Direct Viscosity Enhancement of Carbon Dioxide - American Chemical

Chapter 10

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 14, 2017 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch010

Direct Viscosity Enhancement of Carbon Dioxide 1

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AndrewIezzi ,Robert Enick , and James Brady 1

Department of Chemical and Petroleum Engineering, 1249 Benedum Engineering Hall, University of Pittsburgh, Pittsburgh, PA 15261 Department of Chemistry, 905A Chemistry Building, University of Pittsburgh, Pittsburgh,PA15260 2

A high pressure, visual, falling-cylinder viscometer has been designed for use over a wide range of temperature and pressure, including near-critical and supercritical conditions. The viscometer consists of a precision-bore glass capillary which is contained within a high pressure windowed cell. The terminal velocity an aluminum cylinder falling through this tube was used to determine fluid viscosity. A vent at the bottom of the tube eliminated any pressure differential across the tube wall. This viscometer was used in the evaluation of several additives which were thought to have the potential of enhancing the viscosity of dense carbon dioxide when present in dilute concentrations. Solutions of liquid CO which were saturated with several surfactants did not have viscosities significantly different from pure CO , even when water was introduced into the system to stabilize and swell any micelles. Tri-n-butyltin fluoride forms weakly associating, linear polymers in light alkanes if its concentration is greater than about 0.3 weight percent. Although tri-n-butyltin fluoride is very soluble in pentane (29 weight percent) at ambient temperature, the use of pentane as a cosolvent did not increase the solubility of this compound in CO (0.13 weight percent). High pressure mixtures of the semi-fluorinated alkanes in CO did result in the formation of stable gels due to the microfibrillar morphology of the small excess amount of the semi-fluorinated alkane in the saturated liquid. This class of compound does, therefore, have potential for applications in which dramatic increases in CO viscosity are desired. 2

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0097-6156/89/O406-00122S06.00A) © 1989 American Chemical Society

Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Direct Viscosity Enhancement of Carbon Dioxide 123

The miscible displacement of o i l f r o m porous m e d i a is most e f f e c t i v e when the mobility of the displacing f l u i d , defined as the r a t i o of the r e l a t i v e p e r m e a b i l i t y of the fluid to its viscosity, is less than that o f the fluid it is displacing. A l t h o u g h carbon dioxide is an e f f e c t i v e fluid f o r r e c o v e r i n g o i l , its low viscosity results in an unfavorably high mobility in porous media. T h i s leads to the 'fingering of CO2 through the o i l - b e a r i n g f o r m a t i o n , leaving much of the o i l u n r e c o v e r e d . T h e d i r e c t viscosity enhancement o f CO2 to a l e v e l comparable to the o i l it is d i s p l a c i n g would decrease its mobility, thereby inhibiting this channeling and i m p r o v i n g the v o l u m e t r i c sweep e f f i c i e n c y of this process.


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Many o f these formations consist of l a y e r s of porous m e d i a with v a r y i n g p e r m e a b i l i t y . If a zone is v e r y p e r m e a b l e r e l a t i v e to the rest of the f o r m a t i o n , most of the injected fluids w i l l enter it. E x t r e m e l y viscous CO2 could be used as a d i v e r t i n g agent which would tend to m i n i m i z e flow through a highly permeable l a y e r . In this a p p l i c a t i o n a gel or v e r y viscous liquid would be f o r m e d near the wellbore i n this l a y e r and r e m a i n immobile. Subsequently injected fluids would then flow into the other layers. T h e d i r e c t viscosity enhancement of C O 2 would also improve its p e r f o r m a n c e as a f r a c t u r i n g fluid in low p e r m e a b i l i t y f o r m a t i o n s . A more viscous fluid would be able to transport the proppant p a r t i c l e s into the f r a c t u r e more e f f e c t i v e l y . T h e s e proppants hold the high p e r m e a b i l i t y f r a c t u r e open, increasing the r a t e of fluid r e c o v e r y f r o m the w e l l . LITERATURE

