Influence of reservoir rock composition on crude oil pyrolysis and combustion

Journal of Analytical and Applied Pyrolysis, 21 (1993) 87-95 Elsevier Science Publishers B.V., Amsterdam

Influence of reservoir rock composition pyrolysis and combustion *

87

on crude oil

M. Ranjbar Technical University Clausthal, Agricolastr. IO, D-3392 Clausthal-Z. (Germany) (Received

October

29, 1992; accepted

February

9, 1993)

ABSTRACT The principal objective of this study was to investigate the influence of reservoir rock composition on the pyrolysis and combustion behavior of crude oils in porous media in relation to in situ combustion (KC) as a thermal oil recovery process. Using sand pack of varying mineralogical composition and different crude oils, pyrolysis and combustion tests were performed to examine how the clays affect the amount of fuel and its reactivity during the ISC process. From the experimental results, it appears that clay minerals present in the matrix enhance fuel deposition during the pyrolysis process and also catalyze the oxidation of fuel. Clays; oxidation;

pyrolysis;

thermal

oil recovery.

INTRODUCTION

Thermal processes constitute an important sector of enhanced oil recovery techniques. There are two main types of thermal recovery processe, namely hot fluid injection and in situ combustion. The former generally involves hot water or steam and is particularly applicable to shallow reservoirs. In situ combustion (ISC), commonly known as fire flooding, is a very complex technique for recovering heavy as well as lighter crude oils with a large theoretical potential. It includes some aspects of nearly every enhanced oil recovery method, such as steam flooding, hot and cold water displacement, and gas flooding [ 11. It is well known that several zones exist in an oil formation during the ISC process [2]. The fuel necessary to sustain the combustion front is supplied by the heavy residual material or coke that deposits on the reservoir rock surfaces during distillation, thermal and catalytic cracking, pyrolysis, etc., of the crude oil ahead of the combustion front. An important parameter to be considered in the design of an ISC project is the amount of *Presented at: Pyrolysis ‘92 - Proceedings of the 10th International Conference mental Aspects, Processes and Applications of Pyrolysis, Hamburg, Germany, 28-October 2, 1992. 0165-2370/93/$06.00

0

1993 - Elsevier

Science Publishers

on FundaSeptember

B.V. All rights reserved

M. Ranjbar

88

/ J. Anal.

Appl.

Pyrolysis

27 (1993)

87-95

fuel deposited. Excessive fuel deposition causes a slow rate of advance of the burning front and large air compression costs, and reduces the maximum oil recovery. However, if the fuel concentration per unit bulk volume of reservoir is too low, the heat of combustion will not be sufficient to raise the temperature of the rock and the contained fluids to a level of self-sustained combustion. This leads to an unstable burning front and combustion failure. Therefore, it is necessary to understand the reactions occurring at different tem~ratures as the combustion front moves in the reservoir [3,4]. It is widely believed that the most important controlling factor of fuel formation, deposition and combustion is the nature of the crude oil and reservoir rock minerals [5-lo]. In a recent paper, Ranjbar and Pusch [ 1l] studied the effect of oil composition on fuel formation and combustion. In order to describe the influence of rock composition on the in situ combustion process, this study was conducted to investigate how the clay fractions present in reservoir rock affect the amount and reactivity of fuel. EXPERIMENTAL

The investigations were performed under isobaric conditions (8 MPa) in high pressure differential reactors, as well as in high-pressure thermogravimetric apparatus, for various pyrolysis gases and at temperatures of up to 1100 K. Details of design and equipment used are described elsewhere [ 111. In order to avoid thermal quenching and exposure, the combustion tests were performed on the residues of the pyrolysis tests without cooling the samples to temperatures lower than 623 K. Throughout the experiments, a light oil, a medium oil, an asphaltenic heavy oil and pure asphaltenes were used. The characteristic parameters of oils are listed in Table 1. Samples of unconsolidated sand pack containing different amounts of clay were used as reference porous media. TABLE 1 Characteristic

parameters of crude oils

Parameter

Heavy Oil

Medium oil

Light oil

Density (kg m-“) Viscosity (mPa s) at 323 K Resins (%wt.) AsphaItenes (%wt.) Carbon (%wt.) Hydrogen (%wt .) Oxygen (%wt.) Sulphur (%wt.)

