S'-

COMPONENT PARTITIONING IN COg-HYDROCARBON SYSTEMS:

THE EFFECTS OF CHEMICAL TYPE

A THESIS

SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL

OF THE NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY

by

Matthew Silva

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

MASTER OF SCIENCE

IN PETROLEUM ENGINEERING

May 1984

PRRCUBRARYCOPy

(m)

ACKNOWLEDGEMENTS

I wish to thank Lynn Orr for sharing his insight into the subject

of CO2 flooding and providing encouragement throughout this effort.

I also wish to thank the members of my committee, Joe Taber, John

Heller and Charles Nathan, for their support.

In addition, Joe Franklin, Chris Lien, Kent Wainwright, and Mickey

Pelletier deserve recognition for their special contributions to this

project.

I also wish to thank Dileep Dandge with x^hom I had a number of

discussions about the extraction of organic compounds by carbon dioxide.

A note of thanks also goes to Kathy Grattan for typing this thesis,

to Paula Bradley for her very helpful advice in the area of technical

writing, and to Jessica McKinnis for drafting the figures.

The support that made this research effort possible was provided by

the U.S. Department of Energy, the New Mexico Energy Research and

Development Institute and a consortium of companies and foundations

including the Abu Dhabi Reservoir Research Foundation, American

Cyanamid, the Amoco Foundation, ARCO Oil and Gas, Conoco, Marathon,

Mobil, Shell Development, Sohio, Sun Exploration and Production, and

Tenneco. That support is gratefully acknowledged. In addition, support

was provided by the Petroleum Recovery Research Center.

Crude oil samples were provided by Mobil Research and Development

Corporation, Conoco, Inc., and Pennzoil Exploration and Production

Company.

i

TABLE OF CONTENTS

PAGE

NO.

LIST OF TABLES ii

LIST OF FIGURES iii

ABSTRACT 1

1. INTRODUCTION 2

Role of Phase Behavior in the Development ofMiscibility 4

Slim Tubes and "Minimum Miscibility Pressure" 8

Effects of Oil Composition on MMP 13

Binary C02-Hydrocarbon Systems 16

Multicomponent Systems 26

2. PHASE BEHAVIOR AND FLUID PROPERTY MEASUREMENTS 32

Experimental Procedure 32

C02-Crude Oil Systems 35

C02-Synthetic Oil Systems 46

3. INTERPRETATION OF EXPERIMENTAL RESULTS 58

4. IMPROVEMENT OF CORRELATIONS FOR MINIMUM MISCIBILITY

PRESSURES • 66

Correlating Parameter 66

Preliminary Tests of Correlation Accuracy 69

5. SUMMARY AND CONCLUSIONS 78

REFERENCES 81

ii

LIST OF TABLES

PAGE

NO.

Table 1.1 Extraction of Hydrocarbons by CO2 in BinarySystems 23

Table 1.2 Extraction of Hydrocarbons by CO2 in BinarySystems 25

Table 1.3 Synthetic Oil Compositions for Phase BehaviorStudies Conducted by Monger 28

Table 1.4 Physical Properties of Hydrocarbons Used inSynthetic Oils 30

Table 2.1 Compositions of Gas and Liquid Samples from CMCExperiment for Maljamar Crude Oil at 1400 psiaand 90°F 36

Table 2.2 Comparison of Oil Properties and Run Conditionsfor Continuous Multiple Contact Experiments forMaljamar and Rock Creek Crude Oils 40

Table 2.3 Composition of Mixture of Aromatic HydrocarbonsAdded to Rock Creek Crude Oil 40

Table 2.4 Composition of Rock Creek Crude Oil Mixed withSelected Aromatic Hydrocarbons 40

Table 2.5 Compositions and Molecular Weights of FourSynthetic Oils 49

Table 3.1 Hydrocarbon Components for SyntheticBlend 60

Table 4.1 Experimental Data with Weighted C2+ FractionParameter 70

Table 4.2 Composition of Recombined ReservoirOils 73

Table 4.3 Comparison of Predicted and MeasuredMMPs 76

iii

LIST OF FIGURES

PAGE

NO.

