PHASE IDENTIFICATION AND SATURATION DETERMINATION IN CARBON DIOXIDE FLOODING OF WATER FLOODED CHALK USING X-RAY COMPUTED TOMOGRAPHY

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1 SCA /12 PHASE IDENTIFICATION AND SATURATION DETERMINATION IN CARBON DIOXIDE FLOODING OF WATER FLOODED CHALK USING X-RAY COMPUTED TOMOGRAPHY Ben Niu, Wei Yan, Alexander A. Shapir, Erling H. Stenby Department f Chemical and Bichemical Engineering, Technical University f Denmark This paper was prepared fr presentatin at the Internatinal Sympsium f the Sciety f Cre Analysts held in Nrdwijk, The Netherlands September, 2009 ABSTRA As an effective methd t cpe with green-huse gas emissin, and t enhance il recvery, injectin f carbn dixide (CO 2 ) int water flded petrleum reservirs has btained increasing attentin. In the labratry studies, identificatin f different phases and determinatin f their saturatins by use f X-ray cmputed tmgraphy () is the key prblem t visualize flw and t get insight int the mechanisms f CO 2 injectin. The flding prcess is essentially three-phase, and invlves interactin between il, water and CO 2 (mutual slubility, change f acidity, swelling f the il, etc.). The purpse f ur study is t investigate experimentally the pssibility and ptential prblems f identifying different phases and quantifying their saturatins during CO 2 flding. Labratry experiments n injectin f CO 2 int chalk cres frm the Danish Nrth Sea, saturated with Ispar-L petrleum mixture and distilled water, with varius cmbinatins f dpants, have been carried ut under the pressures varying in the lw t medium range. The experiments utilized imaging t visualize the in-situ saturatins f the fluids. An ptimizatin methd was suggested fr the image data analysis, in rder t crrect the ut-f-range data due t lw energy level scanning and inhmgeneity f chalk. During experiments, strng adsrptin f sme dpant n the chalk cres was bserved. Sensitivity f chalk t different dpants is discussed. Based n recrded data during CO 2 flding, as well as n the image data and prductin prfiles f the different phases, the detailed data analysis was cnducted. The key factrs fr the successful applicatin f the X-ray cmputed tmgraphy t the multiphase flws in a chalk rck were identified. The necessary cnditins fr successful experiments are suggested. INTRODUION The carbn dixide injectin int petrleum reservirs has been cnsidered fr enhanced il recvery (EOR) since early 1950's. Attentin t this prcess is currently grwing due t the glbal warming. This prcess is characterized by cmplex phase behavir and interactin between different phases: il, water and CO 2. The mechanisms f multiphase flw and thermdynamic prperties f varius phases have been well established and discussed by many researchers, bth theretically [1] and experimentally [2]. In realistic cnditins, displacement is affected by presence f the three phases (water, il, and carbn dixide) and chemical interactin between rck and the fluids. Identificatin f all three phases and their distributin in reservir rck by the

