CN115408889B - Method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization - Google Patents

Method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization Download PDF

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CN115408889B
CN115408889B CN202211357181.4A CN202211357181A CN115408889B CN 115408889 B CN115408889 B CN 115408889B CN 202211357181 A CN202211357181 A CN 202211357181A CN 115408889 B CN115408889 B CN 115408889B
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徐建春
孙伟
秦婳婷
李航宇
王晓璞
刘树阳
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China University of Petroleum East China
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Abstract

The invention discloses a method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization, which relates to the technical field of natural gas hydrate resource development and comprises the following steps: s1, establishing a natural gas hydrate reservoir stratum geological model and carrying out grid division; s2, analyzing a seepage field, a temperature field and a chemical field of the geological model, constructing a mass conservation equation, an energy conservation equation, a natural gas hydrate decomposition kinetic equation and a natural gas hydrate generation kinetic equation, establishing a numerical simulation model for developing the natural gas hydrate by three-field coupling, and predicting and analyzing the exploitation effect; the method also comprises the steps of carrying out numerical simulation research and influence factor analysis on the method for exploiting the sea area natural gas hydrate by combining heat injection, fracturing and depressurization. The invention provides a theoretical basis for heat injection-fracturing-depressurization exploitation of the natural gas hydrate reservoir in the sea area; and the influence and the rule of different production factors on the gas production are also analyzed, and a reference value is provided for the exploitation of the sea area hydrate reservoir.

Description

Method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization
Technical Field
The invention relates to the technical field of natural gas hydrate resource development, in particular to a method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization.
Background
Because of the development of society and increasing population, the demand of people on energy is increasing every year, and the natural gas hydrate is widely regarded by governments, energy enterprises and related scientific research institutions all over the world as an energy resource with huge development potential. The hydrate development plan and related scientific research projects are successively set up in China, america, japan, korea, india, european Union and other countries or regions to help develop related researches on hydrates, including basic physical properties, hydrate oil-gas system theory, geological exploration, in-situ resource quantity evaluation, mining methods, potential geological disasters of hydrate distribution areas and other research works, and breakthrough achievements are obtained in multiple aspects of scientific theory, technical development, equipment research and development, engineering construction environment influence evaluation and the like.
Natural gas hydrate deposits are typically distributed in permafrost regions on land, continental shelves in the ocean, and sediments in lakes. The most of the current common exploitation modes achieve the exploitation purpose by changing the phase state equilibrium of the natural gas hydrate, and mainly comprise the following methods: (1) reducing the pressure; (2) thermal stimulation; (3) inhibitor injection; (4) carbon dioxide displacement; and (5) fluidized mining of the solid. Depressurization is currently considered to be the most feasible hydrate reservoir production method due to its economy and high efficiency. At present, a plurality of countries have carried out a plurality of field trial production of hydrates in the world, and China is the only country which successfully carries out trial production in sea area and land area at the same time and realizes trial production of muddy powder sand mold hydrate reservoir stratum for the first time. Despite the many hydrate pilot-mining experiences worldwide, there are many challenges to achieve the goals of long-term, safe, and efficient commercial exploitation of hydrates.
According to the forecast of the natural resources department, the natural gas hydrate resource amount in the sea area of China is about 800 million tons of oil equivalent, which is one of important strategic and successed energy sources in China, and the efficient development of the natural gas hydrate resource amount has great significance for building oceans and strengthening national energy safety in China. Different from sandstone type reservoirs of other countries, the natural gas hydrate reservoir in south China sea mainly takes argillaceous fine silt, has the characteristics of no diagenesis, low permeability, poor cementation and the like, and has higher mining difficulty. The daily gas production of a pilot production single well is low, the stable production time is short, the economic threshold of commercial production cannot be reached only by the traditional depressurization method, a novel production increase mode must be explored, the seepage capacity of a reservoir stratum must be regulated and controlled, and the purpose of improving the yield of the single well is achieved. The single depressurization or thermal stimulation method can not meet the gas production requirement, and the combined method has better economic feasibility.
