Irreducible Water

8/10/2019 Irreducible Water

1/22

IRREDUCIBLE WATER

We have already seen how, on a molecular level, the interaction between clay and

water results in lower resistivity values. Now we will step back somewhat, and

readjust our sights for a microscopicexamination of the pores in a pay zone. At this

level, we will see that water, rather than clay, is a prime factor contributing to low

resistivity pays.

n this section, we will describe a number of inter!related factors, each of which are

intimately tied to the amount of non!producible bound!water that a reservoir can hold

"#igure $%Water at the intergranular scale&. 'hough not produced, this bound!water

is none!the!less detected and measured by resistivity tools, which do not distinguish

between freely produced water and immovable water.

Figure 1
http://www.ihrdc.com/els/ipims-demo/t26/offline_IPIMS_s23560/resources/i/G4108F01.jpghttp://www.ihrdc.com/els/ipims-demo/t26/offline_IPIMS_s23560/resources/i/G4108F01.jpg


2/22

We will start with a brief review of the concepts of porosity and saturation, and will

take a closer look at permeability and capillarity as they relate to bound!water. We

will see that structural position also plays an important role, along with rock!fluid

interactions and fluid!fluid interactions, in determining whether a low!resistivity pay

zone will produce water or hydrocarbons.

Porosity

(orosity is the ratio of pore space in the rock to the bulk volume of the rock. t is

expressed as a fraction or as a percent of the bulk volume. n e)uation form,

where%

* = porosity in fraction

+p pore volume

+b bulk volume

+pand +bcan be expressed in any consistent units.

POROSITYCLASSIFICATION

n terms of production, three types of porosity are recognized%

Total porosityrefers to all pore space in a rock.

Effectiveporosityrefers only to that portion of the total porosity consisting of

interconnected pore spaces- more specifically, effective porosity is that portion

of the total porosity which will allow fluid flow under normal recovery processes

in the reservoir. ffective porosity is a dimensionless )uantity, defined as the

ratio of interconnected pore volume to the bulk volume.

Non-effective porosityis the remaining portion of total porosity which occurs

either as isolated pore spaces or as microporosity.


3/22

'he difference between these porosities may be significant in highly vuggy or

fractured reservoirs, where some vugs or fractures may be isolated.

'he presence of clay also complicates the definition of rock porosity. Although the

layer of closely bound surface water on the clay particle can represent a verysignificant amount of porosity, it is not available as potential reservoir porosity for

hydrocarbons. 'hus, a shale or shaly formation may exhibit a high total porosity,

while actually having a low effective porosity as a potential hydrocarbon reservoir.

/ound water is held by non!effective porosity. When we calculate water saturation

for producibility estimations we are must be sure to use effective porosity.

MICROPOROSITY

Microporosityrefers to pore spaces which are so small in diameter "0 1 or less& that

they trap and hold water immobile through capillary action. 2icroporosity is

commonly associated with authigenic clay minerals whose open structure is able to

trap water. Another example is chalk, which commonly exhibits a large percentage of

microporosity, with very high total porosity but low matrix permeability.

2icroporosity is considered non-effectiveporosity as far as the production potential

of the reservoir is concerned. f it is not recognized as such, microporosity can lead

to optimistic predictions of potential reservoir porosity. 3n the other hand, bound

water associated with extensive microporosity can lower resistivity readings and lead

to pessimistic estimations of water saturation.

Saturatio

4aturation is a measure of the relative volume of each fluid in the pores. 'hus, oil

saturation is defined as the ratio of the volume of the oil in a porous rock to the pore

volume of the same rock. t is expressed in fraction or in percent, and ranges from 5

to nearly $556. Water is always present in all reservoirs, and its saturation is always

greater than zero. n contrast, the oil saturation is zero in gas reservoirs, and the gas

saturation is zero in oil reservoirs when the pressure is above the bubble!point. 3il

or gas saturation is calculated by subtracting the water saturation from unity "in two!

phase reservoirs&.


4/22

IRREDUCIBLEWATERSATURATION

Irreducible water saturation "sometimes called critical water saturation& defines the

maximum water saturation that a formation with a given permeability and porosity

can retain without producing water. 'his water, although present, is held in place by

capillary forces and will not flow. 7ritical water saturations are usually determined

through special core analysis.

