6th International Conference on Gas Hydrates

TECHNICAL LIMITS FOR DEVELOPMENT OF NATURAL GAS HYDRATE DEPOSITS Makogon, Yuri F.; Makogon, Taras Y.; Malyshev, Alexander 2008

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TECHNICAL LIMITS FOR DEVELOPMENT OF NATURAL GAS HYDRATE DEPOSITS                                                                                                          Yuri Makogon Department of Petroleum Engineering Texas A&M University  College Station, TX, 77845 USA  Taras Makogon BP America Inc. - TNK-BP P.O.Box 4381  Houston, TX 77210 USA  Alexander Malyshev Tyumen RUSSIA  ABSTRACT In this work we have formulated the set criteria for cost-effective selection of technologies for industrial production of gas from a hydrate deposit, which rely on the properties of hydrate-bearing rock and the geologic properties of the gas hydrate deposit. For over forty years the world’s energy industry has been trying to effectively master vast unconventional resources of natural gas – the natural gas hydrates [1;3;4]. Specialists have accumulated during this period of time a great deal of knowledge about gas hydrates [8;10]. They established the conditions of hydrate formation in sedimentary rock and the conditions of formation and disappearance of gas hydrate deposits, and offered several classification methods for gas hydrate deposits. Specialists have proposed several methods to locate the gas hydrate accumulations on land and offshore and determined the probable areas where gas hydrate deposits may exist. More than 220 gas hydrate deposits were found to-date, and methods to calculate the amount of gas in a hydrate deposit were developed [1;12]. The principles of gas production from a hydrate deposit were formulated and real experience of commercial natural gas production from a hydrate deposit was gained. However, until now there were no set economic criteria for selection of effective technologies for industrial development of gas hydrate deposits. This results in periodic development of various models not applicable to specific geologic conditions.    Keywords: gas hydrates, development, technical limits                                                         Corresponding author: Phone: +1 979 845 4066 Fax +1 979 845 1307 E-mail: makogon@tamu.edu NOMENCLATURE Cph hydrate heat capacity [kJ/m3/K] Cpp heat capacity of the hydrate bearing rock [kJ/m3/K] dHf heat of dissociation (fusion) of hydrate [kJ/m3]  dT hydrate deposit supercooling [K] Eh energy to dissociate hydrate [kJ]  Fh potential energy of hydrate deposit [kJ]                       Hh total thickness of hydrate bearing layers [m] Qc heating value of gas [kJ/m3] Qh content of gas in hydrate, [m3/m3] Qhd energy to heat hydrate to dissociation, [kJ] Qp energy to heat rock to hydrate dissociation [kJ] Sh area of hydrate deposit, [m2] Vh volume of hydrate in 1 m3 of hydrate deposit [m3] Vhp total volume of hydrated gas [m3] Vp volume of the hydrate bearing rock [m3]  hydrate saturation coefficient φ porosity of hydrate saturated rock INTRODUCTION Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  Over nine thousand research papers were published on gas hydrates. More than two thirds of those deal with gas hydrates in nature. A tremendous knowledge was accumulated on the hydrate deposits formation, stability and dissociation, as well as effective methods of finding a gas hydrate deposit and estimating its size were developed. A number of technologies were proposed for hydrate deposit development. However, these technologies are of particular scope and are designed for development of specific hydrate deposits with defined geologic and thermodynamic parameters [8;9;11].  Clear criteria are unavailable for the selection of commercial hydrate deposit development technique depending on geologic, rock type, saturation and thermodynamic properties of gas hydrate deposits. Gas hydrate accumulations are also not categorized.  One must convert hydrate to gas with subsequent production by conventional means, which is the key characteristic of a gas hydrate deposit development. Conversion of hydrate to gas may be accomplished by a number of ways: pressure drawdown below dissociation pressure, temperature increase above dissociation temperature, equilibrium shift by injection of chemical hydrate inhibitors [4].   CRITERIA Gas hydrate deposits in-situ exist at different pressure and temperature. Fig.1 indicates average parameters for several discovered hydrate deposits [10].  Fi g.  E qu i l i bri um  P - T H y dra t e  di s s oc i a t i on  i n f r e e  s pa s e  w i t h s e a  w a t e r0501001502002503003504004505000 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30T e m p e r a t u r e ,  o CPressure, atmC H 4 - S wM e s s o y h a - N a n k a iB u s h  W h i t eG r e e n  W h i t e  C yO -  N a n k a iO - B l a k e  R i d g eO - B l a c k  s e aO - M e x . S l o p eO - M i s s i s s i p p i  C y nO - O k h o t s kO - P a r a m u s h i rO - M e x . O r c a  B a s i nO - M e x . 2O - P e r u - C h i l e 2O - P e r u . 1O - G w a t e m . 4O - M e x . 1 & 3OO - G u a t e m a l a - 1O - J a p a n  S e aO - C o s t a  R i c aO - A t l a n t . B l a k eO - N a n k a i - J a p a n© M a k o g o nd - 0 . 6G u a t e m a l a - 3 -B u s h  H i l l - OM e s s o y h a - Od = 0 . 6 Fig. 1 Pressure (depth) and temperature for several gas hydrate deposits [2,6,7]  From these data one may see that pressure drawdown of up to several hundred atmospheres may be needed to dissociate gas hydrate in certain deposits. With the use of thermal technique temperature must be increased by several degrees to several tens of degrees C.   