reservoir rock properties i_porosity_module

Introduction toReservoir Rock PropertiesReservoir Rock Properties

Ibrahim Kocabas

Reservoir Engineeringg g

Learning Objectives :D fi iti f t h iDefinition of petrophysicsDefinition of porosityEngineering classification of porosityEngineering classification of porosityGeological classification of porosityFactors affecting porosityg p yPorosity of reservoir rock typesSignificance of porosityAveraging reservoir porosity

Petrophysics

Reservoir rock properties or petrophysics is theReservoir rock properties or petrophysics is the study ofrock properties androck properties, andthe interaction between fluids (oil, gas and brine) within pore space andbrine) within pore space, andthe interaction between fluids and grain surfacessurfaces

PetrophysicsA systematic theoretical and laboratory study of physical properties of petroleum reservoir rocksphysical properties of petroleum reservoir rocks includes:

• Lithology• Porosity• Compressibilityp y• Permeability • Fluid saturations• Capillary characteristics• Capillary characteristics• Rock stress• Rock resistivity• Fluid-rock interaction

RESERVOIR PETROPHYSICS:Course Objectives

By the last day of class, the student should be able to:

1. Define porosity; discuss the factors which effect porosity and describe the methods of determining values of porosity;

2. Define the coefficient of isothermal compressibility ofreservoir rock and describe methods for determining values off ti ibilitformation compressibility;

3. Reproduce the Darcy equation in differential form, explain itsmeaning, integrate the equation for typical reservoir systems,discuss and calculate the effect of fractures and channels, anddescribe methods for determining values of absolutedescribe methods for determining values of absolutepermeability;

RESERVOIR PETROPHYSICS :Course Objectives

4. Explain boundary tension and wettability and their effect on capillary pressure, describe methods of determining values of capillary pressure and convert laboratory capillary pressurecapillary pressure, and convert laboratory capillary pressure values to reservoir conditions;

5 D ib h d f d i i fl id i i i5. Describe methods of determining fluid saturations in reservoir rock and show relationship between fluid saturation and capillary pressure;p y p

6. Define resistivity, electrical formation resistivity factor, resistivity index saturation exponent and cementation factorresistivity index, saturation exponent, and cementation factor and show their relationship and uses; discuss laboratory measurement of electrical properties of reservoir rocks; and d t t th l l ti i l i l b tdemonstrate the calculations necessary in analyzing laboratory measurements;

RESERVOIR PETROPHYSICS: Course Objectives

7. Define effective permeability, relative permeability, permeability ratio; reproduce typical relative permeability curves and show effect of saturation history on relative permeability; illustrate theeffect of saturation history on relative permeability; illustrate the measurement of relative permeability; and demonstrate some uses of relative permeability data.

8. Describe three-phase flow in reservoir rock and explain methods of displaying three-phase effective permeabilities.p y g p p

9. Demonstrate the techniques of averaging porosity, permeability, and reservoir pressure dataand reservoir pressure data.

10. Demonstrate capability to perform calculations relating to all t bconcepts above.

(These are minimum skills to be achieved/demonstrated)

Reservoir rocks are in fact “rock sponges” containing hydrocarbon fluids.

But, the porous structure of rocks is NOT pvisible to our eyes.

In fact, many types of reservoir rocks show no obvious porosity to the naked eye.

Porosity

Reservoir rock are indeed “rock sponges”Reservoir rock are indeed rock sponges containing hydrocarbon fluids.

They contain void spaces in between the grains called porescalled pores.

Th i f ll h llThe size of pores are usually much smaller than we think.

The pores are of micro-meter size and hence we usually see them via electron microscopewe usually see them via electron microscope images.