'he critical water value should be compared to the reservoir8s in!place water

saturation calculated from downhole electric logs. f the in!place water saturation

does not exceed the critical value, then the well will produce only hydrocarbons.

'hese saturation comparisons are particularly important in low permeability

reservoirs, where critical water saturation can exceed 956 while still producing only

hydrocarbons.

USIN!MA!NETICRESONANCETOOBTAINBOUNDWATERSATURATIONS

After describing total, effective, and non!effective porosity "above&, we can now

define the saturation of non!producible bound water in terms of effective and total

porosity through the following e)uation%

Where

4wbis bound water saturation

'is total porosity

is effective porosity

n essence, this bound water saturation e)uation divides non!effective porosity by

total porosity.

'otal and effective porosity measurements can be obtained through magnetic

resonance logging. #or more information on magnetic resonance logging "N2:& in

low resistivity pay zones, see the section of this module entitled Advances in

Logging.


5/22

3stroff, 4horey, and ;eorgi "$


6/22

'he wettability of a surface is determined by the interaction of interfacial energies

that act on the fluids and the surface. #or two immiscible co!existing fluids in a

porous media, the one with the lower interfacial tension is the wettingphase, while

the other is the non-wettingphase. nterfacial tension is a measure of the surface

energy per unit area of the interface between two immiscible fluids, such as water

and crude oil, or oil and gas. 'he lower the solid!fluid interfacial tension, the lower

the surface energy and the higher the tendency for the fluid to wet that surface.

Per'ea"i#ity

(ermeability is a measure of the ability of porous rock to transmit fluid.

PERMEABILITYCLASSIFICATION

(ermeability is further classified as either absolute or effective depending on

whether one or more fluids occupy the pore spaces of the rock !

Absolute permeability occurs when only one fluid is present in the rock. t is

independent of the fluid used in the measurement. 'his assumes that the fluid does

not interact with the rock.

Effectivepermeability is the measured permeability of a porous medium to one fluid,

when other fluids are present. ffective permeability depends on the relative

proportion of the fluids present "fluid saturation&.

7onsider the case of oil and water together in a pore system. Bnder a given

pressure gradient, the oil and water flow through a pore system together. /ased on

Carcy8s e)uation, we find that%

%or oi# (

%or )ater (


7/22

where%

k permeability

) flow rate

fluid viscosity

p pressure differential

D length

A cross!sectional area

#urthermore, we find that the total flow rate ""t,& is expressed by the e)uation%

@t "@oE @w&,

@t is less than the flow rate that either phase would have if it were at $556

saturation. 'hus it appears as though the two phases interfere with each other8s

progress through the pore system. A useful way to )uantify this phenomenon is to

define the relative permeability, "#r&.

RELATI*EPERMEABILITY

Carcy8s definition of permeability was for a porous medium which was $556

saturated with the flowing phase "the phase was water&. =ydrocarbon reservoirs

normally have two and perhaps three phases present% both water and oil- or water

and gas- or water, oil, and gas sharing the pore space of the rock. We have seen

that having more than one phase present in the pores reduces the ability of the rock

to transmit any one of the fluid phases. #or this reason, we define the effective

permeability as the permeability to one phase when there is more than one phase

present in the pore space. ts value decreases as the phases8 saturation decreases.

'here is an effective permeability value for each phase present.

Bsually the effective permeability is expressed as a fraction of the absolute

permeability which is the permeability at $556 saturation of the flowing fluid. 'his

ratio of effective to absolute permeability is termed the relative permeability and can

be displayed as a set of curves as shown in #igure F, for an oil and water system.


8/22

Figure +

'his graph shows that the relative permeability to oil decreases as the oil saturation

decreases and the water saturation increases above its irreducible "or connate&

value. 7onversely, the relative permeability to water increases, reaching a maximum

when the oil saturation is at its residual saturation. 'his same general principle

applies to any two!or three!phase system.

'he graph shows that relative permeability is also a function of fluid saturation.

When multiple, immiscible fluid phases flow in a rock, the sum of the effectivepermeabilities of the various fluids will commonly be significantly less than the

absolute permeability measured with only a single fluid in the rock. A different way of

stating this is that the sum of the relative permeabilities for all the fluids in the roc#

will commonly be less than one.


