Ph.D. Student Ben Niu Supervisor: Ass.Prof. Alexander A. Shapiro Ass. Prof. Wei Yan

1. Phase Identification and Saturation Determination in Carbon Dioxide Flooding of Water Flooded Chalk Using X-ray Computed Tomography Ph.D. Student Ben Niu Supervisor:Ass.Prof. Alexander A. Shapiro Ass. Prof. Wei Yan Prof. Erling H. Stenby

2. Content • Introduction • Aim of the study • Experimental facilities • Experimental procedures • Experimental results • Conclusions • Future plan Ben Niu

3. Introduction CO2 flooding of oil reservoir can potentially enhance oil recovery significantly. At the same time, 1/3~1/2 injected CO2 could be stored during this process. CO2 flooding is characterized by complex phase behavior and geochemical reactions CO2 flooding experiments were performed using CT (Computed Tomography) scanning to determine multiphase in-situ saturations for further understanding the recovery mechanisms. This study is a part of the project ‘EOR through CO2 Utilization’ funded by the Danish National Advanced Technology Foundation and DONG Energy 9/24/2014 Ben Niu Ben Niu 3

4. Aim of the Study Long – term goal: Three phase flooding on chalk reservoir samples at high P and T by using CT scanning, to obtain information on phase saturation. Flooding experiments with CT scanning in the literature Two immiscible phases (many) Two miscible phases (some) Three-phasefloodingat irreducible water saturation (immobile water) at high P and T (several) Three-phase flooding at high P and T (almost none) Short–term objectives: Investigate if different phases could be identified simultaneously Find solutions for identifying the various phases in three phase flooding 9/24/2014 Ben Niu Ben Niu 4

5. Experimental Facilities CT Scanner • Fourth generation Siemens SOMATOM scanner • Three energy levels: 80kV, 120kV, 137kV • X-ray computer tomography may provide information on density, porosity, mineralogy, hetergeneities, phase saturations…… Ben Niu

6. Experimental Facilities Core Holder • Aluminium core holder (Max. 100bar) • Carbon fiber core holder (1000bar,150C) Carbon fiber wrapped coreholder Shematic of experimental setup Ben Niu

7. Experimental Procedures • Four core flooding experiments were carried out to investigate if different phases could be identified simultaneously. • Fluids: • Isopar-L, n-decane • Distilled water • CO2 • Dopants needed to increase contrast of CT numbers • Gas and oil phases have similar densities at high pressure experimental conditions (65bar-100bar); • Oil phase: Iodododecane (C12H25I), Water phase: potassium iodide (KI), sodium tungstate dihydrate (Na2WO4·2H2O). • Two energy levels, 80 kV( 250mA·s) and 120 kV (330mA·s) • Reduce the beam-hardening effects; • Protect the machine (the tube life of CT scanner). 9/24/2014 Ben Niu Ben Niu 7

8. Experimental Procedures Reference scans: dry core, core saturated with each fluid Flooding: inj. rate 0.1 cc/min, scanned at every 2mm 1st Exp. 2nd Exp. 3rd Exp. 4th Exp. 100% Oil No flooding Oil at 1-Swi Oil at 1-Swi Water flooding Water flooding Gas flooding Gas flooding Gas flooding Clean and dry the core 9/24/2014 Ben Niu Ben Niu 8

9. 1st Experiment Core Materials • Perfrom CO2 flooding at residual oil saturation with doped water and doped oil. 100% Oil Wat. flooding Gas flooding Ben Niu

10. 1st Experiment Data Analysis • CT number at different energy levels Ben Niu

11. 1st ExperimentData Analysis • Sarutations are not corrected, negative saturation is calculated. Ben Niu

12. 1st Experiment Data Analysis • Dopant is adsorbed at the inlet of the core. CT and 3-D reconstruction images (120 kV) of a dry core after the experiment Ben Niu

13. 2nd Experiment Core Materials • Test dopant (C12H25I) with various concentrations No flooding Ben Niu

