fundamentals of oil and gas reservoirs handouts

7/31/2019 Fundamentals of Oil and Gas Reservoirs Handouts

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Fund am ent a ls o f Oi l and Gas

Reservo i rs

Oi l and Gas Reservo i rs Ser ies In this first program of the Oil and Gas Reservoirs

Series, Fundamentals, you will learn about the

fundamentals of oil and gas reservoirs, including

sedimentary rock formation, reservoir pressure, and

factors affecting profitability.

2005, Resource Development Corporation. All Rights Reserved.Duplication or modification of this copyrighted material, or any part thereof, is a violation of Federal Law

and will result in civil/criminal penalties.

Oi l and Gas Reservo i r s : Fun dam ent a ls

S u m m a r y

Fundamenta ls is the first program in the Oil and Gas Reservoirs series.

This program covers the nature and formation of the traps in which oil and

gas have collected.

You will learn about the formation of sedimentary rocks, the significance ofporosity and permeability in the development of a reservoir, the migration

of oil and gas during the formation of a reservoir, and the kinds and effectsof reservoir pressures.

Your understanding of the nature of the reservoir will aid you in maintainingreservoir pressure and in interpreting the significance of changes in pressure

and composition of fluid during production.

Sect ion I

I n t rodu c t i on t o Oi l and

Gas Reservoi rs

The Oil and Gas Reservoir

Density

Viscosity

Sec t i on I I

Sed imentary Rock

F o rma t i on

Sedimentary Rocks

Original Porosity

Changed Porosity


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Objec t i ves

Describe the characteristics of an oil and gas reservoir. Describe the density of a fluid. Define the viscosity of a fluid. Describe how sedimentary rocks are formed, compacted, and

cemented. Define "porosity" and describe how it originates in sedimentary

rock.

Describe how porosity in sedimentary rock can be changed. Define "permeability" and explain "effective porosity." Define "hydrostatic pressure" and the effect it has on reservoirs. Describe the effect of expanding fluids on reservoir pressure. Define the effect of pressure and temperature on a flowing well. Describe the effect of compaction on reservoir fluid pressure. Describe how a reservoir's size and depth impact profitability. Describe how impurities affect reservoir productivity. Describe how viscosity, permeability, and pressure of a reservoir

affects profitability.

Permeability

Se ct i o n I I I

Reservo i r PressureHydrostatic Pressure

Reservoir Pressure andTemperature

Pressure From Expanding

Fluids

Pressure from Reservoir Rocks

Sec t i on I V

Factors Af fec t in g

Pro f i t ab i l i t y

Size and Depth

Impurities

Viscosity , Permeability,

Pressure

Overv iew OIL AND GAS RESERVOIRS:

FUNDAMENTALS

In this section, you will learn how oil and gas reservoirs are formed and how

fluid accumulates within them. You will learn about:

Oil and gas reservoirs. Density. Viscosity.

The Oi l and Gas Reservoir OIL AND GAS RESERVOIRS:

FUNDAMENTALS


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An oil or gas

reservoir is aporous rock

formation.The fluids in

the producing

formation aretrapped

between twononpermeable

formations.In the

reservoir, thefluids may

separate outaccording to

their different

densities, with the gas rising above the oil and the water settling below theoil.

The oil, gas, and water trapped in these reservoirs have been compressed andheated over millions of years.

Geological compression and heat cause the

fluids in a reservoir to have stored pressure.

True.

False.

Densi ty OIL AND GAS RESERVOIRS:FUNDAMENTALS

When oil accumulates in a puddle of rainwater, the oil floats on top of the

puddle. This is because oil weighs less than water.

For example, a pint of oil weighs less than a pint of water. Volume is the

amount of space a substance occupies (in this case, the pint is the volume).So when the volume is the same, oil weighs less than water.


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Here's another example. The

container of water and thecontainer of oil each weigh one

pound. But, the container of oilhas a greater volume.

These two containers have the samevolume, but the container of oil weighs

less than the container of water. Thedensity of a substance is its weight per

unit of volume. Because water weighs

more than the same volume of oil, thedensity of water is greater than the

density of oil.

