Bigalke, Nikolaus

Gust, Giselher

Meyer, Andreas

Rehder, Gregor

English

OceanRep

Hydrodynamically constrained flux of in-situ generated methane hydrate dissolving into undersaturated seawaterNiko Bigalke1, Giselher Gust2, Gregor Rehder3, Andreas Meyer21Leibniz Institute of Marine Sciences, IfM-GEOMAR, Kiel, Germany2Technical University Hamburg-Harburg, Dept. of Ocean Engineering, Hamburg, Germany3Baltic Sea Research Institute Warnemünde, Rostock, GermanyHydrate stability in seawaterHydrate dissolutionat controlledhydrodynamicforcing: RationaleDissolution of hydrate – diffusion orreaction controlled ?F = K´A(Csat-Cbckgr)CbckgrCsatzCibulk water(in motion)rd kkK11'1 +=K‘ = kddiffusion controlled:K‘ = krreaction controlled:diffusivesublayer(stagnant)desorption layerhydrate crystalConcentrationkd = D/zolutionat controlledhydrodynamicforcing: RationalekrHydrate dissHydrate dissolution - Tools• Production of a radial flowfield at the hydrate/seawater interface in the flux chamber• Almost constant, adjustable and calibrated friction velocity (u*) overthe entire surfaceInterfacial flux chamber „microcosm“Hydrate dissolutionat controlledhydrodynamicforcing:ExperimentalPeppe et al. (1999)Hydrate dissolution – Exptl. Parameters275 280 285 290T [K]0.00.050.10.150.2CCH4[mol/kg]30 MPaT [°C] ρ[kg/m3]µ[mPas]D/10-5[cm2/s]Csat[mmol/kg]2.0 1042 1.726 9.306 55.3 (209.9)3.7 1041 1.638 9.859 61.4 (204.4)9.0 1040 1.408 11.53 85.8 (189.4)t controlledhydrodynamicforcing:ExperimentalFriction velocities were set to 0.6, 1.0 and 1.2 cm/s for each temperatureCsat: Tishchenko et al. (2005)Hydrate dissolutionaHydrate dissolution – Results I0.6 cm/s 1.0 cm/s 1.2 cm/s9.0 °C3.7 °C2.0 °CCt = Ct0 +  (Csat – Cto) e-(A/V)ktHydrate dissolutionat controlledhydrodynamicforcing: ResultsHydrate dissolution - Results II• Saturation concentration in good agreement with predictionsaccording to Tishchenko et al. (2005)• Change in T results in change of saturation concentration and thus, thermodynamic driving force of dissolution• Friction velocity (u*) has a strong impact on the dissolution raterd kkK11'1 += ( )bckgrsatd CCAkdtdn −=• Results strongly substantiate idea dissolution of methane hydrate in undersaturated seawater is a diffusion-controlled processHydrate dissolutionat controlledhydrodynamicforcing: ResultsHydrate dissolution - Results IIIOpdyke et al. (1987)• Excellent agreement of measured transfer coefficients and those obtained from the dissolution of smooth alabaster plates demonstrates reliability of our data.• Data yield a correlation for the flux of methane from decomposing hydrateoutcrops for a broad range of P, T and u* conditions prevailing in the oceans on the seafloorkd = D/z97.00173.0−∗= uz∗−⎟⎟⎠⎞⎜⎜⎝⎛⋅= uDkd32078.0 ρµHydrate dissolutionat controlledhydrodynamicforcing: ResultsT, P, Csat = ?z = 1 mmComparison with earlier data50 m3Egorov et al. (1999)16.7 m3this studyT = 0°CP = 12 MPaCsat = 53.7 mmol/Lz = 2.3 mmcurrent speed: 1.6 cm/s ->u* = 0.07 cm/sD = 10-5 cm2/sHydrate dissolutionat controlledhydrodynamicforcing: ApplicationComparison with earlier data50 m3Egorov et al. (1999)16.7 m3this studyT = 0°CP = 12 MPaCsat = 53.7 mmol/Lz = 2.3 mmcurrent speed: 1.6 cm/s ->u* = 0.07 cm/sD = 10-5 cm2/s370 µmolz = ?Rehder et al. (2004)current speed: ? ->u* = ?T = 3.5°CP = 10.5 MPaCsat = 69.8 mmol/LD = 10-5 cm2/sT, P, Csat = ?z = 1 mmHydrate dissolutionat controlledhydrodynamicforcing: ApplicationComparison with earlier data50 m3Egorov et al. (1999)16.7 m3this studyT = 0°CP = 12 MPaCsat = 53.7 mmol/Lz = 2.3 