SwellingExtraction Test of a Small Sample Size for Phase Behavior Study

Introduction A swelling/extraction test is a common phase behavior experiment to determine reservoir fluid volume and composition changes due to CO2 dissolution at reservoir temperature

SPE 129728

Swelling/Extraction Test of a Small Sample Size for Phase Behavior Study

J.S Tsau, L.H Bui, and G.P Willhite, SPE, University of Kansas

Copyright 2010, Society of Petroleum Engineers

This paper was prepared for presentation at the 2010 SPE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, USA, 24–28 April 2010

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

Swelling/extraction tests are single-contact phase-behavior experiments to measure the solubility of CO2 dissolved in crude oil and the amount of hydrocarbon that CO2 can extract or vaporize from crude oil The tests are commonly conducted in a visual PVT cell with a large sample size (40-100cc) In this paper, an easy operated apparatus capable of determining phase behavior with a significantly smaller sample size (3 to 14 cc) is described The apparatus consists of a high-pressure view cell, high-pressure and precision syringe pump filled with CO2, a water bath, and accessories to measure the temperature and pressure The device is capable of determining vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibrium commonly observed in a high pressure CO2 enhanced oil recovery process The solubility of CO2 in oil, the expansion volume of oil due

to the dissolution of CO2 as well as the phase transition during the test were quantified with excellent reproducibility The molar volume of oil saturated with CO2 correlated linearly with the mole fraction of dissolved CO2 suggesting ideal mixing in the liquid phase The phase behavior between CO2 and crude oil samples with different composition, temperature and pressure is discussed

Introduction

A swelling/extraction test is a common phase behavior experiment to determine reservoir fluid volume and composition changes due to CO2 dissolution at reservoir temperature The test is usually conducted in a constant volume, high pressure view cell initially filled with a predetermined amount of stock-tank oil CO2 is injected progressively with pressure increase

in a number of discrete steps The change of crude oil volume due to the swelling is measured and the amount of CO2

dissolved in the oil is calculated by assuming that vaporization of crude oil components into the equilibrium vapor phase is neglible until the MMP is approached At a higher pressure, light hydrocarbon components are vaporized or extracted and the hydrocarbon rich oil phase shrinks with increasing pressure Phase behavior at high pressures can be viewed The saturation pressure, solubility of CO2 and swelling factor are commonly used for tuning the equation of state (EOS) for modeling phase behavior

Various sizes of view cells, 140 cc (Hand et al., 1990), 170 cc (Harmon et al., 1988), 190 cc (Orr et al., 1981) and

450 cc from commercial laboratory have been used to study the phase behavior The sample size for the swelling test described by Holm and Josendal (1982) is 30% of the cell volume Thus, the test performed in these cells would require 40cc

to more than 100 cc of liquid sample An inherent difficulty of large volume cells is the length of time required to reach equilibrium after the pressure is changed and the difficulty of mixing large volumes of oil and gas under pressure

This paper describes a small volume high pressure view cell that was developed to investigate the mass-transfer process occurring in swelling/extraction tests when CO2 dissolves in the oil phase The total cell volume is 26 cc with the sample size adjustable for the test This paper summarizes the design of apparatus, experimental procedure and the tests conducted on CO2 with three crude oils produced from reservoirs in Kansas

Experimental Apparatus and Procedure

Apparatus The apparatus consists of a high-pressure view cell, high-pressure and precision syringe pump filled with CO2, a water bath, and accessories to measure the temperature and pressure Figure 1 shows a diagram of the experimental

apparatus setup including an ISCO 100DM syringe pump, a high-pressure view cell, a water bath, a cathetometer,

temperature and pressure measurement devices connecting to a computer data acquisition system (Ren et al., 2008) The

temperature of the water bath is controlled by a Fisher Isotemp Immersion Circulator The high pressure view cell is

equipped with high pressure gauge glass window to allow visual observations of fluids under experimental conditions The

view cell is made of stainless steel The gauge glass window is rated at a maximum temperature of 280oC and pressure of

