OTC-25252-MS【确定南开海槽沉积物中甲烷水合物的相平衡条件】

OTC-25252-MSDetermining Methane Hydrate Equilibrium Conditions in Sediments from the Nankai TroughMichael Nole,and Hugh Daigle,Department of Petroleum and Geosystems Engineering,University of Texas at AustinCopyright2014,Offshore Technology ConferenceThis paper was prepared for presentation at the Offshore Technology Conference held in Houston,Texas,USA,5–8May2014.This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s).Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s).The material does not necessarily reflect any position of the Offshore Technology Conference,its officers,or members.Electronic reproduction,distribution,or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited.Permission to reproduce in print is restricted to an abstract of not more than300words; illustrations may not be copied.The abstract must contain conspicuous acknowledgment of OTC copyright.AbstractOur work shows how pore size affects the quantity of methane dissolved in pore fluid in sediments from the Nankai Trough offshore Japan.Integrating log and laboratory data from shallow sediments in the Nankai Trough,we determined that the range of pore sizes in this region leads to methane hydrate equilibrium temperature depressions of about1–3degrees C at Site C0011,2–5degrees C at Site C0012, and1-5degrees C at Site C0018.Methane solubility was determined to increase by10–30percent at Sites C0011and C0012,while solubility increases at Site C0018were predicted to be between10and50 percent throughout the methane hydrate stability zone.Further,the increase in methane solubility due to the Gibbs-Thomson effect was found to significantly decrease the mass of methane deposited as hydrate as well as decrease the thickness of the methane hydrate stability zone.IntroductionUnderstanding methane availability and distribution is an important component of prospect assessment and completion design in hydrate plays.In particular,constraining methane hydrate phase equilibrium behavior in sediments is instrumental to understanding and predicting subsurface hydrate distributions, particularly in fine-grained sediments in which pores are small enough to have a significant effect on solubility and stability conditions.Preferential Hydrate Growth in Coarse-Grained SedimentsRecent work has explored mechanisms underlying the distribution of methane hydrates in marine sediments,particularly in areas consisting of heterogeneous distributions of coarse-and fine-grained sediments.In many locations worldwide,such as Walker Ridge in the northern Gulf of Mexico(Cook and Malinverno,2013),Hydrate Ridge offshore Oregon(Weinberger and Brown,2006),Blake Ridge offshore South Carolina(Kraemer et al.,2000),the northern Cascadia margin offshore Vancouver Island,Canada (Malinverno,2010),and the eastern Nankai Trough offshore Japan(Tsuji et al.,2008),hydrates have been found to preferentially nucleate in coarser-grained layers where larger pores provide more favorable thermodynamic conditions for nucleation(Clennell et al.,1999).At Blake Ridge,higher concentrations of hydrates were observed in two distinct intervals with greater abundance of grainsϾ5microns in diameter,separated by a region of smaller grain sizes(Ginsburg et al., 2000;Kraemer et al.,2000).In advective systems where long migration is a primary means of methane transport,such hydrate partitioning could be explained simply by easier fluid migration through more permeable sediment layers(Weinberger and Brown,2006).Additionally,when passing through a coarse-grained interval,advecting fluid that is supersaturated with methane can more easily deposit methane in the form of hydrate than when flowing through finer-grained intervals(Kraemer et al,2000).In Hydrate Ridge off the northwestern coast of the U.S.,hydrate was found to have formed in fractured layers of fine-grained sediments as well as in coarse-grained sands(Weinberger and Brown,2006).Such distributions could result from an additional mechanism involving hydraulic fracturing via generation of excess pore pressure during hydrate generation elsewhere(Daigle and Dugan,2010).Gibbs-Thomson Effect and Solubility Impacts in Fine-Grained SedimentsIn any sediment,methane hydrate formation occurs when the concentration of methane dissolved in pore water exceeds its solubility at a specified pressure,temperature,and pore water salinity,regardless of the methane source.Decreasing pore radius increases the surface area to volume ratio of the pore space in sediments,and hydrate crystals that grow in these pores must have correspondingly large surface area to volume ratios.In fine-grained silty and clayey formations,this effect is significant enough that the contribution of interfacial energy to the total Gibbs free energy of the pore-water-methane system cannot be neglected.This in turn leads to an increase in the solubility of methane in the pore space,inhibiting