1. Comparative Evaluation of a New MMP Determination Technique
SPE Paper 99606
Presented at
SPE/DOE Symposium on Improved Oil Recovery
April 22-26, 2006, Tulsa, OK by
S.C. Ayirala and D.N. Rao
The Craft and Hawkins Department of Petroleum Engineering
Louisiana State University and A & M College, Baton Rouge, LA

2. Changing Mix of EOR in US
U.S. EOR Oil: 460,969 BPD
U.S. EOR Oil: 663,451 BPD
Gas injection EOR share increased from 18% to 48%
CO2 miscible gas injection has become the most popular
For process economics, an accurate, quick and cost effective laboratory
determination of gas-oil miscibility conditions is essential

3. Miscibility Measurement Techniques
Definition:
All definitions of miscibility refer to the absence of an interface, and hence to a condition of "zero" interfacial tension, between the injected fluids and the reservoir crude oil (Benham et al., 1965, Stalkup, 1983, Holm, 1987 and Lake, 1989).
Question?
Is this definition satisfied by the experimental techniques presently used to determine miscibility?
Slim-Tube : indirectly from oil recovery (industry standard)
Rising Bubble : visual observation of solvent bubbles in oil
P-X : two-phase envelope
None of these conventional methods deal with gas/oil interfacial tension.

4. Limitations of the Slim-Tube Technique
There is neither a standard design, nor a standard operating procedure, nor a standard set of criteria (Elsharkaway, 1996)
No consensus on slim-tube miscibility definition (Klins, 1984)
Can give misleading results depending on the level of physical dispersion present (Johns et al., 2000)
Does not simulate important reservoir scale mechanisms such as viscous fingering, gravity over ride and heterogeneity (Danesh, 1998)
Oil recoveries cannot be considered as representative of the unit displacement efficiency to expect in reservoir rocks (Stalkup, 1983)
Costly, time-consuming as it may take several weeks to
complete one miscibility measurement (Elsharkaway, 1996)

5. Pressure OR Enrichment
Vanishing Interfacial Tension (VIT) Technique for Gas-Oil Miscibility (Rao, 1997)
Based on zero interfacial tension at miscibility
Quick, accurate and cost-effective
Interfacial Tension
Pressure OR Enrichment
MMP OR
MME
Successfully implemented for two field projects in Canada
Needs to be validated using standard fluid systems
Compositional dependence of this technique still needs to be examined

6. Comparative Evaluation of VIT with Other MMP Techniques
VIT can provide accurate miscibility results even at extreme conditions of
temperature and pressure and is cheaper and less time-consuming

7. Objectives
To further validate the VIT technique against well known standard gas-oil systems
To examine the compositional dependence of VIT technique by varying the gas-oil ratios of fluids
Standard gas-oil systems used:
1. n-Decane-CO2 at 100oF
Known MMP from rising bubble (1280 psi) and slim-tube (1250 psi)
(Elsharkawy et al., 1996)
2. Live decane (25 mole% n-C1, 30 mole% n-C4 and 45 mole%
n-C10) - CO2 at 160o F
Known MMP from slim-tube and phase diagram (1700 psia)
(Metcalfe and Yarborough, 1979)
IFT measurements: Drop Shape Analysis was used for high IFT’s and capillary rise technique was adapted for low IFT’s

8. Drop Shape Analysis (DSA) for Measuring Interfacial Tension
Numerical integration of the Laplace equation of capillarity:
P =  (1/R1 + 1/R2)
where P is the pressure difference across the interface
R1 and R2 are the principal radii of curvature and
 is the interfacial tension
DSA Strategy:
An objective function is constructed for the deviation of the physical profile of the drop/bubble interface from a theoretical curve that satisfies the Laplace equation
This objective function is then minimized numerically
Input data required:
Local gravity
Densities of the fluid phases
Several coordinate points ( usually) that describe the physical profile of the interface

9. Capillary Rise Technique
Gas
Oil r r r r ρ ρ g θ θ h h
Solvent
Oil
Capillary
Capillary ρ ρ L o
Optical Cell
Tube
A contact angle of  = 0o was used as liquids wet the glass
completely in preference to a gas phase

10. Apparatus Used for IFT Measurements at Elevated Pressures and Temperatures
Optical Cell
Ruska Pump
Live Oil Cell
Heating Oven
Digital Camera
Density Meter
Centrifugal Pump
Light Source
System capabilities: P upto 20,000 psi; T up to 400o F
Accommodates both pendent drop and capillary rise techniques

11. Summary of IFT’s in n-Decane-CO2 System at 100oF
It appears that this study is the first ones to successfully use capillary rise technique with complex hydrocarbon fluids at reservoir conditions
Close match between capillary rise and pendent drop techniques
can be seen at 40/60 mole% gas and oil
IFT becomes independent of gas-oil ratio as fluid phases approached equilibrium, which indicates compositional path independence of the VIT

12. Miscibility Determination Using VIT Technique
(n-Decane-CO2 System at 100oF)
MMP of 1150 psi was obtained using the VIT technique
VIT miscibility matches well with slim-tube miscibility (1250 psi) and rising bubble miscibility (1280 psi)
Considering the variabilities reported in the slim-tube and rising bubble measurements, this can be considered as a good match

13. Summary of IFT’s in Live Decane-CO2 System at 160oF
(Capillary Rise Technique with One-hour Aging Period)
As the fluid phases approached equilibrium, IFT is found to be unaffected by gas-oil ratio even in this standard gas-oil system
This again confirms the compositional path independence of VIT technique