4000 psi Pressure in the view cell is measured by a 5000 psi Heise DXD Series 3711 precision digital pressure transducer

Mixing is accomplished by moving a PTFE coated stir bar inside the view cell using a rare-earth magnet in a slot on the

outside of the cell The magnet is raised and lowered manually by a pulley system but could be automated An Eberbach

5160 cathetometer is used to measure the height of the liquid in the view cell

Procedure In a typical swelling experiment, the ISCO pump is filled with CO2 Pressure of the pump is set at the maximum

anticipated pressure with a constant-pressure mode The pump automatically adjusts the volume of CO2 to maintain the

desired pressure Temperatures of the gas lines are maintained above the critical temperature of CO2 to avoid CO2

condensation inside the lines Temperature of the water bath is set at the desired temperature A predetermined volume of

crude oil is carefully injected into the view cell to avoid liquid droplets on the wall of the view cell The view cell is

connected to the gas lines and then immersed into the water bath The height of the liquid sample inside the view cell is

measured using the cathetometer The volume of the liquid sample is calculated using a pre-calibrated curve which correlates

the volume with the measured height

When the system is thermally equilibrated, the gas lines and the view cell are flushed with CO2 at low pressure to

remove any residual gas or air The cell pressure is increased in discrete steps by CO2 injection from the top of the view cell

CO2 injection is stopped when a desired pressure is achieved During pressurization process, the stir bar inside the view cell

is used to mix the liquid and vapor phases, accelerating the mass transfer between the gas and liquid phase When the system

is in equilibrium, the height of the liquid sample in the view cell is measured with a cathetometer Equilibrium between

pressure changes takes from 30-60 minutes after vigourous mixing The pump condition (temperature, pressure & final

volume of CO2), temperature of gas lines and the view cell condition (temperature & pressure) are recorded All the data are

transferred into a specially designed spreadsheet to calculate the solubility, density of the liquid solution, molar volume and

volume expansion As noted earlier, the material balance calculations are based on the assumption that the amount of

hydrocarbon extracted into the vapor phase is negligible The composition of the liquid phase is based on the mass balance

by determining the amount of CO2 dissolved in the liquid This assumption appears to be valid over a wide range of

pressures This method yields high resolution of solubility data (often better than ±0.0001), pressure with accuracy of ±3 psi,

temperature of ±0.01 °C, density up to ±0.4% and volume expansion to ±0.05% (Ren et al., 2008) At the end of the

experiment after depressurization, the view cell is cleaned with methylene chloride, acetone solution and blown dry with

compressed air

Apparatus Verification The apparatus and method were verified by comparing the phase equilibrium data of carbon

dioxide in n-decane at 71.1 °C with published data Figure 2 illustrates the p-x phase equilibrium of CO2 and n-decane binary

system The experimental data of this setup are in excellent agreement with literature data (Nagarajan et al., 1986, Jennings

et al., 1996) The principal assumption in analyzing these data is that the amount of liquid component in the equilibrium

vapor phase is negligible This assumption was verified by Ren et al (2008) who demonstrated that the percentage of decane

in CO2 vapor phase was less than 0.13%

Results and Discussion

Table 1 gives the laboratory measured properties of oil used in this study Figure 3 shows the carbon number distributions

for each of stock-tank oil determined by the gas chromatograph Oil A is the lightest oil among all three oils as its

composition has the least mole fraction of detectable C30+ As the mole fraction of heavy component C30+ increases from

0.031 in oil A to 0.229 in oil C, the API gravity of oil decreases from 39.6 to 24.3