For any rock we talk aboutthree different volumes:three different volumes:1. Bulk volume: Total

volume of rock body.bV

2. Grain or solid volume:Th l f lid VThe volume of solids orsand grains in rockbody.

sV

y

3. Pore volume: Thel f ll

pVvolume of all porescontained in rock body

23/06/2011 Reservoir Rock Properties (Module I) 12

POROSITY DEFINITIONPorosity: The fraction of a rock that is

occupied by pores

Porosity is an intensive property of rocks and• Porosity is an intensive property of rocks and• it is a measure of the fluid storage capacity of a rock

mabp VVVPorosity −

==φ=bb VV

Porosity ==φ=

Pores are classified as:5. Connected pores: Pores that are in communication

with neighboring pores.6. Isolated pores: Pores that are isolated from the

neighboring pores. IsolatedIsolated Void spaces

Interconnectedvoid spaces

Art-micrograph of sandstone with oil


Art-micrograph of sandstone with oil

Engineering Classification of Porosity

Total porosity, φt = V lB lk

Pore VolumeTotal

Effective porosity, φe =VolumeBulk

V lB lkPore SpacectedInterconne

VolumeBulkAbsolute and effective porosity are distinguished by their access capabilities

to reservoir fluids

• Effective porosity – of great importance;

to reservoir fluids

as it contains the mobile fluid

Valuation of PorosityNote that only effective porosity contains fluids that can be produced

Good effective porosity Φ>15%Medium effective porosity 10%<Φ<15%Poor effective porosity Φ<10%

COMPARISON OF TOTAL ANDCOMPARISON OF TOTAL AND EFFECTIVE POROSITIES

• Very clean sandstones : φe → φt

• Poorly to moderately well -cemented i t l t i l φ φintergranular materials: φt ≈ φe

• Highly cemented materials and most• Highly cemented materials and most carbonates: φe < φt

Geological classification of porosity:

Geological classification of porosity: g f f p y

Primary porosity (original)Primary porosity (original)

Secondary (induced) porositySecondary (induced) porosity(Generally more complex thanprimary porosity)primary porosity)

PRIMARY (ORIGINAL) POROSITYPRIMARY (ORIGINAL) POROSITY

• Developed at deposition of sediments

• Typified byyp y– Intergranular pores of clastics or carbonates– Intercrystalline and fenestral pores of carbonatesy p

• Usually more uniform than induced porosity

FOUR MAJOR COMPONENTS OF SANDSTONE

Framework

M t iSand (and Silt) Size Detrital Grains

Matrix

CementSilt and Clay Size Detrital Material

Cement

Material Precipitated Post-Depositionally,During Burial. Cements Fill Pores andReplace Framework Grains

PoresReplace Framework Grains

Voids Among the Above Components

FOUR COMPONENTS OF SANDSTONENote different use of “matrix”1. Framework

Geologist’s Classification

Note different use of “matrix”by geologists and engineers

1. Framework2. Matrix3. Cement4. Pores

Engineering“matrix”

FRAMEWORKPORE

MATRIX(QUARTZ)CEMENT

FRAMEWORK(FELDSPAR)

0.25 mmPrimary porosity in sandstones

SECONDARY (INDUCED) POROSITY( )

• Developed by geologic processes after p y g g pdeposition (diagenetic processes)

• Examples p– Grain dissolution in sandstones or carbonates– Vugs and solution cavities in carbonates– Fracture development in some sandstones, shales,

and carbonates

Geologic processes leading to development of Secondary porosity :• compaction, and cementation,• solution,,• dolomitization,• fracturing, jointing, and fissures development.

DUAL POROSITY IN SANDSTONESandstone Comp. 1. Primary and secondary “matrix” porosity system

2 Fracture porosity system• Framework• Matrix• Cement• Pores

DISSOLUTIONPORE

FRACTURE

2. Fracture porosity system

FRAMEWORKPORE


FRAMEWORK(FELDSPAR)


0.25 mmDual porosity systems

FACTORS THAT AFFECT POROSITYFACTORS THAT AFFECT POROSITY

PRIMARY• Packing (grain arrangement)

• Particle shape (sphericity and angularity)p ( p y g y)

• Sorting (variable grain sizes)

• Cementing materialsSECONDARY (diagenetic)

• Overburden stress (compaction)

• Vugs dissolution and fractures• Vugs, dissolution, and fractures

Upper limit of porosity: cubic packing

In both figures, let’s take the repetitive volume for a single grain.Same equations apply and hence φ remains unchanged.

3 3(2 ) 8bV r r= =34

3srV π

=

38 (1 )6pV r π

= − 1 / 6 0.476p

b

VV

φ π= = − =6 b

Porosity is independent of grain size !