Selecting an effective technique for gas hydrate deposit development does not always depend on economic factors. Hydrate properties and geology of the hydrate-bearing matrix rock are the critical factors for selection of development method in specified conditions.  The most available and economic method for gas extraction from a gas hydrate deposit is reservoir pressure drawdown. This method has been used successfully during the Messoyakha gas hydrate deposit development [6]. However, this technique has its limitations both in the strength of the matrix rock and water coning. Hydrate dissociation in weakly consolidated rock may cause increased sand production, while water coning increases water cut of the produced fluids. Besides these effects, under certain conditions an intensive dissociation of hydrate with pressure drawdown may result in ice formation in rock pores, which sharply reduces permeability. Formed ice can't be dissociated by pressure decrease.  Pressure drawdown method applicability is very limited in highly supercooled gas hydrate deposits, particularly in deepwater ones. For example in order to dissociate gas hydrate in two of the most promising layers of the Blake Ridge deposit drawdown of 125 and 165 atm are needed (Fig.2), which is not possible in reservoir conditions.   E qu i l i bri um  P - T H y dra t e  di s s oc i a t i on  i n f r e e  s pa s e  w i t h s e a  w a t e r0501001502002503003504004505000 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30T e m p e r a t u r e ,  o CPressure, atmC H 4 - S wB u s h  H i l lM i s s .  C y n .O -  N a n k a iB l a k e  R i dg eO - B l a c k  s e aO - M e x . S l o p eO - M i s s i s s i p p i  C y nO - O k h o t s kO - P a r a m u s h i rO - M e x . O r c a  B a s i nO - M e x . 2O - P e r u - C h i l e 2O - P e r u - C h i l e - 1O - G w a t e m . 4O- M e x . 1 & 3O - G u a t e m a l a - 1O - J a p a n  S e aO - C o s t a  R i c aO - N a n k a i - J a p a nS - C - MG u a t e m a l a - 3 - OB u s h  H i l l - OM e s s o y h a - OB l a k e  R i dg e - OABO Fig. 2 Hydrate dissociation equilibria for gas compositions in different gas hydrate deposits.  Blake Ridge GHD [1;2;12] has 9 hydrated layers (Fig. 3) with different thickness of hydrated layers and screens, different hydrate saturation, pressure and temperature. Gas recovery may range from 17% to 51% of original reserve depending on position of hydrated layers.  Fig.3 Cross-section of the Blake Ridge gas hydrate deposit [5].  The main factor in setting the maximum reservoir drawdown is the rock strength gradient and rock solids production from well completion with pressure drawdown. Based on Messoyakha experience a 2-4 atm maximum drawdown is allowed in weakly consolidated sand. Drawdown limits for sandstone are 10-20 atm, for carbonate and shale rock it is up to 10 atm.  0501001502002503003500 5 10 15 20 25C H 4 - S wS a n d s t o n eN e e d  h e a t - d is s o cM a x im . H e a t in gS a n d - h y d r a tW a t e r - C 1P,  a tmT e m p e r a tu r e ,  o COOB l a k e  R i d g eOM e s s o y a h aACabde Fig. 4 Hydrate deposit drawdown and heating limits. А - for layers at the deposit top; В - for layers at deposit bottom.  Fig. 4 above shows the maximum allowable drawdown depending on rock strength, as well as the necessary and allowable limits of heating gas hydrate deposit during its development. We review just two deposits, which contain methane hydrate. Each requires a special design project to account for real geologic characteristics.  Thermal technique effectiveness for gas production from hydrate deposits depends on geologic, rock type and heat capacity, hydrate supercooling, saturation and thermal conductivity properties, especially offshore. Heat loss from a heating fluid circulated in a well to warm up a gas hydrate deposit must also be taken into account, which is a subject of a separate computation. With a heating technique using external heat sources the limit of their effectiveness is at equality of the price of heating value of gas in the hydrate deposit and the price of development and fuel required to heat all hydrate-bearing rock to hydrate dissociation temperature. In order to compare effectiveness of gas extraction from hydrate deposits one may compare just the amounts of energy needed to be supplied to each deposit to dissociate hydrate and produce gas.   The set of source data are required for evaluation of gas production from a hydrate deposit:   water depth above the deposit,   bottom temperature,   pressure and temperature depth profiles,  hydrate stability interval,   depth and thickness of all hydrate saturated layers,   rock porosity,   hydrate saturation in all hydrate saturated layers,  composition and heating value of gas,   heat of dissociation of gas hydrate,   heat capacity of hydrate saturated rock.   Some supporting information is also needed:   presence of impermeable screens over hydrate saturated layers and their stability to pressure drawdown,   presence of free gas below hydrate deposits,   gas saturation of water in neighboring layers,    thermal conductivity and permeability of hydrate bearing and neighboring layers,   permeability of underlaying rock at gas-water contact depth.   Potential energy in hydrate is determined by composition and heating value of gas in hydrate. For example, one cubic meter of methane hydrate at 283 K contains up to 164 m3 of gas. Heating value of methane (Qc) equals 38*103 kJ/m3. Thus each cubic meter of methane hydrate contains 164 * 38*103 = 6.23*106 kJ/m3 of energy. Heat of methane hydrate dissociation at 283 K is 54.2 kJ/mole. One mol of methane hydrate with density of 0.91 g/cm3 equals 124 grams. One m3 of methane hydrate contains 7339 mols. It would take 3.98*105 kJ/m3 to dissociate gas hydrate, which is 6.4% of the energy contained in 1 m3 of hydrate. However such comparison is valid only for deposits, which contain 100% pure hydrate at equilibrium pressure and temperature. Such perfect deposits seldom exist in nature.   