SPE 145036

Case Study: Numerical Simulation of Surfactant Flooding in Low Permeability Oil Field Feng Xu, Xiao Guo, Wanbin Wang, Nan Zhang, Sha Jia, Xiaoqin Wang, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation

Copyright 2011, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Enhanced Oil Recovery Conference held in Kuala Lumpur, Malaysia, 1921 July 2011. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Low permeability reservoir occupies an important position in China's petroleum industry, as an important oil industry resource over a period of time in future, which helps to increase reserves and production. Surfactant flooding is one of the most effective ways to improve development effect in the low permeability reservoirs, which can reduce the injection pressure and increase injection rate, thereby enhancing oil recovery. Common mechanisms that how the surfactants enhance oil recovery are discussed in the paper. On the basis of assumptions, a seepage model of surfactant flooding was built to describe the complex process. Experiments was done to determine the surfactants performance, such as the relationship between surfactant concentration and oil/water interfacial tension, the relationship between surfactant concentration and rock adsorbance, and the relationship between surfactant concentration and water viscosity. Take the Yanchang low permeability reservoir for example, where the study of surfactant flooding is conducted after the optimal water injection plan to evaluate the effect of different surfactant concentration on the development results in the low permeability reservoir. The simulation results show that surfactant flooding has the role of enhancing oil recovery, and that the optimal surfactant concentration is 2%, which can enhance oil recovery by the percentage of 0.22. Introduction The overseas researches of chemical flooding have been carried out for many years and have made a great achievement. But those researches mainly consider the chemical flooding only in medium-high permeability layer, including polymer flooding, alkali-polymer flooding, surfactant-polymer flooding and alkali-surfactant-polymer flooding, and the numerical simulation study about chemical flooding has taken a leading position in theory research. There are some mature commercial numerical simulation software about chemical flooding, such as POLYMER and VIP-POLYMER. So far two kinds of numeric simulation model about alkali-surfactant-polymer flooding have been reported. The first one is modified combination flooding numerical model on the basis of black oil model. This model considers the change of interface tension, phase viscosity, effective permeability and flow behavior in a simplified way as the result of adding chemical agent, such as alkali, surfactant and polymer. And this model doesnt describe all kinds of mechanism and phenomenon of physical- chemical reactions specifically, such as Eclipse. The second one is called combination flooding compositional model which is based on mass conservation equations of all kinds of component. So this model can consider different influences on phenomenon of physical- chemical reactions which are brought out by the concentration changes of various components better than the previous one. The University of Texas takes the leading role in this aspect. BYUYANHE and POPE at al. have added the alkali flooding function to the micelle-polymer flooding model. This model includes four phases: water, oil, microemulsion and gas, as well as a dozen of component, such as active agent, polymer, alkali, monovalent cation, bivalent cation and alcohol. The physical-chemical phenomenon that happened between the chemical agent and rock/fluid are dispersion, divergency, desaturayion, adsorption, interfacial tension force, effective permeability, capillary ,fluid capitation, ion exchange, phase density and viscosity, dissolve and deposit, generation of active surfactant in situ, shear degradation of polymer, inaccessible pore volume, gel consistence, residual resistance factor, trace and temperature. Several researches on chemical flooding have been carried out domestically. ShiyiYuan of the research institute of China Petroleum Exploration and Exploration has established multifunction ASP model. The main characteristic of this model is two-dimension, three-phase, eight-component which includes water, oil, surfactant, alcohol, polymer, monovalent cation, bivalent cation and additive. This model can also reflect the mass-transfer reaction caused by velocity, the component

