1.4- reservoir rock

RR > Reservoir Rock

Sedimentary Rock Cycle, Rock Types, Igneous and Metamorphic rocks,

sedimentary rocks, Clastics, Carbonates, Porosity and Permeability,

Capillary Pressure

1.4- Reservoir Rock

Dr. M. Watfa

1.4 : Reservoir Rock

2 Copyright ©2001-2011 NExT. All rights reserved Material: M. Watfa

2

Sedimentary

Rock

Process

Source Rocks

Igneous Metamorphic Sedimentary

Weathering

Mechanical and Chemical

Deposition Clastics Carbonates Evaporites

Compaction Dissolution Precipitation

Diagenesis

Sedimentary Rock Layers

Sedimentary Rock Cycle



3

The start and end of all rocks is the magma in the mantle.

This is cooled to create igneous rocks.

These can be broken down into sediments.

The sediments are turned into sedimentary rocks.

These can be buried deeper with heat and pressure, turning into metamorphic rocks.

If these are then heated we return to the magma.

Inside this major cycle are sub-cycles. Igneous rocks can be heated to give metamorphic rocks.

Any rocks can be broken into sediments to give sedimentary rocks.




4

Rocks and Rock

Types:

Sedimentary

Characteristics

This chart shows the

relative abundance of

most sedimentary

rocks.

Sedimentary Rock Types- Relative abundance




5

Carbonate

Fraction

130 billions barrels of oil

300 billions barrels of oil

World Oil Reserves



6

Rocks and Rock Types:

There are three main types of rock which are classified as:

igneous,

metamorphic

sedimentary

Igneous rocks: Formed from molten material deep in the earth’s crust. This includes granite

Metamorphic rocks- Modified by high pressure and temperature, such as gneiss.

Igneous and metamorphic rocks are called basement rocks. Only when highly fractured can these rocks serve as a reservoir.

Sedimentary Rocks: Eroded, transported and deposited.

Rock Types



7

These are the most important for the oil industry as it contains most of the source rocks and cap rocks and virtually all reservoirs.

Sedimentary rocks come from the debris of older rocks and are split into two categories

Clastic

and Non-Clastic.

Clastic rocks - formed from the materials of older rocks by the actions of erosion, transportation and deposition.

Non-Clastic rocks - Formed from chemical or biological origin and then deposition.

Sedimentary Rocks

Rock Types



8

Rock Types: Sedimentary

Clastic

– Boulders/Cobbles, Granules(>2mm)

– Sand (0.06 – 2.0 mm)

– Silt (0.004 – 0.04 mm)

– Clay (<0.004 mm)

Carbonate

– Limestone / Dolomite

Evaporite

– Salt / Gypsum

Rock Types



9

The depositional environment

can be

Shallow or deep water.

Marine (sea) and lake or

continental.

This environment determines

many of the reservoir

characteristics

Sedimentary Rocks-

Depositional Environments

Rock Types



11

The structure of a reservoir can

also be determined by

deposition; a river, a delta, a

reef and so on.

This can also lead to

permeability and producibility. of

these properties are often

changed by further events.

The depositional characteristics

of the rocks lead to some of

their properties and that of the

reservoir itself.

The reservoir rock types are

either clastic or non-clastic.

The type of porosity (especially

in carbonates) is determined by

the environment plus

subsequent events.

Sedimentary Rocks-


Rock Types



12

The environment is not

static.

Folding and faulting change

the structure.

Diagenesis (Dissolution and

fracturing) can change the

porosity & permeability.

Sedimentary Rocks-


Rock Types



13

Sediments settle to the

bottom of the sedimentary

basin.

As the sediments

accumulate the

temperature and

pressure increase

This process expels water

from the sediments.

Sedimentary Rocks-

Depositional Environments:

Sedimentations

Rock Types



14

Sedimentary muds become sedimentary

rocks.

Calcareous muds become limestone.

Sands become sandstone.

Another effect involves both the grains in the

matrix and the fluids reacting to create new

minerals changing the matrix and porosity.

Fluids can also change creating a new set of

minerals.

This whole process is called Diagenesis.

Sedimentary Rocks-

Depositional Environments:

Sedimentations

Rock Types



15

Comprise 95% of the Earth's

crust.

Originated from the

solidification of molten material

from deep inside the Earth.

There are two types:

Volcanic - glassy in texture

due to fast cooling.

Plutonic - slow-cooling,

crystalline rocks.

Igneous Rocks

Granite

Igneous & Metamorphic Rocks



16

Igneous rocks can be part of

reservoirs.

Oil could migrate up due to

geometric location

Fractured granites form

reservoirs in some parts of the

world.

