1

Enhanced Oil Recovery with CO2 Injection

Wei Yan and Erling H. StenbyDepartment of Chemical EngineeringTechnical University of Denmark

Contents

Overview

Mechanism of miscibility

Experimental study of gas injection

MMP calculation

Summary

2

Recovery methods

Primary recovery—by depletionSecondary recovery—by water/gas injection for pressure maintenanceTertiary recovery—after primary and secondary

Enhanced Oil Recovery (EOR): “something other than plain water or brine is being injected into the reservoir” (Taber et al., SPE 35385)

EOR methods

A summary by Taber et al.More than 20 methods

3

Trends in EOR with CO2EOR production in the US

The percentage of EOR projects continues to increaseCO2 injection is the only method that has had a continuous increase

CO2 vs. other gases

Supercritical extraction at reservoir conditions

Easier miscibility than N2, flue gas, C1

Cheaper than liquid hydrocarbons

Safer to handle and pressurize than hydrocarbon gases

Reduction of GHG

4

CO2 sequestration + EOR

The biggest barrier for CO2 sequestrationCO2 sequestration cost: 40-60 $/tonCO2 credit: 1-20 $/ton CO2 (?)

EOR can offset the cost and even make it profitableCO2 injected/extra oil produced (mass): 1:1 to 4:1

3:1 is “carbon neutral”Net CO2 storage ratio: 0.17-0.78 tons/barrel oil

CO2 sequestration + EORMaximum permissible cost of carbon dioxide in $/Mscffor the North Sea (Blunt et al., 1993.)

2.50/1.43/0.941.50/0.86/0.570.50/0.29/0.193.7104.50/2.73/1.702.83/1.62/1.071.17/0.67/0.442.269.50/5.43/3.586.17/3.52/2.332.83/1.62/1.07*1.13

302010Mass ratioVolume ratio(Mscf/barrel)

Oil price ($/barrel)Displacement efficiency(CO2/extra oil)

* The three numbers indicate the maximum price for rates of return r = 0/0.1/0.2

A carbon dioxide displacement would be profitable at a 10% rate of return at a gas price of over $3/Mscf (56$/ton).

5

Mechanisms of gas injections

Swelling of the oil phase Lowering of oil viscosityReduction of interfacial tension

Misciblility (no interfacial tension for miscible displacements)

Pseudo ternary system for petroleum mixtures

Three components:Light: C1, CO2, N2Intermediate: C2-C6Heavy: C7+

Useful to illustrate basic concepts

Cannot explain combined mechanism

0.00 0.25 0.50 0.75 1.00

Two phase region

C

(0.20,0.55,0.25)

Single phase region

BA3 1

21.00

0.75

0.50

0.25

0.001.00

0.75

0.50

0.25

0.00

Critical tie line

6

First contact miscibility (FCM)

FCMSingle phase at any proportion

Minimum Miscibility Pressure (MMP)

Fix Comp., change PFCM pressure (FCMP)

Minimum Miscibility Enrichment (MME)

Fix P, change Comp.

P'>P

P

Gas A"

Gas A'

Gas A

Oil B

dilution line

3 1

2

FCMP and swelling test

Experimental/modeling determination of FCMP Easy to perform and provide basic information about gas injection

200

250

300

350

400

450

500

550

600

0.00 0.20 0.40 0.60 0.80 1.00Fraction of Gas

Psat

(atm

)

FCMP

Oil Gas

7

Multicontact miscibility

Gas and oil become miscible by multiple contacts, through which (one or both of) their compositions are changed.

Easier than FCMFor 1D gas injection, 100% recovery if MCMIn reality, >90% recovery for swept areaThree mechanisms

VaporizingCondensing (No such thing in a real reservoir)Combined (Zick, 1986)

Vaporizing mechanismIntermediate components “vaporize” to gas

Miscibility achieved in the displacement front/far from the well

Dry gas/oil with sufficient intermediate components

G2

3 1

2

critical tie line

C

G1

Gas

Oil

System C1/C4/C10 just above MMP

8

Vaporizing mechanism

Study using slimtube simulation

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

Gas

Liquid

Gas

sat

urat

ion

ln K

iD

ensi

ty (k

g/m

3 )

Dimensionless distance

-3-2-101

n-Decane

n-Butane

Methane

0.00.20.40.60.81.0

Gas/oil region

Oil regionGas region

Condensing mechanismIntermediate components “condense” to oil

Miscibility achieved in the displacement rear/near from the well

Heavy oil/enriched gas (with sufficient intermediate components)

System C1/C4/C10 just above MMP

C

critical tie line

O2O1

Oil

Gas

3 1

2

9

Condensing mechanism

Study using slimtube simulation

0

200

400

600Liquid

Gas

Den

sity

(kg/

m3 )

Dimensionless distance

-3-2-101

n-Decane

n-Butane

Methane

0.00.20.40.60.81.0 Gas/oil

region

Oil regionGas region

ln K

iG

as s

atur

atio

n

Condensing mechanism ?Now it is believed that there is no such mechanism in a real reservoir.