The data obtained from the view cell includes solubility of CO2 in oil, density and molar volume of CO2-saturated

liquid solution, and the relative volume change as a result of swelling/extraction at reservoir temperature and specified

pressures It is useful to examine possible correlations between composition and pressure for oil saturated with carbon

dioxide Orr and Silva (1983) correlated the molar density of decane with mole fraction of carbon dioxide for the decane data

at 71 ºC over a wide range of pressures assuming ideal mixing Equation 1 gives the molar volume of a carbon dioxide-oil

solution assuming that ideal mixing occurs when carbon dioxide dissolves in oil and the molar volumes of the carbon dioxide

and oil are independent of pressure at a given temperature

2 2 2

2 2

(1 ) (1) or

( ) (2)

CO CO CO o

o CO o CO

where

2

CO

x is the mole fraction of CO2, and are the molar volume of oil and CO2, respectively V is the molar volume of oil saturated with carbon dioxide Equation 2 suggests that when ideal mixing exists, graph of V versus

o

V

2

CO

V

2

CO

x will

be a straight line with a slope of and an intercept of Figure 4 is a plot of the molar volume of saturated decane versus mole fraction of carbon dioxide The molar volume of decane at 71 °C is 200.65 cc/mol compared to 201 cc/mole determined experimentally The molar volume of carbon dioxide in solution is 42.32 cc/mol

2

( VCO − V )o Vo

Figure 5 is a plot of the molar volume of different crude oils saturated with carbon dioxide at its reservoir temperature The molar volume of crude oil A at 105 ºF is 266.12 cc/mol compared to the measured molar volume of 263.41 cc/mol The molar volume of carbon dioxide in solution is 30.09 cc/mol The molar volume of crude oil B at 110 ºF is 306.99 cc/mol compared to the measured molar volume of 303.44 cc/mol The molar volume of carbon dioxide in solution is 26.23 cc/mol The molar volume of crude C at 78 ºF is 402.52 cc/mol compared to the measured molar volume of 399.20 cc/mol The molar volume of carbon dioxide is solution is 34.39 cc/mol These correlations suggest that carbon dioxide dissolves in oil at constant temperature to form an ideal solution The molar volumes of dissolved carbon dioxide and crude oil are not a strong function of pressure over the range of pressure investigated The correlation of molar volume data for these crude oils suggests that the swelling behavior over a wide range of pressures could be obtained by measuring swelling

at two pressures

Figure 6 shows swelling/extraction curve for the stock-tank-oil B/CO2 system studied at 110 ºF The swelling factor, defined as the ratio of the oil volume at a given pressure to its initial volume at atmospheric condition, increases with the pressure as the dissolution of CO2 in the oil increases The swelling factor of this oil increases to 1.21 with the maximum volume expansion of 21% when 0.728 mole fraction of CO2 was dissolved in it at 1150 psig At a higher pressure, the light components of oil are extracted into CO2 rich phase and the oil starts to shrink and the composition of the liquid phase can no longer be computed by material balance Analysis of either the equilibrium liquid or vapor phases would be necessary to determine the swelling However, it appears that rate of extraction is faster than the rate of swelling as CO2 dissolved in the oil At the end of this experiment, 39.2 volume % of the initial oil has been extracted by carbon dioxide Two phases exist over the pressure range tested At low pressure, the CO2 vapor phase is immiscible with hydrocarbon liquid phase while at moderate high pressure, critical CO2 is equilibrated with hydrocarbon liquid phase

factor are presented in Figure 7 and 8, respectively At a given pressure, the solubility of CO2 decreases with the temperature

as indicated in Figure 7 As a result, the swelling factor of oil decreases with the temperature as less amount of CO2 is dissolved in the oil

Extraction of light hydrocarbons starts at a lower pressure at a lower temperature For example, in Figure 8, the extraction starts (inferered by decrease in the swelling factor) at 1150 psig at 105 °F, whereas it starts at 1350 psig at temperature of 125 °F The ability of CO2 to extract hydrocarbon from the crude oil depends on its density At higher temperature, a higher pressure results in a density equivalent to its density at a lower temperature Holm and Josendal report that the extraction of liquid hydrocarbons into CO2-rich vapor phase occurs when the density of CO2 is at least 0.25 to 0.35 gm/cc The extraction of oil B starts at density of CO2, 0.26 gm/cc at 105 ºF and 1150 psig At 125 ºF, the pressure of CO2

needs to increase to have an equivalent density to start the extraction and it is in the neighborhood of 1300 psia