mmcurrent speed: 1.6 cm/s ->u* = 0.07 cm/sD = 10-5 cm2/s370 µmolz = 0.179 mmRehder et al. (2004)current speed: 1.75 cm/s ->u* = 0.08 cm/sT = 3.5°CP = 10.5 MPaCsat = 69.8 mmol/LD = 10-5 cm2/sHester et al., pers. comm.T, P, Csat = ?z = 1 mmHydrate dissolutionat controlledhydrodynamicforcing: ApplicationComparison with earlier data50 m3 16.7 m3Egorov et al. (1999)this studyT = 0°CP = 12 MPaCsat = 53.7 mmol/Lz = 2.3 mmcurrent speed: 1.6 cm/s ->u* = 0.07 cm/sD = 10-5 cm2/s370 µmolthis studyz = 0.179 mmz = 2.08 mmRehder et al. (2004)current speed: 1.75 cm/s ->u* = 0.08 cm/sT = 3.5°CP = 10.5 MPaCsat = 69.8 mmol/LD = 10-5 cm2/s33.4 µmolHester et al., pers. comm.T, P, Csat = ?z = 1 mmHydrate dissolutionat controlledhydrodynamicforcing: ApplicationComparison with earlier datacylindrical hydratespecimens in cross flowplain surfaces in parallel flowOcean experiment: Lab:Hydrate dissolutionat controlledhydrodynamicforcing: Application• Dissolution experiments demonstrate that hydrate dissolution in undersaturated seawater at P-/T-conditions within the HSF is diffusion and not reaction controlled.• Based on the experimental data, a kd/u* correlation was obtained, whichexcellently agrees with and is thus validated by an earlier correlationobtained from dissolution experiments with alabaster plates. • The validated correlation permits an accurate prediction of the dissolutionrates of smooth and clean methane hydrates exposed to a flow of undersaturated seawater for a broad range of oceanic conditions.• Comparison with earlier data and postulations shows significantdiscrepancies. In one case this was due to a different sublayer thickness, which for lack of available data has been poorly constrained before.• Future studies should address the role of inhibitors such as sediments orbacterial mats covering most natural gas hydrates.Conclusion/outlookHydrate dissolutionat controlledhydrodynamicforcing: ConclusionRelevant Experiments Ialabaster plateEffect of flow:t• Flux of Ca and SO4 determined from mass loss of the alabaster• A diffusive boundary layer model was assumed to explain mass loss.• k = 0.078 Sc-2/3 u*u*u*GRL 1987, 14, 1131-1134CH4 hydratedissolutionat controlledhydrodynamicforcingRelevant Experiments II2h:19min HDTV20h:45min Hi83h:12min HDTV• Results of the field experiment fit well into a diffusive boundary layer model• Dissolution of hydrates appears to bediffusion limited, not by kinteics of a chemical reactionRehder et al. (2004)F(CO2)/F(CH4) = Csat(CO2)/Csat(CH4)F = D/z (Csat-Ct0)z(CO2) = z(CH4),D(CO2) = D(CH4),Ct0=0olutionat controlledhydrodynamicforcingCH4 hydratedissIdea/MotivationCH4 hydratedissolutionat controlledhydrodynamicforcingJ. Greinert, IFM-GEOMAREstimated inventory of hydrate-fixed CH4 in natural gas hydrates0.11101001000100001970 1980 1990 2000 2010Year of estimationCH4 [1015 m3]after SLoan and Koh, 2007Comparison with earlier dataT, P, Csat = ?z = 2.3 mm50 m3 17.4 m3Egorov et al. (1999)this studyT = 2.0°CP = 30 MPaCsat = 57.6 mmol/Lz = 2.3 mmcurrent speed: 1.6 cm/s ->u* = 0.07 cm/sD = 10-5 cm2/s370 µmolthis studyz = 0.179 mmz = 2.08 mmRehder et al. (2004)current speed: 1.75 cm/s ->u* = 0.08 cm/sT = 3.5°CP = 10.5 MPaCsat = 69.8 mmol/LD = 10-5 cm2/s33.4 µmol0.6 µ ol21.7 m3zocean/zlab!=Fluxlab/FluxoceanCH4hydratedissolutionat controlledhydrodynamicforcingzocean/zlab = 11.6Fluxlab/Fluxocean = 11.12 08 mm

Hydrodynamically constrained flux of in-situ generated methane hydrate dissolving into undersaturated seawater

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