Effects of salinity on methane gas hydrate system

Science in China series D: Earth Sciences020◆ SCIENCE IN CHINA PRE∈ssSpringerEffects of salinity on methane gas hydrate systemYANG DingHui1 XU Wen YueDepartment of Mathematical Sciences, Tsinghua University, Beijing 100084, Chinachool of Earth& Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA30332, USAUsing an approximately analytical formation, we extend the steady state model of the pure methanehydrate system to include the salinity based on the dynamic model of the methane hydrate system.Thetop and bottom boundaries of the methane hydrate stability zone(MHSZ) and the actual methane hy-drate zone(MHZ), and the top of free gas occurrence are determined by using numerical methods andthe new steady state model developed in this paper Numerical results show that the MHZ thicknessecomes thinner with increasing the salinity, and the stability is lowered and the base of the MHSZ isshifted toward the seafloor in the presence of salts. As a result, the thickness of actual hydrate occurrence becomes thinner compared with that of the pure water case. On the other hand, since lowersolubility reduces the amount of gas needed to form methane hydrate the existence of salts in seawater can actually promote methane gas hydrate formation in the hydrate stability zone. Numericalmodeling also demonstrates that for the salt-water case the presence of methane within the fleld ofmethane hydrate stability is not sufficient to ensure the occurrence of gas hydrate, which can only formwhen the methane concentratlon dissolved in solution with salts exceeds the local methane solubilityin salt water and if the methane flux exceeds a critical value corresponding to the rate of diffusivemethane transport. In order to maintain gas hydrate or to form methane gas hydrate in marine sedi-ments, a persistent supplied methane probably from biogenic or thermogenic processes, is requlred toovercome losses due to diffusion and advectionmethane gas hydrate, solubility stability of hydrate, salinity, phase equilibriumMethane gas hydrate (MGH)is an ice-like crystalline (pressure)and temperature. Decreasing pressure causedcompound of water and gas molecules.that forms at by lowering the sea level or increasing seawater tem-low temperature and high pressure when the dissolved perature causes the hydrate dissociation, resulting in themethane concentration exceeds the local solubility. geologic hazards of seafloor sloping) and releasingThe formation of methane hydrate can extract water and "greenhouse" gas(main methane gas)including in hymethane gas reserved in porous media. Under suitable drate further resulting in the global climate change.Intemperature and pressure conditions liquid water in a word, changes of temperature and pressure cause thepores and methane dissolved in water may transform transformation between solid hydrate and methane gas,nto solid hydrate. The formation of solid hydrate can which shows a dynamic evolution process of hydrate/increase the strength of sediments and result in lowering methane.porosity and permeability 31. Reversely, when the gas Studies of MGh in marine sediments have focused onhydrate system is not stable, solid hydrate can be de- issues as a potential energy resource, an agent of globalcomposed into water and methane. It is stable under climate change, and a possible role of MGH formationelevated or relatively high pressure and low temperatureReceived December 5, 2006: accepted July 4, 2007conditions such as those found in the marine sediments doi: 1oalong continental margins and permafrost region中国煤化工知cThe gas hydrate stability depends on the water depth NaturalCNMHG4邮Mmwww.scichina.comwww.springerink.comSci China Ser D-Earth Sci l Nov. 2007 I vol 501 no. 11 11733-1745and dissociation in slope instability and other geologic equilibrium between hydrate, free gas, and salt water arehazards, and so on. These are all issues of global im- most important for establishing the stability of methaneportance;consequently there have been significant ad- gas hydrate in marine sedimentsvances in theoretical and laboratory research on MGH The equilibrium methane concentration or solubilityand numerous field studies of MGH in the natural envi- is also important because it determines the minimumronment-7. In recent years the observational data set methane concentration needed for hydrate stability.Theon the in situ conditions associated with gas hydrate oc- gas in solution is limited by the solubility that definescurrences has expanded rapidly 8. Many results of the minimum methane concentration needed to formsuch field-based programs have been a clearer under- hydrate or represents the maximum concentration of gasstanding of in situ porosities, advective flux rates1 l9, that can be dissolved in salt water under equilibriumnatural gas hydrate concentration %, 10,202, dynamic conditions, so that any additional gas is present in theprocess and effects of temperature change on the hydrate hydrate phase Within the stability region of methane gastability222), and hydrate distribution patterns 1.24). hydrate methane solubility increases with both temperaBased on steady state models, some fundamental ques- ture and pressure increasing, and decreases as the salin-tions about gas hydrate formation and stability in marine ity increases"'p. Below