Similar behavior is also observed for oil A Figure 9 shows the swelling/extraction curves of stock-tank oil A with carbon dioxide The extraction starts at 1150 psig at 105 °F whereas it starts at 1100 psig at 98 °F The maximum swelling factor is 1.41 at lower temperature of 98 °F as compared to 1.28 at 105 ºF

observations of phase behavior of CO2 and Maljamar separator oil (Orr et al., 1981) Figure 10 shows the volume fraction of

phases observed in the view cell at pressures between 1100 and 1225 psig At pressure below 1150 psig, liquid-vapor phase coexists with liquid vol% increasing with pressure as the oil swells As the pressure approaches 1160 psig, a CO2-rich middle phase appears The vol% of this middle phase increases with the pressure until 1225 psig wherein it merges with the vapor phase and the system becomes two phases (L+V) This three phase region exists in a narrow pressure range of 50 psi

which is difficult to identify without careful experimental measurements

Oil C was tested at 78 ºF, a temperature below critical temperature of carbon dioxide (87.98˚F) The

swelling/extraction behavior of Oil C is shown in Figure 11 A maximum of 15% of volume expansion was observed before

the system pressure reached the saturation pressure of 930 psig where the CO2 vapor phase became a CO2 rich-liquid phase

Extraction starts at 872 psig at where three phases are observed, 1) Oil-rich liquid phase, 2) CO2-rich liquid phase and 3) CO2

vapor phase The three phases exist in a very narrow pressure range between 872 and 930 psig When the pressure is above

930 psig, the CO2 vapor phase merges with the CO2-rich liquid phase to form two liquid phases, one hydrocarbon rich and

one CO2-rich

Effect of Initial Oil Volume The effect of initial oil volume on the results from swelling/extraction tests is demonstrated in

Figure 12 The comparisons are made among curves at different initial oil volume of 3, 9 and 14 cc which represent 12, 35

and 54 % of total cell volume All curves display both swelling and extraction behavior wherein the rate of extraction is

fastest when the initial sample volume is the smallest The change of relative volume of oil due to the extraction varies with

the initial oil volume The smaller initial oil volume with larger gas-phase volume, the greater amount the hydrocarbon can

be extracted from the oil This similar effect has been reported by Hand et al (1990) and was first noted by Holm and

Josendal (1982) in their work where a 30% of initial volume of sample was used for the swelling test

Estimation of MMP The relationship between the phase behavior observed in swelling/extraction tests and slim-tube

minimum miscibility pressure (MMP) has been investigated by several researchers Harmon and Grigg (1988) reported a

relationship between the pressure required to initiate significant extraction in swelling/extraction tests and the MMP from

slim-tube experiment They proposed that a rapid rise in CO2 upper-phase density of measurement due to the extraction of

hydrocarbon components from the crude oil corresponds to the process by which multiple-contact miscibility is developed

However, Hand and Plnczewski (1990) concluded no such direct relationship between the two tests because the vapor phase

density, dominated by high solvent CO2 concentration, is not a sensitive indicator of the onset of major extraction, or of

MMP In this work, we observed MMP can be graphically derived from the extraction test By examining the extraction test

results with MMP measured from the slim-tube experiment, a relationship exists between these two tests if the initial oil

volume tested in view cell is small (12%) and the relative volume of oil due to extraction falls below 0.8 over the pressure

range investigated Figure 13 present swelling/extraction test curves of oil B/CO2 system at 110 ºF and 125 ºF The rate of

slope changes in two distinct stages in each of the two extraction curves Drawing lines through the major extraction and

secondary stages, the pressure at the intersection of these two lines is close to MMP determined with the slim-tube

