TOWARDS BASIN-SCALE MODELING OF GEOLOGICAL CO 2 STORAGE: UPSCALING OF CAPILLARY TRAPPING

Transcription

1 TOWARDS BASIN-SCALE MODELING OF GEOLOGICAL CO 2 STORAGE: UPSCALING OF CAPILLARY TRAPPING Ruben Juanes MIT Carbon Sequestration Forum VIII Stanford, California, November 13-14, 2007

2 SUMMARY OF RESULTS, Trapping of CO 2 leads to safer sequestration scenarios: immobilization and further dissolution, How to maximize trapping: " Inject deeper " Inject at higher rates " Inject water slugs " Horizontal, smart wells, Word of caution: models will tend to overestimate sweep, trapping and dissolution, We present a new model to upscale capillary trapping to the basin scale R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 2

3 RISKS OF CO 2 STORAGE, Potential leak of the CO 2 into the atmosphere " Activation of faults (overpressurization) " Existing wells (abandoned) " Regional groundwater flow (IPCC Report, 2005), Essential to predict the migration and distribution of the CO 2 in the subsurface at the basin scale, Need proper monitoring to assess the success of the sequestration project at the basin scale R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 3

4 STORAGE MECHANISMS, Mechanisms for CO 2 immobilization " Structural trapping: impermeable cap rock " Solution trapping: dissolution into the brine " Mineral trapping: geochemical binding to the rock by mineral precipitation " Capillary trapping: disconnection of the CO 2 phase into immobile blobs during two-phase flow R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 4

5 BASIS FOR CAPILLARY TRAPPING, Trapping mainly due to snap-off during imbibition, During CO 2 injection (drainage-like process): no trapping, When/where does trapping occur? " after injection (Juanes et al, 2006) " at the trailing edge of the plume during upwards migration (water displacing CO 2 ) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 5

6 CAPILLARY TRAPPING MODEL, Most relative permeability hysteresis models are based on the trapping relation proposed by Land (1968) S gr Sgi = 1+ C S gi, Land s model yields a monotonically increasing initial-residual curve " Higher initial gas sat higher trapped gas R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 6

7 NUMERICAL STUDY, Reservoir model (Juanes, Spiteri, Orr and Blunt: WRR 2006) " Realistic three-dimensional heterogeneous reservoir (PUNQ-S3 synthetic model) " Anticline structure " 8 injectors R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 7

8 RESERVOIR DESCRIPTION, Relative permeability " Data taken from Oak (1990) " Data result in a Land trapping coefficient C ~ 1, PVT properties " Typical of CO 2 at reservoir conditions R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 8

9 EFFECT OF HYSTERESIS, Compare Case 1 (no hysteresis) and Case 2 (with hysteresis), CO 2 saturation after 200 years, The model that accounts for hysteresis predicts " Almost all the CO 2 is trapped " Spread-out distribution of trapped CO 2, as opposed to a concentrated distribution of mobile CO 2 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 9

10 EFFECT OF INJECTION RATE, Compare Case 2 (10 years) and Case 3 (50 years), CO 2 saturation after 200 years, Lower injection rate leads to more mobile CO 2 at the top R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 10

11 EFFECT OF WATER INJECTION, Compare Case 2 (CO 2 injection) and Case 4 (WAG injection), CO 2 saturation after 200 years, Alternating water injection induces more trapping (enhanced imbibition) and reduces the amount of CO 2 that accumulates at the top R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 11

12 EFFECT OF WATER INJECTION, Impact on bottom-hole pressure, Alternating water injection increases the operating BHP " Water injection rate is higher than that of CO 2 " Compressibility of water is much smaller R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 12

13 SUMMARY OF SIMULATION RESULTS (Juanes, Spiteri, Orr and Blunt: WRR 2006), Accounting for trapping and hysteresis of the nonwetting CO 2 phase is essential. Trapping occurs at the trailing edge of the plume after injection stops, Trapping of CO 2 leads to safer sequestration scenarios: immobilization and further dissolution, How to maximize trapping: " Inject deeper " Inject at higher rates " Inject water slugs " Horizontal, smart wells CO 2 water R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 13

14 SCALING OF CAPILLARY TRAPPING, Typical simulation models do not capture: " Gravity override " Channeling and viscous fingering, Simulations are therefore grid dependent, Coarse-grid simulations overestimate trapping! R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 14

15 PREDICTIVE THEORY, Dependence of megascopic trapping coefficient on " Mobility ratio " Gravity number " Subgrid heterogeneity, Grid dependence of block-effective trapping coefficient C eff log h, The same can be done for dissolution, geochemistry, etc. R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 15

16 UPSCALING OF CAPILLARY TRAPPING, Mathematical models that capture gravity override gas injection mobile gas brine water injection trapped gas mobile gas brine water injection trapped gas mobile gas brine " Need to model the injection period " Effect of regional groundwater flow R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 16