0.987 3763

0.896 251

0.826 14

9.7 19.6 84.7 8.4 1.3 3.9

14.3 5.7 84.5 12.3 1.1 0.8

16.4 1.6 84.1 13.2 0.7 0.9

M. Ramjbar 1 J. Anal. Appl. Pyrolysis

RESULTS

AND

27 (1993) 87-95

89

DISCUSSION

EfSect of rock composition

on crude oil pyrolysis

The influence of reservoir rock on the in situ combustion process has been recognized by several authors, such as Geffen [ 121, Chu [ 131 and Vossoughi et al. [14], as an important part of the process; however, none of these studies reflect the importance of the composition of reservoir rock on the kinetics of crude oil pyrolysis (fuel formation). In order to describe how the amount of fuel formed during the pyrolysis process is influenced by the reservoir rock composition, the effect of clays on the pyrolysis kinetics of different oils was investigated. The first step was to identify the influence of clay content on the pyrolysis rate. To determine the pyrolysis rate the following eqn. was used: r =

2

(

(1)

(lOO/mr) >

where r = pyrolysis rate (% fuel min _ ‘), mf = instantaneous concentration of fuel (mg) and t = time (min). Figure 1 presents typical curves of the pyrolysis rate of pure asphaltenes, separated from medium oil, in a sand mixture containing 2% and 30% clay, as a function of the temperature in the dynamic nitrogen purge. Figures 2a-2c show the maximum pyrolysis rate r,,, and its corresponding temperature T(r,) as a function of clay content in porous media for different crude oils. From these results, it can be seen that the addition of clays to the porous media decreases r,,, and causes a shift of T(r,) to a lower temperature range.

0,30

r, wt.% / min ,

Pyrolyaia

Heating

I

Medium: Nitrogen

Rate: 1 K/min

0,lO --

400

500

600

Fig. 1. Typical

TG-curves

500

700

Temperature, of pure asphaltenes

900

K in porous

media.

1000

M. Ranjbar 1 J. Anal. Appl. Pyrolysis 27 (1993) 87-95

90

To study the relationship between clay content in host sand pack and the amount of fuel formed from light, medium, and heavy oil, pyrolysis tests were conducted at two different temperatures (T = 673 K and 773 K) using nitrogen an an inert pyrolysis medium. The results (see Fig. 3) show that a clear dependency of the fuel yield on the clay content in the matrix can be derived. the amount of fuel increases with increasing clay content, and is independent of oil type and pyrolysis temperature. In general, clay fractions of the reservoir matrix are very fine and, therefore, possess the highest specific surface area per gram. This may

0,40

r,n. .., wt.% / min Pyrolysis

<

T kJ...~_ K Medium: Nitrogen

System PreswrB:

4

740

8 MPa

Heating Rate lK/min 0.35

720

0,30 710

0.25

700 0

5

15

20

25

30

35

30

35

Clay Content in Matrix, wt. %

(a)

0.45

10

r,, wt.% / min

Heating Rate lK/min

0,30 0 Cb) Fig. 2 (a), (b).

5

10

15

20

25

Clay Content in Matrix, wt. %

91

M. Ranjbar / J. Anal. Appl. Pyrolysis 27 (1993) 87-95

0,35

T km).

r,nl wt.% / min Pyrolyrir

K

770

Medium: Nitrogen

Syatem Preswre:

8 MPa 760

Heating Rate lK/min 0,30

750

0,25 740

0,20 0

I

I

I 1

5

10

15

I I

25

20

Clay Content

(c)

I

in Matrix,

1

30

730 35

wt. %

Fig. 2. (opposite and above). Maximum pyrolysis rate (r,) ture (T(r,)) vs. clay content in the matrix for (a) medium,

and its corresponding tempera(b) light and (c) heavy oil.

increase the rate of fuel formation and lead to higher fuel deposition. These expected mechanisms were investigated using sand pack of varing specific surface area. Figure 4 shows the relationship between the fuel concentration of medium oil and the specific surface area of the host matrix at three pyrolysis temperatures. It is evident that, independent of the pyrolysis temperature, more fuel is deposited by increasing the surface area of the matrix. This means that the effect of reservoir rock on the pyrolysis rate and Fuel Yield, % Pyrolysis

//*

0

Temperature: T- 773 K

T- 673 K

10

20

30

Clay Content Fig. 3. Dependence

0

10

20

in Matrix,

of fuel yield on clay content

30

% in the matrix.