2 SPE 145036

diffusion by concentration gradation, the mass-transfer between liquid/liquid phase by phase equilibrium transition, the transfer between liquid/solid phases by adsorption/desorption, detention and ion exchange. The extreme value adsorption chemical flooding model which is developed by the Institute of Permeation Mechanics in LanZhou is improved on the basis of POPE model. It has three phases (water, oil and surfactant) and nine components (water, oil, surfactant, polymer, alcohol, negative ion, monovalent cation, bivalent cation and electrolyte). All the above researches are about chemical flooding, and all the mentioned models include surfactant flooding but not specifically talking about it, especially in low permeability layer. In 2004, Professor Daiyin Yin of Daqing Petroleum Institution has established a surfactant flooding numerical model which considers convective diffusion and adsorption and programmed the corresponding numerical simulation software. This model has three dimensions, two phases and three components. When the surfactant interacts with water or oil, this model ignore phase alternation and microemulsion phase and the reservoir only has water phase and oil phase. The different influences on water phase and oil phase which are carried out by different concentrations of surfactant are presented by different effective permeabilities of water phase and oil phase. Such an approach cant precisely describe the physical- chemical reaction between the water phase and oil phase and the flow of water, oil and microemulsion. It is a rough equivalent. So establishing a surfactant flooding numerical model which can precisely describe the physical-chemical reaction between the water phase and oil phase and can also pridict the flow of water, oil and microemulsion and especially for low permeability layer has a great significance. Surfactant EOR Mechanism Investigating characters of surfactant molecule on oil/water contact and the force on resident oil after water flooding, we have studied the effects of surfactant on the resident oil. In the process of surfactant flooding, surfactant is absorbed on the oil/water contact and the rock surface, so that to change the interfacial tension and invoke resident oil, improving the flow capacity of the mixture. Several EOR mechanisms are as follows. Mechanism of reducing the O-W interfacial tension After injecting surfactant into the oil layer, the O-W interfacial tension will reduce sharply (10-3mN/m), and resident oil droplet is easy to deform, then the resistance will reduce as well when it goes through throats. Thus highly dispersed resident oil in water wet rock would flow and be displaced. Sweep efficiency and displacement efficiency are the principal parameters in all factors effecting EOR. Increasing capillary number can improve displacement efficiency and the main way is to decrease interfacial tension. The relation between capillary number and interfacial tension is as follows.

c wN v wo = (1) The more , the less residual oil saturation, and the more displacement efficiency. Increasing andn or decreasing can improve , in which decreasing interfacial tension is the basic surfactant flooding mechanism. is usually 10-9~10-7 in

water flooding. When increases, oil recovery will increase notably. In the ideal state, when is 10-3, oil recovery can be 100%. The quantity degree can change 3 to 4 by decreasing O-W interfacial tension. The O-W interfacial tension is usually 20~50mN/m, ideal surfactant can decrease to 10-2~10-3mN/m, so as to decrease or eliminate the capillary action, to decrease the work of adhesion to scale off the resident oil, and to enhance displacement efficiency.

cNN

wm

cN

wos

c cN

cN

Surfactant EOR mechanism is to decrease the O-W interfacial tension. We know that decreasing the O-W interfacial tension will increase the capillary number. The corresponding relative permeability curve is used for various capillary numbers. For high capillary number, the flow likely to be monophasic flow, and the relative permeability curve is shown in Fig.1. For low capillary number, the relative permeability curve is the laboratory curve. For the capillary number between low and high one, interpolation is used to deal with the relative permeability curve. Large numbers of experiments demonstrate that high capillary number about 10-3 can improve recovery greatly, and if capillary number is lower than 10-5.5, it would not improve recovery. Emulsification Mechanism Surfactant system is highly emulsified to oil. When shearing in two-phase flow, it can disperse and scale off oil from rock surface rapidly, forming oil in water emulsion, thereby improving mobility ratio and sweep efficiency. Due to the adsorption of surfactant, oil droplet is electric and difficult to stick on layer, so it can flow to production well with active water. Wettability reversal mechanism The results show that the displacement efficiency is closely related to rock wettability. Oil-wetted surface results in the poor displacement efficiency, while water-wetted surface results in good one. The suitable surfactant could increase the contact angle of wettability between crude oil and rock, which could also make rock surface transit from oil wettability to water wettability, thereby it would reduce the work of adhesion of oil droplet in rock surface.