14. Miscibility Determination Using VIT Technique
(Live Decane-CO2 System at 160oF)
MMP of 1760 psi was obtained using the VIT technique
VIT miscibility matches well with slim-tube miscibility (1700 psia) and phase diagram (1700 psia)

15. Effect of Gas-Oil Ratio on Dynamic IFT
(Live Decane-CO2 System at 1100 psi and 160oF)
Effect of Gas-Oil Ratio on Dynamic IFT
Similar near equilibrium IFT for both the gas-oil ratios
High rates of mass transfer at 20/80 mole% gas-oil ratio
More amount of solute in oil and less gas to attain saturation quickly
One-hour aging period accounted for % of equilibrium IFT value

16. Proposed Mechanisms for Dynamic Behavior of Gas-Oil IFT
(a) Vaporizing Mode
(b) Condensing Mode
Diffusing component quickly reaches equilibrium composition within gas-liquid interfacial film due to less resistance to mass transfer
Prolonged intra-phase mass transfer within the bulk fluid phases due to smaller concentration gradient resulting in dynamic behavior of gas-oil IFT
Rapid mass transfer within the interfacial film has much higher degree of influence on gas-oil IFT compared to slow mass transfer within the bulk fluid phase

17. Relationship between Miscibility and Gas-Oil IFT
First-Contact Miscibility:
The term “first contact” is used in VIT technique literally to mean the first contact between the gas and the live crude oil in the VIT cell.
This simulates the field situation where the gas injected down the well first meets the oil in the reservoir, it continues to interact with fresh oil residing ahead of the front.
Meanwhile, the fresh gas that is being continuously injected into the well meets the residual oil that has interacted with the gas previously. The addition of fresh gas during VIT experiments simulates this scenario.
The first contact points in VIT technique correspond to the interfacial tension of “first drop” of fresh live oil contacting the gas phase in the cell, without any oil at the bottom.
Since mass transfer is yet to occur when this first drop image was captured, extrapolation of all such first contact IFT data to zero IFT gives raise to the so-called first-contact miscibility condition.

18. Relationship between Miscibility and Gas-Oil IFT
Multiple-Contact Miscibility:
VIT technique involves contacting fresh live crude oil with the injected gas that has interacted and attained “mass transfer equilibrium” with the reservoir live crude oil residing at the bottom of the VIT cell.
This process simulates the dynamic displacement occurring in the reservoir in that the injected gas continually interacts with the residual crude oil and moves forward to contact fresh live oil.
We may choose to describe this process as multiple-contact, but in the reservoir it is a continuous interaction. The VIT experimental procedure has been developed to simulate this continuous interaction of fluids.
Equilibrium IFT being an equilibrium property does not depend on quantities of fluids in the VIT cell and hence can be considered as being independent of compositional paths.
Equilibrium value of IFT might be reached by several compositional paths due to different gas-oil ratios, but all them must lead to a unique value of IFT at equilibrium.

19. Comparison of VIT Results with Other Conventional Experimental Techniques and Calculation Approaches
Close match between VIT and slim-tube miscibilties within 10% absolute
deviation for all the fluid systems except for RKR
The larger deviations of RKR are attributed to “asphaltenes flocculation”
The mass transfer interactions taking place in slim-tube displacements are
well represented in VIT procedure as well

20. Multi-Stages of Contact Involved in VIT Technique
(Using the Data from Rao (1997) for RKR Reservoir)
VIT first contact miscibility: 60 mole% C2+enrichment in gas phase
VIT multiple-contact miscibility: 51.2 mole% C2+enrichment in gas phase
Dynamic behavior of gas-oil IFT in between first- and multiple-contact represents various multi-stages or contacts of injected gas with live crude oil
Dynamic miscibility conditions range from 51.2 – 60.0 mole% C2+enrichment in gas phase

21. Conclusions
Interfacial tension measurements have been conducted in two standard gas-oil systems of n-decane-CO2 and live decane- CO2 using capillary rise and pendent drop techniques.
This study is the first attempt to successfully adapt capillary rise technique for low IFT measurements at reservoir conditions.
For n-decane-CO2 system at 100oF, VIT MMP (1150 psi) matched well with the reported miscibilities from slim-tube (1250 psi) and rising bubble (1280 psi).
A VIT MMP of 1760 psi has been obtained in live decane-CO2 system at 160oF, which is in good agreement with the reported miscibilities from phase diagram and slim-tube (1685 psi).

22. Conclusions
As the fluid phases approached equilibrium, IFT becomes independent of gas-oil ratio, which exemplifies the compositional path independence of the VIT technique.
Related various types of developed miscibility in gas injection EOR, namely, first- and multiple-contact with gas-oil IFT.
VIT is also a multi-stage contact technique, wherein the multi-stages of contact between the injected gas and crude oil are reflected by the dynamic behavior of gas-oil IFT.
Close match of VIT miscibilties with slim-tube for several gas-oil systems indicates that the mass transfer interactions taking place in slim-tube displacements are well represented in VIT.
The results of this study encourage the wide use of VIT for confident determination of gas-oil miscibility conditions in an easy, quick and cost-effective manner.

23. Acknowledgments
United States Department of Energy for funding this study (Award no. DE-FC26-02NT-15323)
Marathon Oil Company for financial support of the equipment
Dr. Jerry Casteel and Dr. Betty Felber of National Petroleum Technology Office (NPTO) of US DOE for continued support and encouragement
Wei Xu and Daryl S. Sequeira of LSU’s Petroleum Engineering Department for the technical help