92 4q

M, Ranjbar / J, Anal. Appl. Pyrolysis 27 (1993) 87-95 Fuel Yield, %

M e d i u m Oil 30

Pyrolysis Medium: Nitrogen System Pressure: 8 MPe 20

10

t

0

I

I

I

I

0,6 1 1,5 2 2,5 Specific Surface Area of Matrix, m2/g

3

Fig. 4. Influence of surface area of the matrix on fuel yield.

fuel yield depends not only on its nature and mineralogical composition but also on the availability of the surface area. Influence o f clay minerals on f u e l combustion

To determine the kinetic parameters of fuel combustion the following equations were used: Rf -

-dCf dt

= k C ~ P'~'~2

(2)

k~ = k P ~ 2

(3)

Rf = -- dCf _ kl Cf" dt

(4)

/ - dCf'~ l o g ~ - - - d ~ ) = log k, + m log Cf

(5)

Details of the kinetic equations are described elsewhere [ 11]. If experimental data are plotted according to eqn. (5), linear regressions give the reaction order m. The values of m obtained are dependent on fuel quality, clay content in the matrix and combustion temperature, and were between 0.64 and 1.12. A typical Arrhenius plot of the combustion of fuel formed from heavy oil and from light oil, used in this study, is given in Fig. 5. It appears that in the temperature range in which the combustion rate is controlled by the kinetics of the chemical reactions (Fig. 5, Region II), the fuel that is formed

93

M. Ranjbar / J. Anal. Appl. Pyrolysis 27 (1993) 87-95

-0,5

-2,5

In K, e" 1 ! -X .... ~ "

pyrolViis Conditions= 'i Injeoted Gas: Nitrogen -':¢'i-"'x---~.,., ~ Tempi)reture: 973 K .............................., , ' ~i l ~.:...................................................i ....................................i.................................~.................................. ........................................... ii........................................... ]. i:! ~Ii i i "'" . . ."~"= . i Comi;ustion InjeCted Gel:C~nditions: Air i

-4,5

..............................................................................

Re, iin

-6,5

!:

"'~" .............................................i

Total Gas Fluxl 1,3 I/rain !'--.

Region"

;

~ ' " '

O Heavy 011 "-~[- Light OII -8,5 i

i

~

. . . . . . . . . . . . . . .

!

"4 I

1,1

|"

1,2

;

i

,

!

1,3

1,4

1,5

1,6

1,7

1/T * 1000, K" 1 Fig. 5. Typical Arrhenius plots for fuel combustion.

f r o m light oil proves to be m o r e reactive t o w a r d s oxygen then that f r o m heavy oil. The activation energies, which were determined f r o m isothermal c o m b u s tion tests, s h o w that the reactivity o f fuel t o w a r d s oxygen is decreased with increasing pyrolysis t e m p e r a t u r e (Fig. 6). This result is due to the different a m o u n t and c o m p o s i t i o n o f fuel, which is f o r m e d at different pyrolysis temperatures. I n d e p e n d e n t o f oil type, the a m o u n t o f fuel decreases with

Activation Energy, kJ/mol

175

150

125

p- 873 K r73 K K

100 0

2

8 12 20 25 30 Clay Content in Matrix, wt. %

Fig. 6. Dependence of activation energy of fuel combustion on clay content in the matrix at three different pyrolysis temperatures.