SPE 145036 3

Mechanism of improving the surface charge density When displacement surfactant is anionic one (anionic or nonionic), they are adsorbed on the surface of oil droplets and rock, it could increase superficial charge density and the electrostatic repulsion between oil droplet and rock surface. It could bring oil droplets easily by displacing medium, and thereby increase the sweep efficiency. Mechanism of coalescence forming oil belt More and more oil droplets washed down from the surface of layer collide with each other when moving forward, and the oil belt could combine with more oil droplets, so oil droplets gather into oil belt .Once the oil belt is formed, it can greatly reduce the driving resistance to displace more residual oil. Mechanism of changing the rheological of crude oil Crude oil contains resin, asphaltene and wax, so it has the nature of non-Newtonian fluid and its viscosity changes with shearing stress. Because resin, asphaltene and paraffin in crude oil are easy to form space reticulation. When oil flows, parts of this structure are destroyed, and the level of destruction relates to flowing velocity. When crude oil is static, it could restore reticulation. Once re-flowing, viscosity will be larger. This non-Newtonian nature of crude oil directly affects the displacement efficiency and sweep efficiency, making the crude oil recovery low. In order to enhance the recovery of these fields, it needs to improve the abnormal rheology of crude oil and reduce its viscosity and limited dynamic shearing stress. When using the aqueous solution of surfactant, parts of the surfactant will dissolve in oil, adsorb on the asphalt particles, increase their stability of the solvent shell, weaken the interaction between particles of asphalt, and weaken the reticulation of large molecules in crude oil. Thereby it could reduce the limited dynamic shearing stress of crude oil, and enhance oil recovery. The seepage model of Surfactant Flooding To simplify mathematical model so that we can solve it conveniently, the following assumptions were made: (1) Water, oil and microemulsion flowing in the porous medium; (2) Six components (water oil, surfactant, alcohol (cosurfactant), chloride and calcium) in reservoirs, and positive and negative

ions are seen as tracer component, the volume ignored; (3) Fluids in reservoir make isothermal movement; (4) Establishment of equilibrium between phases is instantaneous; (5) Volume can be added (i.e., mixed volume of each component is constant) (6) Fluid compressibility is ignored; (7) Effects of convection and diffusion are considered; (8) Adsorption in the rock is considered; (9) Impact of capillary pressure is considered; (10) Effect of gravity is considered. Mathematical Model (a) Mass conservation equation

( ) ( )ii i iC div F D Qt + + =

uur ur (2)

the equation of convection is

1

pn

i jj

ijF V C=

= uur uur (3) the equation of diffusion is

1 1( )

p cn ni

i j kjj k

D S D gradC= =

= ur ij (4) the equation of motion is

( )rjij j jj

K KV gradp g D

= ur

(5)

In this situation the concept of flow potential is introduced, and the following formula is defined,

j j jP gD = (6) Put the Eq. (6) into the motion Eq. (5), we can get

4 SPE 145036

rjij j

j

K KV

= ur (7) (b) The equation of Volume concentration

11

cn

ii

C=

= i=12 cn (8)

1

pn

j ij ij

S C C=

= i=12 pn (9) ii iC C C= + (10)

(c) The equation of saturation

11

pn

jj

S=

= i=12 pn (11) (d) The equation of capillary pressure

cjj j jP P P = j, =12 j pn (12) Definite conditions of the Model Definite conditions of the model include initial conditions and boundary conditions. Boundary conditions include the outer boundary conditions and internal boundary conditions. The following are the models mathematical expression of definite conditions. Initial condition is that the distribution of the reservoir parameters at the specified time (t = 0), for example, the distribution of pressure and saturation.