If you want to compare the densities of oil and gas, you must compare equal

volumes of oil and gas. If you find that the weight of the oil is greater thanthe weight of the gas (and the volumes are the same), you know that the

density of oil is greater than the density of gas.

This diagramrepresents a slice

into the earth's

crust, exposing therock layers to

view. Throughoutthese rock layers,

the gas displacesthe oil downward

and the oil

displaces the water

downward. Oil

and gas are lessdense, so they

keep movingthrough the water

and displacing thewater downward.

Because oil and gas have different densities, they can migrate upward through


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the rocks for many miles. If the openings in the rocks are not filled with

water, oil will settle into low places by gravity. For oil to migrate upward, itmust have water to float upon.

All oil readily migrates through water.

True.

False. Keep in mind that there

are many different typesof oils, displaying a wide

range of density. Highdensity oils are usually

thick and sluggish and

do not flow easily.

2004, Resource Development Company, LLC.

Viscos i t y OIL AND GAS RESERVOIRS:FUNDAMENTALS

Viscosity is a measure of how readily aliquid flows. Fluids with a high viscosity

can be thought of as "thick." For

example, oils are generally more viscousthan water. However, some oils are thin

and can flow like water. These oils have

low viscosity.

Thick lubricating oils have a _______ viscosity.

High.

Low. Thin oils have low

viscosity.

As more gas is dissolved in oil, the density decreases and the oil becomes less


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viscous. The viscosity and density of oil has an important effect on flow. It is

easier to produce oils with:

Low viscosity. Low density. Dissolved gas.

Both the density and viscosity of a fluid affect how difficult it is to produce the

fluid from a well.


Review OIL AND GAS RESERVOIRS:

FUNDAMENTALS

In this section, you learned how oil and gas reservoirs are formed and howfluid accumulates within them, including:

Oil and gas reservoirs. Density. Viscosity.

You learned that density is the weight of a unit of volume of a substance and

that oil displaces water downward because oil is less dense. You also learnedthat:

Oil displaces gas upward because oil is denser than gas. Oil and gas migrate through water-filled rocks for many miles because

of a difference in density.

Oil is more viscous when it moves sluggishly. When oil contains dissolved gas, it is much less viscous.


Sect ion 2 Sed im enta ry Rock Form at ion

The following learning objectives apply to this section:

Describe how sedimentary rocks are formed, compacted, and cemented.

Define "porosity" and describe how it originates in sedimentary rock.

Describe how porosity in sedimentary rock can be changed.

Define "permeability" and explain "effective porosity."


FUNDAMENTALS


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In this section, you will learn about sedimentary rocks, including:

How sedimentary rocks are compacted and cemented. The original porosity of sedimentary rocks. The changed porosity of sedimentary rocks. The permeability of rocks.

Sed imentary Rock s OIL AND GAS RESERVOIRS:FUNDAMENTALS

Sediments are the loose

crystals and grains of

minerals that aredeposited by water or

air. For example,seawater contains

dissolved mineral matter,like salt and lime. When

there is more salt and lime than the water can hold in solution, some of it

crystallizes and is dropped on the ocean floor as sediment.

Waves and currents along shorelines or in rivers also carry grains of mineral

matter, like sand and clay. When there is more sand and clay than the wavescan carry, some of it falls through the water to the bottom as sediment.

Even the dust that drops from air is sediment. And, sand along a beach is

sediment. Sediments form sedimentary rocks after the sediments have been:

Compacted. Cemented.

Com pacted Sed im ent

Compacted sediments are those that are packed together at the bottom of alarge sediment formation. Sediments are deposited in water century after

century, and they continue to pile up in layers. In fact, hundreds of feet ofsediment may be piled up layer upon layer. The weight of the upper layers

creates pressure on the lower layers, packing the lower layers closer andcloser together. As more sediments are packed on top, they apply more and

more pressure to the lower layers, packing the sediments still closer.

Cemented Sed imentCemented sediment are those that are "stuck" together by coated mineral

matter. For example, most sediments are deposited in seawater. So,seawater usually fills all the spaces between the grains or crystals of the

sediment. Cementing occurs when mineral matter in the seawater comes outof solution and coats the individual grains of the sediment.