9/22

:elative permeability is the ratio of the effective permeability of the rock to one

phase divided by the absolute permeability, and it is )uoted at some particular

saturation value%

kro kr k

krw kw k

Re#ati,e Per'ea"i#ity a$ Irre$u&i"#e Water Saturatio

Another important relative permeability concept is that of the irreducible or residual

saturation.

f two fluid phases,Aand $, are flowing in a rock, the relative permeability of fluid

phase A will decrease as the saturation of fluid A decreases. At some non!zero

saturation of fluidA"commonly G6 to 06&, fluidAwill cease to flow, and only fluid $

will continue to flow in the rock. 'he saturation at that point is termed the irreducible

or residual saturationof fluidAfor theA-$two phase flow system in this rock.

:elative permeability to oil at irreducible water saturationis $556 or $, and as water

saturation increases, #rodecreases until it effectively reaches zero at some high

water saturation corresponding to %or, the residual oil saturation.

:elative permeability to water, on the other hand, commences effectively at zerowhen the rock is at irreducible water saturation 4wi, and thereafter increases as %w

increases. t should also be noted that in an oil!wet system, #rois always less at a

given %wthan in a water!wet system. 7onversely, #rwis always greater in an oil!wet

system than in a water!wet one.

2any workers in this field have proposed generalized empirical e)uations to relate

#ro and #rw to %w, %wi, and %or 3f particular note are those cited in =onarpour,

>oederitz, and =arvey "$


10/22

Structural Position

f a well is completed abovethe transition zone where the reservoir is at irreducible

water saturation "#rw 5&, then water will not be produced.

Ca-i##ary Pressure

n everyday experience, water levels in two or more connected containers have the

same level if exposed to the same atmospheric conditions. /ut when it comes to

spaces of capillary size "like those we encounter in porous media&, we cannot take

this rule so literally. 'o illustrate, consider what happens when a tube of capillary size

is dipped in a larger container filled with water "#igure I& 'he water in the capillary

tube rises above the water level in the container to a height that depends on

capillary size.


11/22

Figure .

Although strictly speaking, the water still finds its level, it does so in such a way as to

maintain an overall minimum surface energy.

n this situation, the adhesion force allows water to rise up in the capillary tube while

gravity acts in the opposite direction. 'he water rises until there is a balance

between these two opposing forces. 'he differential force between adhesion and

gravity is the capillary force. 'his force per unit area is the capillary pressure.

7apillary pressure is defined as the pressure difference between two fluid phases"e.g., oil and water& at the same point in the reservoir. t is a measure of the rock!

fluid adhesion and fluid!fluid interfacial tension forces that act to hold one fluid phase

"e.g., water& at a particular location in the reservoir "e.g., above the oil!water contact&

against the force of gravity. 7apillary pressure is a complex function of the nature of


12/22

the contained fluids, the saturation of the fluid phases, the wettability of the rock, and

the pore size distribution of the rock.

As we might surmise from observations of the capillary tube illustrated in the #igure

above, there is a relationship between capillary pressure, (c , and the interfacialtension between the two fluids "in this example !water and air&.

where

(c capillary pressure

wn wettingnon!wetting phase interfacial tension

r radius of the tube

angle of contact between the solid surface and li)uid

CAPILLARYFLUIDRISE

An alternative way to express capillary pressure is in terms of height above a free

water surface. 7apillary pressure is e)ual to the product of the height above the free

water surface and the density difference between the two fluid phases at reservoir

conditions. n a reservoir, the relationship between water saturation and height

above an oil!water or gas!water contact is, of course, strongly dependent on the rock

pore system, as well as on the wettability and interfacial tension properties of the

rock!fluid system.

We will again use #igure Iof the capillary tube to illustrate fluid height. 'he capillary

tube of radius rwill support a column of water of height h. f the density of the air is

aand the density of the water is w, then the pressure differential at the air!water

contact is simply &w -a&h. 'his pressure differential acting across the cross!sectional

area of the capillary is exactly counterbalanced by the surface tension, T, of the

water film acting around the inner circumference of the capillary tube. f the contact

angle is at the interface between the water and the glass face of the capillary tube,

then at e)uilibrium we have%


13/22

Fr' cos "w!a&h . rF

#orce (ressure . Area

/y simplifying and rearranging this expression we see that height is expressed as%

As the capillary tube radius "r & decreases, the height "h& of the water column

increases- therefore, the height of fluid rise above a free water surface in a capillary

tube is inversely proportional to the radius of the tube. We can draw an analogy

between the capillary tube radius and the radii of pore throats in the rock matrix. n

the above example, we can correlate the air to oil, water with water, and the tube

with pore throats.

