NMR Measurement and Viscosity Evaluation of Live Bitumen

1. NMR Measurement and Viscosity Evaluation of Live Bitumen Elton Yang, George J. Hirasaki Chemical Engineering Dept. Rice University April 26, 2011

2. Introduction & Objective • The well log T2 measurements on the live bitumen appear to be significantly longer than the laboratory NMR measurements of dead bitumen sample. This is likely due to the dissolved gas in heavy oil. • Saturate the bitumen sample with three reservoir gases (CO2, CH4, C2H6) at different pressure levels in laboratory. Make NMR and viscosity measurements on recombined live heavy oils. • Correlate the T2, viscosity, and gas content of live bitumen and resolve the differences between the NMR log and laboratory data.

3. Samples and Equipments • Sample: Bitumen Sample #10-19 • Three gases (CO2, CH4 and C2H6) used in this work are provided by Matheson Tri-Gas with product grade of Ultra High Purity. • 2 MHz Maran Spectrometer (Oxford Instrument). • A 40 mm probe with minimum TE = 0.2 msec was employed for all the NMR measurements on bitumen. • Brookfield Viscometer LVDV-III+ (Brookfield Company) for dead oil at different temperatures . • Capillary viscometer for live bitumen at room temperature.

4. T2 Distribution of Bitumen #10-19 at Different T & Corrected T2 with Specified M0 and Lognormal Distribution Model** ** Yang and Hirasaki, JMR,2008

5. Correlation Between Corrected T2 and Viscosity/Temperature Ratio for Three Different Heavy Oils • T2 values are corrected by using lognormal distribution model and specified M0 • Corrected T2 and viscosity/temperature ratio of three dead oil samples closely follow linear relationship on log-log scale. • Data from Brookfield oil deviates from the data of two bitumen samples.

6. Measurements on Live Heavy Oils • The pressure vessel was manufactured by TEMCO and was customized to fit the 40 mm probe. The minimum echo spacing = 0.2 msec. • Pressurized gas was injected into the vessel from top. The gas pressure inside the vessel was monitored during the entire process. NMR measurements were performed periodically. • Convection was generated by rocking the pressure vessel to boost the gas dissolving rate. After equilibrated at the highest pressure, the gas-bitumen system was depressurized to different lower pressure levels. • Viscosity of live bitumen was measured and correlations between T2, viscosity, pressure and gas solubility were established. Generation of Convection

7. Changes of T2 and Pressure of C2H6 Dissolved Bitumen During Pressurization Stage • Bi-modal for the peak of bitumen with C2H6 as C2H6 gradually transfers into bitumen. • Bitumen and gas reached equilibrium after 308 hours.

8. Depressurization of C2H6 to Lower Pressures

9. T2 of C2H6 Saturated Bitumen at Different Pressures • The dissolving of C2H6 in Bitumen significantly changes oil T2. • The T2 of C2H6 saturated bitumen decreases as equilibrated pressure decreases. • The bitumen peak is broad and has fast relaxing components shorter than TE even at the highest saturation pressure. • T2 from regular interpretation > T2 from lognormal distribution model with specified M0. The difference decreases as saturation pressure increases.

10. Corrected Initial Pressures at Different Pressure Levels for Solubility Calculation (Example: C2H6-Bitumen) Pressurization Stage • System would be either heated by pressurization or cooled by depressurization temporarily, and then return to the temperature of air bath (30 oC). • Significant pressure change resulting from the temperature fluctuation would display incorrect P0 for the solubility calculations. • Extrapolation is employed to remove the temperature effect on the initial pressure reading. Depressurization Stage

11. Summary for Live Bitumen with Different Gases • T2 vs P of each reservoir gase is found to be closely linear on semi-log scale and extrapolated near the value of dead oil T2 . • Solubility of CH4 and C2H6 in the bitumen follow the Henry’s law well . • The calculated solubility of CO2 in bitumen is overestimated.

