solution gas drive

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In The Name Of Allah

Solution gas drive in fractured reservoirs

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Table of contents

Abstract...........................................................................................................1

Introduction.....................................................................................................2

Solution gas drive in low pressure decline rate reservoirs...............................4

Material balance for solution gas drive reservoirs...........................................6

Reservoir description during depletion............................................................7

Reservoir zoning under static conditions......................................................

Reservoir zoning under d!na"ic conditions.................................................

Mus#at and pirson "et$ods for solution gas "ec$anis"..............................1%

Solution gas drive "ec$anis" in fractured reservoirs& co"parison between'aze"i and (arren and Root "odels. )case stud! in one of iranian oil

reservoirs*......................................................................................................11

+iscussion and results................................................................................14

References.....................................................................................................1,

2



List of gures

-igure 1 /$e reservoir gas0oil ratio& R vs. recover! is substantiall! lower in

a fractured reservoir 3

-igure 2 /$e rate of pressure decline per unit of oil produced is nor"all! lowin fractured reservoirs 3

-igure 3 Solution gas drive reservoirabove and below bubble point pressure

6

-igure 4 5e$avior of Reservoir -luid roperties 7

-igure , Reservoir zoning under static conditions

-igure 6 Reservoir zones under d!na"ic conditions

-igure 7 oil recover! in +ualporo and +ualporo0+ualper" "odel 13

-igure ressure drop in +ualporo and +ualporo0+ualper" "odel 13

-igure ressure drop at 8nal period of production in +ualporo and

+ualporo0+ualper" "odel 13

-igure 1% as production in +ualporo and +ualporo0+ualper" "odel 14

-igure 11 as saturation in +ualporo and +ualporo0+ualper" "odel 14

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Abstract [1]

In 9-Rs& it is "ainl! t$e "atri:0fracture ;uid interaction w$ic$ governst$e production

-ro" t$e $e reservoir& t$erefore& t$e production "ec$anis"s are focused on

$ow t$e "atri: is depleted and t$e petroleu" is transferred fro" t$e "atri:to t$e fracture )w$ere it will be transported to t$e producing wells*. /$ei"portance of eac$ production "ec$anis" di<ers fro" a Single0porosit!s!ste"& since t$e interaction of t$e two s!ste"s )"atri: and fractures* $asto be /a#en into account. /$e do"inant forces in t$e two distinct s!ste"s can alsobe =uite di<erent. /$e relative i"portance of t$e di<erent production "ec$anis" is basicall!deter"ined b! t$eS$ape of t$e "atri: bloc#s and b! roc# and ;uid properties of t$e s!ste". Int$e conventional

+ual0porosit! "odel t$e production "ec$anis"s in naturall! fracturedreservoirs are "odelled /o calculate a so0called >transfer ter"? w$ic$ represents t$e "atri:0fracture;uid transfer rate. /$ese "ain production "ec$anis"s are

@ -luid e:pansion and solution gas drive@ I"bibition@ ravit! drainage@ iscous displace"ent@ Molecular di<usion

1



Introduction [2]

As depletion goes on& t$e pressure falls below t$e bubble point

ressure in t$e lowest0pressure regions& t$at is& in t$e upper regions of

t$e reservoir and also close to wellbores. as bubbles nucleate wit$in

t$e oil p$ase. ($en t$e bubble point pressure is reac$ed wit$in t$e

"atri: bloc#s& gas bubbles appear wit$in t$e pore networ# of t$e

"atri:. As long as t$ese bubbles grow w$ile re"aining i""obile& an oil

p$ase so"ew$at i"poveris$ed in gas is e:pelled fro" t$e "atri: bloc#s

and conve!ed to t$e production wells. At t$at precise stage& a decrease

in t$e production as0il Ratio )R* "a! t$en be observed if

signi8cant. Actuall! ver! soon& gas bubbles coalesce and for" a "obile

p$ase& for a "ini"u" value of t$e gas saturation& called t$e critical gas

saturation Sgc. As pointed out b! -iroozabadi et al. )12* #nowledge of

critical gas saturation is i"portant for esti"ating recover! in a solution

gas0drive reservoir. Measured values of t$e critical gas saturation in t$e

literature range fro" 2B to 27B . -or solution gas0drive reservoirs&

and particularl! for fractured petroleu" reservoirs& $ig$er values of

critical gas saturation "ean $ig$er oil recoveries. ($en t$e gas is"obilized& gas drive beco"es a prevailing recover! "ec$anis" of t$e