'ranslating this laboratory observation in terms of reservoir conditions, we can see

that water can be drawn up into what would otherwise be a $556 oil column by the

capillary effect resulting from small pores in the rock system. 'hus the maximum

height, h, to which water can be raised is controlled by the following factors%

the surface tension, ', between the two phases "oil and water&

the contact angle, , between the wetting fluid "water& and the rock

the radius of the pore throats "r&

the density difference between phases "w!oin this case&

n summary, the capillary rise will be greater in a rock with smaller pore throats than

in one with large pore throats.

Legt/ o% Trasitio 0oe

;iven the above factors, it is easy to characterize the relative length of a transition

zone in a reservoir. :eservoirs with large pore throats and high permeability have

short transition zones, and the transition zone at a gas!oil contact will be shorter

than that at an oil!water contact simply because of the inter!phase density


14/22

differences involved "#igure 0 &.

Figure

4ince a pore system is made up of a variety of pore sizes and shapes, no single

pore throat radius can be assigned to a reservoir. Cepending on the size and

distribution of the pore throats, certain available pore channels will raise water above

the free!water level. 'he water saturation above the top of the transition zone will

thus be a function of porosity and pore!size distribution.

n a water!wet system, the water will wet the surface of each grain or will line the

walls of the capillary tubes. At the time oil migrates into the reservoir, the capillary

pressure effects will be such that the downward progress of oil in the reservoir is

most strongly resisted in the smallest capillaries. A particular elevation will limit the


15/22

amount of oil that can be expected to fill the pores. Darge!diameter pores offer little

resistance "capillary pressure, 'c, is low because pore radius, r is large&. 4mall!

diameter pores offer greater resistance "'cis high because ris small&. #or a given

reservoir, o and wdetermine the pressure differential that an oil!water meniscus

can support.

Ca-i##ary Pressure a$ Irre$u&i"#e Water Saturatio

2aximum oil saturation is controlled by the relative number of small and large

capillaries or pore throats. 'his maximum possible oil saturation, if expressed in

terms of water saturation, translates into a minimum possible water saturation, and

this is referred to as the irreducible water saturation %wi.

4haly, silty, low!permeability rocks with their attendant small pore throats and high

capillary pressures, tend to have very high irreducible water saturations. Just the

opposite is true for clean sands of high permeability, which have low irreducible

water saturations. #igure Gillustrates this important concept by comparing capillary

pressure curves for four rock systems of different porosity and permeability.


16/22

Figure 2

Stru&tura# Positio )it/i t/e Reser,oir

We all know that gravity segregation causes a natural stratification of reservoir fluids,

with gas on top of oil, and oil over water. n the absence of rock pores, the gas, oil,

and water will form distinct layers, with sharp contacts between each phase. n a

reservoir, however, the contacts between each phase are less distinct, as illustrated

in the #igure 9% (eservoir containing oil and water .


17/22

Figure 3

'his diagram divides the reservoir into three levels. 'he upper level is mainly oil- the

lower level is all water, while the middle level shows ever!increasing concentrations

of water as depth increases. (lotted on the right!hand side is a curve of water

saturation, together with a plot of fluid pressure in the pore spaces.

When a formation is abovethe transition zone, i.e., at irreducible water saturation,

the product of and 4wis a constant. +ariations of porosity are normal on a local

scale, caused both by changes in the depositional environment and by subse)uentdiagenesis. f porosity is reduced locally, then either a greater proportion of pore

throats will be small or there will be simply fewer pore throats. ither way, the mean

radius of the pore throat rwill be smaller- thus 'cwill be larger, and more water can

be held in the pore maintaining the constant%


18/22

. 4wi

After a zone has been analyzed on a foot!by!foot basis for porosity and water

saturation, a plot of versus 4wreveals the presence or absence of a transition

zone.