Disclosure of Invention
In order to solve the technical problem, the invention discloses a method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization. The method is based on a natural gas hydrate three-field coupling numerical simulation method, a heat injection-fracturing-depressurization combined exploitation model is established, a five-point well pattern method is adopted to develop a sea area hydrate reservoir stratum, the gas production rate and the gas production rate of the single depressurization production scheme, the depressurization-heat injection combined exploitation scheme, the heat injection-fracturing-depressurization combined exploitation scheme and the like are analyzed, and the advantages, the disadvantages and the feasibility of the three schemes are compared; meanwhile, the influence and the law of different production factors on the gas production are analyzed.
In order to realize the purpose, the invention adopts the following technical scheme:
a method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization comprises the following steps:
s1, establishing a natural gas hydrate reservoir geological model based on well drilling, well logging and seismic data of a sea area low-permeability natural gas hydrate reservoir, carrying out grid division, and carrying out grid encryption processing on an area near a hydraulic fracture;
s2, analyzing the seepage field, the temperature field and the chemical field of the geological model, constructing a mass conservation equation, an energy conservation equation, a natural gas hydrate decomposition kinetic equation and a natural gas hydrate generation kinetic equation, and establishing a numerical simulation model for developing the natural gas hydrate through three-field coupling.
Optionally, the method further comprises the steps of carrying out numerical simulation research and influence factor analysis on the method for the heat injection-fracturing-depressurization combined exploitation of the sea natural gas hydrate, determining the influence of different production factors on the productivity, and analyzing the yield increasing effects of three modes of depressurization production, depressurization-heat injection combined exploitation and heat injection-fracturing-depressurization combined exploitation.
Optionally, in step S2, the hydrate reaction process obeys the mass conservation law, and the mass conservation equation is:
Figure 453214DEST_PATH_IMAGE001
wherein the content of the first and second substances,kthe components are marked as g, w and h in the formula, and subscripts g, w and h respectively represent methane, water and hydrate;Mexpressed as the mass cumulative term of each component, kg/m 3 ；FAs a componentkMass flux of (2), kg/(m) 2 ·s)；qRepresenting a source and a sink;
the mass conservation equation for methane is:
Figure 846149DEST_PATH_IMAGE002
the conservation of mass equation for water is:
Figure 480393DEST_PATH_IMAGE003
the mass conservation equation of the hydrate is as follows:
Figure 46504DEST_PATH_IMAGE004
wherein the content of the first and second substances,
Figure 348172DEST_PATH_IMAGE005
is the flow velocity, m/s; />
Figure 974325DEST_PATH_IMAGE006
Mass change due to hydration decomposition or formation, kg/(m) 3 ·s)；/>
Figure 33548DEST_PATH_IMAGE007
Is density, kg/m 3 ；SIs the phase saturation; />
Figure 403350DEST_PATH_IMAGE008
Is an inherent porosity; />
Figure 559524DEST_PATH_IMAGE009
Is the gas production rate of the well, m 3 /s；/>
Figure 356579DEST_PATH_IMAGE010
Is the water production rate of the well, m 3 /s。
Optionally, in step S2, the energy conservation equation is:
Figure 965415DEST_PATH_IMAGE011
wherein the content of the first and second substances,
Figure 811011DEST_PATH_IMAGE012
is the flow velocity, m/s; />
Figure 821693DEST_PATH_IMAGE013
Is density, kg/m 3 ；/>
Figure 789649DEST_PATH_IMAGE014
Is the phase saturation; />
Figure 885781DEST_PATH_IMAGE015
For each phase enthalpy, subscripts s, h, g, and w represent the rock skeleton, hydrates, gas, and water, J/mole; />
Figure 597385DEST_PATH_IMAGE016
The heat required for hydrate decomposition or formation, J/(m) 3 ·s)；
The volume-averaging based method describes the heat transfer equation as:
Figure 898791DEST_PATH_IMAGE017
wherein the content of the first and second substances,
Figure 37648DEST_PATH_IMAGE018
is a porosity, is->
Figure 621076DEST_PATH_IMAGE019
Is of thermal conductivity, wherein>
Figure 136371DEST_PATH_IMAGE020
Is the effective thermal conductivity of the deposit, is>
Figure 856065DEST_PATH_IMAGE021
The rock thermal conductivity is J/(mS.K).