Fig. 1.1 Typical pseudo-ternary representation of a crudeoil with carbon dioxide 6

Fig. 1.2 Pseudo-ternary representation of compositionaleffects in the displacement of oil by CO2 7

Fig. 1.3 Effect of pressure on the size of the region oftie line extensions 10

Fig. 1.4 Schematic of a slim tube experiment 12

Fig. 1.5 Qualitative representation of phase behaviorfor binary systems containing CO2 and lightnormal alkanes 18

Fig. 1.6 Qualitative representation of phase behaviorfor binary systems containing CO2 and heavyalkanes 18

Fig. 1.7 PT projection of the critical loci of CO2-tetralin and C02-decalin binary mixtures 21

Fig. 1.8 Critical loci of C02-hydrocarbon binary mixtures ... 22

Fig. 2.1 Continuous multiple contact apparatus 34

Fig. 2.2 Pseudo-ternary representation of phase compositionsof mixtures of CO2 with Maljamar separator oilat 1400 psia and 90°F 37

Fig. 2.3 Pseudo-ternary representation of phase compositionsof mixtures of CO2 with Maljamar separator oilat 1200 psia and 90°F 38

Fig. 2.4 Pseudo-ternary representation of phase compositionsof mixtures of CO2 with Rock Creek separator oilat 1300 psia and 75°F 39

Fig. 2.5 Comparison of partition coefficients for RockCreek and Maljamar crude oils mixed with CO2 41

Fig. 2.6 Pseudo-ternary representation of phase compositionsof mixtures of CO2 with Rock Creek oil containingadded aromatic components at 1300 psia and 75®F .... 43

IV

PAGE

NO.

Fig. 2.7 Comparison of partition coefficients for Rock Creekoil and Rock Creek oil with aromatics added 44

Fig. 2.8 Comparison of extraction of hydrocarbons by denseCO2 for Rock Creek crude oil and Rock Creek oilwith added aromatic components 47

Fig. 2.9 Measured densities for C02-synthetic oil mixtures ... 50

Fig. 2.10 Pseudo-ternary representations of phase compositionsof CO2-hydrocarbon mixtures 51

Fig. 2.11 Partition coefficients in C02-synthetic oilmixtures 53

Fig. 2.12 Effects of changes in tie line slope, solubility ofcomponent, and extraction of component 3 onpartition coefficients 55

Fig. 4.1 Hydrocarbon equilibrium values (with CO2 removed fromthe calculation) for C02-Maljamar crude oil mixturesat 1200 psia and 90°F 68

Fig. 4.2 CO2 density required for miscible type displacementvs. C5-C3Q content 71

Fig. 4.3 CO2 density required for miscible type displacementvs. Z Wi kj 72

1

ABSTRACT

This thesis reviews the relationship between phase behavior and the

development of a miscible type displacement to determine the effects of

hydrocarbon molecular type on phase behavior for C02-hydrocarbon

systems. The continuous multiple contact experiment is used to generate

detailed phase composition and density information for C02-crude oil

systems and four distinctly different C02-synthetic oil systems

(n-alkanes, iso-alkanes, naphthenes, and aromatics). The experimental

results are compared with published data for clues to relate phase

behavior to displacement results observed for systems containing crude

oils. An experiment to resolve unanswered questions about the role of

attached alkyl groups commonly found in reservoir oils is outlined as

well as an improved correlation for predicting displacement pressures

required to achieve miscibility.

1. INTRODUCTION

To date, research into the mechanisms by Xi/hich carbon dioxide

recovers crude oil from a reservoir has established a number of key

relationships. For instance, the efficient recovery of oil relies

directly on the efficient extraction of hydrocarbon components into a

C02-rich phase. Furthermore, the level of hydrocarbon extraction has

been directly related to the density of carbon dioxide. At least one

study, that of Holm and Josendal (1982), produced very strong evidence

that, in addition to CO2 density, the size and chemical type of the

hydrocarbons found in crude oils also influences to some extent the

efficiency of hydrocarbon extraction. This thesis focuses on the

relationship between hydrocarbon molecular type and phase behavior as

part of a larger research effort aimed at developing an improved

correlation for predicting the displacement pressures required to

recover oil efficiently from reservoirs targeted for CO2 floods.