REVIEW

H e l l e r and T a b e r (1) were the first to study and report d a t a i n v o l v i n g the use of a d i r e c t t h i c k e n e r f o r dense c a r b o n dioxide. T h e i r efforts focused on c o m m e r c i a l l y available polymers that would be s u f f i c i e n t l y soluble in CO2 to increase its viscosity a f a c t o r o f 20. F r o m t h e i r study, the authors found that none of the c o m m e r c i a l l y available polymers were able to increase the viscosity of CO2 to the desired l e v e l . H o w e v e r , they were able to make c e r t a i n generalizations on the influence of various p o l y m e r properties on their solubility in liquid or s u p e r c r i t i c a l CO2. H e l l e r and T a b e r (2) then began pursuing three d i f f e r e n t approaches. T h e first approach was the synthesis of amorphous and p r e f e r a b l y a t a c t i c polymers of v a r y i n g molecular weights and with side chains which vary in carbon number. T h e goal was t o c r e a t e a large proportion of disorder and irregularity in such a multlcomponent p o l y m e r so that, when c o m b i n e d with CO2» it would impart high entropy to the system and b e c o m e soluble. Some of the polymers prepared were found to be soluble in C O 2 but d i d not increase its viscosity. Secondly, the synthesis of ionomers f r o m higher alpha-olefin t e r p o l y m e r s was then considered. H o w e v e r , since the work in the first approach had net yet r e a c h e d the l e v e l needed t o study this approach, no conclusions c o u l d be made c o n c e r n i n g these compounds. T h e third approach dealt the the synthesis of o r g a n o m e t a l l i c compounds, s p e c i f i c a l l y , t r i - a l k y l t i n fluorides. These compounds, when dissolved in non-polar solvents, f o r m high m o l e c u l a r weight p o l y m e r s by transient associations between adjacent molecules. A number of such compounds were prepared and were found useful in i n c r e a s i n g the v i s c o s i t y o f dense butane and propane.


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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y


H e l l e r , K o v a r i k , and T a b e r (3), continuing along the same lines of their previous work, reported that additional multicomponent polymers had been synthesized but their molecular weights were too high to a t t a i n sufficient solubilities in CO2. T h e y also began synthesis of diene containing terpolymers. F u r t h e r m o r e , they found that none of the organometallic compounds prepared were soluble enough in CO2 to increase its viscosity. T e r r y et al.(4) presented a novel idea to increase CO2 viscosity. T h e y a t t e m p t e d to increase its viscosity by insitu p o l y m e r i z a t i o n of CO2 miscible monomers. T h e authors found that light olefins c a n be p o l y m e r i z e d in such an environment using c o m m o n l y available initiators. However, no apparent viscosity increases were detected since the polymers were insoluble in CO2 and p r e c i p i t a t e d . T h e use o f entrainers to improve C O 2 mobility c o n t r o l for enhanced oil r e c o v e r y was investigated by L l a v e , C h u n g , and B u r c h f i e l d (5). The candidate entrainers were selected f r o m high molecular weight alcohols and hydrocarbons and ethoxylated compounds including n-decanol, isooctane, 2-ethylhexanol, and an ethoxylated alcohol. T h e y found that these entrainers were appreciably soluble in the CO2 phase and e f f e c t i v e l y increased the phase viscosity and density. However, the viscosity increase could only be attained after the introduction of r e l a t i v e l y large amounts of the e n t r a î n e r . In an a t t e m p t to improve the f r a c t u r i n g capabilities of low temperature liquid CO21 L a n c a s t e r et a l . (6) investigated compounds such as polymers, f u m e d s i l i c a , and amines, which are known to increase the viscosity of liquid alkanes. None of these compounds induced any viscosity change in CO2. R e c e n t l y , H e l l e r , K o v a r i k , and T a b e r (7) reported that the closest approaches to synthesizing p r a c t i c a l direct thickeners for CO2 had o c c u r r e d in the organotin fluorides. A l t h o u g h the solubility of these compounds in CO2 was not high enough to cause any significant change in solution viscosity, the authors felt that this type of associating compound offered a number of promising directions. F o r example, the synthesis of compounds with various types of groups attached to the tin molecules which may increase the solubility of the associating compound in CO2» such as tris-(trimethylsilylpropyl) tin fluoride. T h e authors also reported that they had apparently exhausted a l l possibilities in their search for a t a c t i c , straight chain hydrocarbon polymers with varying side chains that would be soluble enough in CO2 to increase its viscosity sufficiently. EXPERIMENTAL APPARATUS AND PROCEDURE Initial low pressure screening of the proposed "thickeners" was performed by using a f a l l i n g ball v i s c o m e t e r to measure the viscosity of dilute mixtures of these compounds in liquids. T h e liquids chosen to simulate high pressure carbon dioxide were isoctane, pentane, and perfluorinated hexane. These compounds have s i m i l a r densities, dipole moments, solubility parameters, d i p o l a r i t y / p o l a r i z a b i l i t y (8-12) as dense CO2 (Table I). A t t y p i c a l reservoir conditions, the CO2 density ranges between .4 and .7 grams per c u b i c c e n t i m e t e r . Based on the solubility p a r a m e t e r , dipole moment, and the p o l a r i z a b i l i t y / d i p o l a r i t y p a r a m e t e r , the perfluorinated hexane should be the best screening compound. T h e density of the compound, 1.7 grams per c u b i c c e n t i m e t e r , is about 3 times larger than that of CO2. A t higher denisites, .8 - 1.0 grams per c u b i c c e n t i m e t e r , the light alkanes should be the better screening compound based on a l l of the aforementioned p a r a m e t e r s . Compounds which exhibit solubility in these liquids and an increase in solution viscosity were then evaluated in C O 2·


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Direct Viscosity Enhancement of Carbon Dioxide125

IEZZIETAL.