Volcanic tuffs are mixed with

sand in some reservoirs.

Igneous Rocks and

Reservoirs

Granite




17

Metamorphic Rocks are formed by

the action of temperature and / or

pressure on sedimentary or

igneous rocks.

Examples of Metamorphic Rocks

Marble: formed from limestone

Hornfels: from shale or tuff

Gneiss (pronounced- NICE): similar to granite but formed by metamorphosis

Metamorphic Rocks

Gneiss

Schist




Clastics

Consist largely of quartz (silicon oxide SiO2)

Clastic rocks – formed from rock debris

Sand grains cemented to form rock

Commonly contain other silicate minerals: clays, micas, feldspars

Quartz has low reactivity due to very low solubility in brine

Ref: T. Jones / SCR



19

Clastic Depositional

Environments

Alluvial Fan

Lacustrine

Eolian

Fluvial

Delta

Shelf

Marine

Sandstone

Clastics



20

Clastic Rock Clastic rocks are:

sands,

silts and

shales.

The difference is in

the size of the

grains.

Clastics



21

Sediments are transported

to the basins by rivers.

A common depositional

environment is the delta

where the river empties into

the sea.

A good example of this is

the Mississippi and the Niger

Delta.

Clastic Rock- Depositional

Environment - DELTA

Clastics



22

Some types of deposition occur in rivers and sand bars.

The river forms a channel where sands are deposited in layers. Rivers carry sediment down from the mountains which is then deposited in the river bed and on the flood plains at either side.

Changes in the environment can cause these sands to be overlain with a shale, trapping the reservoir rock.

Clastic Rock- Depositional

Environment - RIVER

Clastics



23

High Sphericity

Low Sphericity

Very Angular Angular

Sub- Angular

Sub- Rounded Rounded

Well- Rounded

(Geologists like their sandstones well rounded and with high sphericity)

Roundness and Sphericity of Clastic Grains

Clastics



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Very Well Sorted

Well Sorted

Moderately Sorted

Poorly Sorted

Very Poorly Sorted

Grain-Size Sorting in Sandstone

Clastics



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Change of Composition Change of Size

Change of Shape Change of Orientation

Change of Packing

Sand

Shale

Eolian

Fluvial

Slow Current

Fast Current

River

Beach

Types of Textural Changes Sensed by the Naked Eye as Bedding

Clastics


26 Copyright ©2001-2011 NExT. All rights reserved Material: M. Watfa 26

NMR and FMI in Sandstone

X100-x110: Shaly interval. Low

T2 and hence small pores.

X110-x120: mainly clean

sandstone with small shale as

shown by small T2 values.

X120-x140: FMI shows thin

shale streaks. NMR shows more

low values of T2 confirming the

presence of shale. This is also

confirmed by the high resolution

NMR spikes on the FFV and

BFV.

X140-TD: Tight formation with

low T2 and small pores.

Clastics



Carbonates encompass limestones (largely calcite CaCO3) and dolomites (largely CaMg(CO3)2 )

Formed by carbonate precipitation and aggregation of animal shells

Often associated with evaporite minerals

High reactivity due to relatively high solubility in brines (0.15 grams/litre in 1 molar sodium chloride solution

Wide range of pore sizes, from vugs (~ cm) to micropores (< 1 mm)

Carbonates

Ref: T. Jones / SCR



28

Carbonates form a large

proportion of all permeable

sedimentary rocks ( ≈ 14%).

They consist of:

Limestone.

Dolomite.

Carbonates usually have an

irregular pore structure.

Often, a formation has a mixture

of Limestone and dolomite

Limestone

Dolomite

Carbonates



29

Carbonate Types

Chalk is a special form of limestone

and is formed from the skeletons of

small creatures (cocoliths).

Dolomite is formed by the replacement

of some of the calcium by a lesser

volume of magnesium in limestone.

Magnesium is smaller than calcium,

hence the matrix becomes smaller

and more porosity is created.

Limestone Ca CO3

Dolomite Ca CO3 Mg CO3

Evaporites such as Salt (NaCl) and

Anhydrite (CaSO4) can also form in

these environments.