This chapter opens with the development of a few key concepts

needed to understand how CO2 miscibly recovers crude oil. In

particular, the chapter discusses the role of phase behavior in the

development of miscibility and the use of slim tube displacements to

determine what is commonly referred to as the minimum miscibility

pressure. This discussion is followed by a review of some experimental

investigations which used slim tube displacements to delineate the role

of oil composition in the displacement process. The research of Holm

and Josendal (1982) dominates much of this discussion. In addition to

drawing a clear picture of the effects of hydrocarbon molecular size,

their study produced some surprising results which indicated that

aromatic oils require slightly lower pressures than paraffinic oils to

generate raiscibility with C02- A review of the limited amount of

component partitioning data for binary and mul t ic ompo nent

C02-hydrocarbon systems fails to explain their results and raises a few

additional questions about the behavior of these systems. For instance,

how do hydrocarbon components with widely different molecular structures

(paraffins, naphthenes, and aromatics) partition in the presence of CO2?

Do cyclic molecules act as more efficient co-solvents than paraffinic

molecules as suggested by Holm and Josendal? And, does the attachment

of alkyl groups to hydrocarbon molecules influence their extraction in

multicomponent systems? Finally, does an aromatic crude oil exhibit

phase behavior significantly different from that of a paraffinic crude

oil when contacted by carbon dioxide?

Chapter two systematically addresses each of these questions with a

suite of phase behavior experiments designed to provide detailed

component partitioning information. The novel technique used by the

continuous multiple contact apparatus to generate these data is briefly

reviewed. Phase compositions for three crude oils containing differing

amounts of aromatics are compared. Also compared are measurements of

partitioning in four synthetic oil systems of similar overall molecular

weight but composed of n-alkanes, branched allcanes, naphthenes and

aromatics.

Chapter three examines the almost identical phase behaviors of an

aromatic oil and a paraffinic oil. An indication of the effects of

attached alkyl groups couples with an investigation into typical crude

oil compositions to suggest an explanation. However, conclusive

evidence requires an experiment designed to delineate also the role of

allcyl groups in component partitioning. That experiment is briefly

outlined. The discussion also presents explanations to account for the

differences Holm and Josendal observed between displacements with an

aromatic oil and those conducted with a paraffinic oil. This leads to a

reexamination of their correlation which depends on a rudimentary

characterization of oil composition.

Chapter four presents a recently proposed correlation that uses the

type of component partitioning data generated by the continuous multiple

contact experiment as a basis for determining a parameter representing

oil composition. The merits of the method are discussed, results are

compared with other correlations and experimental data, and the

additional work needed to develop the correlation is outlined.

Chapter five summarizes the results of the investigation and lists

the conclusions that can be drawn from the information presented.

Role of Phase Behavior in the Development of Miscibility

The key to understanding how aromaticity might affect recovery

efficiency first requires a clear picture of the phase behavior

mechanism which allows carbon dioxide to recover oil miscibly. The term

"miscible" is used rather loosely throughout the CO2 flooding

literature. Usually carbon dioxide is not completely miscible with

crude oils at typical reservoir conditions. However, the continuous

process of preferential extraction of lighter hydrocarbons into a

C02~rich phase leads to the development of a very efficient

displacement. Hutchinson and Braun (1961) first used ternary

representations of phase behavior to illustrate the development of

miscibility with methane as the displacing solvent. Similar arguments

can be applied to C02-crude oil systems. Of course, crude oils consist

of hundreds of organic compounds, and the use of ternary diagrams

requires that the oil be described by only two pseudo—components as

shown in Figure 1.1. Orr, Silva and Lien (1983) demonstrated that this

simple representation of phase compositions gives enough information to

make fairly accurate quantitative predictions of one-dimensional

displacements. In general, ternary diagrams offer a convenient method

of representing phase compositions for mixtures containing three

components. Tie lines within the two-phase region connect each upper

phase composition with its corresponding lower phase composition. The

series of tie line end points generates a binodal curve with the plait

point dividing the upper and lower phase composition portions of the

curves.

The type of phase behavior shown here represents but one of several

associated with C02-crude oil mixtures (Orr, Yu and Lien 1981). In this

simple representation, the light hydrocarbon component acts as a

co-solvent for the system. A co-solvent serves to promote miscibility

between two immiscible substances. Francis (1955) showed for several

different ternary systems that the solubility of an oil in carbon

dioxide can be enhanced by many different co-solvents. In the case of

crude oils, as shown in Figure 1.1, the crude oil contains its own

co-solvent in the form of lighter hydrocarbons. Thus, miscibility

between the crude oil and CO2 can be promoted by increasing the

concentration of the co-solvent.