For real deposits one must take into account deposit geology and stratigraphic structure and thickness of screen layers between hydrate-saturated layers, rock porosity and saturation with gas hydrate, fraction of hydrate crystal lattice cavities filled with gas molecules, degree of hydrate supercooling, heat capacity of hydrate bearing rock.  We present two examples of energy balance of development of two real offshore gas hydrate deposits.  Total volume of hydrate in 1 m3 of a hydrate deposit is:  Vh = Hh *   *  * Sh (1)  Total volume of hydrated gas, Vhp is:  Vhp = Hh *   *  * Qh * Sh  (2)  (for methane hydrate at 100 atm and 286 K, Qh=164  m3/m3)  Potential energy of a gas hydrate deposit is:                    Fh = Vhp * Qc  (3)  (for methane Qc = 38*103 kJ/m3)  Part of the gas hydrate deposit potential energy is spent to:  a) heat the hydrate to dissociation:                         Qhd = Vh*Cph*dT  (4)   b) dissociate hydrate:                         Eh = Vh* dHf  (5)  (dHf is approx.3.98*105kJ/m3 for methane hydrate)  c) heat the rock:                         Qp = Vp*Cpp*dT (6)    It is very important to properly determine thickness of rock saturated with hydrate, heating of which would take up part of the energy contained in hydrate gas. For the example the gas hydrate deposit we review (505 m thick) has three hydrate saturated layers with 17 m total thickness, which is 3.4% of the total deposit thickness. Top of the uppermost hydrate layer is at 1141 m depth. Bottom of the lowest hydrate layer is at 1210 m depth. Thickness of the layers to be heated, with account for heating of a part of the surrounding rock is 93 m.   Energy required for heating the gas hydrate deposit rock on a 1 km2 area by 14°C (see Fig.3 above), with account for rock heat capacity (2.5*103 kJ/m3/K), is:  Qp = Vp*Cpp*dT = 93*106 х 2.5*103 х 14 = 3.25*1012 kJ If the hydrate heat capacity equals that of the rock then the energy required to heat hydrate deposit rock does not depend on rock saturation with hydrate; otherwise both heat capacities of hydrate and rock mush be taken into account.  Thus the total energy recovered during gas hydrate deposit development is defined as the difference between the potential heating value of gas contained in the deposit (Fh) and the energy required to recover hydrated gas (heating hydrate Qh), heating rock (Qp), and dissociating hydrate (Eh).  If the quantity Fh exceeds cumulative energy spend Eh + Qh  + Qp                          Fh  >  Eh + Qh  + Qp  (7)          then a project may be started to determine technology for thermal development of this gas hydrate deposit. Otherwise, other gas recovery techniques should be sought.                 When chemicals are used, their concentration at hydrate surface must be up to 15-25 wt%, as seen in Fig.5.  Fig. 5 Concentration of methanol needed to dissociate gas hydrate in reservoir  Injection of chemicals for hydrate dissociation (Fig. 5) into the hydrate deposit is controlled by diffusion of injected fluid to the hydrate dissociation surface, their cost and environmental safety. One should keep in mind that addition of chemicals may remove hydrates from cementing rock and contribute to further destabilizing it thus causing increased sand production.  One of the limiting factors is the allowable pressure upon hydrate dissociation inside the gas hydrate deposit. Figure 6 presents the dependence of pressure after methane hydrate dissociation in a constant volume. High permeability of hydrate bearing rock mitigates the risk of gas hydrate deposit explosion caused by an active flow of gas released from hydrate [8].    E q u i l i b r i u m  a n d  F r e e  G a s  P r e s s u r e  -  T e m p e r a t u r e o f  M e t h a n e  H y d r a t e ,  M p a  -  oK1 . 82 . 67 . 57 3 . 6540743903902301921641401101001000- 1 0 0 10 20 30 40 50T e m p e r a t u r e ,  o CPressure, MpaM e l t i n g  P r e s s u r e  i n  c l o s e d  v o l u m e ,  M P aM e l t i n g  P r e s s u r e  i n  f r e e  v o l u m eABCD Fig.6 Pressure after methane hydrate dissociation in a constant volume  Injection of chemicals for hydrate dissociation (Fig. 5) into the hydrate deposit is controlled by diffusion of injected fluid to the hydrate dissociation surface, their cost and environmental safety. One should keep in mind that addition of chemicals may remove water from cementing rock and contribute to further destabilizing it thus causing increased sand production.   When several techniques are used simultaneously, it is important to control the processes of hydrate dissociation in situ and recovery of free gas from the deposit. A gas hydrate deposit drawn too hard may easily explode along with all engineering and environmental consequences.  Novel hydrate deposit development techniques may include ones that use the properties of hydrate itself. This would be the most profitable way to commercial mastering the gas hydrate resources.   