The generation and decomposition of the hydrate are reversible reaction of methane and water, and the process of the decomposition and generation of the hydrate is as follows:
Figure 103507DEST_PATH_IMAGE022
optionally, in step S2, based on the Kim-bishoni model, the kinetic equation of decomposition of the natural gas hydrate is:
Figure 174231DEST_PATH_IMAGE023
wherein the content of the first and second substances,
Figure 227638DEST_PATH_IMAGE024
as hydrate concentration, gmole/m 3 ；/>
Figure 67418DEST_PATH_IMAGE025
Gmole/(day kPa · m), a hydrate decomposition rate constant 2 )；/>
Figure 548078DEST_PATH_IMAGE026
Surface area per unit volume of hydrate, m 2 /m 3 ；/>
Figure 43781DEST_PATH_IMAGE027
Equilibrium pressure, kPa; />
Figure 635300DEST_PATH_IMAGE028
Gas phase pressure, kPa;
rate constant of hydrate decomposition
Figure 329586DEST_PATH_IMAGE029
Comprises the following steps:
Figure 246727DEST_PATH_IMAGE030
wherein the content of the first and second substances,
Figure 26464DEST_PATH_IMAGE031
is a frequency factor of hydrate decomposition reaction, gmole/(day kPa.m) 2 )；/>
Figure 421673DEST_PATH_IMAGE032
Is the gas constant, J/(mole. K); />
Figure 173728DEST_PATH_IMAGE033
J/mole for activation energy;
hydrate surface area per unit volume
Figure 996191DEST_PATH_IMAGE034
Comprises the following steps:
Figure 263224DEST_PATH_IMAGE035
Figure 462124DEST_PATH_IMAGE036
the natural gas hydrate decomposition kinetic equation is expressed as:
Figure 131003DEST_PATH_IMAGE037
wherein the content of the first and second substances,
Figure 124367DEST_PATH_IMAGE038
is the equilibrium value of hydrate under a certain pressure and temperature; />
Figure 306125DEST_PATH_IMAGE039
Is the gas constant, J/(mole. K); />
Figure 43137DEST_PATH_IMAGE040
Is the specific surface area of the hydrate particles, m 2 /m 3 ；/>
Figure 566523DEST_PATH_IMAGE041
Subscripts g, w, and h represent gas, water, and hydrate, respectively, for phase saturation; />
Figure 996367DEST_PATH_IMAGE042
Is a hydrate dissociation rate constant, < >>
Figure 175675DEST_PATH_IMAGE043
；/>
Figure 450799DEST_PATH_IMAGE044
Is density, kg/m 3 。
Optionally, in step S2, the natural gas hydrate formation kinetic equation is:
Figure 94270DEST_PATH_IMAGE045
/>
wherein the content of the first and second substances,
Figure 429436DEST_PATH_IMAGE046
as hydrate concentration, gmole/m 3 ；/>
Figure 158358DEST_PATH_IMAGE047
Gmole/(day kPa · m) is the hydrate formation rate constant 2 )；/>
Figure 237172DEST_PATH_IMAGE048
Surface area per unit volume of hydrate, m 2 /m 3 ；/>
Figure 672833DEST_PATH_IMAGE049
kPa for equilibrium pressure; />
Figure 444480DEST_PATH_IMAGE050
Gas phase pressure, kPa;
generating a rate constant
Figure 395118DEST_PATH_IMAGE051
Is defined as:
Figure 277624DEST_PATH_IMAGE052
wherein the content of the first and second substances,
Figure 630108DEST_PATH_IMAGE053
gmole/(day kPa · m) is a hydrate formation reaction frequency factor 2 )；
Hydrate surface area per unit volume
Figure 510339DEST_PATH_IMAGE054
Comprises the following steps:
Figure 948273DEST_PATH_IMAGE055
Figure 368891DEST_PATH_IMAGE056
the natural gas hydrate formation kinetic equation is expressed as:
Figure 841460DEST_PATH_IMAGE057
。
optionally, the steps of performing numerical simulation research and influence factor analysis on the method for extracting the sea natural gas hydrate by combining heat injection-fracturing-depressurization include: the method is characterized in that a five-point well pattern method is adopted, a hydrate reservoir is divided into an upper cover layer, a lower cover layer and a hydrate layer, a production well is positioned in the center of the hydrate layer, four water injection wells are distributed at four corners of the hydrate layer, fracture areas are positioned at two sides of the production well, the overall x and y directions are 0.001m x 100m, and the z direction penetrates through the whole hydrate layer.