CUBIC PACKING OF SPHERESPorosity = 48%

Porosity Calculations - UniformPorosity Calculations Uniform Spheres

• Bulk volume = (2r)3 = 8r3• Bulk volume (2r) 8r

Matri ol mer4 3π

• Matrix volume =

l b lk l i l

3

• Pore volume = bulk volume - matrix volume

VolumeBulkVolumePorePorosity =

V lM t iV lB lk

VolumeBulk

VolumeBulkVolumeMatrixVolumeBulk −

=

%6471r3/4r8 33 ππ−( ) %6.4732

1r8 3 =−==

RHOMBIC PACKING OF SPHERESPorosity = 27 %

GRAIN PACKING IN SANDSTONELine of Traverse

(using microscope) 4 Types of Grain Contacts

Tangential ContactPacking ProximityA measure of the extent towhich sedimentary particlesare in contact with their

Cement

Sutured Contact

Long Contact Packing Density

neighbors

A measure of the extent to

Matrix(clays, etc.) Concavo-Convex

Contact

which sedimentary particlesoccupy the rock volume

This Example

Packing Proximity = 40%

(modified from Blatt, 1982)

Packing Density = 0.8


PRIMARY• Packing

• Particle sphericity and angularityp y g y


• Cementing materialsSECONDARY (DIAGENETIC)



ROUNDNESS AND SPHERICITYOF CLASTIC GRAINS

Highty High

L

Poro

sit

Low

VeryAngular Angular Sub-

AngularSub-

Rounded Rounded Well-RoundedAngular g

ROUNDNESS

Porosity


PRIMARY• Packing

• Particle sphericity and angularityp y g y





Packing of Two Sizes of SpheresPorosity = 14%

Grain-Size Sorting in Sandstone

Very WellSorted

WellSorted

ModeratelySorted

PoorlySorted

Very PoorlySortedSorted Sorted Sorted Sorted Sorted

SORTING

Decreasing Porosity


PRIMARY• Particle sphericity and angularity

• Packingg





DIAGENESISDiagenesis is the Post-Depositional Chemical and

CarbonateCemented

Mechanical Changes thatOccur in Sedimentary Rocks

OilSt i d

Some Diagenetic Effects Include

CompactionP i it ti f C tStained Precipitation of CementDissolution of Framework

Grains and Cement

The Effects of Diagenesis MayEnhance or Degrade ReservoirQualityQuality

Whole CoreMisoa Formation, Venezuela Photo by W. Ayers

DUAL POROSITY IN SANDSTONESandstone Comp. 1. Primary and secondary “matrix” porosity system

2 Fracture porosity system• Framework• Matrix• Cement• Pores

DISSOLUTIONPORE

FRACTURE

2. Fracture porosity system

FRAMEWORKPORE


FRAMEWORK(FELDSPAR)


0.25 mm

Factors affecting porosity:fThe grouping of porosity data according to

the reservoir zones, depth profile or graphical di ti l ti l t d i thco-ordination, may reveal spatial trends in the

porosity variation. The recognition of such trends is very important for the developmenttrends is very important for the development of a bulk picture of the reservoir as a porous medium and representation of the reservoirmedium and representation of the reservoir porosity in mathematical simulation models (reservoir characterisation, lateral correlation,(reservoir characterisation, lateral correlation, numerical modelling, etc.) (Zolutkin)

Factors affecting porosity:M h i l di i ( ti ) d h i lMechanical diagenesis (compaction) and chemical diagenesis (cementation) have a profound effect on a sedimentary rock’s porosity. This burial effect is y p yillustrated by the two typical examples of sand and clay deposits in Fig. 7.

TitlepageExamples of primary and secondary Porosity Types:

1 I t l it1 General Aspects

2 Idealised Models

Topic Overview 1. Intergranular porosity

3 Measurements of Porosity

Unfilled interparticle porosity (in Oolite). Porosity is black.

Developers

References Back Next

Titlepage2. Fracture porosity

Fracture porosity is a form of secondary porosity generated

1 General Aspects

2 Idealised Models

Topic OverviewFracture porosity is a form of secondary porosity generated by tectonic fracturing of the rockSuch porosity can develop in any rock, allowing the development of highly productive reservoir in rocks


p g y p

A strongl fract red chalk from an area of onl mild

Developers

References

A strongly fractured chalk from an area of only mild deformation. Such fractures are commonly late diagenetic,

and postdate most other diagenetic features in the rock.