CONCLUSION Energy industry specialists from the whole world have successfully proven, at a relatively low cost of about one billion dollars, that gas hydrate deposits as unconventional potential sources of energy are widespread in nature. A large operations experience was accumulated with the Messoyakha gas hydrate deposit. Over two hundred new hydrate deposits were found. A number of these could be placed economically today into commercial development. Both onshore and offshore deposits to 2000-2500 m water depth can be developed effectively. Effectiveness of a gas hydrate deposit development is determined by the size of the deposit, hydrate saturation of rock, deposit stratigraphy, properties of hydrate and of matrix rock. A gas hydrate deposit has a more favorable economics lf development with a thicker deposit, higher porosity, higher hydrate saturation of rock, and smaller supercooling.  Any possible method of commercial hydrate deposit development has its limitations, determined by the properties of hydrate and by regional geologic factors. Technical deployment of any method requires close monitoring and managing the reservoir and bottomhole pressure. Absence of such management may result in a loss of containment of the hydrate deposit and serious environmental consequences. With a knowledgeable selection and deployment of a gas hydrate deposit development technique, the lifting cost of gas production from a hydrate deposit may be lower than the regional lifting cost of gas production from a conventional deposit.  The criteria discussed in this paper should be evaluated in the following order: 1. Supercooling 2. Rock consolidation 3. Safety and Deposit strength 4. Stratigraphy and impermeable screens 5. Mass transfer 6. Heat transfer 7. Economics  REFERENCES [1] Collett T.S., Ladd Detection of Gas Hydrate Concentration on the Blake Ridge ODP  Rep.V.164, 2000 [2] Hornbah M.J., S.W. Holbrook, A.R. Gorman,  Direct seismic detection of methane hydrate on the Blake Ridge. Geophysics Vol.68. No I-II, 2003 [3] Makogon Y.F. A Gas Hydrate Formation in the Gas Saturated Layers under Low                Temperature. Gas Industry, No.5, 1965 [4] Makogon Y.F. Characteristics of a Gas-Field development in Permafrost. Nedra, Moscow,1966            [5] Makogon Y.F. Natural Gases in the Ocean and the Problems of Their Hydrates. Moscow, VNIIAGasprom.Express-Information,No.11, 1972 [6] Makogon Y.F., F.A.Trebin, A.A.Trofimuk, N.V. Chersky Finding of a Pool of Gas in the Hydrate State. Moscow, DAN SSSR, Vol. 196, No.1, 1971, 197-206  Contact address: makogon@tamu.edu                    [7] Makogon Y.F. Hydrates of Natural Gas (237 pg.) Nedra, Moscow, 1974. PennWell, Tulsa, 1981   [8] Макоgon Y.F.,  Hydrates of Hydrocarbons (1997) Penn Well, Tulsa, USA [9] Makogon Y.F., Holditch S.A., Makogon T.Y. Proven Reserves and Basics for Development of Gas Hydrate Deposits, AAPG, Vancouver, 2004 [10] Makogon Y.F., Holditch S.A., Makogon T.Y. Development of Gas Hydrate Deposits, Oil and Gas Journal No. 7.II.2005, and No. 14.II.2005  [11] Sloan E.D. Clathrate Hydrates of Natural Gases, Sec. Ed. N-Y,1998  [12] Taylor C. Formation Stadies of Methane Hydrates with Surfactants. 2nd International   Workshop on Methane Hydrates. October 2002, Washington.   TECHNICAL LIMITS FOR DEVELOPMENT OF NATURAL GAS HYDRATE DEPOSITS                                                                                                          Yuri Makogon Department of Petroleum Engineering Texas A&M University  College Station, TX, 77845 USA  Taras Makogon BP America Inc. - TNK-BP P.O.Box 4381  Houston, TX 77210 USA  Alexander Malyshev Tyumen RUSSIA  ABSTRACT In this work we have formulated the set criteria for cost-effective selection of technologies for industrial production of gas from a hydrate deposit, which rely on the properties of hydrate-bearing rock and the geologic properties of the gas hydrate deposit. For over forty years the world’s energy industry has been trying to effectively master vast unconventional resources of natural gas – the natural gas hydrates [1;3;4]. Specialists have accumulated during this period of time a great deal of knowledge about gas hydrates [8;10]. They established the conditions of hydrate formation in sedimentary rock and the conditions of formation and disappearance of gas hydrate deposits, and offered several classification methods for gas hydrate deposits. Specialists have proposed several methods to locate the gas hydrate accumulations on land and offshore and determined the probable areas where gas hydrate deposits may exist. More than 220 gas hydrate deposits were found to-date, and methods to calculate the amount of gas in a hydrate deposit were developed [1;12]. The principles of gas production from a hydrate deposit were formulated and real experience of commercial natural gas production from a hydrate deposit was gained. However, until now there were no set economic criteria for selection of effective technologies for industrial development of gas hydrate deposits. This results in periodic development of various models not applicable to specific geologic conditions.    Keywords: gas hydrates, development, technical limits                                                         Corresponding author: Phone: +1 979 845 4066 Fax +1 979 845 1307 E-mail: makogon@tamu.edu NOMENCLATURE Cph hydrate heat capacity [kJ/m3/K] Cpp heat capacity of the hydrate bearing rock [kJ/m3/K] dHf heat of dissociation (fusion) of hydrate [kJ/m3]  dT hydrate deposit supercooling [K] Eh energy to dissociate hydrate [kJ]  Fh potential energy of hydrate deposit [kJ]                       Hh total thickness of hydrate bearing layers [m] Qc heating value of gas [kJ/m3] Qh content of gas in hydrate, [m3/m3] Qhd energy to heat hydrate to dissociation, [kJ] Qp energy to heat rock to hydrate dissociation [kJ] Sh area of hydrate deposit, [m2] Vh volume of hydrate in 1 m3 of hydrate deposit [m3] Vhp total volume of hydrated gas [m3] Vp volume of the hydrate bearing rock [m3]  hydrate saturation coefficient φ porosity of hydrate saturated rock INTRODUCTION Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  Over nine thousand research papers were published on gas hydrates. More than two thirds of those deal with gas hydrates in nature. A tremendous knowledge was accumulated on the hydrate deposits formation, stability and dissociation, as well as effective methods of finding a gas hydrate deposit and estimating its size were developed. A number of technologies were proposed for hydrate deposit development. However, these technologies are of particular scope and are designed for development of specific hydrate deposits with defined geologic and thermodynamic parameters [8;9;11].  