17 MATHEMATICAL MODEL INJECTION h g gas injection Q H, Main features of the mathematical model " Sharp interface approximation " Vertical equilibrium (Dupuit approximation) p = p p I I ρg( h z), + ( ρ + Δρ) g( h z), if if z z < > h h " Incompressible flow Q g + Q = u h + u ( H h ) = Q w g g w g 0 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 17

18 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 18 * k rg, Formulation " Darcy s law endpoints of relative permeabilities " Fractional-flow formulation " Mass balance MODEL INJECTION PERIOD ( ) x h f h g k Q f Q g g g g g g Δ = ) (1 * λ ρ 0 ) ) (1 ( = + x Q t h S g g wc φ = x h g x p k k u g I g rg g ρ μ *

19 MODEL INJECTION PERIOD, Dimensionless parameters " Plume height: " Time: τ d " Length: hg h = H t =, T = injection time T x QT ξ =, L = L Hφ " Viscosity ratio: μ g M = μ " Effective gravity number: w N gd = * kkrgδρg μ Q H g H QT Hφ k k v h R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 19

20 MODEL INJECTION PERIOD gas injection Q h, Equation in dimensionless form ((1 S τ d wc ) h) + ξ f ( h) N gd D( h) h ξ = 0 fractional flow function gravity diffusion function " This is a nonlinear advection diffusion equation R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 20

21 MODEL POST-INJECTION PERIOD water injection UH trapped gas h mobile gas brine, Now we can have drainage or imbibition, Dimensionless parameters " Time: t τ i =, Tc = T T Q UH " Effective gravity number: Q N gi = N gd = UH c N g * k rw R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 21

22 MODEL POST-INJECTION PERIOD water injection UH trapped gas h mobile gas brine, Equation in dimensionless form ( θ ( h) h) τ i + ξ f ( h) N gi D( h) h ξ = 0, Seemingly the same equation, but: " Scaling of gravity forces is different " The constitutive relations are discontinuous! R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 22

23 MODEL POST-INJECTION PERIOD water injection UH trapped gas h mobile gas brine, Storage coefficient: (1 S θ ( h) = (1 S ), S if h ), if h τ i > 0 τ < 0, Fractional flow function, and Gravity diffusion function wc wc gr i R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 23

24 ANALYTICAL SOLUTIONS, Full analytical solutions are possible for " Injection period " Early post-injection period (detached fronts) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 24

25 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 25, Analytical solutions to the hyperbolic model " Late post-injection period (continuous drainage/imbibition) ANALYTICAL SOLUTIONS, For the full model, we resort to numerical solutions 0 ) ( ) ( ) ) ( ( = + ξ ξ τ θ h h D N h f h h gi i 0

26 NUMERICAL SOLUTIONS, High-viscosity gas (M = 0.5, N g = 0), Low-viscosity gas (M = 0.05, N g = 0) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 26

27 FOOTPRINT OF THE PLUME, ξ max depends on M, N g (for a given trapping coefficient C), When does the plume stop? Critical length scale L v,max L " Vertical v,max = rt, Bo Bo = Δρgk σ L h,max L " Horizontal rt Ca Ca h,max =, = μwu σ, In our analysis, we chose ε v L v,max = H < 0.01 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 27

28 FOOTPRINT OF THE PLUME, We want to match the footprint, but cannot resolve gravity override, Equivalent to solving a stable flow (M = 1), with different values of the megascopic trapping coefficient <C> <C> = 1 <C> = 5 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 28

29 FOOTPRINT OF THE PLUME, Calibration curve ξ max vs megascopic trapping coefficient <C> R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 29

30 MEGASCOPIC TRAPPING COEFFICIENT, From ξ max (M,N g ) and the calibration curve ξ max (<C>), we obtain the response surface <C>(M,N g ) Almost insensitive to N g for values of M < 0.1 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 30

31 BLOCK-EFFECTIVE TRAPPING COEFF, What is C eff if we partially resolve gravity override? " Reduced gravity number " More favorable viscosity ratio N z = N z = R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 31

32 BLOCK-EFFECTIVE TRAPPING COEFF, The trapping coefficient is larger for coarse grids " Highly refined grids laboratory measurement C lab " Very coarse grids megascopic value <C> M = 0.1, N g = 10, C = 1 <C> C lab R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 32

33 VALIDATION WITH EXPERIMENTS R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 33

34 VALIDATION WITH SIMULATION M = 0.1, N g = 10, C = 1 <C> C lab R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 34

35 THE BASIN SCALE, When all is said and done, this is the important scale (100 s 1000 s km) 100 km ( An order-one scientific problem " Open boundaries! Natural recharge discharge areas " The issue is not (only) where the CO 2 goes, but where the displaced brine goes " Need to upscale, upscale, upscale (injection & trapping) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 35