Sedimentary rocks are formations that have been compacted and cemented.


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When layers of sediments are packed together,

they are being:

Cemented.

Compacted. Cementation is the

process of binding thesediments together after

compaction hasoccurred.

Because sediments arecemented in layers,

sedimentary rocks are

formed in layers.


Orig inal Poros i ty OIL AND GAS RESERVOIRS:FUNDAMENTALS

Porosity is a measure of the percentageof openings within a rock. Sedimentary

rocks have pores (holes or openings),

and sometimes water, oil, and gas fillthese holes. The pores originate when

the sedimentary rock is formed. Thepores occur between the crystals and the

grains that have been compacted andcemented together.

A porous rock is a rock that has many

pores (openings). Porosity in rocksranges from 0% to 40%, but most rocks

have porosity between 5% and 25%. Soa rock that has 25% porosity has good porosity. In the oil industry, good

porosity means that there are many openings in the rocks and that they arelarge enough for oil and gas to flow into and out of them.

Most oil-bearing rocks are originally deposited as layers of sediment in the

ocean and at the mouths of rivers. Water fills the pores in the sedimentarydeposits from the start, so even the smallest pores are filled with water.

While the sediments are being changed into rock by compaction andcementation, the pore space is reduced. Most, but not all of the water is

squeezed out. Grains of rock attract water and draw it to them. So eventhough oil and gas later displace most of the water, a thin film of water still

clings to the grains. This means that the total pore space is not available for


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the storage of oil and gas.

Sandstone is a sedimentary rock made ofcemented sand grains. It has many, many

pores between the grains of sand. Round

grains of sand can be packed together in

different ways. For example, A has grains that are directly above one anotherand has larger pores than B.

These grains of sand are not round,

but angular.

When angular grains are packedunder pressure, their edges

crush. The pieces help fill the

pores. So, rocks made ofangular grains tend to become

less porous than rocks made ofround grains.

When grains of different sizesoccur together, the smallergrains tend to fill in the pores

between the larger grains anddecrease porosity.

A rock made up of large grains has _______

pores than a rock made up of smaller grains.

Larger.

Smaller. The size and shape of

the pores vary becauseof two different factors:

The way thegrains are packedtogether.

The shape and


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size of the grains.

Cementing materials canfill the pores and

decrease the porosity.

And, if clay is depositedwith sand grains, there

may be little or noporosity. Clay particles

are so tiny that thepores between them are

too small for oil, water,and gas to flow into and

out of them.


Changed Poros i ty OIL AND GAS RESERVOIRS:FUNDAMENTALS

The original porosity of a rock can bechanged by different factors, including:

Settling and movement of theearth's crust.

Vug formation. Dolomitization. Deposited mineral matter.

Whenever different factors change the original porosity, the shape, size, anddistribution of the original pores are changed.

Set t l ing and Mov in g o f th e Ear th 's Crus t

All rocks are breakable and can be fractured by settling or movement of the

earth's crust. The fractures and joints increase the porosity of the limestone.

Vug Fo r m a t ion

A "vug" is an enlarged pore. Ground water circulating through a limestone

formation enlarges the fractures, joints, and pores. In other words, whenwater that is not already saturated with dissolved minerals flows through a

limestone formation, the fractures and joints expand. Sometimes, the

fractures and joints become caves and caverns because so much of thelimestone has dissolved.

Fossil shells in rocks dissolve quite easily. When they dissolve, they leavevugs in the rocks that contained them.

Do lom i t i za t i on

Dolomitization is the process of limestone chemically changing to dolomite.


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Limestone dissolves easily and also changes chemically when its calcium is

exchanged for the magnesium in seawater. Dolomite is a rock that containsboth calcium and magnesium.

When a dolomite particle replaces a limestone particle, there is some space

leftover because the dolomite particle takes up less space than the limestone

particle. When dolomite replaces many limestone particles, many spaces areleft over. So, dolomitization results in increased porosity.