#igure ? shows a log!log plot, where points at irreducible saturation plot along a

straight line "the red line denoting Kero Water (roduction in this graphic&, and the

points in the transition zone plot to the right of the irreducible line.

Figure 4

/y plotting the product of . 4wi, it is possible to predict certain production

characteristics. #or points below irreducible saturation, some portion of water
http://www.ihrdc.com/els/ipims-demo/t26/offline_IPIMS_s23560/resources/i/G4108F08.jpghttp://www.ihrdc.com/els/ipims-demo/t26/offline_IPIMS_s23560/resources/i/G4108F08.jpghttp://www.ihrdc.com/els/ipims-demo/t26/offline_IPIMS_s23560/resources/i/G4108F08.jpg


19/22

production is to be expected, depending on the mobility ratio, "k w1oko1w&, for the

particular fluids present. 3n the other hand, in a low-porosity low-permeability

formation, we again see that surprisingly high water saturations can be tolerated

without fear of water production. 7onversely, in other formations that exhibit good

porosity and permeability, even moderate values of 4w will mean that water

production should be expected.

Again, since both capillary pressure and relative permeability data are a strong

function of pore size distribution and geometry "among other factors&, they will, in

turn, often fall into groups that correlate with specific reservoir facies "2organ and

;ordon $


20/22

Figure 5

'he difference in the measured relative permeabilities illustrates the importance of

pore system configuration in determining fluid flow characteristics.

3ne important observation to be derived from evaluating capillary pressure and

relative permeability for individual geologic facies is the determination of the

producing oil!water "or gas!water& contact elevation throughout various parts of a

reservoir. 'his elevation can vary across a reservoir, and can be substantially

different from the free water surface elevation. 'he free water surface should be at a

constant elevation throughout the reservoir, providing the reservoir is in a state of

gravity!capillary e)uilibrium with no significant flow occurring. 'he producing oil!

water contact is often taken as the highest elevation where $556 water is produced.

T/is $e-t/ )i## ot e&essari#y "e t/e sa'e at a## -oits i t/e reser,oir6 e,e

u$er e7ui#i"riu' &o$itios8 It )i## strog#y $e-e$ o #o&a# &a-i##ary


21/22

-ressure 9)ater saturatio ,ersus /eig/t a"o,e t/e %ree )ater sur%a&e: a$ t/e

re#ati,e -er'ea"i#ity &/ara&teristi&s o% t/e ro&;8

Ca-i##ary Pressure a$ !eo#ogi& Fa&ies

A relationship between geologic facies and capillary pressure curves is illustrated

graphically in #igure


22/22

Puttig It A## Toget/er ( !eo#ogy a$ F#ui$s

We have just reviewed a number of factors which influence the rreducible Water

4aturation within a formation. 'hese factors include interactions between fluidsand

roc#, as well as interactions between different fluids. /y examining porosity, relative

permeability, and capillary pressure relationships, along with rock texture and

structural position, it is possible to determine whether a well having high 4w

calculations will actually produce water, or instead, will produce water!free.

#or example, as we move toward the top of a fining!upward se)uence, the decrease

in sand grain diameter will produce a corresponding decrease in pore throat radius.

'his decrease in pore throat radius is accompanied by an increase in capillary

pressure, thus increasing the amount of water that can be imbibed into the system. f

we add clay or silt to this example, we can expect that microporosity will constitute a

substantial percentage of total porosity. 4uch a setting is bound to produce high

values of irreducible water saturations.

/oth irreducible water and residual hydrocarbon saturations are strongly influenced

by rock texture, which is controlled by depositional environment. #ine!grained

sediments, usually characteristic of low!energy depositional environments, tend to

have high irreducible water with high residual hydrocarbon saturations- coarse!

grained sediments, characteristic of high!energy environments, tend to have low

irreducible water saturation and low residual hydrocarbon saturation. n addition,

fine!grained sediments tend to have lower permeability than coarse!grained

sediments.

'hese factors must all be considered together when analyzing low resistivity pay

zones. (orosity, capillarity, relative permeability, structural position and grain size will

all influence the final evaluation of irreducible water saturation in a low resistivity pay

zone.

Documents

Irreducible Water