NextBack

Titlepage3.Micro- porosity

1 General Aspects

2 Idealised Models

Topic Overview Micro-porosity is that part of the pore space that has a characteristic dimension less than 1 micronIn general, this includes not only very small pores


but also the porosity associated with surface roughnessThe water in this pore space is part of the capillary-bound water and the small-pore water capillary-bound water and the small-pore water. Water in micropores is not expected to flow on productionThe term is also defined as porosity that cannot be p yseen at magnifications less than 50x

Developers

References Back Next

Titlepage

4. Vugular porosity

1 General Aspects

2 Idealised Models

Topic Overview Vugular porosity is the pore space consisting of cavities or vugsVugular porosity can occur in rocks prone to dissolution, such as limestone in which case is secondary porosity3 Measurements

of Porositysuch as limestone, in which case is secondary porosity

Developers

References

Vuggy porosity. Probably solution enlarged. Porosity is black.

Back Next

Titlepage5. Intragranular porosity

1 General Aspects

2 Idealised Models

Topic Overview


Unfilled intraparticle porosity (within a large coral fragment).

Developers

References

p p y ( g g )Porosity is black.

Back Next

Types of Reservoir Rocks

S d t k A lid t d i kSandstone rock: A consolidated reservoir rockcomposed of sand particles.Carbonate rock: A reservoir rock of chemical origin and

d f l i b tcomposed of calcium carbonate.Dolomite rock: A reservoir rock composed of calciummagnesium carbonates.S fShale Rock: It consists of compacted clay material. Ithas very high porosity (40%). It serves as a cap orbase rock.


27. Homogeneous porosity distribution: Reservoir rockporosity is constant.

28. Heterogeneous porosity distribution: Reservoir rockg p yporosity is not constant, but rather changes fromone point to the next.

29. Isoporosity map: A map showing lines (contours) ofconstant porosity. The increment between any twoneighboring lines is constant called contourinterval. The contour lines do not intersect. Theyare continuous and terminate at the edge of map orare continuous and terminate at the edge of map orreservoir. Two adjacent contour lines have thesame value only when there is reversal in directionto reflect a minimum or maximum value.to reflect a minimum or maximum value.


30 Hydrocarbon volume in a

Significance of Porosity30. Hydrocarbon volume in a

reservoir: The volume of oil and gas that exist in the pores of a reservoir rock is in the units of Reservoir Barrels (RB). ( )

HCHC SAhV φ7758=

(feet)essnet thicknreservoiraverage(acres)extension arealreservoir

(RB)reservoir in n volumehydrocarbo

===HC

hAV

(fraction) saturationn hydrocarbo average(fraction)porosity effective average

(feet)essnet thicknreservoir average

===

HCS

hφ


Significance of Porosity31. Initial oil-in-place (N): The amount of oil in a

reservoir in units of Stock Tank Barrels(STB).

oo BSAhN /7758 φ=

f 3614 91(RB/STB).factor volumeformation oil =oB

ft35.61459=1B


32. Initial gas-in-place (G): The amount ofSignificance of Porosity

g p ( )natural gas in a reservoir in units of standardcubic feet (scf).

/7758= gg BSAhG φ

or

(RB/scf)factor volumeformation gas where =gB

(ft3/scf)factorol meformationgashere

/560,43o= gg

B

BSAhG φ

(ft3/scf)factor volumeformation gas where =gB


33 Average porosity: The average porosity of n rock

Porosity Averaging33. Average porosity: The average porosity of n rock

samples is given by general average equation:

nnnn

:equationporosity average General

∑∑∑∑==== ====

n

iiiii

n

iiii

n

iib

n

ip hLwhAVV

ii1111

φφφφ

∑∑∑∑====

n

iiii

n

iii

n

ib

n

ib

avg

hLwhAVVii

1111

φ


Porosity Averaging

∑

∑== n

n

iib

avg

Vi

1 :porosity averaged Volumeφ

φ

∑

∑=

n

ii

ib

A

Vi

1

φ

∑=

==

n

n

ii

iavg

A1

1 :porosityaveraged Area φ

∑

∑== n

ii

iii

avg

h

h

1

1 :porosity averaged Thicknessφ

φ

∑=

=

=n

iiavg

i

n 1

1

1 :porosity average Arithmetic φφ


reservoir rock properties i_porosity_module

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