Lecture 3_a Porosity

7/23/2019 Lecture 3_a Porosity

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PorosityBy: Dr. Pavel Spirov

Petroleum geology


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The principal goal of reservoir characterization is to

three dimensional images of petrophysical propert

basic denitions and laboratory measurements of tpetrophysical properties porosity, permeabilitypermeability, capillarity, and saturation. Pore

distribution is presented as the common lin! bet"eproperties.


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#haracterisation of reservoir ro

Petrophysics is study of the physical properties of the roc!s. $or roc! to form


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#haracterisation of reservoir ro

Petrophysics is study of the physical properties of the roc!s. $or roc! to form


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Porosity

% &bsolute porosity% '(ective porosity


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Porosity


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&bsolute porosity

- is dened as the ratio of the total pore space in the roc! bul! volume.

- & roc! may have considerable absolute porosity and yetconductivity to )uid for lac! of pore interconnection


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'(ective porosity

is the percentage of interconnected pore space "it

to the bul! volume


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Petroleum Geology Anatomy of oil & gas fields

SANDSTONES: consist og quartz,

feldspar grains and clay minerals.

Porosity mostly intergranular in

!et"een t#e sand grains$

%&'ESTONES:

(onsist of calcit

generated !y !iol

acti)ity. (an

c#emically !e c#ainto dolomite.

Porosity types:

&ntergranular !e

grains$

&ntragranular in

grains*fossils$

Dissolution )ugs

ca)erns$

Reservoir types


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Porosity

Porosity is determined by core analysis or bwell logging


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Porosity

Porosity is determined by visual methods and la

measurements.

*isual methods of measuring total porosity are ebest because the amount of porosity visible depmethod of observation: the higher the magnica

more pore space is visible.

Porosity is commonly estimated by visual inspeclabs using a lo"po"er microscope


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Determination of porosity

Porosity is determined by visual methods and laboratory measurem

Porosity is commonly estimated by visual inspection of core slabs po"er microscope

+easurement of porosity of roc! samples in the laboratory reuires!no"ing the bul! volume of the roc! and either its pore volume or tvolume of the matri- material mineral volume/

Bul! volume is usuallymeasured by volumetric displacement of a strongly non"etting )uidmercury, or by direct measurement of a regularly shaped sample


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Determination of porosityPore volume can be obtained in a number of "ays.

0f the mineralogy is !no"n, mineral volume can be calculated from grain density and the"eight1 pore volume is bul! volume minus mineral volume.

+ethods of measuring:

The most accurate method of measuring porosity is the 23 helium expansion method:

& dried sample is placed in a chamber of !no"n volume and the pressure is measured

"ith and "ithout the sample, !eeping the volume of gas constant di(erence in pressure pore volume.

4/ The injection of mercury under very high pressure theporosimeter/ is also used to measure porosity.

The complete removal of all )uids is critical for accurate measurements of porosity. &ny )

not removed "ill be included as part of the mineral volume, resulting in porosity values


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'-ercise

#alculate density of the roc! #alculate the porosity

p5 of "ater


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'-ample:

& clean and dry core sample "eighing 647 g "assaturated "ith 2.8 specific gravity ;/ brine. Th"eight is 67< g. The core sample is 24 cm in len6 cm in diameter.

#alculate the porosity of the roc! sample.


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#ore analysis*t total volume*s solid volume*p pore volume

*t totalvolume


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#ore +easurements

These samples can provide

useful details of the lithology,petrology, porosity,permeability, and hydrocarboncontent of the formation

&ll of these measurements helpgeologists, engineers anddrillers better understand theconditions of the "ell and itspotential productivity.


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$actors controlling porosity

a/ =rain Diameter% =rain size% =rain size distribution Sorting/% =rain shape

b/ Depth >#ompaction?c/ Pac!ingd/ Primary porosity @ Secondary


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Porosity


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The porosities of petroleum reservoirs range from 79 to


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Controls on porosity

S&AD

=&0A

Packing ! " #$%& ' "()%*

+orting

C

c

Th e factors gover ning the magnitude of porosity inclastic sediment are as follo"s:


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&3 Cniformity of grain size:Cniformity or sorting is the gradation of grains.0f small particles of silt or clay are mi-ed "ith larger sand grains, the e(ective intercommunicbe considerably reduced. These reservoirs are referred to as dirty or shaly. Sorting depends onfactors: size range of material, type of deposition, current characteristics, and the duration of tprocess.

b3 Degree of cementation or consolidation:

The highly cemented sandstones have lo" porosities, "hereas the soft, unconsolidated roc!s have high porosities. #emeboth at the time of lithication and during roc! alteration by circulating ground"ater. The procthat of lling void spaces "ith mineral material, "hich reduce porosity. #ementing materials incarbonate, magnesium carbonate, iron carbonate, iron suldes, limonite, hematite, dolomite cclays, and many other materials including any combination of these materials.

The factors governing the magnitude of porosity in cla

are as follo"s:


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c3 &mount of compaction during and after deposition:

#ompaction tends to lose voids and sueeze )uid out to bring the mineral particles closetogether, especially the nergrained sedimentary roc!s. This e-pulsionof )uids by compaction at an increased temperature is the basic mechanism for primary migration of petrto reservoir roc!s. Ehereas compaction is an important lithifying process in claystones, shales, and negroc!s, it is negligible in closely pac!ed sandstones or conglomerates. =enerally, porosity is lo"er in deepee-ceptions to this basic trend are common. +any carbonate roc!s sho" little evidence of physical compac

d3 +ethods of pac!ing:

Eith increasing overburden pressure, poorly sorted angular sand grains sho" a progressive change from rcloser pac!ing. Some crushing and plastic deformation of the sand particles occur s.