94

M. Ranjbar / J. Anal. AppL Pyrolysis 27 (1993) 87 95

TABLE 2 Activation energy of fuel combustion in porous media; influence of clays in the temperature range 623-823 K Clay content

Activation energy (EA) (kJ mol ~) of fuel "

(%wt.) 0 2 8 12 20 25 30

Heavy oil

Medium oil

Light oil

147.9 145.2 141.5 137.6 134.3 129.7 128.4

139.4 138.1 135.2 132.5 127.8 122.7 120.2

127.6 126.8 125.7 122.4 119.3 117.9 116.7

Pyrolysis conditions: T = 823 K, gas = nitrogen.

increasing pyrolyis temperature (Fig. 3). However, fuel composition is also influenced by the pyrolysis temperature. For example, at a pyrolysis temperature of 673 K the fuel which is formed from heavy oil contains 41.3% coke and 58.7% organic compounds. At 873 K the fuel consists only of coke. Higher amounts of coke and lower quantities of organic fractions cause the lower reactivity of the fuel. Figure 2 and the results given in Table 2 also show a significant reduction in the activation energy of fuel combustion resulting from the addition of clay to the sand mixture. This means that fuel combustion during the ISC process can be catalyzed by the clay fractions of the reservoir rock. Activation energy and Arrhenius constant values decrease with an increase in the amount of clay in porous media. A reduction in activation energy is caused by the increasing solid surface area and the catalytic effect of the clay fractions. Since the clays used here are of the solid acid type, their catalytic activities are related to their acid site density and acid strength. The reduction of activation energy is only one effect of the clay fractions in the reservoir matrix. The other effect is the increase of the combustion heat, which leads to a higher reaction velocity. Therefore, the influence of matrix composition on fuel combustion is an important part of the ISC process. It needs to be investigated for each project in conjunction with the oil type. CONC LUS I ONS

In view of the results in this paper, it can be pointed out that the most important controlling factor of fuel formation and combustion is the nature of the crude oil and reservoir rock minerals. The presence of clay in the

M. Ranjbar / J. Anal. Appl. Pyrolysis 27 (1993) 87-95

95

m a t r i x c a n influence the m a x i m u m pyrolysis rate, its c o r r e s p o n d i n g t e m p e r a t u r e a n d the kinetic p a r a m e t e r s o f fuel c o m b u s t i o n . T h e a d d i t i o n o f clays to the p o r o u s m a t r i x significantly affects fuel d e p o s i t i o n a n d c o m b u s t i o n . T h e a m o u n t o f fuel increases a n d the a c t i v a t i o n e n e r g y o f the c o m b u s t i o n process decreases with increasing clay c o n t e n t in the matrix. This is caused by the high surface a r e a o f clay f r a c t i o n s in the m a t r i x a n d their catalytic activities o n fuel c o m b u s t i o n . REFERENCES 1 J. Burger, P. Sourleau and M. Combarnous, Thermal Methods of Oil Recovery, Editions Technip, Paris, 1985, pp. 266-273. 2 C. Wu and P.F. Fulton, Soc. Pet. Eng. J., 11 (1971) 38 3 I.S. Bousaid and H.J. Ramey, Soc. Pet. Eng. of AIME, Pap., 243 (1968) 137-14. 4 M.K. Dabbous and P.F. Fulton, Soc. Pet. Eng. J., 14 (1974) 253-262. 5 J.G. Burger and B.C. Sahuquet, Soc. Pet. Eng. J., 12 (1972) 410-421. 6 M. Ranjbar, Ph.D. dissertation, TU Clausthal, Germany, 1990. 7 R. Kharrat and S. Vossughi, Soc. Pet. Eng. J., 8 (1985) 1441. 8 M.R. Fassihi, W.E. Brigham and H.J. Ramey, Soc. Pet. Eng. J., 24 (1984) 399-40. 9 F. Behar and R. Pelet, J. Anal. Appt. Pyrolysis, 7 (1984) 121-135; 8 (1985) 173 187. 10 F.P. Miknis, Fuel, 71 (1992) 731-738. 11 M. Ranjbar and G. Pusch, J. Anal. Appl. Pyrolysis, 20 (1991) 185-196. 12 T.M. Geffen, Oil Gas J., 71 (1973) 66-76. 13 C. Chu, J. Pet. Technol., 34 (1982) 19-36. 14 S. Vossoughi, G.W. Bartlett and G.P. Willhite, Soc. Pet. Eng. J., 10 (1985) 656-664.