Clear criteria are unavailable for the selection of commercial hydrate deposit development technique depending on geologic, rock type, saturation and thermodynamic properties of gas hydrate deposits. Gas hydrate accumulations are also not categorized.  One must convert hydrate to gas with subsequent production by conventional means, which is the key characteristic of a gas hydrate deposit development. Conversion of hydrate to gas may be accomplished by a number of ways: pressure drawdown below dissociation pressure, temperature increase above dissociation temperature, equilibrium shift by injection of chemical hydrate inhibitors [4].   CRITERIA Gas hydrate deposits in-situ exist at different pressure and temperature. Fig.1 indicates average parameters for several discovered hydrate deposits [10].  Fi g.  E qu i l i bri um  P - T H y dra t e  di s s oc i a t i on  i n f r e e  s pa s e  w i t h s e a  w a t e r0501001502002503003504004505000 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30T e m p e r a t u r e ,  o CPressure, atmC H 4 - S wM e s s o y h a - N a n k a iB u s h  W h i t eG r e e n  W h i t e  C yO -  N a n k a iO - B l a k e  R i d g eO - B l a c k  s e aO - M e x . S l o p eO - M i s s i s s i p p i  C y nO - O k h o t s kO - P a r a m u s h i rO - M e x . O r c a  B a s i nO - M e x . 2O - P e r u - C h i l e 2O - P e r u . 1O - G w a t e m . 4O - M e x . 1 & 3OO - G u a t e m a l a - 1O - J a p a n  S e aO - C o s t a  R i c aO - A t l a n t . B l a k eO - N a n k a i - J a p a n© M a k o g o nd - 0 . 6G u a t e m a l a - 3 -B u s h  H i l l - OM e s s o y h a - Od = 0 . 6 Fig. 1 Pressure (depth) and temperature for several gas hydrate deposits [2,6,7]  From these data one may see that pressure drawdown of up to several hundred atmospheres may be needed to dissociate gas hydrate in certain deposits. With the use of thermal technique temperature must be increased by several degrees to several tens of degrees C.   Selecting an effective technique for gas hydrate deposit development does not always depend on economic factors. Hydrate properties and geology of the hydrate-bearing matrix rock are the critical factors for selection of development method in specified conditions.  The most available and economic method for gas extraction from a gas hydrate deposit is reservoir pressure drawdown. This method has been used successfully during the Messoyakha gas hydrate deposit development [6]. However, this technique has its limitations both in the strength of the matrix rock and water coning. Hydrate dissociation in weakly consolidated rock may cause increased sand production, while water coning increases water cut of the produced fluids. Besides these effects, under certain conditions an intensive dissociation of hydrate with pressure drawdown may result in ice formation in rock pores, which sharply reduces permeability. Formed ice can't be dissociated by pressure decrease.  Pressure drawdown method applicability is very limited in highly supercooled gas hydrate deposits, particularly in deepwater ones. For example in order to dissociate gas hydrate in two of the most promising layers of the Blake Ridge deposit drawdown of 125 and 165 atm are needed (Fig.2), which is not possible in reservoir conditions.   E qu i l i bri um  P - T H y dra t e  di s s oc i a t i on  i n f r e e  s pa s e  w i t h s e a  w a t e r0501001502002503003504004505000 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30T e m p e r a t u r e ,  o CPressure, atmC H 4 - S wB u s h  H i l lM i s s .  C y n .O -  N a n k a iB l a k e  R i dg eO - B l a c k  s e aO - M e x . S l o p eO - M i s s i s s i p p i  C y nO - O k h o t s kO - P a r a m u s h i rO - M e x . O r c a  B a s i nO - M e x . 2O - P e r u - C h i l e 2O - P e r u - C h i l e - 1O - G w a t e m . 4O- M e x . 1 & 3O - G u a t e m a l a - 1O - J a p a n  S e aO - C o s t a  R i c aO - N a n k a i - J a p a nS - C - MG u a t e m a l a - 3 - OB u s h  H i l l - OM e s s o y h a - OB l a k e  R i dg e - OABO Fig. 2 Hydrate dissociation equilibria for gas compositions in different gas hydrate deposits.  Blake Ridge GHD [1;2;12] has 9 hydrated layers (Fig. 3) with different thickness of hydrated layers and screens, different hydrate saturation, pressure and temperature. Gas recovery may range from 17% to 51% of original reserve depending on position of hydrated layers.  Fig.3 Cross-section of the Blake Ridge gas hydrate deposit [5].  The main factor in setting the maximum reservoir drawdown is the rock strength gradient and rock solids production from well completion with pressure drawdown. Based on Messoyakha experience a 2-4 atm maximum drawdown is allowed in weakly consolidated sand. Drawdown limits for sandstone are 10-20 atm, for carbonate and shale rock it is up to 10 atm.  0501001502002503003500 5 10 15 20 25C H 4 - S wS a n d s t o n eN e e d  h e a t - d is s o cM a x im . H e a t in gS a n d - h y d r a tW a t e r - C 1P,  a tmT e m p e r a tu r e ,  o COOB l a k e  R i d g eOM e s s o y a h aACabde Fig. 4 Hydrate deposit drawdown and heating limits. А - for layers at the deposit top; В - for layers at deposit bottom.  