the MHSZ, methane solubil-sediments have also been investigated by Xu and Rup- ity increases with decreasing temperature -. Recently,pel"and davie and buffett It makes that our undeDavie et al. 2)suggested a practical method of calculat-standing of hydrate systems is clearer. However, the ing the solubility profile within the MHSZ in marinequantitative estimates given by Xu and Ruppel do not sediments for a given water depth, seafloor temperatureinclude the effect of salinity on the hydrate system. Ac- and geothermal gradient. But this method is based ontually, it is an important factor for predicting the occur- phase equilibrium calculations of Zatsepina and Buf-rence, distribution, and evolution of methane gas hydrate feti and an extension of simple parametric modelsto consider the effect of salinity on the hydrate system in into the MHSZ 2 The extension may produce big er-the seafloor. Therefore, the present study extends the rors at the intersection points(the boundaries of thesteady state model suggested by Xu and Ruppell2to MHSZ and the MHz). Our present calculations of solu-include the salinity and presents some numerical results bility at any depths are based on the extension of ana-showing the effects of salinity on the hydrate systemlytic model suggested by Xu and ruppel 2,which in-Addition of simple salts to water previously has been cludes the effects of multi-factors on the solubility. Inshown to decrease the stability of gas hydrate such that other words, the solubility in our present model isfor certain pressure ranges the dissociation temperature treated as a function of pressure, temperature, and salin-is depressed by a constant amount relative to the pure ity of the liquid solution.water system2.8. For the pressure range of 2.75-10.0 The main purpose of this paper is to investigateMPa, at any given pressure, the dissociation temperature role of salts in the methane gas hydrate system. For thisof methane hydrate is depressed by approximately we incorporate the salinity into a steady state model for1. 1C relative to the pure methane-pure water sys-the formation of hydrate below the seafloor, and thentem29). The phase equilibria of hydrate under conditions determine distributions of MHSz and MHz and inves-that are relevant to marine environments have been also tigate the influence of salinity on the MHSZ and thenree-phase equilibriuneenMHZ by using the quasi analyticalfree gas, and seawater 29-31]. The conditions for two- the bases of mHSz and Miz and the top of free gas inphase equilibrium between hydrate and salt water werethe pure-methane pure-water and pure-methane saltater cases are numerically compared. Our calculationalso investigated by Zatsepina and Buffet /2n. Such con- shows that the effect of salinity on the hydrate system isditions are important for determining the deepest deptof actual occurrence of gas hydrate in marine sedimentsvery significantwhich often coincides with a mark of the bottom simulating reflector(BSR)(9 32 but occurs subtantially deeper Basic theorythan the bases of the mhZ and the mhsz in some set- 1.1tings"2. Consequently, the conditions for three-phase中国煤化7modeto incoporateCNMH1734YANG DingHui et al. Sci China Ser D-Earth Scil Nov 2007 I vol 50I no. 1111733-1745the influences of salinity on the MGH formation and sponding phases in our present study.stability. For this we consider such a MGH system con- Using Xu's notations 2l and assuming that Darcy'staining four components, which are methane, salt water law describes the conservation of momentum for multi(subscript w), and solid porous sediment of porosity phase fluid flow through oceanic sediments that areand permeability k, and five phases: liquid solution, free moving downward at a velocity uy, the mass fluxes ofgas, solid gas hydrate, solid salt, and solid sediment liquid(subscript 1), gas(subscript g), hydrate(subscript(subscript s, density =P,). Assuming that salt is com- h) and solid sediment phases in eqs. (1)-(4)are givenpletely solved in liquid solution, for the case of our con- byideration the goveming equations can be derived from底P,the thermodynamic model developed by xu 2, whichP+p)+9S1p正are based on the mass conservations(including methanemass conservation, salt mass conservation and totalP(P+P3)+0P可,mass conservation )and heat conservation, and be simplyandGNoFV-qC1+qg g+qn Ch-os,D V(p. CD]=Qe=4SAp,豆=(-φ)p,,where the vector g is the gravitational acceleration, P( denotes pressure, H is fluid viscosity, and k, and kg ared(gpX)dr +V(,X +9, h-oS, D, V(e, X )]=0,(2) the relative permeabilities of the liquid and gas phases,respectively. The saturations of liquid, gas, salt(sub-0(9)+1++4+SDV+script x), hydrate satisfy(D-D)V(P,C)(D-Dw)v(p, Pll=Qe, ( which are determined by the phase equilibrium calcula-LpH+(-o)P,H,1+v (91H,+qH,+tion1.2 An approximate steady state model豆Hn+互Hx+,H2-vT)=0,(4) Time-dependent equations stated above show that thewhere eqs. ()-()describe the conservations of mass hydrate and bubble volumes evolve into steady statesfor methane gas, salt mass, and total mass for the gas after approximately several million years For the one-drate system, respectively, and eq (4)represents the dimensional case steady state solutions can be obtainedconservation of heat energy. De, D and Dw in eqs. (1)- by integrating the time-dependent equations( 1)-(4)(3)are