12. Correction for Deviation of CO2 Solubility in Bitumen L-L-V Three-Phase-Equilibrium could have formed inside the pressure vessel

13. Correlation Between T2 and Viscosity/Temperature Ratio for Bitumen and Brookfield Oil Brookfield Oil Bitumen • Regardless of the gas type used for saturation, the live oil T2 correlates with viscosity/temperature ratio on log-log scale. • The changes of T2 and viscosity/temperature ratio caused by gas saturations in oil follows the same trend of those caused by temperature variations on the dead oil.

14. Comparing with Previous T2 vs Viscosity Data Relaxation time and viscosity/temperature ratio are normalized with respect to 2 MHz as shown below**: ** Hirasaki, Lo and Zhang, Magnetic Resonance Imaging, 2003

15. Conclusion • The live bitumen T2is significantly larger than T2of dead bitumen, even at the lowest pressure level in this work (~100 psia). • The relationship between live bitumen T2 and equilibrium pressure / solubility is linear on semi-log scale for all three reservoir gases. • Regardless of the gas type used for saturation, the live bitumen T2 correlates with viscosity/temperature ratio on log-log scale. • More importantly, the changes of T2 and viscosity/temperature ratio caused by solution gas follows the same trend of those caused by temperature variations on the dead oil.

16. Appendix A • The method for computing solubility from pressure data is described as follows: (1) Pressurization stage: (2) Depressurization stage: • sg,i is the solubility at current pressure level. sg,i-1 is the solubility at previous pressure level right before the depressurization. • Vg is the volume of vapor phase inside the pressure vessel. Voil is the volume of oil sample inside the pressure vessel. Assuming the swelling effect of oil in this work is negligible, both Vg and Voil are constant. • P0 and Peq are system pressure at beginning and pressure at equilibrium after each pressurization/depressurization, respectively. • z0 and zeq are compressibility at beginning and compressibility at equilibrium after each pressurization/depressurization, respectively.

17. Back-up Slides

18. Collected data in CPMG Mo from FID Approach to Compensation for T2 Information Loss • Determine initial magnetization M0 from FID. • Supplement M0 into the regular CPMG data and assume lognormal distribution for bitumen.

19. Changes of T2 and Pressure of CO2 Dissolved Bitumen During Pressurization Stage

20. Depressurization of CO2 to Lower Pressures

21. T2 &T1 of CO2 Saturated Bitumen at Different Pressures • The dissolving of CO2 in Bitumen significantly changes oil T2. • T2 from regular interpretation > T2 from lognormal distribution model with specified M0. The difference decreases as saturation pressure increases. • The change of T1 with pressure is much less significant, comparing to the corresponding T2. • The change of bitumen viscosity has much more effect on the T2 response rather than T1.

22. Changes of T2 and Pressure of CH4-Bitumen at Different Pressure Levels Pressurization Stage Depressurization Stage

23. T2 of CH4 Saturated Bitumen at Different Pressures • The change of bitumen T2 resulting from the saturation of CH4 is obviously less significant than that observed in the case of CO2 or C2H6 • The T2 of C2H6 saturated bitumen decreases as equilibrated pressure decreases. • The minor peaks between 100 msec and 1 sec are from CH4 in the vapor phase. As pressure decreases, the gas peak moves to the smaller values and peak area shrinks. • T2 from regular interpretation > T2 from lognormal distribution model with specified M0. The difference decreases as saturation pressure increases.

24. Re-adjustment of z factor of CO2 to Correct the Calculated Solubility to Follow Henry’s Law • Adjustment of z0 at the initial pressure gives the re-evaluated value of z factor (z0*) to be 0.96, which is very unlikely for the compressibility factor of CO2 at 745 psia. • Adjustment on of ze at the equilibrium pressure shows that, the corrected value of z factor (ze*) needs to move down to 0.55 at 709 psia. • The calculated mole fraction of CO2 in vapor phase is 0.54, and the mole fraction in CO2-rich liquid phase is 0.46. Correspondingly, the volume fraction of CO2 in either vapor phase or CO2-rich liquid phase is calculated to be 0.82 and 0.18, respectively.