2 SCA /12 applicatin f X-ray cmputer tmgraphy () may prvide useful practical infrmatin fr understanding the prcess f CO 2 injectin. X-ray has been applied t cre analysis in the il industry ver thirty years [3]. It has als been applied t recvery studies, sample characterizatin, frmatin damage, etc. [4]. Fr example, Zitha et al. [5] and Du et al. [6] recently investigated CO 2 rhelgy in prus media. Izgec et al. [7] studied the chemical reactins assciated with injectin and strage f CO2 in carbnate frmatins. Jikich et al. [8] measured the srptin istherms f CO 2 in cal cres. It shuld be nted that mst f the recent studies deal with tw-phase prcesses. A few papers n three phase identificatin either d nt illustrate in-situ saturatin measurements r cnduct the experiments at lw pressure cnditin t keep gas as lw density vapr phase, which cannt represent reservir cnditins. Vinegar et al. [9] discussed the mechanisms f X-ray scanner, prperties f the dpants, and three phase flw study, but withut detailed analysis f the three phase in-situ saturatins. Chatzis et al. [10] applied X-ray t study the physical prperties f prus media and saturatin prfiles during tw- and three-phase flw in gravity-assisted displacement prcesses, with qualitatively visualizatins f three phase fluid distributins. Lackner et al. [11] investigated X-ray energy levels fr three-phase saturatin measurements and cnducted three phase experiments under lw pressure, withut analysis f phase saturatins. There are few discussins n the three phase measurements in the chalk cre with CO 2 flding. In ur study, fur chalk cres frm the Danish Nrth Sea are used as material. Key factrs n the successful applicatin f X-ray n the identificatin f three-phase flws under CO 2 flding are discussed based n the experimental data btained. EXPERIMENT Equipment The scanner used in ur study is a furth generatin Siemens SOMATOM scanner (figure 1) with 1200 statinary detectrs. The X-ray tube rtates arund the bject in a 360 circular path. As the beam passes thrugh the bject, the lwer energy phtns are preferentially absrbed. The attenuatin cefficient is usually scaled and reprted as a number. numbers are generally presented in terms f an internatinally standardized number scale subdivided int Hunsfield units (HU). The scale is linear, with numbers fr air and water defined as 1000 and 0 respectively. The scanner has three different energy levels: 80kV, 120kV, and 137kV, and a higher energy level prvides better reslutin. The maximum number is 3071 at all energy levels. An individual scan prduces a digital 2-D image map f the X-ray attenuatin in a tmgraphic slice thrugh the bject, and the image infrmatin is stred in a 512 by 512 matrix. The lighter clr n the image indicates higher numbers. A series f 2- D images can be used t cnstruct a 3-D image. All image analysis were perfrmed by special graphical sftwares FPImage ( and ImageJ ( Tw kinds f X-ray cre hlders were used in the experiments: an aluminum cre hlder with a maximum peratin pressure f 100 bar, and a carbn fiber wrapped cre hlder with a maximum peratin pressure f 1000 bar and a maximum temperature f 150 C. The schematic f experimental setup is shwn in figure 2.

3 SCA /12 Experimental Prcedure Fur similar cre flding experiments at rm temperature and different pressures were carried ut t investigate if different phases culd be identified simultaneusly. Table 1 prvides an verview f all the experiments. A light mineral il, Ispar-L and n- decane were used as il phase individually in the different experiments. Tw kinds f dpant were selected fr distilled water. One was ptassium idide (KI), which is widely used in cre analysis with X-ray scanner. The ther was sdium tungstate dihydrate (Na 2 WO 4 2H 2 O), which had been applied in the previus experiments with chalk cres [12]. Table 2 shws the prperties f fluids used in this wrk withut dpant. CO 2 is a liquid at 65 bar and 100 bar. The density f CO 2 appraches t that f n-decane and Ispar-L as pressure increases. This indicates that at least tw phases are required t be dped t btain substantial number difference between different phases under high pressure. T identify the in-situ saturatins, dual energy levels, 80 kv and 120 kv, were chsen, with cmpsite factrs (ma s), 250 ma s and 330 ma s respectively. These tw cmpsite factrs are the largest values which can be chsen in each energy level t reduce the beam hardening effects. At 137kV, the scanner can nt perfrm cntinuus scanning fr mre than 20 images due t the high temperature f the tube, althugh the image quality can be imprved a little cmpared with that at 120 kv. Our chice f the energy levels was based n the image quality as well as the tube life f X- ray scanner. The cre hlder was installed hrizntally n the platfrm. In rder t guarantee accurate image psitining, the cre hlder was fixed during the flding and nly mved fr scanning between different fldings. The injectin rate was 0.1 cc/min. The images were taken every tw mm alng a cre. All fur experiments share a similar general prcedure: 1. Scan dry cre, CO 2 saturated cre, dped il saturated cre, and dped water saturated cre with dual energy. If necessary, clean the cre with tluene and ethanl befre changing the saturating fluid; 2. Fld the cre with dped il until S wi is reached; 3. Fld the cre with dped water until S r is reached; 4. Fld the cre with CO 2 until n fluid is prduced; 5. Clean and dry the cre. A dual energy scan was taken at the end f each step. Cmparisn between the scanning data f the dry cre befre and after the experiment is imprtant fr errr estimatin. During the flding prduced il, water and gas were separated at atmspheric cnditin and their vlumes were recrded. Saturatin Determinatin Saturatin determinatin based n the measured numbers has been well established, fr tw phases, ws w S w (1)