"atri: oil in t$e oil zone. Cowever& t$e oil0to0gas "obilit! ratio& t$at is

alread! unfavorable because of t$e low gas viscosit!& rapidl! gets worse

because of a rapidl!0increasing gas0to0oil relative per"eabilit! ratio& due

to gas saturation increase. In addition& for reservoirs subDected to

convection p$eno"ena& t$e di<usion of "atri: solution gas to t$e

fractures causes a s$rin#age of t$e oil p$ase wit$in t$e "atri: bloc#s

w$ic$ furt$er +ecreases t$e "atri: oil p$ase saturation and "obilit!.

Eventuall!& t$e oil recover! fro" solution gas drive alone generall!

re"ains low. At t$at ti"e& t$e evolution of well eFuent di<ers according

to t$e reservoir structure and fracture intensit!

2



G in t$e case of $ig$0dip& t$ic# and intensel!0fractured reservoirs suc$ as

Iranian 8elds& t$e gas e:pelled b! t$e "atri: bloc#s can segregate

wit$in t$e fracture networ# and for" a secondar! gas0cap. /$e latterwill e:pand via t$e fracture networ# as production goes on& and initiate

gas0oil gravit! drainage of t$e gas0surrounded "atri: bloc#s. In t$at

situation& t$e production R sta!s at a lower value t$an in

conventional reservoirs& -ig 1 and -ig 2.

G /$e previous situation t!pical of well0fractured reservoirs "a! not

occur in t$e presence of a low0relief reservoir wit$ a poorl!0connected

andHor low0per"eabilit! fracture networ#. Suc$ reservoirs t$en be$ave

li#e conventional ones and rapidl! deliver a $ig$0R eFuent. Suc$ a

production be$avior calls for a strateg! of pressure "aintenance&

denoted as >secondar! recover!?& t$at involves t$e )re*inDection of a

;uid p$ase& eit$er water or gas.

Solution gas drive is generall! regarded as an ine<ective

recover! process for fractured reservoirs& e:cept for $ard0to produce

8elds w$ere ot$er recover! "ec$anis"s& driven b! capillarit! andHor

gravit! forces& are ine<ective. /$is is t$e case for tig$t& viscous and oil0

wet reservoirs w$ere t$e "atri: can neit$er be i"bibed b! water nor

e<ectivel! drained b! gas.

3



igure 1! /$e reservoir gas0oil ratio& R vs. recover! is substantiall! lower in afractured reservoir

igure 2! /$e rate of pressure decline per unit of oil produced is nor"all! low infractured reservoirs

Solution gas drive in lo" #ressure decline rate reservoirs[$]

Botset and Muskat (1939) made a series of solution gas drive experiments by

varying the rate of pressure drop using small size permeable cores and fluid composed of

kerosene and essentially methane !ith different bubble point pressure" #he volume of free

gas saturation occupying the pores !as measured at the end of each experiment !hile the

rate of pressure drop varied from $"% to &$$ psi'min" or the range of 1"% to $"% psi'min

they measured a free gas saturation of about 1$ of the pore volume" *lthough $"% psi'min

corresponds to about +&3$$$ psi'year ho!ever they concluded that pressure drop belo!