the diffusivities of methane, salt, and water in from t=0 for a sufficiently long time. But the treatmentliquid solution, respectively. @c represents the rate of in in computational efficsitu methane production, a is effective thermal conduc- provements are possible if the problem is reformulatedtivity, and Sy denotes the liquid saturation. P, C, X, andh to solve directly for the steady state. One method is thedenote the composite density, concentration, salinity, and approximate analytical method under some suitable asenthalpy, respectively. The vector q with subscripts L,g, h, sumptions, and the other is the discretization methodx,and s are the fluxes of liquid, free gas, hydrate, salt, such as finite-difference and finite-element methods inand solid sediment, respectively. Variables C(concentra- which we first discretize eqs. (1)-(4)and then solve thetion), p(density), X(salinity), and H(enthalpy) with resulting finite-difference equations. We note that thesubscripts /(liquid), g(gas), h(hydrate), x(salt), and s goal of this paper is to gain fundamental physical in-(sediment) are the properties of individual phases (liquid sights into the effects of salinity on the natural gas hy-gas,solid gas hydrate, salt, and solid drate systems in natural environments through thesediment). For example, HA Hg, H,, Hr and Hn represent plication of a simple model. For this like Xu and Rup-enthalpies of liquid, gas, solid sediment, salt, and hy- pel'we assumethat the hydrate system isdrate, respectively. Similarly, we can define other varboundary conditions and salts areables with subscripts. These variables are either con- compl中国煤化工e samestants or functions of P, T, and salinity of the corre- alsoCN MH Gate and salt in poreYANG DingHui ef al. Sci China Ser D-Earth Sc!I Nov 2007 I vol, 50 I no. 11I1733-1745space is sufficiently small that the liquid phase occupies and these relations between pressure and temperaturealmost the entire available porosity, and the advectiveq-q1C070and diffusive effects of solid hydrate are negligiblekpr+p1gFollowing the treatment suggested by Xu and ruppela, Cio(qe-q CroTwith these assumptions the governing equations for theone-dimensional system reduce approximately from egsP=P⊥A8(T-T),(1)-(4)todz+P18=constant,(5) where Po and To are seafloor pressure and temperaturerespectivelyqe=q C1oT-A=constant,When eq.()is coupled with methane hydrate stabiity curves that are treated as a function of pressure andam=a C-pD-a acsalinity and is regressed from the data obtained usingCSMHYD software(Appendix), the pressure Pr(andwhere Co is specific heat capacity of liquid water, C is PB)and temperature Tr(and TB)at both the top and baseconcentration of methane, g and qe stand for constantof the mhsz can be determined we can determine thefluxes of total fluid mass and energy in all regions below locations of the MHSZ by substituting the values of Trthe seafloor, respectively. S, equals I based on our pre- and TB into(8), respectivelyvious assumptions, and qm represents depth-dependent ()Top and bottom of MHZ. The locations of MHZmethane flux, which is a subsection function. In otherboundaries can be determined by solving the intersec-words, below the base of the mHZ, qm is a constant tions of the solubility curve with concentration curve forgiven by the corresponding boundary condition. In the the steady state system. Within the MHZ solubility orregion above the top of the MHZ, m reduces to a con-the dissolved methane mass fraction can be written as astant and equals the flux gmr at the top of the MHZ and function of pressure and temperature. Above the topgm is not a constant within the MHz.of the MHz, integrating(7) with the seafloor boundarycondition Cl(a=0)=Co from the top of MHZ to the1 Hydrate stability and actual zone of hydrate seafloor, we can determine the top depth of the mhz asfollows(i) Locations of MHSZ boundaries. The phase bounda-Cries for methane gas hydrate have been determined ex-qr≠0perimentally for pure water- and salt-watersystemsIn theory, the intersection of the local geotherm with gasPDCs(z)-colhydrate stability constraints defined the base of the gashydrate zone. However, in dynamic systems, the posi- where Co is the concentration of methane at the seafloortions of the MHSZ boundaries depend on the flux of boundary, Cs,(ar) is the solubility of methane at the topvarious system components and must be written as func- boundary(zr)of the MHZ. From the computational extions of the mass and energy transport. In other words, pression stated above we can see that eq.(10)is theve first determine the actual geotherm via the approxi- same as Xu and Ruppel's results 2 for the pure-watemate model in section 1. 2 and then solve the intersection case except that the solubility(Cs(zr)in eq.