( ) ( )zyxPtzyxPt

,,,,, 00 == (13)

( ) ( )zyxStzyxSt

,,,,, 00 == (14) Boundary conditions mean what the geometric boundaries are in the state of the process in the exploitation of oil and gas reservoir.Outer boundary conditions (Neumman boundary) is

1oP C

n

= (15)

2wP Cn

= (16) And internal boundary conditions (Direchlet boundary) are

Constant bottom hole pressure wr r

P const= = (17)

Constant production wr r

Q con= = st (18) Cases study Parameters selection for surfactant flooding The relationship between surfactant concentration and oil/water interfacial tension is shown in Table 1. When the concentration of surfactant reach up to 1kg/m3, the oil/water interfacial tension will be constant(110-6N/m). The relationship

SPE 145036 5

between surfactant concentration and rock adsorbance is shown in Table 2, when the concentration of surfactant reach up to 1kg/m3, the rock adsorbance will be constant(0.0005kg/kg). The relationship between surfactant concentration and water viscosity is shown in Table 3.The relationship between capillary number and interpenetration curve is shown in Table 4. The interpenetration curve with low capillary number will be used if the capillary number is less than or equal to 10-5.5, and we should select the interpenetration curve with high capillary number when it is more than 10-3. Otherwise, the interpenetration curve will be gained through interpolation. Cases design for surfactant flooding In order to demonstrate the development effect of surfactant flooding, 7 cases have been designed. The basic case F1 is the optimal case of water flooding, whose water injection and oil production both are twice as much as original. Based on this, surfactant flooding cases F2 to F7 also have been designed, whose parameters are the same as F1 except that their surfactant concentration are different, and the details are shown in the Table 5. The above seven cases are simulated in the numerical software and the predicting time is 20years. The following figures (Fig.2-Fig.6) show the simulation results of the 7 cases and the Table 6 shows the final production index. From the figures and the table, we can see that surfactant injection can improve the recovery, but the increased percentage is not ideal. The reason is that there are not many injection wells in the block and those non-injected zones are not affected by surfactant. While in injected zones, the recovery increased marginally because the injection-production system is not perfect and the water injection pattern is not precise, making the surfactant effecting area small. To sum up, the extent of water flooding is limited, not meeting the requirements of EOR. Compared with water flooding, when the concentration of injecting surfactant equals to 1%, it could increase the recovery by 0.12%. When the concentration of injecting surfactant is 2%, it could increase the recovery by 0.1% than that surfactant concentration at 1%. When the surfactant concentration is 3%, it can increase the recovery by 0.06% than that surfactant concentration at 2%. The concentration of injection surfactant at 4% could increase the recovery by 0.06% comparing with 3% surfactant concentration. When the injection surfactant concentration is at 5%, it can increase the recovery by 0.04% than that when surfactant concentration is 4%. And when the surfactant concentration equals to 6%, it increases the recovery by 0.03% than that when surfactant concentration is 5%. From the analysis above, the optimum concentration of surfactant is 2%. Once this optimum concentration exceeds, the recovery would reduce significantly. Seen from the figures and tables, surfactant flooding has the role of enhancing oil recovery, which is conducted after the optimal injection water case, but the effect on enhancing oil recovery is not ideal. When the optimal surfactant concentration is 2%, it can enhance oil recovery by the percent of 0.22. Conclusions (1) Surfactant flooding is one of the most effective ways to improve development effect in the low permeability reservoir, which can enhance oil recovery by reducing the injection pressure and increasing injection rate. When the optimal surfactant concentration is 2%, it can enhance oil recovery by the percent of 0.22. (2) From the case studied in Yanchang oil field, we can see that surfactant injection can improve the recovery, but the increased percentage is not ideal. The reason is that there are not many injection wells in the block and those non-injected zones are not affected by surfactant. While in injected zones, the recovery increased marginally because the injection-production system is not perfect and the water injection pattern is not precise, making the surfactant effecting area small. To sum up, the extent of water flooding is limited, not meeting the requirements of EOR. Acknowledgement The authors wish to express acknowledgement to all the participants from Southwest Petroleum University. The work was supported by the Yanchang oil Co. Ltd. Nomenclature

cN capillary number; v flooding velocity, m/s;

w viscosity of displacing fluid, mPa.s; wo interfacial tension between oil and displacing fluid, mN/m;

porosity; iC total concentration of component I;

iQ the pore volume injected or produced per unit pore volume; V seepage velocity, m/s;

ijC the concentration of component i in the motion phase j;

6 SPE 145036

jS saturation of phase j; ikjD the diffusion coefficient between component j and component k in phase I;

the phase density, g/cm3; K the absolute permeability, mD;

rjK the relative permeability of phase j; the phase viscosity, mPa.s;

iC the component adsorbed in solid phase; wr wellbore radius, m;

P pressure, MPa; Q production rate, m3/d.