experiment As shown in this figure, the pressures at the intersection point are 1340 psig and 1640 psig at 110 ºF and 125 ºF,

respectively The MMP determined from slim-tube for oil B/CO2 were 1350 psig and 1650 psig (Bui et al., 2010) Figure 14

shows another swelling/extraction curve for oil A/CO2 at 105 ºF The MMP determined graphically from this test is 1240

psig while MMP measured from slim-tube experiment was 1250 psig

For oil C/CO2 system, the MMP can not be determined graphically from the plot (see Figure 11) as it lacks of

secondary stage of extraction as measured from this setup In addition, the relative volume measured is 1.0 indicating very

low concentration of extractable hydrocarbons MMP was not determined for this oil, however, the slim-tube experiment

showed a low recovery efficiency of 50% at pressure of 1000 psi (Tsau et al., 2008)

Conclusions

1 The swelling of crude oil due to the dissolution of CO2 was determined accurately in a new apparatus using small sample

sizes

2 Swelling of crude oil by dissolved CO2 correlated with the molar volume of crude oil and apparent molar volume of CO2

over the range of pressure where neglibile oil was extracted into the CO2 phase

3 The pressure at which extraction starts by CO2 depends on the initial volume of oil and temperature tested

4 A three phase region (VLL) equilibrium was identified with the apparatus and occurred over a narrow range of pressure

5 Using 12 % of cell volume sample size, the MMP estimated by the swelling/extraction test graphically is close to what

determined from the slim-tube experiment

Acknowledgements

The authors wish to acknowledge the funding support by Research Partnership to Secure Energy for America (RPSEA) small

producer program, RPSEA Contract DE-AC26-07NT42677/Subcontract 07123-03, and Scott Ramskill of TORP for help in

design and manufacturing of the apparatus

References

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SPE 129710 to be presented at the Improved Oil Recovery Symposium, Tulsa, OK April 24-28, 2010

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1215-1220

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87-98

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Nonane and C 9 Alkylbenzens,” J Chem Eng Data, 41, 831, 1996

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Systems, 2 CO2 + n-Decane,” J Chem Eng Data, 31, 168 1986

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480-492

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Multiple-Contact Experiment,” SPEJ (April 1983) 272-280

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2007

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Table 1 Properties of crude oil used in this study

Viscosity at reservoir

CO 2 + n-Decane 71.10 o C (160 o F)

0 400 800 1200 1600 2000

Mole fraction of CO 2 in the liquid phase

This work #1 This work # 2 Nagarajan & Robinson Jr.

Jennings & Schucker

Figure 2 Phase equilibrium data of n-decane with carbon dioxide

Figure 1 Experimental setup of swelling/extraction test

Figure 3 Carbon number distributions of crude oils

Figure 4 Molar volume of carbon dioxide saturated n-decane

at 71.1 ºC

Figure 5 Molar volume of carbon dioxide saturated crude oils

at different temperature

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Pressure, psi

0.0 0.2 0.4 0.6 0.8 1.0

Swelling Factor CO2 solubility

Figure 6 Swelling factor and solubility of carbon dioxide in crude oil B at 110 ºF

0 200 400 600 800 1000 1200 1400

Mole fraction of CO 2 in the liquid phase

105F 110F 115F 120F 125F

Figure 7 Effect of temperature on solubility of carbon dioxide

in crude oil B

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Pressure, psi

105F 110F 115F 120F 125F

Figure 8 Effect of temperature on swelling factor of crude oil

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Pressure, psi

14 cc

9 cc

3 cc

Figure 12 Effect of initial sample volume on swelling/extraction of crude oil B

Figure 9 Effect of temperature on swelling/extraction of crude

oil A

Figure 13 Estimation of MMP from extraction test of crude oil

B Figure 10 Three-phase region LLV of crude oil A at 98 ºF

Figure 11 Three- phase region LLV of crude oil C at 78˚F

(25.56˚C) Figure 14 Estimation of MMP from extraction test of crude oil A