36 BACK-UP SLIDES R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 36

37 HUMAN IMPACT, Human activities are interacting with the planet at a global scale, Atmospheric accumulation of CO 2 and other greenhouse gases (CH 4, NO 2 ) " Global warming " Reduction of ph of upper ocean, Reductions in emissions are needed to stabilize concentrations at a level double the pre-industrial level (IPCC Third Assessment Report, 2001), Challenge: balancing emissions and meeting energy demands R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 37

38 NO SILVER BULLET, A portfolio of technologies must be deployed to meet this challenge (there is no silver bullet) " Improved efficiency (cars, buildings, power plants) " Nuclear energy " Renewable energy sources (solar, wind, biofuels) " Forestation " Accelerated ocean uptake " Carbon capture and storage ( putting the carbon back ), Carbon capture and storage is one of the stabilization wedges (Pacala & Socolow, Science 2004) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 38

39 GEOLOGICAL CO 2 STORAGE, What is CO 2 sequestration? Capture (at the surface) and storage (in the subsurface) of anthropogenic CO 2, Target formations for geological CO 2 storage Coal-bed methane formations Deep (saline) aquifers Depleted oil and gas reservoirs Ocean sediments R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 39

40 STORAGE IN OIL AND GAS RESERVOIRS, Familiar scenario for the oil industry, In oil and gas reservoirs, CO 2 is injected for storage and EOR " The CO 2 is injected, displacing the oil " Well-to-well flow from injector to producer " Typically, miscible floods (high local displacement eff.) " Fingering, channeling and gravity override limit sweep Source: IPCC Report, 2005 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 40

41 STORAGE IN OIL AND GAS RESERVOIRS Advantages A geologic seal exists Detailed reservoir characterization is available Considerable experience in CO 2 floods for enhanced oil recovery (EOR) Dissolution of the CO 2 in both the resident oil and formation water Extension of the life of the field possibility to offset costs Disadvantages Unless it is damaged by prior operation The CO 2 has low viscosity compared to oil subject to viscous fingering and permeability channeling Opportunity to co-optimize EOR and CO 2 storage (water-alternating-gas strategies) Well-defined regulatory structure Oil and gas reservoirs are not always near the areas where anthropogenic CO 2 is generated R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 41

42 STORAGE IN COAL BEDS, Storage mechanism of CO 2 in coal " In many coalbeds, CH 4 is adsorbed onto the coal surface " When CO 2 is injected, it replaces the CH 4 " This allows for methane recovery (ECBM) Source: Seto, 2004 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 42

43 STORAGE IN COAL BEDS, Adsorption in coal " CO 2 and N 2 adsorb preferentially (compared with CH 4 ) " All gases display hysteresis CO 2 adsorption is stable Source: Kovscek, 2004 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 43

44 STORAGE IN COAL BEDS Source: Orr, 2004, Transport of CO 2 in coal " Dual porosity system " Fast flow through fractures, slow diffusion into matrix Source: Seto, 2005 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 44

45 STORAGE IN COAL BEDS Advantages Stable adsorption desorption isotherm: permanent storage Enhanced coalbed methane recovery possibility to offset costs Unminable coalbeds may be available near coal-fired power plants Possibility to co-inject with other contaminant gases Disadvantages Physics of coalbed systems not very well understood complex interplay of flow, permeability variation, diffusion and adsorption ECBM predictive modeling and field experience are limited Storage volumes are uncertain R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 45

46 STORAGE IN SALINE AQUIFERS Advantages Deep saline aquifers are widely distributed Several trapping mechanisms may immobilize the CO2 including capillary trapping Opportunity to optimize the CO 2 injection scheme (injection location and rate; water-alternating-gas strategies) Success story: Sleipner Disadvantages A geologic seal may not exist; potential leakage through faults and abandoned wells Detailed reservoir characterization is usually not available Costs cannot be offset by by-products (as in EOR and ECBM) The CO 2 has low viscosity compared to water may limit the reservoir volume contacted by CO 2 Regulation is still not well-defined R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 46

47 STORAGE IN SALINE AQUIFERS, Structural trapping " The CO 2 remains mobile at the top of the formation, Solution trapping Source: Hesse, 2005 " Diffusion of CO 2 into brine creates denser brine at the top " Gravity-driven fingers develop, enhancing mixing Source: Riaz et al, 2006 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 47

48 STORAGE IN SALINE AQUIFERS, Mineral trapping " CO 2 dissolution in water produces a weak acid " This acid reacts with rock minerals, leading to precipitation of solid carbonate minerals " It is arguably the most permanent form of storage R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 48