4 SCA /12 with slutin S w S 1 (2) S w w w (3) Fr three phases, the crrespnding system f equatins is with slutin S S S (4) w1 w 1 g1 g wg1 S S S (5) w2 w 2 g2 g wg2 Sw S Sg 1 (6) S ( wg1w 1)( g 2 w 2) ( wg 2 w 2)( g1w 1) ( )( ) ( )( ) 1 w1 g2 w2 2 w2 g1 w1 (7) S g ( wg1w 1)( 2 w 2) ( wg 2 w 2)( 1w1) ( )( ) ( )( ) 2 w2 g1 w1 1 w1 g2 w2 (8) In eqs. 1 t 8, the phase saturatins S i are expressed in terms f the measured numbers i f varius fluid saturated cres. Subscripts, w and g represent il, water, and gas respectively. Their cmbinatins, w and wg suggest the existence f varius phases in the cre. Subscripts 1 and 2 indicate the tw different energy levels, 80 kv and 120 kv in ur experiments respectively. The dependence f a calculated saturatin n numbers can be evaluated in tw ways. One way is t differentiate eqs. 3 and 7 with respect t each f the numbers. Fr example, fr tw phases Fr three phases, S w w 2 ( w) (9) S (10) ( ) ( ) ( ) ( ) ( ) wg1 w1 g 2 w2 wg 2 w2 g1 w1 w2 g ( 1w1) ( g2 w2) ( 2 w2) ( g1w 1) The ther way is t evaluate the cefficient matrix. Equatins 1-2 and 4-6 are linear equatins, and expressed in the general frm, Fr tw phases, Ax B (11)

5 SCA / S w A 2 w, x 2 S, and B 2 1 Fr three phases, by substituting eq. 6 int eq. 4 and 5 t eliminate S w, ne btains w w a a w g1 w1 g A 3 a21 a, x w2 S and g2 w2 S B 3 wg1 w1 g1 w1 wg 2 w2 g2 w2 The determinant r the cnditinal number f cefficient matrix A 2 and A 3 indicate whether eq. 11 is well- r ill-cnditined. RESULTS AND DISCUSSION Experiment 1 In the first experiment, selectin f dpants was in accrdance with the study in [12]. Figure 3 shws the number in the plug frm psitin 2 mm t 40 mm (the numbers at the very ends f the plug are mitted due t the nise created by the aluminum filter). Since the penetrating pwer f X-rays thrugh the material increases with the ptential difference acrss the tube, the number at 80kV is always higher than that at 120 kv frm the fluids used in ur experiments. It shuld be nted that at bth energy levels, the numbers fr different fluids are nn-distinguishable arund the inlet, and apprach the upper limit f the scanner, The phase saturatins calculated by eqs. 7 and 8 are shwn in figure 4. The average gas saturatin, 64.4%, was clse t the value, 61%, calculated frm mass balance. But the average water and il saturatins were nt in a reasnable range, exhibiting even negative values. As shwn in the and 3-D recnstructin images (figure 5), the numbers at the inlet f the dry cre after the experiment are much higher than thse befre the experiment (figure 3) and almst reach the upper limit f the scanner. This indicates that strng adsrptin f the dpant happens during the experiment, which spils its results. Experiment 2 T avid adsrptin, lwer Na 2 WO 4 H 2 O cncentratins at 1 and 3 wt% were tried. Figure 6 shws the numbers f the plug saturated with different fluid. The numbers f Ispar-L and CO 2 are clse t each ther with an average difference f arund 9. An implicatin f this bservatin is that the three-phase flding can be treated as a pseud tw phase flding in visualizatin, with the dped water as ne phase and CO 2 /Ispar-L as anther. Figure 6 als indicates that water dped with 1 wt% Na 2 WO 4 H 2 O can nt prvide enugh cntrast in number and 3 wt% Na 2 WO 4 H 2 O was finally used. Hwever, adsrptin in the chalk was bserved again and it was cncluded that Na 2 WO 4 H 2 O was nt adequate t ur chalk samples.