Reference 1. Hu,W.R.: The present and future of low permeability oil and gas in China, Engineering Sciences .2009, 11(8), pp.29-37 2. Ye, Z.B. Enhanced oil recovery; Petroleum Industry Press: Beijing, 2007. 3. Seethepalli, Adibhatla, K.K.Mohanty. Wettability alteration during surfactant flooding of carbonate reservoirs. SPE89423, presented at

SPE/DOE Symposium on Improved Oil Recovery, Tulsa, Oklahoma,17-21 April, 2004 4. Liu F., Gao,S.Z.;Liao,X.C.: Application of Surface Active Agent in Oil Recovery, Fine Chemicals. 2010, 17(12), pp.696-699 5. Ge,J.J.; Zhang G.C.;Jiang.P.: Development of Surfactants as Chemicals for EOR, Oilfield Chemistry. 2007, 24(3), pp.287-292 6. Wang,Y.F.;Zhao,F.L.: The Salt Tolerance of nonionic anionic surfactants, Oilfield Chemistry. 1999, 16(4), pp.336-440. 7. Baviere M.: Alpha sulfonated fatty acid ester for surfactant at high salinity, Preprints(Div. Pet. Chem.ACS),1988,33(1), pp.158~162 8. Chiu Y C and Hwang H J.: The use of carboxy methyl lethoxylates in enhanced oil recovery, Colloids Surf., 1987,28(1),pp. 53~62 9. Standard Oil Co . Enhanced oil recovery . US4811788-A., 1989-3-14. 10. Zhao,L.Y.; Fan,X.J.: The New Development of Surfactant Oil Displacement Systems, Journal of Xian Shiyou University. 2000,

15(2), pp. 55-58. 11. Daiyin Yin.A Numerical Simulation Study on Surfactant Flooding and its Field Application in Daqing Oilfield. SPE112424, presented

at Europec/EAGE Conference and Exhibition, Rome, Italy, 9-12 June ,2008 12. Ma,T.;Zhang,X.F.: Progresses in research on surfactant used as oil displacement agents, Speciality Petrochemicals. 2008, 25(4), pp.

78-82. 13. Guo,D.H.: Flooding Mechanism and Application of Surfactant Flooding, Advances in Fine Petrochemicals. 2002, 29(6), pp. 36-41. 14. Kazemi H, Gilman J R, El-Sharkaway A M.: Analytical and numerical solution of oil recovery from fractured reservoirs u-sing

empirical transfer functions. SPE Reservoir Engineering, 1992, 7(2), pp.8-11 15. Zhang,F.L.: Numerical simulation of surfactant flooding for low-permeability reservoirs, Journal of Daqing Petroleum Institute.

2007, 31(1), pp. 31-34. 16. Zhu,W.Y.: An improved chemical flooding compositional model simulator, Acta Petrolei Sinica. 1992, 13(1), pp. 79-89. 17. Reppert, T.R., Bragg, J.R.: Second Ripley Surfactant Flood PilotTest, SPE20219, presented at SPE/DOE Enhanced Oil Recovery

Symposium, Tulsa, Oklahoma, 22-25 April, 1990. 18. J.H.Base,C.B.Petrick.: Glenn Pool Surfactant Flood Pilot Test: Comparison of Laboratory and Observation Well Data, SPE

Reservoir Engineering, 1986, 1(6), pp. 593-603.