hydrate growth(Clennell et al.,1999).An increase in methane solubility thereby necessitates a decrease in the equilibrium triple point of methane in the pore space with decreasing pore radius,known as the Gibbs-Thomson effect.Thus,methane hydrate can be expected to require lower temperatures for nucleation in smaller pores than in larger pores,all else remaining constant.Methane Hydrate Stability Conditions in the Nankai TroughWe analyzed data from three sites in the Nankai Trough offshore Japan in order to determine stability conditions for methane hydrates in the region.Although the three sites modeled are not hydrate bearing due to lack of sufficient geochemical conditions,they are representative of the subsurface conditions in the strata approaching the subduction front throughout the Nankai Trough and provide useful analogs to prospective hydrate-bearing areas to the northeast(Expedition333Scientists,2012).Fig.1illustrates the locations of the boreholes offshore Japan’s Kii Peninsula.The sites themselves vary slightly in sampling depth and the characteristics of the sediments they penetrate.Adjacent Sites C0011and C0012,depicted in Fig.2,sample regions of similar lithology,though Site C0011seems to exhibit an expanded lithologic distribution compared to a compressed Site C0012. Both Sites C0011and C0012represent sediments unaffected by subduction(Expedition333Scientists, 2012).Site C0018is located within a slope basin above the accretionary wedge.It therefore contains complex structural features related to wedge deformation associated with convergence in the Nankai Trough.The site exhibits southeast-dipping beds,shear zones,and chaotic bedding,all of which result in complicated hydrate stability conditions.Fig.3depicts the lithologic units sampled at Site C0018.All three sites analyzed are made up predominantly of fine-grained sediments(median grain diameter Ͻ3microns;Daigle and Dugan,in review),so it is expected that small pore radii,via the Gibbs-Thomson effect,will significantly affect methane hydrate stability ing measurements of grain size and pore size along with gamma ray logs,we determined the methane triple point temperature depressions and subsequent methane solubility increases that characterize hydrate stability conditions in fine-grained marine sediments within the Nankai Trough.MethodsWe used logging-while-drilling (LWD)and laboratory sample data from three Integrated Ocean Drilling Program (IODP)sites in the Nankai Trough to determine the impact of pore throat sizeson Figure 1—An (a.)aerial view of Sites C0011,C0012,and C0018in the Nankai Trough offshore Japan,as well as (b.)a columnview.Figure 2—Lithologic columns of sites C0011and C0012methane solubility in the region.We used the LWD gamma ray log to determine shale volume,which we correlated with grain size measurements to determine a relationship between shale volume and gran size.We then used shipboard helium pycnometer porosity data to determine the effective electrical pore size from the grain size.We used heat flow and porewater salinity measurements to determine hydrate stability conditions as well as the change in equilibrium temperature and solubility caused by the Gibbs-Thomson effect.Shale Volume DeterminationIn order to determine the grain sizes within the methane hydrate stability zone (MHSZ),we correlated grain size measurements with LWD gamma ray data.As gamma ray measurements are useful for providing an estimate of the quantity of clay or shale in a formation,we began by calculating the percentage of shale at each depth within the sample,V shale .Areas of low gamma ray response were deemed sandy regions (of larger grain size),while regions of high gamma ray response were assumed to be shaly.Once pure sand and shale endpoints were chosen for the gamma ray logs of each site,we calculated shale percentage via a linear interpolation of the gamma raydata:where z is the depth coordinate.Fig.4depicts the gamma ray data for each borehole,while Fig.5depicts their corresponding shale volume calculations.Electrical Pore Throat Radius DeterminationThe relationship between clay content and grain size is,in general,non-linear (e.g.,Dewhurst et al.,1999).However,the median grain sizes (D 50)in the sediments we analyzed in this study fell in a very narrow range,from about 1to 3microns.Therefore,we approximated the relationship between grain size and shale content as linear.Matching the depths corresponding to each grain size measurement with the interpolated values of shale content at the same depths,we fit a linear trend to our grain radius data(eachFigure 3—Lithology of Site C0018Figure 5—V shale data for boreholes 11A,12H,and18BFigure 4—Gamma Ray data for boreholes 11A,12H,and 18Bof the measured D 50values divided by 2)using a root mean square method,shown in ing this linear trend line,we computed grain sizes throughout the depth of the MHSZ via matching shale