Optionally, the water injection temperature of the water injection well is 50 ℃ and the injection speed is 200m 3 /day。
The beneficial effect of the invention is that,
the invention constructs a heat injection-fracturing-depressurization combined exploitation numerical simulation model, contrastively analyzes the feasibility of the schemes of single depressurization production, depressurization-heat injection combined exploitation, heat injection-fracturing-depressurization combined exploitation and the like, provides a theoretical basis for heat injection-fracturing-depressurization exploitation of the sea natural gas hydrate reservoir and has good application value; and the influence and the rule of different production factors on the gas production are also analyzed, the production scheme is further optimized, and a reference value is provided for the production of the sea area hydrate reservoir.
Drawings
FIG. 1 is a flow chart of a method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization, which is provided by the invention;
FIG. 2 is a schematic geological model of a sea area natural gas hydrate reservoir produced by combining heat injection, fracturing and depressurization in the embodiment of the invention;
FIG. 3 is a gas production curve for producing a hydrate reservoir by different stimulation methods in an embodiment of the present invention, in which a solid line represents daily gas production and a dotted line represents accumulated gas production;
FIG. 4 is a gas production curve of the sea area natural gas hydrate reservoir at different injection speeds in the embodiment of the invention, wherein a solid line represents daily gas production and a dotted line represents accumulated gas production;
FIG. 5 is a water production curve of different injection speeds of a sea natural gas hydrate reservoir in an embodiment of the invention, wherein a solid line represents a gas-water ratio, and a dotted line represents an accumulated water yield;
FIG. 6 is a gas production curve of different dimensionless fracture conductivity of a sea area natural gas hydrate reservoir in the embodiment of the present invention, where a solid line represents daily gas production and a dotted line represents accumulated gas production;
FIG. 7 is a water production curve of different dimensionless fracture conductivity of a sea natural gas hydrate reservoir in the embodiment of the invention, wherein a solid line represents a gas-water ratio, and a dotted line represents accumulated water production.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
A method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization is shown in figure 1 and comprises the following steps:
s1, establishing a geological model of the natural gas hydrate reservoir and carrying out grid division on the geological model based on drilling, logging and seismic data of the sea area low-permeability natural gas hydrate reservoir;
s2, analyzing a seepage field, a temperature field and a chemical field of the geological model, constructing a mass conservation equation, an energy conservation equation, a natural gas hydrate decomposition kinetic equation and a natural gas hydrate generation kinetic equation, and establishing a numerical simulation model for developing the natural gas hydrate by three-field coupling;
the hydrate reaction process obeys the mass conservation law, and the mass conservation equation is as follows:
Figure 954910DEST_PATH_IMAGE058
wherein the content of the first and second substances,kthe components are marked as g, w and h in the formula, and subscripts g, w and h are respectively substitutedEpi-methane, water and hydrates;Mexpressed as the mass cumulative term of each component, kg/m 3 ；FAs a componentkMass flux of (2), kg/(m) 2 ·s)；qRepresenting a source and a sink;
the mass conservation equation for methane is:
Figure 316359DEST_PATH_IMAGE059
the conservation of mass equation for water is:
Figure 540667DEST_PATH_IMAGE060
the mass conservation equation of the hydrate is as follows:
Figure 602163DEST_PATH_IMAGE061
wherein the content of the first and second substances,
Figure 886514DEST_PATH_IMAGE062
is the flow velocity, m/s; />
Figure 299041DEST_PATH_IMAGE063
For mass change due to hydration decomposition or formation, kg/(m) 3 ·s)；/>
Figure 264723DEST_PATH_IMAGE064
Is density, kg/m 3 ；SIs the phase saturation; />
Figure 446306DEST_PATH_IMAGE065
Is an inherent porosity; />
Figure 635979DEST_PATH_IMAGE066
Is the gas production rate of the well, m 3 /s；/>
Figure 535801DEST_PATH_IMAGE067
Is a wellWater production rate of m 3 /s；
The energy conservation equation is as follows:
Figure 39595DEST_PATH_IMAGE068
wherein the content of the first and second substances,
Figure 341263DEST_PATH_IMAGE069
is the flow velocity, m/s; />
Figure 701838DEST_PATH_IMAGE070
Is density, kg/m 3 ；/>
Figure 88957DEST_PATH_IMAGE071