49 STORAGE IN SALINE AQUIFERS, Deep saline aquifers are prime candidates for CO 2 storage " Huge storage capacity, widely distributed " Predominantly water-wet injection period post-injection period water drainage CO 2 and connate water imbibition R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 49

50 A LOOK AT THE PORE SCALE, Fluid arrangement according to wettability " CO 2 in the pore centers " Water in the pore crevices (Lenormand et al, 1983) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 50

51 THE DRAINAGE PROCESS, CO 2 displaces water in a piston-like fashion, one duct at a time CO 2 water, This process is called invasion percolation, Water remains at pore crevices, coating the solid, Both water and CO 2 form connected clusters, so they are mobile (Lenormand et al, 1983) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 51

52 THE IMBIBITION PROCESS, Now water displaces CO 2, Initially also piston-like, one duct at a time CO 2 water, But water films swell and disconnect the CO 2 ( snap-off ) (Lenormand et al, 1983) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 52

53 RESIDUAL CO 2, Now water displaces CO 2, Initially also piston-like, one duct at a time, But water films swell and disconnect the CO 2 ( snap-off ) CO 2 water, Clusters of CO 2 are left behind, immobile, Residual (or capillary) trapping of CO 2 (Lenormand et al, 1983) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 53

54 CAPILLARY TRAPPING, At the pore scale, the CO 2 is disconnected by snap-off during imbibition P c drainage, Macroscopically, this leads to hysteresis (residual CO 2 ) in: imbibition residual (trapped) saturation " capillary pressure P c " relative permeability k r of CO 2 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 54

55 LAND S HYSTERESIS MODEL, The basis for Land s hysteresis model is to express the imbibition relative permeability at a given saturation as the drainage relative permeability evaluated at a different flowing saturation () i ( d) krn ( Sn ) = krn ( Snf ), At any bulk saturation Σ ν : D Sn = Sn - Snf, The trapped saturation DΣ ν is equal to the maximum trapped minus what is flowing but will eventually be trapped D S = S ( S )- S ( S ) n nr ni nr nf R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 55

56 GEOPHYSICAL MONITORING, Appropriate space resolution is a challenge Time-lapse seismic data at Sleipner (IPCC report, 2005) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 56

57 ARE WE DONE?, Some outstanding issues " Small (well) scale " Intermediate (pilot/reservoir) scale " Large (basin) scale R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 57

58 THE WELL SCALE INJECTION, CO 2 injection is an inherently unstable process: " CO 2 is less dense (gravity fingering) " CO 2 is less viscous (viscous fingering) " Heterogeneity variations (channeling), Leads to a patchy, nonuniform, finger-like distribution of CO 2 water, Can we capture the average behavior with a coarse numerical model? CO 2 and connate water R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 58

59 THE WELL SCALE POST-INJECTION, Where is trapping taking place? " Leading (top) edge counter-current flow " Trailing (bottom) edge co-current flow CO 2 fingers counter-current flow? co-current flow?, How much is trapped? " Depends on degree of patchiness/instability " No model exists currently, Need for experiments! R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 59

60 THE BASIN SCALE CAN WE SOLVE IT? R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 60

61 (terpsichore.stsci.edu) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 61

62 ISSUES IN CO 2 STORAGE, Capacity of different target formations " Very uncertain " Lack of geophysical information and laboratory data " Depend on the economic model Geologic formation Oil and gas fields Coal seams Saline aquifers Lower estimate (GtCO 2 ) Upper estimate (GtCO 2 ) Source: IPCC Report on Carbon Capture and Storage, 2005 R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 62

63 STORAGE IN SALINE AQUIFERS, The time scales of the various mechanisms can be very different t struct ~ t capil << t dissol << t miner (10 s yrs) (100 s yrs) (1000 s yrs) (IPCC Report, 2005) R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 63

64 SETUP OF NUMERICAL SIMULATIONS, The formation is initially filled with brine. A total of 0.15 pore volumes of CO 2 are injected, Four different scenarios Case number Description No hysteresis Hysteresis Hysteresis Hysteresis - WAG Injection time 10 yrs 10 yrs 50 yrs yrs, Three observation points " Block (13,18,1) near the top " Block (7,21,1) slightly lower " Block (11,11,1) slightly lower still R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 64

65 EFFECT OF HYSTERESIS, Evolution of CO 2 saturation at observation points, When hysteresis is ignored " Accumulation of CO 2 at the top of the formation " CO 2 travels through without leaving residual saturation, When hysteresis is included " Lower CO 2 saturation at the top of the structure " CO 2 migrates leaving residual saturation R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 65

66 EFFECT OF INJECTION RATE, Evolution of CO 2 saturation at observation points, Higher injection rates " Larger areal extent of the CO2 plume at the end of injection " More trapping during the vertical ascent of the plume R. Juanes, Nov 2007 Carbon Sequestration Forum VIII 66