content values to corresponding grain radii.Invoking the idea of an effective electrical pore throat radius,defined in the presence of a thin conductive grain coating that differs in conductivity from pore fluid (Johnson et al,1986),we proceeded to link grain radius to pore radius in our samples.When the pore fluid conductivity is much larger than the surface conductivity of the sediment grains,Revil and Cathles (1999)showed that effective electrical pore radius could be calculatedaswhere ⌳is the effective electrical pore throat radius,R is the characteristic grain radius,m is the cementation exponent,and F is the formation factor (Revil and Cathles,1999).We calculated the formation factor using Archie’sequation:where a ϭ1and m ϭ2.4,determined from previous work in the Nankai Trough area (Kinoshita et al.,2008).During IODP Expedition 322,no LWD porosity measurements were taken,so instead we calculated the porosity distribution with depth,⌽,using shipboard moisture and density (MAD)values.This method is a simplification assuming surface conductivities are small compared to pore fluid conductivity,and,following Revil and Cathles,R values are used as characteristic grain radii,equal to one half of the grain diameter measurements,D 50.Gibbs-Thomson EffectUpon calculating pore throat radii throughout the MHSZs in each site,we assessed the extent to which the equilibrium triple point temperature of methane is expected to depress.It is important to note here that this should yield a near-maximum temperature depression (as well as maximum solubility increase),as throat radius is used in these calculations,while pore bodies are expected to be larger in radius thanthe Figure 6—V shale and grain radius data,with a linear trend linethroats.The triple point depression due to constrained pore size,assuming cylindrical pores,is calculated via the Gibbs-Thomson equation (Anderson et al.,2009):where T mb is the bulk melting temperature of methane,␴hl is the solid-liquid interfacial energy between hydrate and liquid,␸is the hydrate wetting angle,H f is the hydrate bulk enthalpy of fusion,and ␳h is the density of methane hydrate.The initial equilibrium triple point of methane immersed in saline pore water is determined following the method of Bhatnagar et al.(2007),using the salinity effect calculated by Duan et al.(1992)and assuming a constant geothermal gradient for each sample.Fig.7depicts the initial three-phase equilibrium temperature,the triple point depression,and the geotherm for each site.The depressed triple point temperature near the base of the MSHZ is lower than the in situ temperature,indicating that hydrate may not form as a stable phase near the base of the MHSZ due to the effect of pore size.Effect on SolubilityBy depressing its equilibrium triple point in saline pore water,reservoir methane will necessarily see an increase in solubility at constant reservoir temperatures and ing the method of Davie and Buffett (2001),we calculated the equilibrium concentration of methane throughout the MHSZ relative to both the initial triple point temperature as well as the depressed triple point temperature using the followingequation:where T 3is the three phase equilibrium temperature and ␶is a constant,set to 15.3(Davie and Buffett,2001).The solubility increase due to the Gibbs-Thomson effect is therefore calculated as a percent change between the equilibrium methane concentration at the initial triple point and that at the depressedtriple Figure 7—Equilibrium three-phase methane concentration curves,equilibrium triple point depressions,and geotherms for Sites 11A,12H,and 18B.point.Fig.8depicts the percent increase in solubility predicted at depths throughout the MHSZ at all three sites.DiscussionThroughout Sites C0012and C0018,solubility is observed to increase most near the base of the MHSZ. This is due to a decrease in pore radius with depth throughout the hydrate stability zone in these areas(Fig.9),which also correlates with a decrease in porosity with depth.At Site C0011,the solubility increase is relatively consistent throughout the MHSZ due to anomalously high porosity values near the base of the stability zone(Expedition333Scientists,2012).Relatively constant values of porosity imply a fairly consistent distribution of pore throat sizes and greater homogeneity within the sediment(Griffiths and Joshi,1989),so it makes sense that the solubility increase is somewhat constant throughout Site C0011.MICP measurements taken on samples from the same cores(Daigle and Dugan,in review)yield values very similar to the pore radius values calculated using effective electrical pore radius theory,validating our approach and results(see Fig.10).The magnitude of the median pore throat radii observed in the sediments from each site should yield an increase in methane solubility relative to its free fluid solubility due to the Gibbs-Thomson effect,which in turn indicates