Depos i ted M inera l Mat te r

As water saturated with dissolved minerals deposits mineral matter as it

circulates through the pores of the rocks, the original porosity decreases. Insome oil wells, what starts out as good porosity later becomes clogged with

residues, precipitates, or deposits which fill the pores and decreaseproduction.

Which change results in a decrease in porosity?

Dolomitization.

Mineral deposits.

Fractures and joints.

Vug formation.

Dissolving, fracturing,

and dolomitization ofrocks results in an

increased porosity.Mineral deposits

decrease porosity.Ground water can also

reduce or destroy

porosity by depositingmineral matter in the

pores of the rocks.

When rocks are buried deeply, they have:

High porosity.

Low porosity. After rocks are formed,they can still be

compacted by a verygreat weight, decreasing

their porosity.



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Permeab i l i t y OIL AND GAS RESERVOIRS:

FUNDAMENTALS

Permeability is the potential for the movement of fluids. A rock is permeable

when:

The openings are large enough to permit the flow of fluid. The openings are connected so that oil, gas, or water can flow from

one to another.

If a rock only has a few openings that are not connected, the rock has poor

permeability and porosity. Some rocks may have good porosity, but if thepores are unconnected, they have no permeability.

Porosity refers to the potential for the _______

of fluids.

Storage.

Movement. Porosity refers to thepotential for the storage

of fluids. Permeability

refers to the potentialfor fluid movement.

Some rock formations have

good permeability withoutmuch porosity. The fractures,

or cracks in the nonporousrock permit oil movement

through it.


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This drawing of a block of sandstone shows

the rounded grains of sand and the pore spacebetween them. Sandstone has good

permeability.

If a rock has pores that are connected but too

small for fluid to flow through, the rock:

Is permeable.

Is not permeable. For example, shale is a

rock made up of tiny

clay particles so closelypacked together that the

pores between the clayparticles are too small

for oil and gas to flowthrough them. Shale

may have good porosity,

but it has poorpermeability.

Ef fec t ive Poros i t y

When pores have passages from one to another that permit flow, the rock

formation has effective porosity. It is only the effective porosity, or porositywith permeability, that permits the migration of oil and gas. For migration, oil

and gas use only the effective porosity of a rock formation.

The factors that affect porosity, like compaction, also effect permeability.

Rocks that are very deeply buried are under such pressure that the effective

porosity is greatly reduced. The pressure at depths tends to close fracturesand joints. Without permeability, oil and gas cannot migrate through the

rocks.



FUNDAMENTALS

In this section, you learned about sedimentary rocks, including:


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How sedimentary rocks are compacted and cemented. The original porosity of sedimentary rocks. The changed porosity of sedimentary rocks. The permeability of rocks.

You also learned that sea water contains minerals in solution. When there is

more mineral matter than the water can keep dissolved, the minerals comeout of solution and are deposited on the bottom.

Waves and currents deposit grains of sand and clay on the bottom. Loosemineral grains or crystals deposited in layers are called "sediments." The

weight of layers of sediment results in compaction and sediments may becemented together by mineral matter coming out of solution. The rocks

resulting from these processes are called sedimentary rocks. Sedimentaryrocks are sediments which have been compacted and cemented, and are

usually layered.

The percentage of opening in the rock is the porosity, and good porosity

means the rock contains many openings, which are large enough for oil andgas to enter. The shape and size of the pores depends on the shape and sizeof the grains and the way they are packed together.

Original porosity can be changed from settling rocks, dissolving limestone,

dolomitization, and mineral deposits. All pores are filled with water becausethe sediment was formed in water. When oil and gas displace water from the

pores, a thin film of water remains on the surface of the grains. Whenopenings in the rock are connected so that oil and gas can flow through the

rock, the rock is permeable:

Sandstone has good permeability.

Shale has pores that oil and gas cannot enter, so it has poorpermeability.

Permeability is the potential for fluid to move, porosity is the potentialfor fluid to be stored. Oil production depends on effective porosity,which is both porosity plus permeability.


Sect ion 3 Reservo i r Pressur e

The following learning objectives apply to this section:

Define "hydrostatic pressure" and the effect it has on reservoirs.

Define the effect of pressure and temperature on a flowing well.

Describe the effect of expanding fluids on reservoir pressure.