(10)isof the geotherm with hydrate stability constraints to get calculated as a function of pressure, temperature, andthe position of the MHSZ. For the steady state system, salinity of liquid solution, The above intersection pointintegrating to eqs. (5)and (6), we can obtain the follow- (top of the MHZ)(ar)of the solubility and methaneconcentration curves yields 251λ,9-qC070C(ar)=Cu(t) ac,(z)dce(tT)(11)qe-q Corso from()we havez=-(7-T)中国煤化工dC()(12)mMhg dYANG DingHui et a. Sci China Ser D-Earth SciI Nov. 2007 I vol 501 no. 1111733-1745Knowing methane solubility C, and its derivative at z Table 1 Physical parameters used in calculationsZr, the position of the top of the MHZ can be calcu- Symbollated from(10)for the steady state system. In practical g gravitational accelerationTo temperature at seafloor℃calculations, we can calculate iteratively to determine zrur sedimentation rateby using eqs. (10)and(12)The base of the MHZ (zB) may often be shalloweD. methane diffusivity in seaway13×109kgm-stthan the base of the MHSZ (MHSZB) due to the de-Ce methane concentration at seafloor 1X10kg kgpendence of the MHZ base on the rate of methane sup-l×0‘kgm2sply. At the location of the base of the MHZ(MHZB), g, total energy fluxne following relations are satisfied byhydrate flum-28-1C(zB)=Cs(zB),0. rate of methane gas productiondz(13)A density of liquid-phase fluidqm=mB =q, Ca(aB)-D,s, acg(zB)(14)8.87X10 kg ms-ldX salinity0035ke keKnowing the methane flux m at the bourlocation of the mHZB can be determined iteratively using(14). Using(12)and (14)together yields the thick- 2.1 Effects of salinity on the MHSZof the MHZ, which depends obviously on a combi- On the basis of the approximate steady state modelnation of the rates of fluid flux energy flux qe, meth- given in section I, the previous calculations" includedane flux qm, salinity, and the water depth at the seafloor. only effects of various parameters such as the rates ofFor a given rate of fluid flux, increasing methane flux m energy flux qe, methane flux qm, and the water depth atresults in a thicker MHZ up to the critical point, which the seafloor on the MHSZ and do not account for theoccurs when the mhz coincides with the MHSZB for influence of salinity on the MHsz. we investigate ef-a pure -methane and pure-water system. For the pure- fects of salinity on the MGH system. By including salinmethane and saltwater system we shall further show the ity, the methane hydrate stability is a function of pres-results for the critical flux in subsection 2. 4sure and salinity, which is obtained by the regressedmethod from these data obtained(see Appendix)2 Numerical resultsEffects of salinity are shown in Figure 1 using theconstant salinity S=0.035 kg/kg that is comparable toThe basic theory stated above can be used to derive the that of seawater. For comparison, the top and base of therelations between the mhZ, the mHz and theseMHSZ for the pure-water pure-methane case are alrameters such as the rates of fluid flux qs energy flux ge shown in Figure l. Figure I shows that the MHSZBmethane flux qm, the salinity, the water depth at the sea(MHSZB-1)for the pure-methane seawater case (Stafloor, and so on. In this section, numerical results are biliy-1)is shallower than that base(MHSZB-2)for theused further to demonstrate the influence of different pure-methane pure-water case(Stabillity-2), which isparameters such as salinity, water depth, energy flux, identical with previous results,30,36,37. In other wordsand methane flux on the mgH system with saltwater. water in marine gas hydrate systems contains dissolvedFollowing Xu and Ruppel, we here use observational salts, which reduces the stability of gas hydrate. Thedata to constrain the physical parameters given in sec- shallower MHSZB results in the shallower MHZB intion 1. Table 1 lists the physical properties of the hydrate gas hydrate systems with salts(MHZB-1) comparedsystem used in our present calculations. These parame- with that in pure-methane pure-water systems(MHZB-2)ters coincide basically with those of the standard MGH ( see Figure 2).system. Some values in Table I have been chosen to Figure 2 shows effects of salinitycrudely represent those that might be applied to marine MHS中国煤化工fthesystems like the Blake Ridges.CNMHGpure-methane sea-YANG DingHui et al. Sci China Ser DEarth Sci I Nov 2007 I vol 50 I no. 1111733-17452700rthe MGH system with salts are shallower than those forSeafloorthe pure-methane pure-water system, and shoal as thesalinity increasesMHSZ2.2 Effects of salinity on the miz目In order to understand the effects of salinity on the topII MHSZB-Iand base of the MHZ, we chose the same model pa-in Table 1. In fact, from Figure 2we have seen the influence of salinity on the MHz. Inthe following we give again more numerical results tofurther demonstrate the effect of salinity on gas hydrateFigure 3 shows that the thickness of the MHz deFigure 1 Comparison of effects of salinity on the MHSZ betwepends on a combination of the rates of energy flux qestems with salts and without salts for an assumed seafloormethane flux gm, salinity, and the water depth at the sea-responding to 2800 m water depth. Curves stability-I andresent the stability temperatures for gas hydrate systems withfloor for the pure-methane seawater hydrate system. Forwithout salts, respectively, and curves MHSZB-1 and MHSZB-2 show the a given rate of fluid flux, increasing methane flux resultsbases of the MHSZ for the saltwater and pure-water systems, respectively. in a thicker MhZ up to the critical point denoted by theattainment of constant MHZ thickness in Figure 3. Thcritical point occurs when the MHZB instantaneouslycoincides with the MHSZB and the GAsT. Beyond thecritical point, further increases in methane flux rate willnot produce a thicker MHZ (see Figure 3). This is iden-tical with results given by Xu and ruppel for the pure-methane pure-water system. Figure 3 also shows thatfor