Is the phase saturation; />
Figure 458758DEST_PATH_IMAGE072
For the enthalpy of each phase, the subscripts s, h, g, and w represent the rock skeleton, hydrates, gas, and water, J/mole; />
Figure 287037DEST_PATH_IMAGE073
The heat required for hydrate decomposition or formation, J/(m) 3 ·s)；
The volume-averaging based method describes the heat transfer equation as:
Figure 84092DEST_PATH_IMAGE074
wherein the content of the first and second substances,
Figure 692927DEST_PATH_IMAGE075
is a porosity, is->
Figure 866420DEST_PATH_IMAGE076
Is of thermal conductivity, wherein>
Figure 316249DEST_PATH_IMAGE077
Efficient heat transfer to depositsConductivity ratio>
Figure 284205DEST_PATH_IMAGE078
Is the rock thermal conductivity, J/(m.s.K);
the generation and decomposition of the hydrate are reversible reaction of methane and water, and the process of the decomposition and generation of the hydrate is as follows:
Figure 380337DEST_PATH_IMAGE079
based on a Kim-Bishinoi model, the natural gas hydrate decomposition kinetic equation is as follows:
Figure 91941DEST_PATH_IMAGE080
wherein the content of the first and second substances,
Figure 957129DEST_PATH_IMAGE081
as hydrate concentration, gmole/m 3 ；/>
Figure 33669DEST_PATH_IMAGE082
Gmole/(day kPa · m), a hydrate decomposition rate constant 2 )；/>
Figure 617097DEST_PATH_IMAGE083
Surface area per unit volume of hydrate, m 2 /m 3 ；/>
Figure 866813DEST_PATH_IMAGE084
Equilibrium pressure, kPa; />
Figure 852087DEST_PATH_IMAGE085
Gas phase pressure, kPa;
rate constant of hydrate decomposition
Figure 161845DEST_PATH_IMAGE086
Comprises the following steps:
Figure 170253DEST_PATH_IMAGE087
wherein the content of the first and second substances,
Figure 223659DEST_PATH_IMAGE031
is a frequency factor of hydrate decomposition reaction, gmole/(day kPa.m) 2 )；/>
Figure 63439DEST_PATH_IMAGE088
Is the gas constant, J/(mole. K); />
Figure 481782DEST_PATH_IMAGE089
J/mole for activation energy;
hydrate surface area per unit volume
Figure 774223DEST_PATH_IMAGE090
Comprises the following steps:
Figure 631321DEST_PATH_IMAGE091
Figure 325607DEST_PATH_IMAGE092
the natural gas hydrate decomposition kinetic equation is expressed as:
Figure 977169DEST_PATH_IMAGE093
wherein the content of the first and second substances,
Figure 458703DEST_PATH_IMAGE094
is the equilibrium value of hydrate under a certain pressure and temperature; />
Figure 853913DEST_PATH_IMAGE039
Is the gas constant, J/(mole. K); />
Figure 402706DEST_PATH_IMAGE095
Is the specific surface area of the hydrate particles, m 2 /m 3 ；/>
Figure 490747DEST_PATH_IMAGE096
Subscripts g, w, and h represent gas, water, and hydrate, respectively, for phase saturation; />
Figure 695464DEST_PATH_IMAGE097
Is a hydrate dissociation rate constant, < >>
Figure 894364DEST_PATH_IMAGE098
；/>
Figure 297663DEST_PATH_IMAGE099
Is density, kg/m 3 ；
The natural gas hydrate generation kinetic equation is as follows:
Figure 556606DEST_PATH_IMAGE100
wherein the content of the first and second substances,
Figure 310936DEST_PATH_IMAGE101
as hydrate concentration, gmole/m 3 ；/>
Figure 985631DEST_PATH_IMAGE102
Gmole/(day kPa · m) is the hydrate formation rate constant 2 )；/>
Figure 509016DEST_PATH_IMAGE103
Surface area per unit volume of hydrate, m 2 /m 3 ；/>
Figure 938860DEST_PATH_IMAGE104
Equilibrium pressure, kPa; />
Figure 914907DEST_PATH_IMAGE105
Gas phase pressure, kPa;
generating a rate constant
Figure 393292DEST_PATH_IMAGE106
Is defined as:
Figure 36763DEST_PATH_IMAGE107
wherein the content of the first and second substances,
Figure 371930DEST_PATH_IMAGE053
gmole/(day kPa · m) is a hydrate formation reaction frequency factor 2 )；
Hydrate surface area per unit volume
Figure 100851DEST_PATH_IMAGE108
Comprises the following steps:
Figure 618814DEST_PATH_IMAGE109
Figure 116791DEST_PATH_IMAGE110
the natural gas hydrate formation kinetic equation is expressed as:
Figure 622859DEST_PATH_IMAGE111
；
s3, carrying out numerical simulation research and influence factor analysis on the method for extracting the sea natural gas hydrate by combining heat injection, fracturing and depressurization, wherein a five-point well pattern method is adopted, namely a production well is positioned in the center of a hydrate layer, four water injection wells are distributed at four corners of the hydrate layer, fracture areas are positioned at two sides of the production well, the overall x and y directions are 0.001 mx 100m, and the z direction penetrates through the whole hydrate layer; the injection temperature of water in the water injection well is 50 ℃, and the injection speed is 200m 3 Day, determining the influence of different production factors on the productivity, and analyzing the pressure reduction production, the pressure reduction-heat injection combined mining, and the heat injection-fracturing-pressure reduction combinedAnd (3) exploiting the yield increasing effect of three modes.