Describe the effect of compaction on reservoir fluid pressure.


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FUNDAMENTALS

In this section, you will learn about reservoir pressure, including:

Hydrostatic pressure: Hydrostatic pressure results from the weight ofa column of water extending from the water table down to the water in

the reservoir. Hydrostatic pressure is greater in deep wells than inshallow wells.

Compaction: Loose reservoir sediments that are compacted by theweight of overlying sediments or rocks increases pressure.


Hydros ta t ic Pressure OIL AND GAS RESERVOIRS:FUNDAMENTALS

A reservoir is a porousand permeable rock

formation in which oil,gas, and water

accumulate. Onesource of reservoir pressure is the weight

of water in the ground and rocks.

This diagram shows the level of the water

table a short distance below the groundsurface. A fresh water stream is anexample of the water table touching the

surface. When drilling a water well, youmust drill below the water table.

Except for oil and gas reservoirs, wherever there are openings or pores in the

rocks below the water table, these openings are filled with water. The rocksin the deepest wells are saturated with water. The heavy weight of the water

creates downward pressure. The weight of the water above creates pressureon the water below. Imagine a wide column of ground water extending from

the water table deep into the earth. The weight of this column of waterincreases with depth. So, the water pressure increases with depth.

Hydrostatic pressure is the pressure applied by the weight of any column of

liquid. When a reservoir is under hydrostatic pressure, there is usually somekind of permeable path to the surface. A column of ground water cannot

press down on the water below if there is no path for it to flow down from thesurface. The permeable path to the surface may be miles and miles long, and

it may be very crooked. When it is straight, gases may escape to the surface.In some reservoirs, a complex system of fractures and joints provide the

permeable path to the surface. These are called open reservoirs.


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Open reservoirs:

Are under hydrostatic pressure from the weight of the water columnextending down from the water table.

Have the weight of the ground water applying pressure on the water inthe reservoir, which in turn transmits pressure to the oil above it.

Closed reservoirs:

Do not have a permeable path to the surface. Are not under hydrostatic pressure.

Hydrostatic pressure is approximately 45pounds of pressure per square inch for each

100 feet of water column.

A reservoir 10,000 feet deep has a hydrostaticpressure of _______ pounds per square inch.

An open reservoir at 9,000 feet is under

_______ pressure than an open reservoir at6,000 feet.

Less.

Greater.

The weight of the column of oil and gas in the well-bore also applieshydrostatic pressure. Hydrostatic pressure varies with the density of a fluid ina column. In other words, hydrostatic pressure depends on the depth of the

fluid column and the density of fluid.


Reservoir Pressure and Temperat ure OIL AND GAS RESERVOIRS:


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FUNDAMENTALS

Reservoirs behave predictably regarding both:

Pressure Temperature

Pressure

Reservoir fluids tend to move from areas ofgreater pressure to areas of lower pressure.

Typically, you are accustomed to seeingliquids flow downhill, because of gravity. But

at great depths, the pressure sometimes

forces liquids to flow upward into areas oflower pressure.

When a well is drilled into a reservoir, the wellitself becomes an area of low pressure, and

the gas, oil, and water all migrate toward thewell. The oil flows toward the well because of

the:

Low pressure area created by the well-bore.

The gas above expands and pushesdown on the oil.

The water below pushes up on the oil.Then, the gas and oil enter the well and are often pushed all the way to the

surface by the reservoir pressure. A flowing well is one that can push

reservoir fluids to the surface.

In the early years of oil production, some wells would spout high into the air

because of the great pressure in the reservoir. Eventually, reservoir pressuresdecline so that there is no longer enough pressure to push the fluids to the

surface. Then, pumps or other mechanical devices are used to bring the oil tothe surface.

The reservoir pressure may force the gas into the oil solution. When the

pressure is very high, more gas is forced into solution. So, more gas is forcedinto solution when the reservoir is very deep. When the oil is brought toward

the surface, the pressure is greatly reduced, and much of the dissolved gasseparates from the oil. Then, the oil becomes more viscous and movessluggishly.