$"% psi'min leaves the same percentage of free gas saturation" ,lkins (19%3) reported t!o

similar experiments using -praberry core !ith a porosit! of .1, B and

per"eabilit! of 1.1 "d. /$e sa"ple $ad a water saturation of 2., B in

4



test 9 1 and 13.4 B in test 92 and reservoir oil of about 2%%% psi

bubble point pressure. /$e "easured free gas saturation in t$e core

after it reac$ed at"osp$eric pressure were respectivel! about ,2 and

2 B of t$e pore volu"e w$en /$e pressure was dropped at t$e rate of

2%% psiH"in and 1%% psiHda! in t$e test 91 and 92. /$e results of

t$ese e:peri"ents are considerabl! di<erent fro" t$ose "easured b!

t$e previous aut$ors. /$ere is a ratio of about 7 between t$e 1%%

psiHda! case )%.%7 psiH"in* and t$at e:trapolated b! t$e previous

aut$ors. /$e t$eor! developed b! /arner )144* and Mus#at )14,* for

calculating free gas saturation b! solution gas drive process& is based on

co"plete e=uilibriu" conditions between gas and oil and t$eassu"ption t$at t$e laborator! "easured relative per"eabilities are

co"parable to t$ose ta#ing place in t$e reservoir. It was due to t$e

above unrealistic e:peri"ental wor#& wit$ suc$ wide divergence& and

t$e concept of solution gas drive "ec$anis" t$at was "asterl!

developed b! /arner and Mus#at t$at "ade t$e petroleu" industr! sta!

asleep for so long& to t$e e:tent t$at even t$e alar"ing wor# of +u"ore

)17%* did not wa#e up t$e industr!. Jonse=uentl! t$e "et$od of

"easuring gas0oil relative per"eabilit! also su<ered. /$at is& it was

assu"ed t$at t$e concept of t$e above solution gas drive "ec$anis"

$ad a well0establis$ed basis and for e:a"ple t$e laborator! "easured

critical gas saturation of 1% B is a fairl! t!pical one. /$eir concept and

laborator! e:peri"ents "a! be correct under ver! special conditions

but t$is is often not t$e case. In fact t$e oil recovered fro" Spraberr!

8eld wit$ a "a:i"u" rate of pressure drop of about 1 psiHda! was not

all b! solution gas drive. As could be noted& w$en a well $ad been s$ut0

in for so"e da!s& t$e R and I of t$at well i"proved considerabl!

w$en it was put bac# into production. It can be said t$at in "ost of

reservoirs produced under natural depletion& w$en t$e reservoir

,



pressure dropped "oderatel! larger gas volu"e and "ore dispersed gas

distribution were calculated t$an e:isted in t$e Reservoirs. In t$e /arner

and Mus#at "et$od of calculating free gas saturation due to t$e solution

gas drive process& t$e "icroscopic e<ects of t$e rate of pressure drop&

di<usion& interfacial tension between gas and oil& and t$e degree of

in$o"ogeneit! of reservoir roc# on t$is process were co"pletel!

ignored. /$erefore& it was assu"ed t$at free gas saturation developed

b! t$e solution gas drive "ec$anis" was independent of t$e

para"eters "entioned above. In fact& t$is is true onl! w$en dHdt is

large& e.g. over a few $undred psiH!ear.

/$e classic solution gas drive "et$od for calculating free gassaturation in reservoirs& developed b! Mus#at and /arner& lac#s two

i"portant ele"ents. /$ese are na"el!

a. /$e separation of gas fro" solution ta#es place in an in8nitesi"all!

s"all volu"e )volu"e of a pore*& w$ic$ is in8nitel! s"aller t$an a /

cell. /$erefore& t$e evolution of gas fro" solution follows a certain

p$!sical law. In t$e / cells t$is dependenc! can be ignored.

b. /$e oil and its solution gas in t$e pores are under al"ost =uiescent

conditions&

unli#e t$e / cell in t$e lab.

Knder reservoir conditions& t$ese two ele"ents "ainl! e<ect t$e

interfacial tension between oil and t$e supersaturation pressure& in

w$ic$ t$e latter is related to t$e rate of pressure drop. /$ese can give a

co"pletel! di<erent picture fro" w$at is usuall! #nown as t$e solution

gas drive "ec$anis". As indicated above& w$en t$e rate of pressure

drop is low t$e solution gas drive "ec$anis" can result in a S g of a few

per cent& even in a reservoir wit$ $ig$ bubble point pressure. Suc$ a

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volu"e of gas saturation can be far below t$e oil s$rin#age in t$at

reservoir and t$us& a considerable volu"e of oil will be lost in t$at part

of t$e reservoir& rat$er t$an oil recover! b! solution gas drive

"ec$anis" as could be e:pected.