The application case is as follows:
1. hydrate reservoir model establishment
In order to research the adaptability of the heat injection-fracturing-depressurization combined production method to natural gas hydrate, analyze the influence of the heat injection-fracturing-depressurization combined production on the production dynamics of a gas well, establish a three-dimensional geological conceptual model as shown in figure 2, and carry out numerical simulation research according to the model, wherein the model belongs to a III-type hydrate reservoir, the hydrate reservoir is divided into an upper cover layer, a lower cover layer and a hydrate layer, the length of the model is 500m, the width of the model is 500m, the height of the model is 130m, the thickness of the hydrate layer is 50m, and the thicknesses of the upper cover layer and the lower cover layer are 40m respectively; setting a grid of 50X 18, fracture area x and y directions of 0.001m X100m, a water injection well position as shown in figure 2, an injection temperature of 50 ℃ and an injection speed of 200m 3 /day。
2. Comparison of different production modes
Comprises 3 conditions of single depressurization production, depressurization-heat injection combined mining and heat injection-fracturing-depressurization combined mining. The cracks are uniformly set to be 100m long, penetrate through the whole hydrate layer in the vertical direction, and the non-dimensional crack flow conductivity is 10. Four water injection wells are respectively arranged at four corners of the hydrate reservoir, and the water injection amount of the water injection wells is 200m 3 Day, water injection temperature 50 ℃.
FIG. 3 shows the effect of three different stimulation modes on gas production. Although the effect of gas production in the early stage of the pressure reduction-heat injection combined production is not as good as that of single pressure reduction production, the gas production is greater than that of single pressure reduction production after the peak value of the gas production is reached. Meanwhile, the gas production rate of heat injection-fracturing-depressurization combined mining is highest, and the effect is best.
3. Production factor sensitivity analysis
3.1 Influence of the injection speed
The injection speed of each water injection well is set to be 100m 3 /day、200m 3 /day、300m 3 Day and 400m 3 The injection temperatures were 50 ℃ per day. The fracture vertically penetrates through the whole hydrate layer, the non-dimensional fracture conductivity is 10, the bottom hole pressure is 4.5MPa, namely the pressure reduction amplitude is 0.67P i The number of days for simulated production is 5000 days.
Fig. 4 and 5 are gas and water production curves for different injection rates. It can be seen that the faster the injection speed, the lower the gas production speed in the early production stage, and the higher the gas production speed in the later production stage, but the higher the accumulated gas production.
3.2 Influence of non-dimensional fracture conductivity
Setting the non-dimensional crack flow conductivity to be 0.1, 1 and 10 respectively, and setting the injection speed of a water injection well to be 200m 3 The injection temperature is 50 ℃, the bottom hole pressure is 4.5MPa, namely, the pressure reduction amplitude is 0.67P i The number of days for simulated production is 5000 days.
Fig. 6 and 7 are gas and water production results without influence of the flow conductivity of the dimensional fracture. The gas production is increased along with the increase of the flow conductivity of the crack, the accumulated water is not changed greatly, and the gas-water ratio is gradually increased at the early stage. It is evident that the time to peak is progressively shorter and the peak is progressively higher.