6 SCA /12 Experiment 3 In this experiment, Ispar-L was deliberately nt dped t take advantage f its similar numbers t the high pressure CO 2 at 65 bar and 15 C. Since Ispar-L and CO 2 can be treated as ne pseud phase in visualizatin, nly the water saturatin was measured here and the ther tw saturatins were nt determined. Step 3 in the general prcedure was deliberately skipped in this experiment t investigate if CO 2 can mbilize the irreducible water. The difference f numbers between CO 2 and Ispar-L induces at maximum ± errr in the CO 2 /Ispar-L saturatins calculated by eq. 9. Figure 7 shws fluid saturatins calculated by using numbers at 120 kv, whse average value is in a gd agreement with thse calculated frm mass balance. Cmpared with the water saturatin befre CO 2 flding, the water saturatin at the end f the cre has increased by arund Meanwhile, n water prductin was bserved during the CO 2 flding. It seems that the water mbilized by the injected CO 2 is captured at the end f the cre by capillary pressure. Figure 8 shws the distributin f pixel numbers in the regin f interest n the image (The regin f interest excludes bright circle area, which is characteristic f high number and induced by the beam hardening effect.). Bth sftware FPImage and ImageJ prvide similar results. The histgrams frm experiments 1, 2 and 3 indicate that the number distributin is very clse t nrmal distributin, which is usual fr a hmgeneus sample. Hwever, sme deviatins ccur at the areas f hetergeneity (figure 9). Bth the 3-D image and 2-D image at 120 kv indicate that cre 3 cntains high-density nn-prus inclusins, which d nt cntribute t the flw and shuld be excluded fr calculating the average values. As indicated in figures 3 and 6, by using dpant, a decent cntrast in numbers fr different fluids is btained at 120 kv. Hwever, at 80 kv, there is a danger that sme numbers will be beynd the upper limit f the scanner, as illustrated in figure 9. ImageJ des nt accunt fr that effect and thus prvide errneus average numbers. T btain crrect numbers, an ptimizatin methd, the nnlinear Huber estimatin [13], was used t fit the numbers n an image. Cmpared with the Levenberg- Marquardt methd, this methd has the advantage f cnfining wild pints in fitted data and faster cnvergence. Figure 9 als shws the applicatin f the nnlinear Huber estimatin. The differences between the average numbers frm ImageJ and frm ur methd are 25 and 96 fr tw cases respectively. Figure 10 cmpares the water saturatin determined using data at 80 kv and 120 kv. It can be seen that if the 80 kv data were nt crrected using ur methd, a significant deviatin culd be resulted. Therefre, re-estimatin f the average number using the nnlinear Huber estimatin is necessary when there are a lt f data pints beynd the upper limit f the scanner. Experiment 4 In this experiment, the back pressure was increased t 100 bar where full miscibility can be achieved. The flding prcedure simulates CO 2 injectin int a water flded reservir as described in the general experimental prcedure.