A dolomite is formed when one

magnesium ( Mg) molecule replaces a

Calcium (Ca) molecule

Carbonates



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Porosity Types- Carbonates Interparticle: Pores between particles or grains

Intraparticle: Pores within individual particles

Moldic Pores formed by dissolution of an individual grain crystal in the

rock

Fracture: Formed by a planar break in the rock

Vug Large pores formed by indiscriminate dissolution of cements and

grains

Carbonates



31

Dunham Carbonate Rock Classification

Depositional Texture Recognizable Depositional Texture

Not Recognizable

Mudstone Wackestone Packstone Grainstone Boundstone Crystalline Carbonate

Grain Supported

Lacks Mud, Grain-

Supported

Components Not Bound Together During Deposition

Mud Supported

Contains Mud (clay and silt size particles

<10 % Grains

>10 % Grains

Original Components Bound Together

During Deposition

(modified from Dunham, 1962)

Carbonates



33

Carbonate Porosity - Example

Thin section micrograph - plane-polarized light

Smackover Formation, Alabama (Photograph by D.C. Kopaska-Merkel)

Moldic

Pores

• Due to dissolution

and collapse of ooids

(allochemical particles)

• Isolated pores

• Low effective porosity

• Low permeability

Blue areas are pores.

Calcite

Dolomite

Moldic

Pore

Carbonates



35

Cross section

showing complex

facies relations in a

carbonate reef

setting. Reservoir

quality varies with

facies.

Carbonate Depositional Environment

Reef System

Carbonates



36

Carbonate Rock Distribution

Reef; Shelf Carbonate, Deep Carbonate; Carbonate Oil Province

Carbonates



37

Sedimentary Rock Characteristics

Porosity

– The percentage of pore volume or void space

that can contain fluids

Permeability

– The measure of how easily fluid moves through

a rock, typically measured in Darcies or

millidarcies

Sorting

– Range of sedimentary grain sizes that occurs in

sedimentary rock

Matrix (lithology) - major constituent of the rock

Porosity and Permeability



38

How permeability is formed will

depend on many factors.

For the same porosity, a wide

range of permeabilities can

develop.

These various controls can be

grouped under various facies

categories

Effects of various controls on

Porosity & Permeability




39

1- Definition of Porosity

Sand

Anhydrite

Shale

The Matrix could be complex Lime-Dol




40

A rock can be made up of small grains or large grains but have the same porosity.

Porosity depends on grain packing, not the grain size.

In a clastic rock the grain size ( same size grains ) does not affect the porosity.

A sand, a silt and a shale can have the same porosity .

Differences come in permeability where the grain size has a direct effect, large grains meaning higher permeability.

Porosity in sandstones: Grain Size

Different grain size- same porosity




41

The porosity of a sandstone depends on the packing arrangement of its grains. The system can be examined using spheres.

In a Rhombohedral packing, the pore space accounts for 26% of the total volume.

With a Cubic packing arrangement, the pore space fills 47% of the total volume. In practice, the theoretical value is rarely reached because:

the grains are not perfectly round, a

the grains are not of uniform size.

Porosity in sandstones: sorting




42

The environment can also involve

subsequent alterations of the rock

such as Chemical changes.

Diagenesis is the chemical alteration

of a rock after burial.

An example is the replacement of

some of the calcium atoms in

limestone by magnesium to form

dolomite.

Mechanical changes - fracturing in a

tectonically-active region.

Porosity in Carbonates: Diagenesis and secondary porosity




43

Mouldic porosity: Pores created by

the dissolution of shells, etc.

Interparticle porosity:

Each grain is separated, giving a

similar pore space arrangement as

sandstone.

Intergranular porosity:

Pore space is created inside the

individual grains which are

interconnected.

Intercrystalline porosity:

Produced by spaces between

carbonate crystals.





44

Vuggy porosity:

Created by the

dissolution of fragments,

but unconnected.

Fracture porosity:

Pore spacing created by

the cracking of the rock

fabric.

Channel porosity:

Similar to fracture

porosity but larger.





45

Intergranular porosity is called

"primary porosity".

Porosity created after deposition is

called "secondary porosity".

The latter is in two forms:

Fractures

Vugs.

Definition of Porosity Carbonate Porosity Types




46

Fractures are caused when a

rigid rock is strained beyond its

elastic limit - it cracks.

The forces causing it to break

are in a constant direction,

hence all the fractures are also

aligned.

Fractures are an important

source of permeability in low

porosity carbonate reservoirs.

Fractures

Porosity in Carbonates: Fractures




47

Vugs are defined as non-

connected pore space.

They do not contribute to the

producible fluid total.

Vugs are caused by the

dissolution of soluble material

such as shell fragments after

the rock has been formed.

They usually have irregular

shapes.

Porosity in Carbonates: Fractures




48

Carbonate

Dissolution

Cavity:

Carbonates

have dissolution

cavities- but not

as large as this

cave.

Vugs

Courtesy Schlumberger




49

The rate of flow of a liquid

through a formation depends on:

The pressure drop.

The viscosity of the fluid.

The permeability.