The depressurization method reduces the local pressure condition to be below the phase equilibrium curve of the natural gas hydrate, so that the hydrate can be automatically decomposed in a state that the hydrate can not stably exist. Low permeability can adversely affect the movement of the drawdown profile in the formation due to the manner in which hydrates are consolidated in the reservoir and due to geological factors. In actual low-permeability-layer exploitation, the depressurization method only can affect a near-wellbore zone of a perforation area, automatic decomposition of the natural gas hydrate can be continuously carried out only by providing a large amount of heat by an external environment, the heat of a lower cladding layer is difficult to supplement to a hydrate layer in time through original formation pores due to low permeability of the formation, depressurization and decomposition of the hydrate are difficult to continue, and the reason that high yield cannot be obtained by single thermal stimulation exploitation can also be considered.
Compared with a single depressurization or thermal stimulation method, the heat injection-depressurization combined mining method has a certain effect of improving the yield, but cannot obviously improve the yield on the premise of failing to solve the low permeability of the stratum, because the flow channel is narrow and even blocked due to the low permeability. The hydraulic fracturing is an effective means for modifying a hypotonic reservoir, has been widely and successfully applied to the development of shale oil gas and compact oil gas in recent years, and is greatly helpful for improving the yield of the hypotonic reservoir. The hydraulic fracturing technology is applied to a low-permeability hydrate reservoir, and a flow channel with high flow conductivity is formed near a shaft, so that the problems that a pressure drop profile is difficult to move during the pressure reduction method exploitation and the pressure is suppressed due to flow blockage of injection heat flow in an area near the shaft of an injection well during the heat stimulation method exploitation can be effectively solved. The reservoir stratum after hydraulic fracturing can achieve the expected yield increasing effect of two methods of heat injection and pressure reduction, and the method has certain guiding significance for optimizing the development of low-permeability natural gas hydrate reservoirs and promoting the commercial exploitation of hydrates.
The invention establishes a heat injection-fracturing-depressurization combined exploitation model based on a natural gas hydrate three-field coupling numerical simulation method, develops a sea area hydrate reservoir by adopting a five-point well pattern method, analyzes the gas production rate and the gas production rate of the schemes of single depressurization production, depressurization-heat injection combined exploitation, heat injection-fracturing-depressurization combined exploitation and the like, compares the advantages and disadvantages and feasibility of the three schemes, and simultaneously analyzes the influence and law of different production factors on the gas production rate.
It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make modifications, alterations, additions or substitutions within the spirit and scope of the present invention.

Claims (1)

1. A method for exploiting sea natural gas hydrate by combining heat injection, fracturing and depressurization is characterized by comprising the following steps:
s1, establishing a natural gas hydrate reservoir geological model, carrying out grid division, and carrying out grid encryption in a hydraulic fracture area;
s2, analyzing a seepage field, a temperature field and a chemical field of the geological model, constructing a mass conservation equation, an energy conservation equation, a natural gas hydrate decomposition kinetic equation and a natural gas hydrate generation kinetic equation, establishing a numerical simulation model for developing the natural gas hydrate by three-field coupling, and performing predictive analysis on the heat injection-fracturing-depressurization combined mining effect by using a numerical simulation method;
the method also comprises the steps of carrying out numerical simulation research and influence factor analysis on the method for exploiting the sea natural gas hydrate by combining heat injection, fracturing and depressurization; the influencing factors comprise injection speed and dimensionless fracture conductivity;
in step S2, the mass conservation equation is:
Figure FDA0004026671160000011
wherein k is a component identifier, g, w and h are expressed in the formula, and subscripts g, w and h respectively represent methane, water and hydrate; m is expressed as the mass accumulation term of each component, kg/M 3 (ii) a F is the mass flux of component k, kg/(m) 2 S); q represents a source and sink item;