14. 2nd Experiment Data Analysis • Three-phase can be treated as pseudo-two phase flooding. Fig.7 CT number at different energy levels (Experiment 2) Ben Niu

15. 2nd Experiment Data Analysis • Adsorption happens even at low concentration of dopant. Ben Niu

16. 3rd Experiment Core Materials • Determine water saturation under three phases conditions Oil at 1-Swi Gas flooding Ben Niu

17. 3rd Experiment Data analysis • CT number at different energy levels Ben Niu

18. 3rd Experiment Data analysis • Water saturations from mass balnce and CT scanning agree well Ben Niu

19. Challenges in Data Analysis • In a homogenous chalk, the CT number of the pixels follows a normal distribution. The mean value and standard deviation obtained from different software are correct (FPImage and Image J). CT number distribution in the region of interest of the dry core Ben Niu

20. Challenges in Data Analysis • Software Challenges: • Inhomogeneity of the core • High density objects in the core, • give high CT numbers • Mean value and standard deviation are affected by the noise Ben Niu

21. Challenges in Data Analysis • Maximum CT number CTmax • The maximum CT number is 3071, and any pixels with a higher value will be set to 3071. • By using dopant, decent contrast of CT numbers can be obtained at 120kV. But CT numbers at 80kV may exceed the upper limit. • Can we estimate the correct mean value if some of the CT values are higher than CTmax? Ben Niu

22. Challenges in Data Analysis • An optimization method (nonlinear Huber estimation) is used to meet the two challenges, correcting the CT number, thus avoid the influence from inhomogeneity of the core and compensate for overflow of CT values. Challenge 1 Ben Niu

23. Challenges in Data Analysis • Optimization changes the mean value by 25 for Challenge 1 and 101 for Challenge 2 • ΔCT between oil and water is around 100 Challenge 2 Ben Niu

24. Challenges in Data Analysis • An example for the corrected data at 80kV (3rd Experiment), in which ImageJ does not work well. • Saturation calculated based on optimization is better than that from ImageJ. (Results form 120kV as reference) • When oil and water both are doped, the accuracy of saturation calculation could be enhanced Ben Niu

25. 4th Experiment Core Materials • Identify three phases saturation with application of optimization method Oil at 1-Swi Wat. flooding Gas flooding Ben Niu

26. 4th Experiment Data analysis • CT number at different energy levels Ben Niu

27. 4th Experiment Data analysis • Three phase saturation and mass balance Ben Niu

28. 4th Experiment Data analysis • Calculation of three-phase saturatons Subscripts o oilphase w water phase g gas phase wog three phases 1, 2 energy level 1, 2 • The CT numbers of different phases increase similarly when the energy level changes from 80 to 120 kV, making the coefficient matrix A3 ill-conditioned (the smallest determinant is 0.018) and calculated saturation extremely sensitive to CT numbers (1 unit error in CTo1 leads to a maximum deviation of 0.127 in the calculated So) • If the entries in the second column of A3 have opposite signs, A3 can be well-conditioned. Possible dopant with Kedge between 120 kV and 80 kV could be the choice (rarely used). Ben Niu

29. Conclusions Comparison of the dry core images before and after experiment is important to reveal the problem of dopant adsorption High pressure three-phase flooding can be visualized as pseudo two-phase, where the most interesting phase can be identified at a single energy level. CT numbers in a homogeneous chalk sample follow normal distribution at 80 kV and 120 kV. The nonlinear Huber estimation is a useful tool to correct the average CT number at 80 kV. The major reason for the failure in simultaneous determination of the three-phase saturations is the is ill-conditioned saturation equation. 9/24/2014 Ben Niu Ben Niu 29

30. Future Plans The concept of ‘Pseudo two-phase’ flooding will be applied in the future experiments with the Danish chalk cores. More dopants, with suitable properties, will be tested under the two scanning energies. 9/24/2014 Ben Niu Ben Niu 30