that less hydrate can be formed in these sediments.Further,the decrease in pore throat radii with depth at Sites C0012and C0018will yield lower amounts of methane hydrate approaching the base of the MHSZ.Ignoring the effect of pore size on solubility,the equilibrium concentration of methane(depicted in Fig.11)throughout the MHSZ would range from8ϫ10-4to3ϫ10-3kg CH4per kg of pore water for SiteC0011.With an average sediment porosity of0.54,seawater density of1024kg per m3,and MHSZthickness of241.16m,Site C0011could contain between about1.06ϫ108and4.0ϫ108kg CH4persquare kilometer within the stability zone.With the Gibbs-Thomson effect accounted for,the actualsolubility of methane is closer to1.1ϫ10-3to3.3ϫ10-3kg CH4per kg of pore water,yielding1.47ϫ108to4.4ϫ108kg CH4per square kilometer and a constant increase in dissolved methane ofapproximately4.05ϫ107kg CH4per km2.Figure8—Solubility impact due to the Gibbs-Thomson effect at Sites C0011,C0012,and C0018.Figure9—Pore throat radius throughout MHSZFigure10—Measured MICP throat radius puted throat radiusSite C0012contains sediment with an average porosity of 0.55,a hydrate stability zone thickness of 145.35m,and initial methane equilibrium concentrations ranging from 8.5ϫ10-4to 2.9ϫ10-3kg CH 4per kg of pore water (Fig.11).In the absence of pore size effects,this yields between 6.96ϫ107and 2.37ϫ108kg CH 4per km 2.Including the Gibbs-Thomson effect,the equilibrium concentration jumps to between 1ϫ10-3and 4ϫ10-3kg CH 4per kg of pore water,allowing for between 8.19ϫ107and 3.27ϫ108kg CH 4per km 2,for an average increase of 5.11ϫ107kg CH 4per km 2.The average porosity at Site C0018is about 0.56,while the hydrate stability zone is the thickest of all three sites,at 322.97m.Initial equilibrium methane concentrations range from 8ϫ10-4to 2.75ϫ10-3kg CH 4per kg of pore water,and this range is increased to between around 1ϫ10-3and 4.25ϫ10-3kg CH 4per kg of pore water (Fig.11).Therefore,an average increase of 1.57ϫ108kg CH 4per km 2is expected at Site C0018due to the Gibbs-Thomson effect.ConclusionAt Site C0011,the equilibrium methane concentration was determined to increase by 4.05ϫ107kg CH 4per km 2when accounting for the Gibbs-Thomson effect.Similarly,Site C0012was determined to contain increased amounts of methane in dissolved water by about 5.11ϫ107kg CH 4per km 2,and Site C0018should contain more dissolved methane by nearly 1.57ϫ108kg CH 4per km 2.An increase in dissolved methane decreases the methane locked in hydrate form at a particular concentration above equilibrium,meaning a lower potential hydrate yield during efforts to extract methane hydrates from sediments.In the Nankai Trough,while there are significant reservoirs of methane hydrate beneath the surface,the limitations on hydrate growth imposed by the Gibbs-Thomson effect are non-negligible when estimating potential hydrate production quantities.At all of the sites,some of the equilibrium triple point values near the base of the MHSZ depress to lower values than the corresponding reservoir temperature.While at Site C0011most of this seems to be attributable to scatter in the data,Sites C0012and C0018exhibit regions where this effect is pronounced.If the triple point temperature of methane is lower than the reservoir temperature,hydrate is not able to form,and instead of solid methane hydrate,these regions will contain solely methane gas and dissolved methane.This illustrates the fact that the Gibbs-Thomson effect decreases the thickness of the methane hydrate stability zone,which can have important implications to reservoir quality and production designs.This work is significant in that it allows us to better assess the distribution of methane hydrates in Nankai Trough sediments,which in turn will allow for more effective and efficient methaneproduction Figure 11—Initial free water methane solubility and solubility due to the Gibbs-Thomson effectTable1—Constants affecting the properties of the MHSZ at each sampled siteSite Seafloor Depth(m)Seafloor Temp(K)Geothermal Gradient(K/m)Base of MHSZ(mbsf)C00114079274.90.091241.16C00123550276.00.135145.35C00183084274.60.063322.97from hydrates in this area.Further,this work provides a method of determining phase equilibrium behavior from log data,which provides valuable information for hydrate exploration and production in the Nankai Trough and other potential methane hydrate plays around the world.This method may be used to predict methane hydrate stability conditions from wireline or LWD data.With more data on grain size distributions at each site and with more sites to analyze,solubility impact estimates using this method can be significantly improved.ReferencesAnderson,R.,B.Tohidi,and J.B.W.Webber,2009,Gas hydrate growth and dissociation in narrow pore networks:Capillary inhibition and hysteresis phenomena:Geological Society of London Special 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