7 SCA /12 Data frm psitin 0 mm t 28 mm was pltted and analyzed (Table 3) fr illustratin. Fr the results at 80 kv, t accunt fr pixels with numbers higher than the upper limit f the scanner, the Huber estimatin was used t get the crrect average numbers. In Table 3, the calculated saturatins are clearly unsatisfactry since sme values are ut f the range [0, 1]. The large errr can be explained by the determinant f matrix A in eq. 11. The system f equatins is well-cnditined if the determinant is far frm zer. This can be easily achieved in tw-phase scanning where is much smaller than w. Furthermre, the scanning is ften perfrmed at the high energy level, 120 kv, where the number can be mre accurately measured and the upper limit f the scanner is nt a prblem. Fr three-phase scanning, hwever, the determinant becmes far frm zer if a 12 and a 22 are largely different frm each ther. Althugh i1 and i2 (i=, w, g) are strngly different at tw energy levels, the differences between a 12 and a 22 are nt that large, especially fr liquefied CO 2. As shwn in table 3, det(a 3 ) appraches t zer, which indicates that eq. 11 is ill-cnditined. In additin, the numbers btained at 80 kv have larger errrs cmpared t thse at 120 kv due t the upper limit f the scanner as explained in Experiment 3 and the prer image quality at lwer energy scanning. This further affects the determinatin f three-phase saturatins By differentiating eq. 7 with respect t w1, 1, and g1, similar t eq. 10, the sensitivity f S t the individual numbers can be calculated (table 3). It is clear that S is mst sensitive t the change f 1, e.g., if, the maximum abslute change n the S can be as large as The errr estimatin reveals the reasns fr inaccurate three-phase saturatins. Vinegar and Wellingtn suggested selecting a K edge dpant fr determining three-phase saturatins [8]. scanning just abve the K edge energy will see a sudden increase in the attenuatin cefficient. Therefre, if tw dpants are used and ne f the dpants has K edge between the tw scanning energy levels, it is pssible t make ( - w ), r a 12 and a 22, have ppsite signs at tw energy levels. And if the abslute values f a 12 and a 22 are nt t clse t zer, A 3 will becme well-cnditined In the pssible dpants with K edge between ur energy levels 120 kv and 80 kv [8], mst f them are cmpunds cntaining heavy elements, such as lead nitrate (Pb(NO 3 ) 2 ) and thallium fluride (TlF). Thse dpants are rarely in use in cre analysis perhaps due t txicity. And ne als shuld be aware f the ptential adsrptin prblem if a new dpant is used. CONCLUSION Imprtant cnclusins frm these experiments are: 1. Since Na 2 WO 4 2H 2 O tends t be adsrbed in ur chalk samples, even at lw cncentratin, KI was finally selected t dpe the water phase. Cmparisn f the dry cre images befre and after experiment is imprtant t reveal the adsrptin prblem. 2. High pressure three-phase flding can be visualized as pseud tw-phase flding, where the gas and il phases have similar numbers and are treated as ne phase. This prvides an alternative fr future three-phase experiments, where the mst interested phase can be identified under a single energy level.

8 SCA /12 3. number in a hmgeneus chalk sample shws nrmal distributin at bth energy levels 80 kv and 120 kv. The nnlinear Huber estimatin has prved t be an useful tl t btain the crrect average number at 80 kv, where a nn-negligible prtin f numbers are beynd the upper limit f the scanner. 4. The numbers f the idddecane dped il phase, the KI dped water phase, and the high density CO 2 phase increase similarly when the energy level changes frm 120 t 80 kv, which makes eq. 11 ill-cnditined and calculated saturatins sensitive t smaller errrs in numbers. This is the majr reasn fr us t give up the simultaneus determinatin f the three-phase saturatins in the current study. 5. T make eq. 11 well-cnditined, ne pssible way is t use a dpant with K edge between tw energy levels. ACKNOWLEDGEMENT This study is carried ut under the prject Enhanced Oil Recvery thrugh CO 2 Utilizatin funded by the Danish Natinal Advanced Technlgy Fundatin. REFERENCES 1. Zick A.A., A Cmbined Cndensing/Vaprizing Mechanism in the Displacement f Oil by Enriched Gases, Paper SPE presented at the SPE Annual Technical Cnference and Exhibitin, New Orleans, Luisiana, 5-8 Octber E. A. Turek, R.S. Metcalfe, R.E. Fishback, Phase Behavir f Several CO 2 /West- Texas-Reservir-Oil Systems, SPE Reservir Engineering, (1988)3, 2, p A. Kantzas., Investigate f Physical Prperties f Prus Rcks and Fluid Flw Phenmena in Prus Media using Cmputer Assisted Tmgraphy, In Situ, (1990)14, 1, p Withjack E.M., C. Devier, and G. Michael., The Rle f X-Ray Cmputed Tmgraphy in Cre Analysis, Paper SPE presented at the SPE Western Reginal/AAPG Pacific Sectin Jint Meeting, Lng Beach, Califrnia, May Zitha P.L.J., Q.P. Nguyen, and P.K. Currie., Effect f Flw Velcity and Rck Layering n Fam Flw: an X-ray Cmputed Tmgraphy Study, Paper SPE presented at the SPE Asia Pacific Oil and Gas Cnference and Exhibitin, Jakarta, Indnesia, 9-11 September Du D.X., P.L.J. Zitha, and M.G.H. Uijttenhut, Carbn Dixide Fam Rhelgy in Prus Media: A Scan Study, SPE Jurnal, (2007)12, 2, p Izgec O., B. Demiral, H. Bertin, and S. Akin, CO 2 Injectin in Carbnates, Paper SPE presented at the SPE Western Reginal Meeting, Irvine, Califrnia, 30 March-01 April Jikich S.A., R. McLendn, K. Seshadri, G. Irdi and D.H. Smith, Carbn Dixide Transprt and Srptin Behavir in Cnfined Cal Cres fr Carbn Sequestratin, SPE Reservir Evaluatin & Engineering, (2009)12, 1, p Vinegar H.J. and S.L.Wellingtn, Tmgraphic Imaging f Three-Phase Flw experiment. Rev.Sci.Instrum., (1987)58, 1, p Chatzis I., A. Kantzas, F.A.L. Dullien, On the Investigatin f Gravity-Assisted Inert Gas Injectin Using Micrmdels, Lng Berea Sandstne Cres, and Cmputer-Assisted Tmgraphy Paper presented at the SPE Annual Technical Cnference and Exhibitin, Hustn, Texas, 2-5 Octber 1988.