%aterial balance for solution gas drive reservoirs [4]

-olution gas drive reservoirs are assumed to be volumetric due to the absence of

!ater influx and gas caps" .n determining the material balance for this type of drive

mechanism t!o phases can be distinguished as sho!n in igure +"+ (a) !hen the reservoir

oil is under saturated and (b) !hen the pressure is fallen belo! the bubble point and a free

gas phase exists in the reservoir (/ake 190)"

igure $! Solution gas drive reservoir )a* above bubble point pressure li=uid oil )b*5elow bubble point oil plus liberated solution gas

Below bubble point pressure (saturated oil): -aturated reservoir is one

that originally ,xists at its bubble point pressure" 2nce the pressure falls belo! the bubble

point solution as is liberated from the oil leading in many cases to a chaotic and largely

uncontrollable situation in the reservoir !hich is the characteristic of !hat is referred to as

the solution gas drive process" *ssuming that the !ater and rock expansion term , f! 4 $ or

negligible in comparison !ith the expansion of solution gas the general MB, may be

expressed by5

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N = NpBo+(Gp− NpRs )Bg(Bo−Boi)+( Rsi− Rs)Bg

*s pressure decline rate is lo!er in fractured reservoirs for same 6 p Bg and Bo !ill beestimated higher and because segregation is better due to fractures p is lo!er than

conventional reservoirs" #herefore applying the conventional methods for simple

expansion and solution gas7drive of a fractured reservoir gives high 22.8 or an extremely

efficient !ater7drive for history match"

igure &! 5e$avior of Reservoir -luid roperties

'eservoir descri#tion during de#letion [5]

racture net!ork is divided into a number of zones each of them practically

saturated !ith only one phase !hile inside each zone the matrix block may be saturated



!ith one t!o or even three phases" * given reservoir zonation !ill already exist before

reservoir production begins (under static euilibrium) and another zonation !ill result from

reservoir production conditions during field exploitation (dynamic state)"

'eservoir (oning /ivision of a reservoir into zones depends essentially on the fractured net!ork

saturation" -ub7zones may also develop during the production of the reservoir as a result

of fluid euilibrium inside the matrix block as !ell as fluid exchange bet!een matrix and

fracture" #he extension of zones and subzones is continuously changing during reservoir

production"

Reservoir zoning under static conditions:

igure )! Reservoir zoning under static conditions

Reservoir zones under dynamic conditions:

Main zones:

0 (ater0invaded zone between (L and (L

0 as0invaded zone between L and L

0 il zone between (L and L

Sub0zones:

As a result of pressure variation wit$ dept$ t$e oil zone "a! be



+ivided into two additional zones

0 /$e gassing zone& between L and s 5pp

0 /$e under saturated zone& between s 5pp and (L

igure *! Reservoir zones under d!na"ic conditions

a. as0invaded zone: ravitational drainage displace"ent

b. assing zone: Liberated gas e:pansion N buo!anc! N i"bibition N

convection "ec$anis"s

/wo sub0zones "a! be developed as a result of critical gas saturation vs.

e<ective gas saturation in t$e "atri:. /$e circulation of liberated gas in

fractures saturated wit$ oil& as well as t$e contact between t$e $eavier

oil of fractures wit$ t$e lig$ter oil re"aining in t$e "atri:& develops

"ore co"ple: transfer processes. In t$e case of non0unifor" distribution

of "atri: pores and low decline rate of reservoir pressure&

supersaturation pressure p$eno"ena can occur.

c. Kndersaturated zone: Si"ple e:pansion drive "ec$anis" t$e

e:pansion drive "ec$anis" will be bigger if t$e co"pressibilit! and

pressure decline rate are $ig$er w$ile production rate increases if bloc#

di"ensions are s"aller.

1%



d. (ater0invaded zone: ravitational N capillar! i"bibition

il recovered fro" t$e "atri: pores as a result of progressive E:posure

of t$e "atri: pores to a water environ"ent in t$e fractures& is rate

sensitive to rate of water table advance"ent

0 If t$e a=uifer is li"ited or non0e:istent& t$e oil will be produced as a

result of t$e e:pansion of gas liberated fro" solution in t$e oil zone& and

of gravit! drainage in t$e gas0invaded zone.