constant rates of fluid and methane fluxes increasingenergy flux produce a thinner MHZ.For comparisons, Figure 4 shows the thickness of theMHZ for the gas hydrate systems with salts(lines 1 and3)and without salts(lines 2 and 4). In Figure 4 the basesSalinity (%)of the MHZ for the hydrate system with salts shown byFigure 2 Comparison of depths versus salinity between the MHZB and lines 1 and 3 are shallower than those for the hydrthe MHSZB and the top of the free gas zone for the pure-methane salwa- system without salts shown by lines 2 and 4. This sug-ter and pure-methane pure-water systems, which the depths are calculated gests that the presence of salts reduces the thickness ofas a function of salinity X for the constant methane flux q=2.7x10-kgsmand energy flux 40 mW m" at the water depth of 2700 m Wherethe Mhz. The tendency is also shown in Figure 2 and isdashed curves MHZB-L, MHSZB-l, and GAST-I represent the bases of identical with previous results/20.291the MHZ und the MHSZ, the top of the free gas zone for the saltwaterIn order to further illustrate the influence of salinitysystem, respectively, and solid curves MHZB-2, MHSZB-2, and GAST-2 on the thickness of the mhz, Figure 5 presents changesrepresent the bases of the MHZ and the MHSZ, and the top of the freezone for the pure-water system, respectively.of thickness of Mhz with salinity. Forfluid flux and methane flux, increasing salinity results inwater and pure-methane pure-water cases shown in Fig- a thinner MHZ up to the critical depth that the base andure 2 indicates that the MHSzB for the MGH system the top of the mhz become coincident in Figure 5. Thewith salts is generally shallower than that of the pure- critical salinity, which results in no gas hydrate to formmethane pure-water case except for salinity equal to within the MGH zone, decreases with increasing energyzero, and the degree of MHSZ deepening increases with flux For a given salinity, the top of the actual hydratedecreasing salinity. This trend can also be observed on occurrence tends to a shallower location as the heat fluxdepths below seafloor(mbsf) versus salinity curvesYHa中国煤化工 affect the top of thegiven in Figure 2, where the MHZB and the GAST for MHZCNMHG1738YANG DingHui et al. Sci China Ser D-Earth Sci I Nov 2007 I voL 50 I no. 1111733-1745200253035450(a)10000L.5202.53.01.02.53.0Methane flux(10 s ' m 2)lux(I0-kgs" 'm2)==c)3000mMethane flux(10-kg")Figure 3 The top and base of the MHZ as a function of methane flux rates at constant water depths of (a)1000 m, (b)2000 m, and(c)3000 m at thethe mhz while three curves located at the bottom show the base of the MHZ, corresponding to heat flux rates of 50, 40, and 30 mw m? show the top ofseafloor, assuming a constant fluid flux rate of 10 kg sm(about 0.3 mm a). Where three curves located at the top in(a), (b), and(c)show the top ofline 1. 40 mw m 2line 3. 30 mw m-line 4. 30 mW m-000m3.000020040060080.100120.14016018020Methane flux(i0- kg s-m)Comparison of the thickness of the MHZ between gas hydrate Figure 5 Enfluence of salinity on the top and the base of the actual zoneith salts (lines I and 3)and without salts(lines 2 and 4). The of gas hydrate occurrence(MHZ)for the total methane flux m=IxI0-lis calculated as a function of the methane flux ga for constant kg s-m"andenergy fluxes of 50, 40, and 30 mw m"ar theenergy fluxes of 40 and 30 mWm-2at the water depth of 1000m.water depth of 2000 m2.3 Effects of salinity on the top of free gascentration exceeds the solubility.. For methane flfree gas is present in the deeper sediments, while gas rates中国煤化工he gasthydrate occurs at shallower depths when the gas con-MHSZ, and the mhzCNMHGYANG DingHu et al. Sci China Ser D-Earth Scil Nov. 2007 I voL. 50 I no. 1111733-17451739is separated from the free gas zone by a layer of sedi- 2.4 Critical rates of methane supplyment containing neither gas hydrate nor free gas">1.The mhzb does not coincide with the mhszb in theFigure 2 has shown the relationships between the GAST pure-methane pure-water system". For the gas hydrateand the bases of the MHZ/MHSZ from Figure 2 we can setting with salts, similar conclusion can be obtained. Insee that the free gas zone exists deeper than both the fact, the MHZB is only equivalent to the MHSZB whenMHZ and the MHSz, and the GAST shoals gradually methane mass flux gm exceeds a critical value that prowith the salinity increasing. We can also observe that theduces a concentration excess of methane solubility inGAST for the pure-methane saltwater case is shallower salt water within all MHSZ. Figure 7 shows the resultsthan that of the pure-methane pure-water case except for of calculations to determine this critical methane fluxthe limit case of salinity equal to zero. In the following value as a function of water depth at the seafloor andexample presented in this subsection, we would like to various fluid fluxes of for the given energy flux 40use the same calculation method stated previouslymw m The result shown in Figure 7 demonstrates thatinvestigate the effects of salinity on the GAST for dif- the critical methane flux rate necessary for the MHZB toferent energy fluxescoincide with the MHSZB is strongly dependent on theIn the numerical calculation, we similarly chose the total fluid flux. The critical methane flux rate increasesphysical parameters given in Table I. The predicting