Gas will continue to dissolve in oil until the oilcannot hold any more gas. At that point, the

oil is saturated with gas. It takes more gas to


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saturate oil in ________ reservoirs.

Shallow.

Deep. Gas is more likely to

separate out of oil takenfrom deep reservoirs.

If the oil is saturated with gas, the extra gas that cannot be dissolved in the

oil accumulates above the oil in a gas cap. So the very presence of a gas capindicates that the oil below is saturated with gas. And, the oil is as thin (less

viscous) as it can be.

If a gas cap is on top of the oil, the oil is:

Undersaturated with gas.

Saturated with gas. If the oil is

undersaturated with gas,

there is no gas capabove the oil.

Temperature

Temperature increases toward the center of the earth. Deep coal mines arenoticeably warmer than the air near the surface. For example, the Gulf Coast

increases by 1 F for every 60-foot increase in depth. In a reservoir 12,000feet deep, the rocks are about 200 F warmer than the average surface

temperature. If the average surface temperature is 70 F, then the reservoirtemperature at 12,000 feet would be 270 F. This is much higher than the

boiling point of water at the surface.

When oil is heated, it becomes:

Less viscous.

More viscous. Heat decreases theviscosity of oil and

water. So, the heat in areservoir makes the oil

and water less viscous.


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Pressure From Ex panding Flu ids OIL AND GAS RESERVOIRS:FUNDAMENTALS

A closed reservoir experiences pressure fromthe expansion of reservoir fluids. For example,

sandstone lenses are usually buried in shale.

Water cannot flow through shale, so sandstonelenses are closed reservoirs. In a closed

reservoir, pressure results from the expansionof reservoir fluids. Reservoir fluids include

gas, oil saturated with dissolved gas, and saltwater or brine saturated with dissolved gas.

Gases are always compressible and liquids are

only slightly compressible.

Liquids in the reservoir are made more

compressible than ordinary liquids dissolved bygas. In a closed reservoir, the fluids have

been compressed under very high pressure byfluids driven in from the surrounding

formation.

Fluids are squeezed out of sediments as the sediments are compacted into therock. When the sand in a lens is compacted into the sandstone and the

surrounding clay is compacted into shale, some of the fluids in the clay aresqueezed into the sandstone lens.

The fluids that are forced into the sandstone lens from the surrounding shale

increase the compression of the reservoir fluids. This compression ofreservoir fluids provides reservoir pressure.


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A reservoir with no permeable path to the

surface is a(n) ______ reservoir.

Open.

Closed. A closed reservoir hasno permeable path to

the surface and is not

affected by thehydrostatic pressure of

the ground water.

In reservoirs under

hydrostatic pressure,expanding reservoir

fluids help maintain

pressure while the oil

and gas are beingremoved.


Pressure From Reservoir Rock s OIL AND GAS RESERVOIRS:

FUNDAMENTALS

Normally, reservoir rocks are thoroughly compacted and cemented and cansupport the weight of the overlying rocks. Typically, reservoir fluids do not

help support the weight of the rocks. However, in young reservoirs that may

be undergoing compaction, the reservoir fluids do help support the weight ofthe rocks.

In a closed reservoir, this places abnormally high pressure on the reservoirfluids and builds up high reservoir pressure.

When is reservoir fluid pressure abnormally

high?

When the rocks are undergoing

compaction.

When the rocks are thoroughly

compacted.


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FUNDAMENTALS

In this section, you learned about reservoir pressure. You learned that:

Reservoir pressures force oil, gas, and water to flow upward into areasof lower pressure (like those created by the well-bore). And, as oil andgas are removed from the reservoir, reservoir pressure is graduallylowered.

Reservoir pressures force varying amounts of gas into solution in theoil.

A gas cap accumulates above the oil after the oil is saturated with gas. The oil in deep reservoirs has more gas in solution because the

reservoir pressure is greater.

As rocks are buried deeper and deeper, there is an increase inreservoir temperature. Deeply buried reservoirs contain oil that is less

viscous than oil at the surface.

When oil is brought to the surface, it becomes cooler, its pressuredecreases, the dissolved gas content decreases, and it becomes moreviscous.

fundamentals of oil and gas reservoirs handouts

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