%us+at and ,irson methods for solution gas mechanism [1]

As t$e pressure drops in t$e fracture s!ste"& as a conse=uence

of production& ;uid e:pands and ;ows out fro" t$e "atri: to e=uilibrate

t$e "atri: pressure wit$ t$e surrounding fracture pressure. Also t$e

co"pressibilit! of roc# or t$e secondar! co"paction can gain

i"portance speciall! low ;uid co"pressibilit! or low porosit!. /$e pore

co"pressibilit! for "atri: and -racture is in "ost of t$e cases =uite

di<erent.

5elow t$e bubble point pressure t$e solution gas liberates and

e:pands. /$e eOcienc! of t$e "atri: recover! under solution gas

displace"ent can be calculated in a standalone wa!& using t$e well0

#nown Mus#at or irson "et$ods. According to irson& t$e e=uations t$at

describe t$e perfor"ance of t$e reservoir in case of solution gas drive

reservoir can be written in 8nite di<erence for" of t$e "aterial balance

∆ [ Np N ]=[1− Np

N ]∆ [ BoBg− Rs]− [1+mg,o ]Bob ∆ 1

Bg

[Bo

Bg− Rs]i+1+ R

/$e instantaneous gas0oil ratio is

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R= Rs+Bo

Bg . Krg

Kro . μo

μg

/$e oil saturation is

So=[1− Np N ] . BoBob(1−Sw)

Mus#at used also a di<erential for" of t$e "aterial balance e=uation to

evaluate t$e

erfor"ance of a reservoir wit$ internal gas0drive )solution gas drive*.

/$e c$ange in t$e oil Saturation is

So λ ( p )+So Kg

Ko η ( p )+(1−Sw−So)ε ( p)

ω( p , So)∆So=∆ p¿

($ere λ ( p )η ( p ) ε ( p) and ω ( p , So ) are special Mus#at functions

λ ( p )=BgBo

dRsdp

ε ( p )=Bg d

dp( 1

Bg)

η ( p )= 1

Bo

μo

μg

dBo

dp

ω ( p, So )=1+ Kg Ko

μo μg

/$e cu"ulative a"ount of produced oil is t$en

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Np=Vp∑ Pi

P

∆( SoBo )=Vp[( SoBo )i−( SoBo )]

Solution gas drive mechanism in fractured reservoirs-com#arison bet"een .a(emi and /arren and 'oot models0ase Stud3 in one of Iranian oil reservoirs4[*]

In fractured reservoirs ;uid is ;owing between two di<erent

"ediu"s& "atri: and fracture t$at is "ore co"plicated t$an

conventional reservoirs. /o predict t$e future of fractured reservoirs we

need to "odel t$e".

+espite conventional reservoirs t$at we need to #now onl!

reservoir data for "odeling& in si"ulation of fractured reservoirs weneed to #now also t$e "odel t$at ;uid is ;owing in t$e "ediu"s. /wo

#nown "odels for fractured reservoirs are 'aze"i and (arren and root.

EJLISE uses two "odels of +ualporo )dual porosit!* and +ualporo0

+ualper" )dual porosit!& dual per"eabilit!* t$at +ualporo is (arren and

root and +ualporo0+ualper" is 'aze"i "odel.

In t$is case stud! oil recover! and pressure drop rate is co"pared for

t$ese two "odels in one of Iranian fractured reservoirs t$at was ;owing

under solution gas drive "ec$anis". /$e results s$ow dual porosit!

"odel gives 3B "ore recover! and lower pressure drop for longer

periods of production. /$e reservoir data are

9o. of bloc#s )P&Q&* 3%:3%:

il viscosit! )cp* 1.,4

Reservoir t$ic#ness

)ft.*

4%% il for"ation volu"e factor

)bblHstb*

1.,6

Matri: porosit! 2,B Solution gas ratio )scfHstb* 1%41-racture porosit! %.,B 5ubble pressure )psi* 4%42Matric per"eabilit!