Reason: the multicomponent system (reservoir fluid) contains both light intermediate and heavy intermediate. Gas tends to extract heavy intermediate, leaving the oil saturated with light and light intermediates, which are hard to be miscible with the gas.

The exchange of components is “two-way”, both vaporizing/condensing can happen. This leads to the combined mechanism.

10

Combined mechanism15 comp. (N2, C1, CO2, C2, C3, iC4, nC4, iC5, nC5, C6 and 5 C7+ comps).

0.0 0.2 0.4 0.6 0.8 1.00

200400600800

1000

Gas

sat

urat

ion

ln K

i

Near miscible zone

Vaporizing segment Condensing segment

Gas

Den

sity

(kg/

m3 )

Dimensionless distance

-8-6-4-202

0.00.20.40.60.81.0

Gas region

Gas/oil regionOil region

Liquid

Experimental study

Swelling testEasy to perform

Forward- and backward-contact

Slimtube experiment

Rising bubble apparatus

11

Forward contact

Simulate vaporizing processProvide phase and volumetric data for the processMiscibility can be achieved if P>MMP

Oil

GasOil1

Gas1

Removed

…Gas1

Oil

Backward contact

Simulate condensing process

Injection gas

Oil

Injection gas

Oil1

Gas1

Oil1

Removed

…

12

Slimtube experimentPhysically simulates gas injection into a 1D reservoir

Standard method to determine MMP

1.2 Pore Volume Injection (PVI) at different pressures

Recoveries measured

Time consuming

Slimtube experiment

MMP is determined as the pressure corresponding to the break point

55

60

65

70

75

80

85

280 300 320 340 360 380 400 420

Pressure (atm)

Rec

over

y %

MMP

13

Rising bubble apparatusQuick but only for vaporizing mechanism

Pressure Gauge

Windowed PressureVessel

“Flat” GlassTube

Gas Bubble

Needle

Air Bath

OIL

GAS

PUMP

MMP calculation methodEmpirical correlations

Limiting tieline method

Single cell simulation

Slimtube simulation (multicell/cell-to-cell simulation)

Global approach by key tieline identification (semi-analytical method based on intersecting tie lines)

14

Experimental correlations

Many suggestions found in the literature

Expressed, e.g., as functions of pseudo critical properties of gas, specific gravity of gas…

Easy to use, fast predictions

Accurate for “reference” system

Inaccurate for other systems

Limiting tie line method“Negative” flash to find the P when the injection tie line or the initial tie line become “critical”

Fast, but without stability analysis

only for pure vaporizing /condensing

C1

C2-C6

C7+

•

•Gas

Oil•

Critical point

Initial tie-line

Injection tie-line

15

Single cell simulation

Jensen and Michelsen, 1990Correponding to forward/backward contact (vaporizing/condensing mechanisms)

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80 100 120 140 160

Number of contacts

LInitial tie-line

Onecell simulation

∑=

−=nc

iii yxL

1

22P < MMP

Multicell (slimtube) simulation

Multicell (cell-to-cell) simulation—physical description

Slimtube simulation—mathematical description

Cell 1 Cell 2 Cell n

Injection gas Production

Batch i

( )nki

nki

nki

nki FF

ztCC 1,,,

1, −+ −

∆∆

−=n = time stepk = grid block

iC

iFOverall molar composition

Overall molar flux

16

Assumptions in slimtube simulation

The porous medium is homogenous and incompressible

Instantaneous thermodynamic equilibrium

Small pressure gradient compared to total pressure

Capillary forces and gravity are neglected

The flow is isothermal and linear

Mass transfer by diffusion/dispersion is neglected

Slimtube (multicell) simulation

Directly simulate slimtube experimentGive correct MMP

Time consumingNumerical dispersion if grids are too few

Simulation time proportional to Ngrid2

Extrapolation to infinity Ngrid needed, for example, determine RF∞(P) by plotting RF(P) vs. 1/sqrt(Ngrid) and extrapolating to zero.