Figure 1.2, first presented by Orr (1983), combines a ternary

representation of phase behavior with a simple model of a porous medium

to show how this is accomplished. This explanation of the diagram

TWO PHASE

REGION

LOWER PHASE

COMPOSITIONS

HEAVY

HYDROCARBONS

CO.

SINGLE

PHASE

REGION

UPPER PHASE

COMPOSITIONS

PLAIT

POINT

LIGHT

HYDROCARBONS

(Cg-C,^)

Figure 1.1 Typical pseudo-ternary representation of acrude oil with carbon dioxide.

HEAVY lightHYDROCARBONS HYDROCARBONS

a. PSEUDO-TERNARY DIAGRAM

STEP

1 I

2 I

3 I

0 0 0 0 0 0

V,L|

0—

0— 0 0

— 0

VR,

LR|—

V2

L20 0 — 0 0

VRg ^2 V2 +0 0 0 n

, LR2 L3\J

b. SEQUENCE OF MIXING CELLS

Figure 1.2 Pseudo-ternary representation of compositional effectsin the displacement of oil by CO2.

closely follows that given by Orr. Time step 0 shows that prior to CO2

injection the cells contain only crude oil. In the first time step

carbon dioxide is injected into the first mixing cell until the

resulting overall composition lies at point A. The equilibrated mixture

splits into tv7o phases with compositions and The C02-rich phase,

has a significantly lower viscosity than L|̂ because has a higher

concentration of CO2 and preferentially-extracted lighter hydrocarbons.

In an actual porous medium the less viscous phase, would be expected

to travel to the front of the transition zone and contact more fresh

oil. This simple model, composed of a series of discrete mixing cells,

assumes that only the less viscous phase is mobile. As more CO2 is

injected into the first cell, the upper phase, Vj^, proceeds to the

second cell and combines with fresh oil to form phases V2 and L2.

Meanwhile, the components in the first cell have partitioned into an

upper phase richer in carbon dioxide and light hydrocarbon, VR^, and a

lower phase, LRj^, which contains the residual heavier hydrocarbons and

some dissolved carbon dioxide. With continued CO2 injection, phase V2,

which is now directly miscible with the original reservoir oil, moves to

the leading edge of the transition zone to contact more fresh oil. The

molecular weight of the residual hydrocarbons continues to increase at

the trailing edge as the lighter components are preferentially extracted

and carried away.

Slim Tubes and "Minimum Miscibility Pressure"

Clearly, phase behavior determines whether or not miscibility

develops. Factors which can influence phase behavior include

temperature, pressure, and oil composition. The temperature and oil

composition are, of course, reservoir characteristics that cannot easily

be changed. Figure 1.3 couples two ternary diagrams to illustrate

qualitatively the effects of pressure on phase behavior and consequently

displacement efficiency. In both diagrams the complex composition of

the reservoir oil is again represented as two pseudo-components. The

top diagram is at a lower pressure than the bottom one. The extended

critical tie line in each passes through the plait point adjacent to the

binodal curve. In the top diagram the critical tie line intersects the

baseline to the right of the original oil composition so that the

original oil composition lies within the region of tie line extensions.

As the displacement proceeds, the composition at the front of the

transition zone again travels down the binodal curve, but in this case

it encounters a limiting tie line before miscibility develops. The

upper phase composition remains fixed as it contacts additional fresh

oil. Thus, the displacement remains immiscible. Increasing the

pressure as shown in the bottom figure alters the component

partitioning. The increase in hydrocarbon extraction into the C02-rich

phase combines with the increase in CO2 solubility in the oil-rich phase

to decrease the size of the binodal curve and to shift the tie line

slopes in a favorable way. In this case, the region of tie line

extensions does not include the original oil composition and, hence, oil

can be "miscibly" recovered by the same mechanism described for Figure

1.2.

Rather than measure phase behavior directly, the studies of Holm

and Josendal (1982) and Yellig and Metcalfe (1980) relied on slim tube

displacements to measure the effects of temperature and oil composition

on recovery efficiency. Slim tube displacements are usually conducted