Fig. 4 above shows the maximum allowable drawdown depending on rock strength, as well as the necessary and allowable limits of heating gas hydrate deposit during its development. We review just two deposits, which contain methane hydrate. Each requires a special design project to account for real geologic characteristics.  Thermal technique effectiveness for gas production from hydrate deposits depends on geologic, rock type and heat capacity, hydrate supercooling, saturation and thermal conductivity properties, especially offshore. Heat loss from a heating fluid circulated in a well to warm up a gas hydrate deposit must also be taken into account, which is a subject of a separate computation. With a heating technique using external heat sources the limit of their effectiveness is at equality of the price of heating value of gas in the hydrate deposit and the price of development and fuel required to heat all hydrate-bearing rock to hydrate dissociation temperature. In order to compare effectiveness of gas extraction from hydrate deposits one may compare just the amounts of energy needed to be supplied to each deposit to dissociate hydrate and produce gas.   The set of source data are required for evaluation of gas production from a hydrate deposit:   water depth above the deposit,   bottom temperature,   pressure and temperature depth profiles,  hydrate stability interval,   depth and thickness of all hydrate saturated layers,   rock porosity,   hydrate saturation in all hydrate saturated layers,  composition and heating value of gas,   heat of dissociation of gas hydrate,   heat capacity of hydrate saturated rock.   Some supporting information is also needed:   presence of impermeable screens over hydrate saturated layers and their stability to pressure drawdown,   presence of free gas below hydrate deposits,   gas saturation of water in neighboring layers,    thermal conductivity and permeability of hydrate bearing and neighboring layers,   permeability of underlaying rock at gas-water contact depth.   Potential energy in hydrate is determined by composition and heating value of gas in hydrate. For example, one cubic meter of methane hydrate at 283 K contains up to 164 m3 of gas. Heating value of methane (Qc) equals 38*103 kJ/m3. Thus each cubic meter of methane hydrate contains 164 * 38*103 = 6.23*106 kJ/m3 of energy. Heat of methane hydrate dissociation at 283 K is 54.2 kJ/mole. One mol of methane hydrate with density of 0.91 g/cm3 equals 124 grams. One m3 of methane hydrate contains 7339 mols. It would take 3.98*105 kJ/m3 to dissociate gas hydrate, which is 6.4% of the energy contained in 1 m3 of hydrate. However such comparison is valid only for deposits, which contain 100% pure hydrate at equilibrium pressure and temperature. Such perfect deposits seldom exist in nature.   For real deposits one must take into account deposit geology and stratigraphic structure and thickness of screen layers between hydrate-saturated layers, rock porosity and saturation with gas hydrate, fraction of hydrate crystal lattice cavities filled with gas molecules, degree of hydrate supercooling, heat capacity of hydrate bearing rock.  We present two examples of energy balance of development of two real offshore gas hydrate deposits.  Total volume of hydrate in 1 m3 of a hydrate deposit is:  Vh = Hh *   *  * Sh (1)  Total volume of hydrated gas, Vhp is:  Vhp = Hh *   *  * Qh * Sh  (2)  (for methane hydrate at 100 atm and 286 K, Qh=164  m3/m3)  Potential energy of a gas hydrate deposit is:                    Fh = Vhp * Qc  (3)  (for methane Qc = 38*103 kJ/m3)  Part of the gas hydrate deposit potential energy is spent to:  a) heat the hydrate to dissociation:                         Qhd = Vh*Cph*dT  (4)   b) dissociate hydrate:                         Eh = Vh* dHf  (5)  (dHf is approx.3.98*105kJ/m3 for methane hydrate)  c) heat the rock:                         Qp = Vp*Cpp*dT (6)    It is very important to properly determine thickness of rock saturated with hydrate, heating of which would take up part of the energy contained in hydrate gas. For the example the gas hydrate deposit we review (505 m thick) has three hydrate saturated layers with 17 m total thickness, which is 3.4% of the total deposit thickness. Top of the uppermost hydrate layer is at 1141 m depth. Bottom of the lowest hydrate layer is at 1210 m depth. Thickness of the layers to be heated, with account for heating of a part of the surrounding rock is 93 m.   Energy required for heating the gas hydrate deposit rock on a 1 km2 area by 14°C (see Fig.3 above), with account for rock heat capacity (2.5*103 kJ/m3/K), is:  Qp = Vp*Cpp*dT = 93*106 х 2.5*103 х 14 = 3.25*1012 kJ If the hydrate heat capacity equals that of the rock then the energy required to heat hydrate deposit rock does not depend on rock saturation with hydrate; otherwise both heat capacities of hydrate and rock mush be taken into account.  Thus the total energy recovered during gas hydrate deposit development is defined as the difference between the potential heating value of gas contained in the deposit (Fh) and the energy required to recover hydrated gas (heating hydrate Qh), heating rock (Qp), and dissociating hydrate (Eh).  If the quantity Fh exceeds cumulative energy spend Eh + Qh  + Qp                          Fh  >  Eh + Qh  + Qp  (7)          then a project may be started to determine technology for thermal development of this gas hydrate deposit. Otherwise, other gas recovery techniques should be sought.                 