0

The 'C' numbers indicated the number of carbon

atoms in the molecular chain.


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Reservoir Fluids

Hydrocarbon Structure

The major

constituent of

hydrocarbons

is paraffin.


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Reservoir Fluids

Hydrocarbon Classification

Hydrocarbons are also defined by their weightand the Gas/Oil ratio. The table gives some

typical values:

GOR API Gravity

Wet gas 100mcf/b 50-70

Condensate 5-100mcf/b 50-70

Volatile oil 3000cf/b 40-50

Black oil 100-2500cf/b 30-40

Heavy oil 0 10-30

Tar/bitumen 0


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Reservoir Fluids

Hydrocarbon Gas

Natural gas is mostly (60-80%) methane, CH4.

Some heavier gases make up the rest.

Gas can contain impurities such as Hydrogen

Sulphide, H2S and Carbon Dioxide, CO2.

Gases are classified by their specific gravity

which is defined as:

"The ratio of the density of the gas to that of air

at the same temperature and pressure".


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Reservoir Fluids

Reservoir Pressure

Reservoir Pressures are normally controlled by

the gradient in the aquifer.

High pressures exist in some reservoirs.


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Reservoir Fluids

Reservoir Pressure Calculation


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Reservoir Fluids

Reservoir Pressure Example


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Reservoir Fluids

Reservoir Temperature Gradient

The chart shows three possible temperaturegradients. The temperature can be determined if

the depth is known.

High temperatures exist in some places. Local

knowledge is important.


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Reservoir Fluids

Fluid Phases

A fluid phase is a physically distinct state, e.g.:

gas or oil.

In a reservoir oil and gas exist together at

equilibrium, depending on the pressure andtemperature.

The behaviour of a reservoir fluid is analyzed

using the properties; Pressure, Temperature and

Volume (PVT).

There are two simple ways of showing this:

Pressure against temperature keeping the

volume constant.

Pressure against volume keeping the

temperature constant.


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Reservoir Fluids

PVT Experiment


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Reservoir Fluids

Phase Diagram -single

componentThe experiment is conducted at different

temperatures.

The final plot of Pressure against Temperature is

made.

The Vapour Pressure Curve represents theBubble Point and Dew Point.

(For a single component they coincide.)


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Reservoir Fluids

Phase diagram Oil

The Pressure/Temperature (PT) phase diagram for an oil

reservoir:

Point 'A' is the initial reservoir condition of pressure and

temperature.

If the reservoir is produced at a constant temperature

until the fluid reaches the wellbore, the line to Point 'B'

is drawn. This represents the flow of fluid from the

reservoir to the borehole. The fluid travelling to surface now

drops in both temperature and pressure arriving at he "separator

conditions" (s) with a final volume of oil and gas.


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Reservoir Fluids

Phase Diagram Condensate/Gas

Point 'C' is at the initial reservoir conditions. The

reservoir is produced at a constant temperature

from C to D. Fluids flowing up the well now drop

in temperature and pressure, crossing the Dew

point line and liquid condenses out.

At separator conditions (s) the result in both

liquid and gas on the surface.


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Reservoir Fluids

Gas Reservoir

In a gas reservoir the initial point is A. Producing

the well to separator conditions B does not

change the fluid produced.

The point B is still in the "gas region" and hencedry gas is produced.


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Reservoir Fluids

Hydrocarbon Volumes

Fluids at bottom hole conditions produce

different fluids at surface:

Oil becomes oil plus gas.

Gas usually stays as gas unless it is a Condensate.

Water stays as water with occasionally somedissolved gas.


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Reservoir Fluids

FVF Oil and Gas

There is a change in volume between downhole

conditions and the surface.

The volume of the fluid at reference conditions is

described by the Formation Volume Factor:

FVF =

Bo = formation volume factor for oil.

Bw = formation volume factor for water.

Bg = formation volume factor for gas.

Volume at downhole Conditions

Volume at reference Conditions

R i Fl id


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Reservoir Fluids

Saturation

Formation saturation is defined as the fraction of

its pore volume (porosity) occupied by a given

fluid.

Saturation =

Definitions

Sw = water saturation.

So = oil saturation.Sg = gas saturation.

Sh = hydrocarbon saturation = So + Sg

Saturations are expressed as percentages orfractions, e.g.

Water saturation of 75% in a reservoir with

porosity of 20% contains water equivalent to

15% of its volume.

Volume of a specific fluid

pore volume

R i Fl id


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Reservoir Fluids

Saturation Definition

Reservoir Fluids


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Reservoir Fluids

Wettability

The wettability defines how a fluid adheres to the

surface (or rock in the reservoir) when there are

two fluids present, e.g. water and air.

The angle measured through the water is the

"contact angle".

If it is less than 90 the rock is water wet; greater

than 90 the rock is oil wet.

Most reservoir rocks are water wet.

Reservoir Fluids


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Reservoir Fluids

Irreducible Water Saturation

In a formation the minimum saturation induced

by displacement is where the wetting phase

becomes discontinuous.