The factors governing the magnitude of porosity in cla

are as follo"s:


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'A=0A''0A= #F&SS0$0#&T0GA G$PGGS0TH During sedimentation and lithication, some of the pore spaces initially dev

became isolated from the other pore spaces by various diagenetic and cata

processes such as cementation and compact ion $rom this , many of the pores "ill be interconnected, "hereas others "ill be

isolated.

This leads to t"o distinct categories of porosity, namely , total absolute/ an

The di(erence bet"een the total and e(ective porosities is the isolated or nporosity

&bsolute porosity is the ratio of the total void space in the sample to the buthat sample

0n order to recover oil and gas from reservoirs, the hydrocarbons must )o"several hundred feet through the pore channels in the roc! before they reacproducing "ellbore. 0f the petroleum occupies non connected void spaces, iproduced and is of little interest to the petroleum engineer. Therefore, e(ecis the value used in all reservoir engineering calculations


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='GFG=0#&F #F&SS0$0#&T0GA G$PGGS0TH

& common method of classifying porosity of petroleum reservoirs is ba"hether pore spaces in "hich oil and gas are found originated "hen th"ere laid do"n primary or matri- porosity/, or if they "ere formed thrsubseuent diagenes is e.g. ,dolomitization in carbon ate roc!s /, cat earth stresses, and solution by "ater )o"ing through the roc! secondporosity/

Primary Porosity

Secondary Porosity


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Primary Porosity

-ntercrystalline voids:*oids bet"een cleavage planes of crystals, voids bet"een individual crystals, and voids in crystal latthese voids are subcapillary, i.e., pores less than 8.884 mm in diameter. The porosity found in crystabet"een mudsized particles is called>microporosity?

-ntergranular or interparticle voids:

*oids bet"een grains, i.e., interstitial voids of all !inds in all types of roc!s. These openings range frosupercapillary size voids greater than 8.7 mm in diameter/.

'edding planes voids:

*oids of many varieties are concentrated parallel to bedding planes. The larger geometry of many peiscontrolled by such bedding planes. Di(erences of sediments deposited, of particle sizes and arrangeenvironments of deposition are causes of bedding plane voids

.iscellaneous sedimentary voids:

2/ *oids resulting from the accumulation of detrital fragments of fossils, 4/ voids resulting from the


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Secondary PorositySecondary porosity is the result of geological processes diagecatagenesis/ after the deposition of sediment. The magnitudeand interconnection of the pores may have no direct relation original sedimentary particles. 0nduced porosity can be subdivthree groups based on the most dominant geological process

Solution porosity:

Dolomitization:

$racture porosity:

+iscellaneous secondary voids:


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+olution porosity:

#hannels due to the solution of roc!s by circulating"arm or hot solutions1 openings caused by "eathering, such as enlarged oints andcaverns1 and voids caused by organisms and later enlarged by solution

olomiti0ation:

& process by "hich limestone is transformed into dolomite according tofollo"ing chemical reaction:

Some carbonates are almost pure limestones, and if the circulating por

contains signicant amounts of magnesium cation, the calcium in the roe-changed for magnesium in the solution. Because the ionic volume ofmagnesium is considerably smaller than that of the calcium, "hich it rethe resulting dolomite "ill have greater porosity. #omplete replacementcalcium by magnesium can result in a 242


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1racture porosity:

Gpenings created by structural failure of the reservoirroc!s under tension caused by tectonic activities such as folding andfaulting. These openings include oints, ssures, and fractures. 0n somereservoir roc!s, such as the 'llenburger carbonate elds of Eest Te-as,fracture porosity is important. Porosity due to fractures alone in the carbusually does not e-ceed 29

.iscellaneous secondary voids:

2/ Saddle reefs, "hich are openings atthe crests of closely folded narro" anticlines1 4/ pitches and )are openings formed by the parting of beds under gentle slum


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0n carbonate reservoirs, secondary porosity is muchimportant than primary porosity: dolomites comprisI89 of Aorth &merican hydrocarbon reservoirs.

Primary porosity is dominant in clastic also calleddetrital or fragmental sedimentary roc!s such as

sandstones, conglomerates, and certain oolitic lime5o"ever, it is important to emphasize that both typporosity often occur in the same reservoir roc!.


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*0SC&F D'S#0PT0GA G$ PGGS0TH 0A #&BGA&T

The development of "ell logging technology has provided the petrole

industry "ith e(ective and direct methods to measure the in situ porformation

The visualdescription of the pore geometry:

2/ the grain size

4/ the amount of interparticle porosity


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Fucia presented eld classication of carbonate ropore spacebased on the visual description of petrophysicalparameter s of a large number of samplesThis sho"s t"o commontypes of particle sizesbased on articiallyprepared samplescontaining various !indsof carbonate particles:large sandsized particlessuch as those found inpac!stone or grainstonedeposits, smallsilttoclaysized particlessuch as mudstone or"ac!estone


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The concept of support indening particle size indolomites is illustrate. 0fthe dolomite crystals form

a continuous, supportingnet"or!, their size controlsthe connected pore size.The dolomite crystal size isof primary interest "hen itis the same or larger thanthe sediment particlesize, such as observed indolomitized limestone or"ac!estone roc!s.


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Ae-t K Fucia classication

Lecture 3_a Porosity

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