9 SCA / Lackner A.S., G. Haaskjld, and O. Trsaeter, Selecting X-Ray Energy Levels fr Three-Phase Saturatin Measurements, Paper SPE presented at the SPE Latin American and Caribbean Petrleum Engineering Cnference, Ri de Janeir, Brazil, June S.Y. Yu. I. Akervll, O. Trsaeter, J.A. Stensen, J. Kleppe and S.H. Midtlyng, Histry Matching Gas Injectin Prcesses with In-Situ Saturatin Measurements and Prcess Hysteresis, Paper SPE presented at the SPE Internatinal Cnference and Exhibitin in China held in Beijing, China, 2-6 Nvember, Madsen K. and H.B. Nielsen, Finite Algrithm fr Rbust Linear Regressin, BIT Numerical Mathematics, (1993)30, p Table 1. Overview f cre flding experiments Plug sample prperties P and T Fluids and dpant Name Length Diameter Prsity k water P T Oil and dpant Water and dpant (mm) (mm) (%) (md) (bar) ( C) (wt %) (wt%) Ispar-L Distilled water Cre# % 7% Na Idddecane 2 WO 4 H 2 O Cre# Cre# Cre# Ispar-L withut dpant Ispar-L withut dpant n-decane, 5% Idddecane Distilled water 1 % Na 2 WO 4 H 2 O 3 % Na 2 WO 4 H 2 O Distilled water 5 wt% KI Distilled water 3 wt% KI Gas CO 2 CO 2 CO 2 CO 2 Table 2. Fluid prperties at labratry temperature 15 C Ispar-L n-decane Water CO 2 Pressure (bar) Density (g/ml) Viscsity (cp) Density (g/ml) Viscsity (cp) Density (g/ml) Viscsity (cp) Density (g/ml) Viscsity (cp) Atm E Table 3. Three phase saturatin and errr estimatin fr experiment 4 Psitin (mm) S Sg det(a 3 )

10 SCA /12 Figure 1. X-ray Scanner Figure 2. Schematic f experimental setup Figure 3. number at different energy levels (Experiment 1): (1) 80kV (2) 120kV Figure 4. Three phase saturatin (Experiment 1) Figure 5. and 3-D recnstructin images (120 kv) f a dry cre after the experiment

11 SCA /12 Figure 6. number at different energy levels (Experiment 2): (1) 80kV (2) 120kV Figure 7. Results frm Experiment 3: (1) number at 120kV (2) In-situ Saturatin Figure 8. number distributin in the regin f interest

12 SCA /12 Figure 9. Limitatin f ImageJ: (1) inhmgeneity f the cre (2) upper limit f scanner Figure 10. Cmparisn f water saturatin (Experiment 3)