SPE 145036 7

Table 1 The relationship between surfactant concentration and oil/water interfacial tension

Cs(kg/m3) wo N/m 0 0.05

0.1 0.0005

0.5 1.00E-05

1 1.00E-06

30 1.00E-06

100 1.00E-06

Table 2 The relationship between surfactant concentration and rock adsorbance

Cs(kg/m3) Ssc(kg/kg)

0 0

1 0.0005

30 0.0005

100 0.0005

Table 3 The relationship between surfactant concentration and water viscosity

Cs(kg/m3) Vw(CP)

0 0.61

30 0.8

100 1

Table 4 The relationship between capillary number and interpenetration curve

Log(CAPN) Fm

-10 0

-5.5 0

-4 0.5

-3 1

2 1

8 SPE 145036

Table 5 Final production index compared at different surfactant concentrations

Case names Surfactant concentration (%)

Cumulative oil production m3

Recovery ratio (%) Increased oil

recovery %

F1 0.0 709049 14.98

F2 1.0 714596 15.10 0.12

F3 2.0 719075 15.20 0.10

F4 3.0 722329 15.26 0.06

F5 4.0 724946 15.32 0.06

F6 5.0 726954 15.36 0.04

F7 6.0 728362 15.39 0.03

Table 6 Detailed annual index of the optimal case F3 Production

years Daily oil production(t) Annual oil

production(104t) Cumulative oil

production (104t) Recovery ratio

(%) Annual production

rate (%) 0 69.97 2.03 13.26 3.38 0.43

1 136.47 4.34 17.60 4.48 0.92

2 121.42 3.86 21.47 5.47 0.82

3 108.24 3.45 24.91 6.34 0.73

4 98.94 3.11 28.03 7.14 0.66

5 91.94 2.87 30.89 7.87 0.61

6 86.10 2.68 33.58 8.55 0.57

7 80.52 2.51 36.08 9.19 0.53

8 75.40 2.35 38.43 9.79 0.50

9 71.38 2.21 40.64 10.35 0.47

10 67.82 2.10 42.75 10.88 0.44

11 64.42 2.00 44.74 11.39 0.42

12 61.58 1.91 46.65 11.88 0.40

13 59.13 1.82 48.47 12.34 0.38

14 57.06 1.75 50.22 12.79 0.37

15 55.26 1.70 51.92 13.22 0.36

16 53.60 1.64 53.56 13.64 0.35

17 52.14 1.60 55.16 14.04 0.34

18 50.68 1.55 56.71 14.44 0.33

19 49.25 1.51 58.21 14.82 0.32

20 47.86 1.47 59.68 15.20 0.31

SPE 145036 9

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0Sw

Kr

KrwKro

Fig.1 Relative Permeability Curve for High Capillary Number

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0 2000 4000 6000 8000 10000 12000Time, Days

Rec

over

y,D

imen

sion

less

F1 F2 F3

F4 F5 F6

F7

Fig.2 The recovery curve of the 7cases in the prediction time of 20 years

10 SPE 145036

0

100000

200000

300000

400000

500000

600000

700000

800000

0 2000 4000 6000 8000 10000 12000Time, Days

Cum

ulat

ive

Oil

Prod

uctio

n,m3

F1 F2

F3 F4

F5 F6

F7

Fig.3 The cumulative oil production curve of the 7cases in the prediction time of 20 years

0

20

40

60

80

100

120

140

160

180

0 2000 4000 6000 8000 10000 12000Time, Days

Dai

ly P

rodu

ctio

n R

ate,

m3/d

ay

F1 F2

F3 F4

F5 F6

F7

Fig.4 The daily oil production curve of the 7cases in the prediction time of 20 years

SPE 145036 11

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2000 4000 6000 8000 10000 12000Time, Days

Wat

er C

ut, D

imen

sion

less

F1 F2

F3 F4

F5 F6

F7

Fig.5 The water cut curve of the 7cases in the prediction time of 20 years

0

10

20

30

40

50

60

0 2000 4000 6000 8000 10000 12000Time, Days

Fom

atio

n Pr

essu

re, B

ar

F1 F2 F3

F4 F5 F6

F7

Fig.6 The formation pressure curve of the 7cases in the prediction time of 20 years