with increasing the water depth(or pressure)at seafloor,results for the GAST are shown in Figure 6 for two gas and the slope of critical methane flux rate profile alsohydrate systems with and without salts. In Figure 6 lines gradually becomes larger with increasing the fluid flux1, 3, and 5(solid lines)correspond to energy fluxes of rate. Figure 7 also shows numerically that marine sedi-50, 40, and 30 mW m for the gas hydrate system with ments characterized by high advection rates require asalts,while lines 2, 4, and 6(dashed lines)correspond- high rate of methane supply if the actual hydrate occur-ing to the three cases represent the tops of the MHz for rence is to extend to the MHSZB. This result has importhe pure-methane pure-water system. Figure 6 shows for tant implications for assessment of the resource potentialdifferent energy fluxes that the GAST in the gas hydrate of methane gas hydrate deposits in marine sedimentssystem with salts is shifted upward as compared to thatin the gas hydrate system without salts. This suggeststhat the presence of salts increases the thickness of the2. mma-free gas zone if the abundance of gas in marine sedi-ments can be provided From Figure 2 we also have the 3same conclusionline 400100015002000250030003500water depth(m)lineigure 7 Critical methane flux 4m for the mhz to coiMHSZB, which is calculated as a function of water depth(or pressure)atseafloor. The four curves correspond to the fluid flux rates of 2 mm/a, 14mm/a, 0.8 mm/a, and 0.3 mm/a, respectively8502.5 Effects of salinity on the solubility and saltwaterMethane flux(10-"m)Figure 6 Comparison of the top of the free gas zone between gas hy- The existence of salts in pore water shifts the mhszb toirate systems with salts(ines 1,3, and 5)and without salts(lines 2, 4, and shallower depths, which is shown in Figure 1, and6), the top boundaries are calculated as a function of the methane flux gmfor constant energy fluxes of 50, 40, and 30 mw m at the water depth of caus中国煤化工 ility of methane2000mCNMHGbeen calculated byYANG DingHui et al. Sci China Ser D-Earth Scil Nov. 2007 I vol 50 I no 11 11733-1745Table 2 Comparison of depths(m)of base and top of the MHSZ, the MHZ, and the free gas zone for the saltwater(case 1)and pure-water(case 2)casesTop of the MHZBase of the mhzTop of the free gas zoneCase I112338292777l1234257855423atsepina and Buffett using 0.6-mol solution of Nacl Figure 8 correspond to these results of cases 1 and 2,that is approximately equivalent to 0.035 kg/kg. Their respectively. For comparison, the locations of top andcalculations for the solubility show that a linear de- base of the MHZ and the MHSZ, and the location of thependence on the salinity X. Recently, Davie et al. 2 GAST for two cases are also listed in Table 2. From Ta-suggested a calculation formula of the solubility asble 2 and Figure 8(left) we clearly see that the MHZBC,(T, P, X)=(1-Bx)cpm(T, P). (15)(MHZB-1)for case I is deeper than that of case 2(MHZB-2) It demonstrates that accounting for the inCs( P)is the solubility at the base of the MHSZ in fluence of salinity on both the solubility and the stabilitythe pure water case, which is calculated by two steps. (case 1)results in more thickness of the mhZ as com-The solubility at the conditions of three-phase equilib- pared with the pure-water case(case 2), but the gASTrium is first approximated by a linear function of tem- (GAST-1)for case 1 becomes shallower than that of theperature and pressure and then extended this solubilitypure-water (GAST-2). Figure 8 also shows that theinto the MHSZ using simple parametric equationMHSZB does not coincide with the MHzB because up-The calculation may produce big errors for the solubilityward flux of methane(qm)does not exceed the criticalCl(T, P)at the intersection points. In this paper, value for the saltwater system. The free gas zone onlycalculations of the solubility at any depths are based on exists at depths for which the concentration of methanethe steady state model presented in subsection 1.2, CI exceeds methane solubility Cst below the MHSZBwhich is treated as a function of pressure, temperature, Figure 8 ). Figure 8 shows that the free gas zone liesand salinity of the liquid solution. The computational below the MHSZB and is separated from the MHsz byeps are givenan intervening layer of sediment lacking both gas hyWe first calculate the solubility at any depths from the drate and free gas for every case. The intervening layerMHSZB to the seafloor, using the following formulabetween the MHSZB (MHSZB-1)and the GAStC(T, P)=dexp(a+bln P+cln P)xexpla(T-T ), (GAST-1)for the pure -methane system with salts(case 1)(16) is thinner that for the pure-methane pure-water systemwhere T, is the stability temperature for the saltwater (case 2)(see Figure 8). This may be a significant implicase, and coefficients of a= -26.7534, b=1.9848.c= cation to explain why the MHSZB in many situations-0.0448, d=0.8904, and a= 0.07 are determined bycoincides with the Gast because the salinity affectsusing experimental data( 34.38nd then we modifysimultaneously both the stability and the solubility innatural marine environmentsthe solubility by using the following formula:On one hand, in Figure 8(night) or Figure 1a-BXCa(T, P), XXo(17 presses the temperature to form gas hydrate in marinesediments. The physical meaning is