the mass conservation equation for methane is:
Figure FDA0004026671160000012
the conservation of mass equation for water is:
Figure FDA0004026671160000013
the mass conservation equation of the hydrate is as follows:
Figure FDA0004026671160000014
wherein v is the flow velocity, m/s;
Figure FDA0004026671160000017
for mass change due to hydration decomposition or formation, kg/(m) 3 S); rho is density, kg/m 3 (ii) a S is the phase saturation;
Figure FDA0004026671160000015
is the inherent porosity; q. q.s g Is the gas production rate of the well, m 3 /s；q w Is the water production rate of the well, m 3 /s；
In step S2, the energy conservation equation is:
Figure FDA0004026671160000016
wherein v is the flow velocity, m/s; rho is density, kg/m 3 (ii) a S is the phase saturation; h is the enthalpy of each phase, and the subscripts s, H, g, and w represent the rock framework, hydrate, gas, and water, J/mole; q. q.s h The heat required for hydrate decomposition or formation, J/(m) 3 ·s)；
The volume-averaging based method describes the heat transfer equation as:
λ c ＝λ s (1-φ)+φ(λ h H h +λ g H g +λ w H w )
where φ is porosity and λ is thermal conductivity, where λ c Is the effective thermal conductivity of the deposit, is the thermal conductivity of the rock, J/(m.s.K);
in the step S2, based on the Kim-Bishinoi model, the natural gas hydrate decomposition kinetic equation is as follows:
Figure FDA0004026671160000021
wherein the content of the first and second substances,
Figure FDA0004026671160000022
as hydrate concentration, gmole/m 3 ；k d Gmole/(day kPa · m) is the hydrate decomposition rate constant 2 )；A d Surface area per unit volume of hydrate, m 2 /m 3 ；p e Equilibrium pressure, kPa; p is a radical of g Gas phase pressure, kPa;
hydrate decomposition rate constant k d Comprises the following steps:
Figure FDA0004026671160000023
wherein the content of the first and second substances,
Figure FDA0004026671160000024
gmole/(day kPa · m) is the hydrate decomposition reaction frequency factor 2 ) (ii) a R is a gas constant, J/(mole.K); e is activation energy, J/mole;
hydrate surface area per unit volume A d Comprises the following steps:
Figure FDA0004026671160000025
Figure FDA0004026671160000026
the natural gas hydrate decomposition kinetic equation is expressed as:
Figure FDA0004026671160000027
wherein K (p, T) is a hydrate equilibrium value under a certain pressure and temperature; r is a gas constant, J/(mole.K); a. The HS Is the specific surface area of the hydrate particles, m 2 /m 3 (ii) a S is phase saturation, and subscripts g, w and h respectively represent gas, water and hydrate; lambda [ alpha ] d Is the hydrate decomposition rate constant, (gmole/m) 3 ) -1 (ii)/kPa; rho is density, kg/m 3 ；
In step S2, the natural gas hydrate formation kinetic equation is:
Figure FDA0004026671160000031
wherein, the first and the second end of the pipe are connected with each other,
Figure FDA0004026671160000032
as hydrate concentration, gmole/m 3 ；k f Gmole/(day kPa · m) is the hydrate formation rate constant 2 )；A f Surface area per unit volume of hydrate, m 2 /m 3 ；p e Equilibrium pressure, kPa; p is a radical of g Gas phase pressure, kPa;
generating a rate constant k f Is defined as:
Figure FDA0004026671160000033
wherein the content of the first and second substances,
Figure FDA0004026671160000034
gmole/(day kPa · m) is a hydrate formation reaction frequency factor 2 )；
Surface area per unit volume hydrate A f Comprises the following steps:
Figure FDA0004026671160000035
Figure FDA0004026671160000036
the natural gas hydrate formation kinetic equation is expressed as:
Figure FDA0004026671160000037
the method for the combined exploitation of the sea natural gas hydrate by heat injection, fracturing and depressurization is subjected to numerical simulation research and influence factor analysis, and comprises the following steps: adopting a five-point well pattern method, namely, a production well is positioned in the center of a hydrate layer, four water injection wells are distributed at four corners of the hydrate layer, fracture areas are positioned at two sides of the production well, the overall x and y directions are 0.001 mx 100m, and the z direction penetrates through the whole hydrate layer;
the injection temperature of water in the water injection well is 50 ℃, and the injection speed is 200m 3 Day, hydrate production by heat injection-fracturing-depressurization combined;
the hydraulic fracturing technology is applied to a low-permeability hydrate reservoir, and a flow channel with high flow conductivity is formed near a shaft, so that the problems that a pressure drop profile is difficult to move during pressure reduction method exploitation and the flow is blocked due to the injection of heat flow in the area near the injection shaft during heat stimulation method exploitation are solved.
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