17

Slimtube simulation (example)

-12

-10

-8

-6

-4

-2

0

2

4

0 100 200 300 400 500

Grid number

ln (K

)

0

0.2

0.4

0.6

0.8

1

1.2

Vap

or m

olef

ract

ion

Recovery curves from slimtubesimulations (numerical dispersion)

0

0.2

0.4

0.6

0.8

1

1.2

100 150 200 250 300 350Pressure (atm)

RF

at 1

.2 P

VI

FD (100 grid blocks, 1200 time steps)FD (500 grid blocks, 6000 time steps)FD (5000 grid blocks, 60000 time steps)

18

A MMP calculation method is needed

Can correctly account for the injection mechanismWrong mechanism leads to overestimation

FastUnlike slimtube

No numerical artifacts like numerical dispersion

Global approach by key tielineidentification

Fast, semi-analtyical based on intersecting key tielines

Based on the analysis of 1D multicomponent two-phase dispersion free flow using the Method Of Characteristics (MOC)

0=∂∂

+∂∂

xF

tC ii nci ,..,1=

19

Main results from the analysis (I)In the composition space, the analytical solution forms a composition path starting from the injection gas composition to the initial oil composition.

The composition path must travel through a sequence of key tielines.

For a nc component system, there are nc-1 key tielines, including

The initial tie line and the injection tie linenc-3 crossover tielines

Main results from the analysis (II)At MMP, one of the key tie lines become critical

vaporizing and condensing mechanisms are special cases when the initial key tie line and the injection key tie line become critical

The composition path can have discontinuities known as shocks. When the path consists ONLY of shocks (the usual case), the key tie lines will intersect pairwise.

For other situations (solution consisting of not only shocks but also rarefactions), intersection of key tielinesis a good approximation

20

Illustration of the concepts

C10

Initial oilInitial tie line

CH4

Injection tie line Crossover tie line

CO2 Injection gas

C4

Solution path

Semi-analytical 1D Solutions

T,P fixed

nc-1 key tie linesS

z0 1

Details: find intersection key tielines

CO2

C4

C10

True point of intersection

Wang and Orr (1997)

Jessen et al. (1998)

Tie-line extending through injected Gas

Tie-line extending through initial Oil

OilGas

Critical point

21

Details: mathematical models (I) Intersection equations

Isofugacity criterion

⎩⎨⎧

−==

=−1,1

,1 , 0ˆˆ

ncjnci

yx vi

ji

li

ji ϕϕ

( ) 0)1(1 21

212121 =−−−+− +

−−+

jj

ijj

ijj

ijj

i yxyx αααα

1,1 −= nci 2,1 −= ncj

1,1 0)1(

0)1( 11

11

−=⎭⎬⎫

=−−−=−−−

−− nciyxzyxz

injnciInj

nci

Inji

OiliOiliOili

ββββ

Specification of Initial and Injection composition

Details: mathematical models (II)

∑=

−==−nc

i

ji

ji ncjyx

1

1,1 , 0

Summation of mole fractions

Total number of equations

)1(2 2 −= ncNequations

Newton-Raphson iteration scheme.

0 =+∆ FJ

22

Details: structure of Jacobianmatrix (nc=4)