that the existence ofwhere17.094 is determined from the results given salts depresses the stability of gas hydrate. As a result,by Zatsepina and Buffett 20l, and Xo=0.0058 kg/kg is the position of the MHZB is shallower than that of theapproximately equivalent to O I molpure-water system(see Figure 4)as we only accountFigure 8 shows the comparison of the relationship influence of salinity on the pure-methane saltwateramong the MHZ, the mhsz, and the free gas zone for tem. On the other hand, the salinity decreases thetwo cases of including simultaneously effects of salinity bility from theoretical formulae( 15)and(17)fur(=0.025 kg/kg) on both the stability and the solubility resul中国煤化工 nimum methane concase 1)and x=0(case 2). Labels 1 and 2 shown incentrCNMHGSo when consideringYANG DingHui ef a. Sci China Ser D-Earth SciI Now. 2007 I vol. 50l no. 11 I1733-17451741290Seafloorop of MHz3]003200MHZ3300MHZB3400MHSZB-2GAST-23600370037Water+ tree gas00010000200.0030Figure 8 Comparison of effects of salinity on the MHZ, the MHSZ, and the free gas zone between methane gas hydrate systems with constant salinity2.5%and without salts for the constant methane flux rate of gm=2.71xl0 s"m-and an assumed seafloor pressure corresponding to 3000 m waterdepth, Where curves Cl and Si represent the methane concentration and the solubility including simultaneously cffects of salinity on both the stability andthe solubility in natural marine environments, and curves C2 and S2 show the methane concentration and the solubility for the methane gas hydrate withoutsalts. Below the MHSZ the solubility is approximated by a constant solubility equal to the solubility at the base of the MHSZ. Curves MHZB-I andMHZB-2 show the bases of the MHZ for two cases, while GASTI and GAST-2 present the tops of the free gas zone for two cases, respectively. Stabilityand MHSZB-1, stability-2 and MHSZB-2 show the stability curves and bases of the MHSZ for the gas hydrate systems with salts and without salts, respecsimultaneously effects of salinity on both the stabilityand the solubility, our numerical results(figures 8 (letand 9 in the following)show that the salinity increasesactually the thickness of the mhz within the MHSZalthough the MHSZB for the saltwater case is shallower 2 300than that for the pure-water caseIn order to further investigate the effect of salinity on 3501pure-methane saltwater system under this case of simul-taneously considering influences of salinity on both hy-drate stability and solubility, in the final example the500model parameters are chosen by the total energy flux ge60 mW m, total methane flux qm= 2.71 mW m101214161820Salinity(%e)floor temperature To=2(C), seafloor depth of 3050m, and the salinity from 0.6 to 20 % Rest parameters the MHSZB, and the GAST for the pure-methane saltwater system in-are listed in Table 1. The numerical results are displayed cluding simultaneous effects of salinity on both the hydrate stability andin Figure 9, where the solid and dashed curves represent the solubility and for the pure-methane pure-water system, of whichthe bases of MHZ and MHSZ, and the GAST for the flux ga 271xlo-lkg s-'m2, energy flux 60 mw m" and seafloorpure-water and saltwater cases, respectiveltemperature To=2 C at the water depth of 3050 m. Dashed curvesFigure 9 shows that the MHSZB and the GASt forS7B-L and GAST-l represent the MHZB, the MHSZB, andthe saltwater case are shallower than those for the pure- mh2中国煤化工tve, and solid curvesMHZB, the MHszB, and thewater case in the chosen range of salinity and shoal with GCNMHG1742YANG DingHui et al. Sci China Ser D-Earth Sci I Nov. 2007 I vol. 50 no. 1111733-174increasing the salinity, and relationships of MHZB be- methane solubility is determined by(16)and(17),andtween the pure-water and saltwater cases is different in temperature and pressure in(16)and(17) are calculateddifferent salinity range. In other words, the MhzB for by the steady state model presented in subsection 1. 2.the pure-methane saltwater system is deeper than that We apply our calculations to determining the bases offor the pure-methane pure-water system as the salinity is the MHSZ and the mhz, and the gast for the pure-smaller than about 3.5% in the chosen range of salinity, methane saltwater system. Numerical results indicatewhereas it is shallower than that for the pure-methane that the MHSZB for the pure-methane saltwater case ispure-water case when the salinity is greater than 3. 5%. shallower than that base for the pure-methane pure-waThe numerical experiment also illustrates that for the ter case, while the top of the MHz is independence ofmarine sediments with salts the MHSZB may be shal- the salinity(Figures 5 and 8)but depends on the energylower than the predicted mhZB by the pure-water flux and shoals with increasing the energy flux( Figuresmodel when the salinity greater than 6.3% and the top of 3-6). The shallower MHSZB may result in the shal-free gas may be shallower than the MHzB for the lower MHZB in gas hydrate systems with salts when wepure-water system as the salinity over 7.3%. Meanwhile, only account to the effect of salinity on the stability. Thefrom Figure 9 we can observe that for the saltwater sys- salinity decreases directly the thickness of the MHZ astem the MHSZB is near to the GAST as compared with shown in Figures 2 and 4. Changes in salinity