When chemicals are used, their concentration at hydrate surface must be up to 15-25 wt%, as seen in Fig.5.  Fig. 5 Concentration of methanol needed to dissociate gas hydrate in reservoir  Injection of chemicals for hydrate dissociation (Fig. 5) into the hydrate deposit is controlled by diffusion of injected fluid to the hydrate dissociation surface, their cost and environmental safety. One should keep in mind that addition of chemicals may remove hydrates from cementing rock and contribute to further destabilizing it thus causing increased sand production.  One of the limiting factors is the allowable pressure upon hydrate dissociation inside the gas hydrate deposit. Figure 6 presents the dependence of pressure after methane hydrate dissociation in a constant volume. High permeability of hydrate bearing rock mitigates the risk of gas hydrate deposit explosion caused by an active flow of gas released from hydrate [8].    E q u i l i b r i u m  a n d  F r e e  G a s  P r e s s u r e  -  T e m p e r a t u r e o f  M e t h a n e  H y d r a t e ,  M p a  -  oK1 . 82 . 67 . 57 3 . 6540743903902301921641401101001000- 1 0 0 10 20 30 40 50T e m p e r a t u r e ,  o CPressure, MpaM e l t i n g  P r e s s u r e  i n  c l o s e d  v o l u m e ,  M P aM e l t i n g  P r e s s u r e  i n  f r e e  v o l u m eABCD Fig.6 Pressure after methane hydrate dissociation in a constant volume  Injection of chemicals for hydrate dissociation (Fig. 5) into the hydrate deposit is controlled by diffusion of injected fluid to the hydrate dissociation surface, their cost and environmental safety. One should keep in mind that addition of chemicals may remove water from cementing rock and contribute to further destabilizing it thus causing increased sand production.   When several techniques are used simultaneously, it is important to control the processes of hydrate dissociation in situ and recovery of free gas from the deposit. A gas hydrate deposit drawn too hard may easily explode along with all engineering and environmental consequences.  Novel hydrate deposit development techniques may include ones that use the properties of hydrate itself. This would be the most profitable way to commercial mastering the gas hydrate resources.   CONCLUSION Energy industry specialists from the whole world have successfully proven, at a relatively low cost of about one billion dollars, that gas hydrate deposits as unconventional potential sources of energy are widespread in nature. A large operations experience was accumulated with the Messoyakha gas hydrate deposit. Over two hundred new hydrate deposits were found. A number of these could be placed economically today into commercial development. Both onshore and offshore deposits to 2000-2500 m water depth can be developed effectively. Effectiveness of a gas hydrate deposit development is determined by the size of the deposit, hydrate saturation of rock, deposit stratigraphy, properties of hydrate and of matrix rock. A gas hydrate deposit has a more favorable economics lf development with a thicker deposit, higher porosity, higher hydrate saturation of rock, and smaller supercooling.  Any possible method of commercial hydrate deposit development has its limitations, determined by the properties of hydrate and by regional geologic factors. Technical deployment of any method requires close monitoring and managing the reservoir and bottomhole pressure. Absence of such management may result in a loss of containment of the hydrate deposit and serious environmental consequences. With a knowledgeable selection and deployment of a gas hydrate deposit development technique, the lifting cost of gas production from a hydrate deposit may be lower than the regional lifting cost of gas production from a conventional deposit.  The criteria discussed in this paper should be evaluated in the following order: 1. Supercooling 2. Rock consolidation 3. Safety and Deposit strength 4. Stratigraphy and impermeable screens 5. Mass transfer 6. Heat transfer 7. Economics  REFERENCES [1] Collett T.S., Ladd Detection of Gas Hydrate Concentration on the Blake Ridge ODP  Rep.V.164, 2000 [2] Hornbah M.J., S.W. Holbrook, A.R. Gorman,  Direct seismic detection of methane hydrate on the Blake Ridge. Geophysics Vol.68. No I-II, 2003 [3] Makogon Y.F. A Gas Hydrate Formation in the Gas Saturated Layers under Low                Temperature. Gas Industry, No.5, 1965 [4] Makogon Y.F. Characteristics of a Gas-Field development in Permafrost. Nedra, Moscow,1966            [5] Makogon Y.F. Natural Gases in the Ocean and the Problems of Their Hydrates. Moscow, VNIIAGasprom.Express-Information,No.11, 1972 [6] Makogon Y.F., F.A.Trebin, A.A.Trofimuk, N.V. Chersky Finding of a Pool of Gas in the Hydrate State. Moscow, DAN SSSR, Vol. 196, No.1, 1971, 197-206  Contact address: makogon@tamu.edu                    [7] Makogon Y.F. Hydrates of Natural Gas (237 pg.) Nedra, Moscow, 1974. PennWell, Tulsa, 1981   [8] Макоgon Y.F.,  Hydrates of Hydrocarbons (1997) Penn Well, Tulsa, USA [9] Makogon Y.F., Holditch S.A., Makogon T.Y. Proven Reserves and Basics for Development of Gas Hydrate Deposits, AAPG, Vancouver, 2004 [10] Makogon Y.F., Holditch S.A., Makogon T.Y. Development of Gas Hydrate Deposits, Oil and Gas Journal No. 7.II.2005, and No. 14.II.2005  [11] Sloan E.D. Clathrate Hydrates of Natural Gases, Sec. Ed. N-Y,1998  [12] Taylor C. Formation Stadies of Methane Hydrates with Surfactants. 2nd International   Workshop on Methane Hydrates. October 2002, Washington.   