In normal water-wet rocks, this is the irreducible

water saturation, Swirr.Large grained rocks have a low irreducible water

saturation compared to small-grained formations

because the capillary pressure is smaller.

Reservoir Fluids


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Reservoir Fluids

Capillary Forces

In a simple water and air system the wettability

gives rise to a curved interface between the two

fluids.

This experiment has a glass tube attached to a

reservoir of water. The water "wets" the

glass. This causes the pressure on the concave

side (water) to exceed that on the convex side(air). This excess pressure is the capillary

pressure.

Pc = capillary pressure.

W = surface tension.

q = contact angle.

rcap = radius of capillary tube.

Reservoir Fluids


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Capillary Forces and Rocks

In a reservoir the two fluids are oil and water

which are immiscible hence they exhibit capillary

pressure phenomena.

This is seen by the rise in the water above the

point where the capillary pressure is zero.

The height depends on the density difference and

the radius of the capillaries.

Reservoir Fluids


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Transition Zone

The phenomenon of capillary pressure gives rise

to the transition zone in a reservoir between the

water zone and the oil zone.

The rock can be thought of as a bundle of

capillary tubes.The length of the zone depends on the pore size

and the density difference between the two fluids.

Reservoir Fluids


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Relative Permeability

Take a core 100% water-saturated. (A)

Force oil into the core until irreducible water

saturation is attained (Swirr). (A-> C -> D)

Reverse the process: force water into the core

until the residual saturation is attained. (B)During the process, measure the relative

permeabilities to water and oil.

Reservoir Fluids


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Relative Permeability

Experiment

Reservoir Fluids


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Drive Mechanisms

A virgin reservoir has a pressure controlled by

the local gradient.

Hydrocarbons will flow if the reservoir pressure

is sufficient to drive the fluids to the surface

(otherwise they have to be pumped).As the fluid is produced reservoir pressure drops.

The rate of pressure drop is controlled by the

Reservoir Drive Mechanism.

Drive Mechanism depends on the rate at which

fluid expands to fill the space vacated by the

produced fluid.

Main Reservoir Drive Mechanism types are:

Water drive.

Gas cap drive.

Gas solution drive

Reservoir Fluids


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Water Invasion 1

Water invading an oil zone,

moves close to the grain

surface, pushing the oil out

of its way in a piston-

like fashion.

The capillary pressure

gradient forces water to

move ahead faster in the

smaller pore channels.

Reservoir Fluids


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Water Invasion 2

The remaining

thread of oil

becomes smaller.

It finally breaks

into smaller pieces.

As a result, some

drops of oil are left

behind in the

channel.

Reservoir Fluids


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Water Drive

Water moves up to fill the "space" vacated by

the oil as it is produced.

Reservoir Fluids


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Water Drive 2

This type of drive usually keeps the reservoir

pressure fairly constant.

After the initial dry oil production, water maybe produced. The amount of produced water

increases as the volume of oil in the reservoir

decreases.

Dissolved gas in the oil is released to form

produced gas.

Reservoir Fluids


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Gas Invasion

Gas is more mobile than oil and takes the path of

least resistance along the centre of the larger

channels.

As a result, oil is left behind in the smaller, less

permeable, channels.

Reservoir Fluids


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Gas Cap Drive

Gas from the gas cap expands to fill the space

vacated by the produced oil.

Reservoir Fluids


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Gas Cap Drive 2

As oil production declines, gas production

increases.

Rapid pressure drop at the start of production.

Reservoir Fluids


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Solution Gas Drive

After some time the oil in the reservoir is below

the bubble point.

Reservoir Fluids


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Solution Gas Drive 2

An initial high oil production is followed by a

rapid decline.

The Gas/Oil ratio has a peak corresponding to

the higher permeability to gas.

The reservoir pressure exhibits a fast decline.

Reservoir Fluids


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Drives General

A water drive can recover up to 60% of the oil in

place.

A gas cap drive can recover only 40% with a

greater reduction in pressure.

A solution gas drive has a low recovery.

Reservoir Fluids


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Drive Problems

Water Drive:

Water can cone upwards

and be produced through

the lower perforations.

Gas Cap Drive:

Gas can cone downwards

and be produced through

the upper perforations.

Pressure is rapidly lost as

the gas expands.

Gas Solution Drive:

Gas production can occurin the reservoir, skin

damage.

Very short-lived.

Reservoir Fluids


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Secondary Recovery 1

Secondary recovery covers a range of techniques

used to augment the natural drive of a reservoir

or boost production at a later stage in the life of a

reservoir.

A field often needs enhanced oil recovery (EOR)

techniques to maximise its production.

Common recovery methods are:

Water injection.

Gas injection.

In difficult reservoirs, such as those containing

heavy oil, more advanced recovery methods are

used:

Steam flood.Polymer injection. .

CO2 injection.

In-situ combustion.

Reservoir Fluids

S 2


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Secondary Recovery 2

water

injection

gas injection

(2) a-Reservoir Fluids

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