X . . . X . . . X . . . . . . . . . . . . . . . . . . . . .. X . . . X . . X . . . . . . . . . . . . . . . . . . . . .. . X . . . X . X . . . . . . . . . . . . . . . . . . . . .X X X X . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . X X X X . . . . . . . . . . . . . . . . . . . . . .X X X X X X X X . . . . . . . . . . . . . . . . . . . . . .X X X X X X X X . . . . . . . . . . . . . . . . . . . . . .X X X X X X X X . . . . . . . . . . . . . . . . . . . . . .X X X X X X X X . . . . . . . . . . . . . . . . . . . . . .X . . . X . . . . X . . . X . . . X . . . . . . . . . X . .. X . . . X . . . . X . . . X . . X . . . . . . . . . X . .. . X . . . X . . . . X . . . X . X . . . . . . . . . X . .. . . . . . . . . X X X X . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . X X X X . . . . . . . . . . . . .. . . . . . . . . X X X X X X X X . . . . . . . . . . . . .. . . . . . . . . X X X X X X X X . . . . . . . . . . . . .. . . . . . . . . X X X X X X X X . . . . . . . . . . . . .. . . . . . . . . X X X X X X X X . . . . . . . . . . . . .. . . . . . . . . X . . . X . . . . X . . . X . . . X . X .. . . . . . . . . . X . . . X . . . . X . . . X . . X . X .. . . . . . . . . . . X . . . X . . . . X . . . X . X . X .. . . . . . . . . . . . . . . . . . X X X X . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . X X X X . . . .. . . . . . . . . . . . . . . . . . X X X X X X X X . . . .. . . . . . . . . . . . . . . . . . X X X X X X X X . . . .. . . . . . . . . . . . . . . . . . X X X X X X X X . . . .. . . . . . . . . . . . . . . . . . X X X X X X X X . . . .. . . . . . . . . . . . . . . . . . X . . . X . . . . . . X. . . . . . . . . . . . . . . . . . . X . . . X . . . . . X. . . . . . . . . . . . . . . . . . . . X . . . X . . . . X

Details: search for MMPDisplacement of Zick[1] Oil by Gas A

0

0.1

0.2

0.3

0.4

0.5

0.6

120 130 140 150 160

Pressure (atm)

Tie-

line

Leng

th

Displacement of Zick [1] Oil by Gas B

0

0.1

0.2

0.3

0.4

0.5

0.6

120 135 150 165 180 195 210

Pressure (atm)

Tie-

line

Leng

th

Tie-line length equals 0 at MMP( )∑=

−=nc

iii yxd

1

2

23

Details: validation of the algorithm

Method / Oil Zick-A Zick-B SVOC SVOD SVOC+D

Multicell [2] - - 514.2 231.9 310.9

Slimtube [2]* - - 512 ± 7 228 ± 10 302 ± 10

Slimtube [1]** 152 213.8 - - -

Louis Bleriot*** 157 211 524 216 298

Key tie line 156.74 211.0 519.3 217.3 295.7

Time (seconds) 0.7 0.7 1.9 1.7 1.6

Comparison of different results from literature. P (atm)

*Eclipse simulation, ** Experimental, *** Multicell

[1] Zick, 1986; [2] Høier, 1997

Details: validation of the algorithm

150

250

350

450

550

150 250 350 450 550Multicell Simulator MMP (atm)

Cal

cula

ted

MM

P (a

tm)

24

Influence of gas composition on MMP

EyEyy solventgasinj +−= )1(

Gas enrichment study when two gases are availableThe rich gas is treated as solvent

Monotonic Non-monotonic

Extension: semi-analytical solution to 1D two-phase gas injection

Identification of key tielines

1D solution to fullly self-sharpening systems (only of shocks)

1D solution to systems also having rarefactions

3D streamline based compositional reservoir simulation

MOC

MOC

Streamline method

25

Example

0.00

0.20

0.40

0.60

0.80

1.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40

Wave velocity (z/t)

Vol

ume

fract

ion

of g

as (S

)

MOCNumerical (100,450)Numerical (1000, 4500)Numerical (10000, 45000)

A near miscible displacement at 365 atm and 387.45 K.

0.9 sec

4.4 sec

5.4 min

7.8 hr

Besides phase equilibrium...

Viscosity instabilityCO2 viscosity: 0.02-0.05 cPReservoir fluids: 0.5-5 cPInherently unstable

Gravity segregationCO2 desnity: 1/2-3/4 water density, close to oil

Reservoir heterogeneityChanneling

26

SummaryEOR with CO2 provides double benefits in terms of sequestering CO2 and improving oil recovery

EOR with CO2 injection is mainly attributed to multicontactmiscibility. Three mechanisms for MCM are discussed, only two ofthem (the vaporizing and the combined) are realistic

In experimental study of CO2 injection, swelling test is the easiest one to perform while only the slimtube experiment can correctly determine MMP (also the standard method).

SummaryMany MMP calculation methods are available, but only two (the slimtube simulation and the intersecting tieline method) can capture the correct mechanism. The first one is time consuming and needsextrapolation, while the second one gives quick and correct solution.

A useful extension of the intersecting tie line method is the semi-analytical solution to 1D two-phase gas injection, which can be further used in streamlined based reservoir simulation

MMP (phase equilibrium) only determines local displacement efficiency, sweep efficiency are related to other aspects (viscosity, gravity, rock heterogeneity) which must be taken into consideration.