The pressure drop is a

reservoir property.

The viscosity is a fluid

property

The permeability is a measure of the ease at which a fluid can flow through a formation.

Relationships exist between permeability and porosity for given formations, although they are not universal.

A rock must have porosity to have any permeability.

The unit of measurement is the Darcy.

Reservoir permeability is usually quoted in millidarcies (mD).

Permeability Definition




50

The flow of fluid of viscosity m

through a porous medium was first

investigated in 1856 by Henri

Darcy.

He related the flow of water

through a unit volume of sand to

the pressure gradient across it.

In the experiment the flow rate can

be changed by altering the

parameters as follows:


Darcy Experiment Q = f(P1-P2); Q = f (1/L); Q = f( A), Q= f (1/µ)

Q = Constant . A . (P1-P2)/ (µ . L)

Q = K . A . (P1-P2)/ (µ . L)

P1

P2

Area: A

L




51

K = permeability, in Darcies.

L = length of the section of rock,

in centimeters.

Q = flow rate in centimeters /

sec.

P1, P2 = pressures in bars.

A = Pore area, in cm2.

µ = viscosity in centipoise.


Parameters

P1

P2

Pore Area: A

L

K = Q. µ . L / { A . ( P1 - P2 ) }




52

Production rate

Radial flow rate is most important

Require values for the following

– ko = Permeability

– h = Net Pay

– Pe = Reservoir

– Pw = Bottom hole pressure

– μ = Fluid viscosity

– Bo = Formation volume factor

– re/rw = Drainage & wellbore radii

Radial Flow Rate

qo= 7.08 ko h (Pe – Pw)

μ Bo ln (re / rw)

rw

re




53

In formations with large grains,

the permeability is high and the

flow rate larger.

In a rock with small grains the

permeability is less and the flow

lower.

Grain size has no bearing on

porosity, but has a large effect

on permeability.


Permeability and Rocks




54

50 m-Darcy 10 m-Darcy

> 25 Darcy

5 m

m M

arb

les

10cm Diameter cup

>5 Darcy 1

.5 m

m M

arb

les

( B

ea

ch

Sa

nd

) 500 m-Darcy

1 mm

Sam

e po

rosi

ty Φ

≈ 2

5 %




55

Fractures Compaction

& Cementing

Compaction

& Leaching

Porosity

Per

mea

bili

ty

The relationship

between porosity and

permeability for

various carbonate

rocks.

Permeability in

Carbonates

After R. Nurmi- 1986

Courtesy Schlumberger




56

Free Fluid Index In Carbonates

Classical example of carbonates

with bi-modal porosity.

A free fluid index cutoff of 100

msec was used based on core

centrifuge.

Below X415: Sw ≈100%. NMR

shows low T2 distribution and

mainly BFV.

This is confirmed by the low

permeability values.

This allowed perforations to be

made to be made as low as X410

without producing high water cut.

This essentially improves the

hydrocarbon recovery. Courtesy Schlumberger




Surface Tension

Interaction between hydrocarbons and water in the

reservoir depends on the surface tension between them

Surface tension is the apparent film which separates two

immiscible fluids

A pressure difference exists across any curved interface

Capillary Pressure



Apparent Surface Film caused by

Imbalance of Molecular Forces

Capillary Pressure



Pressure in a bubble

r

P1

P2

P1 – P2 = 2 . σ / r in dynes

Where:

P2 = Pressure inside Bubble dynes / cm2

P2 = Pressure outside Bubble dynes / cm2

r = Bubble Radius cm

σ = surface tension dynes/cm

1 psi = 68,948 dynes / cm2

Capillary Pressure



Water 72.6 dynes/cm

Benzene 28.9 dynes/cm

Cyclohexane 25.3 dynes/cm

Surface tensions between some common fluids and air at 20° C

Interfacial tension between water

and oil at 20° C 30 dynes/cm

Interfacial tension between a liquid

and its vapor decreases with

temperature increase until at the

critical point, surface tension is

zero and differentiation between

fluid/vapor phases ceases to exist.

Contact Angle as a

measure of wettability

Capillary Pressure



Contact Angle as a Measure of Wetting

Capillary Pressure



Capillary Rise Fluid Rise in a Capillary

Tube Bundle

Capillary Pressure



Shape of the Capillary Pressure vs.

Saturation Curve

Capillary Pressure



Capillary Pressure

Shape of

Capillary

Curve

and Grain

Size

Distribution

1.4- reservoir rock

Documents

sedimentary rocks rock

cap rocks

basement rocks

granite metamorphic

nonclastic rocks

main types of rock

sedimentary characteristics

materials of older rocks