):&!&z*&"d

%.1 Matri:0fracture transfer

s$ape factor )*

%.%%

4-racture per"eabilit!

):&!&z*&"d

13%% il densit! )pcf* ,4

I of t$e "odel 1%42%% (ater densit!)pcf* 63.%

13



)stb* %% 2oil gravit! )AI* 32 as densit!)pcf* %.%7

%2Lowest wf )psia* 2%%% Roc# co"pressibilit! at

36%% psi )1Hpsi*

4:1%

06

ressure at %%%ft

)psia*

,,%% Scg %.%3

EJLISE is used for t$is stud! and a producing well is placed on bloc#

1,:1, )P& Q* t$at was perforated on all la!ers. (ell was ;owing at wf of

1%% psi and Rs of 1%41 scfHstb. After 7%2% da!s of production& t$e oil

recover! is 8:ed and "odeling is done till t$is date.

-ig07 s$ows oil recover! wit$ dualporo "odel is about 2B t$atgives 3B "ore recover! t$an dualporo0+ualper" "odel. +ualporo

"odel produces onl! t$roug$ fracture despite +ualporo0+ualper" "odel

t$at produces t$roug$ bot$ fracture and "atri:. So in earl! ti"es& oil

recover! in +ualporo is lower t$an +ualporo0+ualper" but as "atri:

per"eabilit! is ver! low& t$e di<erences is not signi8cant. In t$e 8rst

!ear of production wit$ dualporo0+ualper" "odel& t$e oil recover! was

47317 stb and 473%, stb for +ualporo "odel. In -ig0 and -ig0

pressure drop is s$own. At earl! ti"es of production till bubble pressure&

t$at no solution gas was liberated& t$e pressure drop is fast and after

t$at due to liberation of solution gas t$e rate decreased. In +ualporo0

+ualper" "odel per"eabilit! is $ig$er t$at results $ig$er gas

production )-ig01%* and lower gas saturation )-ig011* but as +ualporo

"odel $as a lower per"eabilit! t$e gas production is lower and it will

give $ig$er reservoir pressure )lower pressure drop* and $ig$er oil

recover!.

14



igure 5! oil recover! in +ualporo and +ualporo0+ualper" "odel

igure 6! ressure drop in +ualporo and +ualporo0+ualper" "odel

igure 7! ressure drop at 8nal period of production in +ualporo and +ualporo0+ualper" "odel

1,



igure 18! as production in +ualporo and +ualporo0+ualper" "odel

igure 11! as saturation in +ualporo and +ualporo0+ualper" "odel

9iscussion and results

In +ualporo0+ualper" due to $ig$er per"eabilit!& gas can "ove

better even in low per"eabilit! "atri:. So gas production is $ig$er

and gas saturation is lower.

5ecause "ore gas will re"ain in +ualporo "odel& reservoir

pressure will be #ept $ig$er.

16



In earl! ti"es of production wit$ +ual0poro& recover! is lower t$an

+ualporo0+ualper" but wit$ "ore production as reservoir

pressure is $ig$er in +ual0poro& its 8nal recover! is also $ig$er.

'eferences

T1U Mo$a""ad /ag$i A"ir!& -ebruar! 2%14. Modeling ;ow be$avior innaturall! fractured reservoirs.

T2U . Le"onnier and 5. 5ourbiau: oil V gas science and tec$nolog!& I-&vol. 6,& 2%1%. Si"ulation of naturall! fractured reservoirs& p$!sical"ec$anis"s and si"ulator for"ulation.

T3U Ali M.Saidi 16. Reservoir engineering of fractured reservoirs.

T4U +orcas 'ari#ari& +ece"ber 2%1%. (ell perfor"ance in solution gasdrive reservoirs.

T,U IE0K/& funda"entals of fractured reservoir engineering. roduction"ec$anis" of a fractured reservoir.

T6U Ma$"ood S$a#iba& Masood Riazi& Aban 132. Solution gas drive"ec$anis" in fractured reservoirs& co"parison between #aze"iand warren and root "odels.

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solution gas drive

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