canpure-water systrongly affect the formation of actual methane gas hydrate. Increasing salinity decreases the thickness of the3 Discussion and conclusionsMhZ and increases the thickness of the free gas zone ifThese factors such as temperature, pressure, salt, sedi- the abundance of gas in marine sediments can be pro-mentation rate, source of methane supply, and so on af. vided(Figures 2 and 9). Measured salinity profiles, infect strongly the formation and dissociation of gas hy. Conjunction with the depth of MHZ and MHSZ, providea complementary constraint on the quantity and distribu-drate in marine sediments. Many studies have demon- tion of hydrate and free gas in the sedimentsstrated that the temperature, pressure, and salinity areOn one hand, the salinity reduces the stability of gasthe most important factors for the formation and the sta- hydrate system( see Figures 1 and 8)further resulting in2021,293334】the shallower MHZB as compared with the pure-meth-creases,MGH may be dissolved into seawater, whereas ane pure-water case(Figures 2 and 4). On the other hand,more MGH may be formed as a result of the decrease in since the salinity lowers the methane solubility from(I7)solubility that occurs when the salinity increases20211further resulting in the reduction of the amount of gasEffects of salinity on the stability also result in affecting required to form hydrate, the presence of salts in naturalto form gas hydrate because the presence of salts in ma- settings may actually promote hydrate formation withinrine gas hydrate systems depresses the stability of gas the MHSZ Speaking more details, the MHZB for thehydrate/20,29,36,37. Although the obtained results have pure-methane saltwater system is deeper than that formade great progress in study about the role of salts in the pure-water case as the salinity is smaller than 3.5%marine gas hydrate systems, no quantitative links be- whereas it is shallower than that for the pure-water casetween the gas hydrate and the salinity, which is based on when the salinity is greater than 3. 5%(Figure 9). It sug-the extension of the quasi-steady state model suggested gests that we can estimate the position of the MHZ fromby Xu and Ruppel 2, were established in these studies. the predicted results by the pure-water model and theIn this study, we incorporate the salinity into a steady salinity of the marine sediment. The curves(Figure 9),state model presented in subsection 1.2 to establish the indicating that the MHSZB and the GAST change withlinks between the gas hydrate system and the salinity, salinities, show that the MHSZB for the saltwater sysand to investigate the influences of salinity on the mGh tem may be shallower than the predicted MHZB by thestability and the actual gas hydrate occurrence in marine pure-water model when the salinity greater than 6.3%sediments By including the salinity into a unified model, and the gaST may be shallower than the MHZB for thethe methane hydrate stability is a function of pressure pure-'g over 7.3%. Besidesand salinity, which is obtained by the regressed methodfrom these data obtained by Sloan(see Appendix).TheYHa中国煤化工 is near to the GAsCNMH Gstem(Figures 8andYANG DingHui et al. Sci China Ser D-Earth Soil Nov. 2007 I vol. 50 no 11 11733-174517439). This is why the GAST is near or equal to the bases of Appendix: Stability functionthe MHSZMHZ in many practical marine sediments When we consider the influence of salinity on the lbecause there exists salt in real marine sediments and thesystem, the methane hydrate stability is treatedsalinity increases the MHZB and decreases the GAST.function of pressure and salinity and is obtained by re-Our numerical results also reveal an obvious explana- gressing from data obtained using the CSMHYD softtion for the disparity in the depths to the mHZB, the ware 2 as followsMHSZB, and the GAST shown in Figures 8 andTuability =A(X)In P+B(X)In P+ C(X)InP,(Al)When the methane solubility is a function of independ-ence of salinity treated approximately as Xu and rup.25 for certain methane flux rates a free gas zoneA(X)=1262236066X+107890X-11080.07X6153487X-1858518X5+231861.14X6,could develop far below the MHSzB and be separatedB(X)=-2,0780+3243X-74052X2+85069X3from the MhzB by a zone of marine sediments con-539943X+17516733X5-216507.56X,taining neither free gas nor gas hydrate. However, whencX=0341855X+15432X2198504Xncorporating the effect of salinity on the methane solu+1385528X4-46855685+585543x6,bility into calculations of the solubility, we find that the where X denotes the salinity and P is pressure. If X=0,thickness of the separated zone between the MHSZB then Stability denotes the stability temperature( ow)forand the gast becomes thinner(see Figures 8 and 9)the pure water caseI Makogon Y F, Hydrates of hydrocarbons, Penn Well, Tulsa, Okla-Drilling Program), 2000, 164: I-459homa, USA, 1997. 1-2113 Xu W, Lowell R P, Peltzer E T. Effect of seafloor temperature and2 Sloan E D Jr Clathrate Hydrates of Natural Gases, 2nd ed, Revisedpressure variations on methane flux from a gas hydrate layer: Comparison between current and Late Paleocene climate conditions.3 Song H B. Geophysical Researches on Gas Hydrates(in Chinese).Geophys Res,2001,106:26413-26423Beijing: Ocean Press, 2003. 224-22914 Yu XG, LiJ B, Gong J M, et al. 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