Table I. Characteristics of CO2 and Several Liquids Suggested for Low-Pressure Screening

Compound Density Solubility Parameter p(g/cm ) (cal/cmfy Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 14, 2017 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch010

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H

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H

" 5 12 - 6 14 i CgH C F C F 18

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1 4

7

1 6

.4 .5 .6 .7 .8 .9 1.0 .626** ^β"" .684** · ** .692** 1.69** 1.73*** 6 8 4

Dipole Moment Polarizability/ μ (debye) Dipolarity ir

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3.1 3.8 4.7 5.2 6.0 6.7 7.4 7.0 7.4 6.9 5.9 6.0

0 0 0 0 0 0 0 0 0 0 0 0

-.25* -.18* -.12* -.08* -.04* -.01* 0.0* -.08 -.04 -.04 -.40 -.39

•at 50 °C from reference 8. Over this range, values increase with density. Reported values from other references (9,10) increase from -.6 to 0 over this density range. ••at 20 °C •••at 25 °C A novel, high pressure, visual falling cylinder viscometer developed by Barrage (13) was employed to measure the viscosity of the solution formed when these compounds were combined with dense CO2. The apparatus is shown in Figure 1. The two major components are an autoclave mixing chamber and a viscometer. After the thickener was charged into the system, CO2 was compressed in until the desired pressure, 1000-2500 psia, was attained* Mixing and rapid equilibration was accomplished by the rotating impellers of the mixing cell. Any compound not solubilized into the dense phase settled into a trap in the bottom of the mixing cell. A high pressure, visual rotameter has been modified to become a viscometer by replacing the original tapered tube and ball with a straight, precision bored tube and aluminum cylinder (see Figure 2). A pressure vent at the bottom of the tube permits pressure drop across the tube wall to be eliminated. The pressure distortion characteristic of sapphire crystal viscometers, which have high pressure CO2 on the inside of the tube and ambient pressure on the outside, is thereby eliminated. There is not a vent at the top of the tube, therefore flow occurs only through the inside of the tube. The cylinder was machined from aluminum to minimize the density difference between the CO2 and the cylinder, thereby decreasing the terminal velocity. Extremely small gap sizes could also be attained (0.113 mm) in order to decrease the terminal velocity. This enabled precise measurements of low viscosity fluids to be attained. w

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SUPERCRITICAL FLUID SCIENCE AND T E C H N O L O G Y

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1 — Nitrogen Cylinder 2 — Carbon Dioxide Cylinder 3 — Pressure Regulators 4 — Air Driven Gas Booster 5 — Carbon Dioxide Cylinder with Diptube 6 — Autoclave 7 — Vacuum Guage 8 — Pressure Guage 9 — Flowmeter ( Viscometer ) 10 — Back Pressure Regulator 11 — Sample Port

12 — Vacuum Pump 13 —Wet Test Meter 14 to -On/Off Valves 23 24 — Regulating Valve 25 — Needle Valve 26 — Air Supply 27 —To Hood 28 —To Manometer 29 —To Hood

Figure 1. Experimental Apparatus


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Direct Viscosity Enhancement of Carbon Dioxide


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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y

The impellers were also used to develop fluid flow through the v i s c o m e t e r . T h e upward flow c r e a t e d by the impellers f o r c e d the s m a l l aluminum c y l i n d e r to the top of the pressure equilibrated tube. F l o w through the v i s c o m e t e r was then t e r m i n a t e d by closing the valves leading to it. T h e cylinder then f e l l through a stationary column of the solution. Viscosities were c a l c u l a t e d f r o m the t e r m i n a l velocities. T h e viscometer was c a l i b r a t e d with a fluids of known density and viscosity. T h i s p a r t i c u l a r geometry, a c o a x i a l cylinder and tube, also allowed the a n a l y t i c a l derivation of the c a l i b r a t i o n constants (13) since the fluid displaced by the f a l l i n g cylinder flowed through the annulus between the c y l i n d e r and tube wall. T h e specific parameters of the viscometer were r =2.000 m m , r^= 1.887 mm and p =2.7 g / c m . Note that neither the length of the c y l i n d e r , nor the tube length, a f f e c t e d the viscosity measurements as long as the tube was long enough for t e r m i n a l v e l o c i t y to be attained. End effects were negligible since the r a t i o of cylinder length to annulus gap width was 70:1. A comparison of experimental CO2 viscosities obtained over a wide range of temperatures and pressures with that of previously r e p o r t e d v i s c o s i t é s (14) taken over that same range is illustrated in F i g u r e 3. T h e three different experimental curves shown correspond to a p a r t i c u l a r c a l i b r a t i o n fluid, water or CO2. or the constant derived from the N a v i e r Stokes equation. o

3

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(P

(1)

- P ) ( t - tj) f

C

K

Direct Viscosity Enhancement of Carbon Dioxide - American Chemical

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