GAS HYDRATE STABLE ZONE in ONSHORE (Makogon, 1965)   VI IGHC, Vancouver - 2008Cross section of Blake Ridge FieldBlake Ridge, well 997; Water depth=2790mEffect of development Blake Ridge GHD by traditional technology Shebelinka-18.II.1957Effect of development Blake Ridge GH Lens by traditional technology  Equilibrium and Free Gas Pressure - Temperature of Methane Hydrate, Mpa - oK1.82.62.67.573.6540540743903902301921641401101001000-1001020304050Temperature, oCPressure, MpaMelting Pressure in constant volume, MPaMelting Pressure in free volume, MPaABCDTraditional technologies for development of GHD:Decrease reservoir PressureIncrease reservoir TemperatureInjection of chemical “Inhibitors” Combination of the above  - Makogon, 1965Dr. Makogon Y.F. TAMUDr. Makogon T.Y. BPDr. Malyshev A.G. SNGTECHNICAL LIMITS FOR  THE DEVELOPMENT OF NATURAL GAS HYDRATE DEPOSITS  TECHNICAL LIMITS FOR THE DEVELOPMENT OF NATURAL GAS HYDRATE DEPOSITS Dr. Makogon Y.F. TAMU Dr. Makogon T.Y. BP Dr. Malyshev A.G. SNG  VI IGHC, Vancouver - 2008  Traditional technologies for development of GHD:  Decrease reservoir Pressure Increase reservoir Temperature Injection of chemical “Inhibitors” Combination of the above - Makogon, 1965 GAS HYDRATE STABLE ZONE in ONSHORE (Makogon, 1965)  Bl ake Ri dge, wel l 997; Wat er dept h=2790m  Crossection through Structure -2 (Andaman) Distance, km 0  2  4  6  8  10  12  14  900 925 950 975  Depth, m  1000 1025 1050 1075 1100 95-09 99-25  1125 1150  Cross section of Blake Ridge Field  99-09 Open Str.  1175 1200  Ef f ect of devel opment Bl ake Ri dge GHD by t r adi t i onal t echnol ogy Ef f ect of devel opment Bl ake Ri dge GH Lens by t r adi t i onal t echnol ogy  Potential Energy of Hydrates (m^3)  7.00E+08  6.16E+08  Development Energy Needed (m^3)  2.62E+09  Energy Lost to Pressurisation (m^3) Energy lost to Transportation (m^3)  100%  6.00E+08  100%  Available Energy (m^3)  2.50E+09  Energy required for Heat Unproducible  5.00E+08  3.27E+08  4.00E+08  53%  3.00E+08  Production, 27*109 m3  2.00E+09  Energy requared for Transportation Hydrate Energy (m 3 /km 2 )  Hydrate Energy (m 3 /km 2 )  Potential Energy of Hydrates (m^3) Development Energy Needed (m^3) Energy Lost to Pressurisation (m^3) Energy lost to Transportation (m^3) Available Energy (m^3)  Resource, 53*109m3  3.00E+09  1.39E+09  Shebel i nka- 18. I I . 1957 1.50E+09  51%  1.00E+09  2.00E+08 9.77E+07 1.00E+08  16%  5.09E+08  1.05E+08  8.62E+07  3.57E+08 5.00E+08  17%  14%  3.67E+08 20%  14%  0.00E+00  15%  0.00E+00  1  1  Tem perature, oC  Temperature, oC 0  2  4  6  8  10  12  14  16  18  20  22  24  26  28  0  30  2  4  6  8  10  12  14  16  18  20  22  24  26  0  0  Bush Hill O-Okhotsk O-Paramushir Messoyha-O Bush Hill-O O-Mex.Slope O- Nankai  100  300  O-Blake Ridge  O-Mex.2  S-C-M  O-Japan Sea O-Guatemala-1 Blake Ridge-O  350  O-Peru-Chile2  450  10  15  0  a 100  300  O-Mex.2  S-C-M  Guatem ala-3-O  Messoyaha O  O-Japan Sea O-Guatem ala-1 300  A  O-Mex.2  B  300  Blake Ridge 350  C  O  O-Japan Sea O-Guatem ala-1  O  B  O-Peru-Chile2  450  O-Nankai-Japan O-Peru-Chile-1  100  Equilibrium P-T Hydrate dissociation in free spase with sea water  Equilibrium and Free Gas Pressure - Temperature o of Methane Hydrate, Mpa K1000  N  C  A  90  B C  80  H  D  D  70  60  50  100  140  Melting Pressure in constant volume, MPa 230 192 164  74 Melting Pressure in free volume, MPa 7.5  Heating  F  0  390  390  Cooling  E  M  540  73.6  10  20  e  28  S-C-M  O-Costa Rica  Methane Hydrate Formation-Dissociation in a Porous Media  30  d  26  500  250 b  24  Blake Ridge-O  40  A O  22  Guatemala-3-O  400  O-Nankai-Japan  Pressure, kg/cm2  200  20  A  350  O-Peru-Chile2  25  150  18  O-Mex.1&3 O-Gw atem .4  Blake Ridge  O-Costa Rica  Temperature, oC  P, atm  16  Miss. Cyn.  O-Mex.Orca Basin  250  Blake Ridge-O  CH4-Sw Sandstone Need heat-dissoc Maxim.Heating Sand-hydrat Water-C1  50  14  O-Okhotsk O-Param ushir Messoyha-O Bush Hill-O O-Mex.Slope O- Nankai Mississippi Cyn O  Equilibrium P-T Hydrate dissociation in free spase with sea water  20  12  Bush Hill  O-Peru-Chile-1  5  10  CH4-Sw  O-Black sea  500  Equilibrium P-T Hydrate dissociation in free spase with sea water  8  200  Blake Ridge O  450  500  0  100  O-Mex.Orca Basin  250  400  O-Nankai-Japan O-Peru-Chile-1  6  150  350  O-Costa Rica  400  O-Black sea  50  Miss. Cyn.  O-Mex.1&3 O-Gw atem .4  200  Guatemala-3-O  O-Mex.Orca Basin  250  4  0  O-Mississippi Cyn  150  Pressure, atm  Pressure, atm  O-Black sea  O-Okhotsk O-Param ushir Messoyha-O Bush Hill-O O-Mex.Slope O- Nankai  Miss. Cyn.  O-Mex.1&3 O-Gw atem.4  200  2  Bush Hill  50  O-Mississippi Cyn  150  0  Pressure, Mpa  100  Temperat ure, oC 30  CH4-Sw  CH4-Sw 50  28  A  Equilibrium  G  1.8  B  2.6  1 2  4  6  8  10  12  14  16 Тem perature, C  18  -10  0  10  20  30  40  50  Temperature, oC  Over $1 Bn allocated for gas hydrate research O roduced Over ver 7 7B Bn nm m33 P P roducedgas gasfrom fromM Messoyakha essoyakhagas gashydrate hydratedeposit deposit ((cost hydrates 0.15 cost of of gas gasfrom fromgas gashydrates: hydrates: US S$ $0.15/m 0.15/m /m33)) hydrates:: U 0.15/m 3 Potential O Bn mB gas production Bidge lake gas Ridge hydrate deposit Over ver 20 15,000 n m3 Potential gas from Bfrom lake R hydrate deposit  C ost $1. C ost of of natural natural gas gasto toa aconsum consumer er at at home homein in T Texas approx. U US S$1.2/m $1.2/m 2/m33 Texas exasapprox. $1.2/m  Recommendations: • To effectively produce gas from hydrate deposits use the technologies based on the natural properties of gas hydrates. • Do not wait for further uncontrolled growth in energy cost before starting to explore and develop gas hydrate deposits.  30  

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