System for realizing LNG (Liquefied Natural Gas) gasification by utilizing geothermal energy and design method thereof

Abstract

The invention discloses a system for realizing LNG (Liquefied Natural Gas) gasification by utilizing geothermal energy and a design method thereof. The design method comprises the steps that a physical model of a natural gas-U-shaped well gasification system for LNG gasification through geothermal energy of a U-shaped geothermal well, a formation temperature model of the physical model, an in-well fluid heat transfer model, a pressure model and a natural gas physical property parameter calculation model are established; and all the models are coupled and solved, and design parameters of the U-shaped geothermal well are obtained. The abandoned well can be transformed into the U-shaped geothermal well, LNG is introduced into the U-shaped geothermal well, the low-temperature characteristic of LNG is utilized, the heat utilization rate of geothermal energy can be greatly increased, and the LNG gasification cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of LNG gasification methods, and in particular to a method for gasifying LNG using geothermal energy. Background Art

[0002] Liquefied Natural Gas (LNG) is a liquid formed by cooling gaseous natural gas at normal pressure to -162°C. It is a clean and efficient energy source that is safe, reliable, clean, environmentally friendly, economical, efficient, flexible and convenient. At the same time, liquefaction can greatly save storage and transportation space, improve safety performance and transportation efficiency, reduce the cost of gas supply facilities, and can be used for power generation after gasification.

[0003] The existing LNG gasification method is achieved through LNG vaporizers. Some common LNG vaporizers include air-temperature vaporizers, open-frame vaporizers, submerged combustion vaporizers, etc. Among them, air-temperature vaporizers use room temperature air as a heat source to exchange heat between liquefied natural gas through a series of surface heat exchangers. The airflow cooling outside the heat exchanger is controlled by the natural buoyancy (convection) of high-density air or the installation of forced ventilation ambient air fans. Air-temperature vaporizers are most suitable for areas with higher ambient temperatures, but in cold weather conditions, auxiliary heating systems are required, and their use can easily cause water vapor in the air to condense into ice, forming frost in the system, reducing the thermal efficiency of the system; open-frame vaporizers often use seawater as a heat source, LNG is heated and vaporized by seawater from bottom to top in the tube bundle. However, because the temperature of LNG is very low, it needs to absorb a large amount of heat to vaporize it. Therefore, the open-frame vaporizer requires a large amount of seawater, and the construction cost of seawater pumps and heat exchange pipelines is very high. In addition, the temperature of the discharged seawater is relatively low, which will cause environmental damage. The submerged combustion vaporizer is usually composed of a stainless steel vaporizer coil immersed in a water tank. The hot water obtained by heating the high-temperature gas after the combustion of oil or fuel gas (such as the boil-off gas of the liquefied natural gas storage tank) transfers heat to the LNG to vaporize it. The submerged combustion vaporizer needs to continuously burn fuel during operation, with high operating costs and a small amount of vaporization.

[0004] On the other hand, geothermal energy is a renewable energy source with abundant reserves, wide distribution, stability and reliability. However, most geothermal energy has low grade and low energy conversion efficiency, so it is difficult to use and has poor economic benefits. At the same time, the degree of attention paid to it in the process is also low, which wastes the abundant geothermal resources. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present invention is to provide an LNG gasification system and a design method thereof, which can apply geothermal energy to LNG gasification and significantly improve the utilization efficiency of geothermal energy and reduce the cost of LNG gasification. The system has a simple application process and low equipment requirements, and can achieve efficient utilization of geothermal energy in LNG gasification.

[0006] The technical solution of the present invention is as follows:

[0007] A method for designing a system for realizing LNG gasification using geothermal energy, comprising:

[0008] S1 Establish a physical model of a natural gas-U-well gasification system for LNG gasification using geothermal energy from a U-shaped geothermal well;

[0009] S2, establishing a formation temperature model according to the physical model of the natural gas-U-shaped well gasification system;

[0010] S3, establishing a heat transfer model and a pressure model of the fluid in the well according to the physical model of the natural gas-U-shaped well gasification system;

[0011] S4 establishes a calculation model for the physical property parameters of natural gas in liquid, gas and gas-liquid mixed states;

[0012] S5 couples the formation temperature model, the heat transfer model and pressure model of the fluid in the well, and the natural gas physical property parameter calculation model to obtain the temperature and pressure model of the natural gas-U-shaped well gasification system, solves it, and obtains the design parameters of the U-shaped geothermal well.

[0013] According to some preferred embodiments of the present invention, the physical model of the natural gas-U-shaped well gasification system is established as follows: it includes: an LNG storage tank, a booster pump, a U-shaped geothermal well, an insulation layer and a throttle valve, wherein one end of the U-shaped geothermal well is connected to the LNG storage tank through the booster pump, and the other end thereof is provided with the insulation layer and connected to the natural gas user through the throttle valve.

[0014] According to some preferred embodiments of the present invention, the formation temperature model is established as follows:

[0015] T d =T0+aH (1)

[0016] Among them, T d represents the formation temperature; T0 represents the ground temperature; a represents the formation temperature gradient; H represents the depth.

[0017] According to some preferred embodiments of the present invention, the well fluid heat transfer model and pressure model include:

[0018]

[0019] in:

[0020]

[0021]

[0022] Where z represents the axial coordinate; ρ represents the density of the fluid; u represents the flow rate of the fluid; C represents the specific heat capacity of the fluid; T f represents the temperature of the fluid in the wellbore; λ represents the thermal conductivity of the fluid; R1 represents the sum of the thermal resistances per unit length between the fluid and the formation; λ1, λ2, λ3, and λ4 represent the thermal conductivity of the casing, cement sheath, formation, and insulation layer, respectively; d1 and d2 represent the inner and outer diameters of the casing; d3 represents the outer diameter of the cement sheath; d4 represents the outer diameter of the insulation layer; h represents the convective heat transfer coefficient between the inner wall of the casing and the working fluid; T D represents dimensionless temperature; t D represents dimensionless time; τ represents the cumulative contact time between the working fluid and the formation.

[0023] According to some preferred embodiments of the present invention, the natural gas physical property parameter calculation model includes: a density calculation model for liquefied natural gas, a viscosity calculation model for liquefied natural gas, a constant-pressure specific heat calculation model for liquefied natural gas, and a thermal conductivity calculation model for liquefied natural gas; a density calculation model for gaseous natural gas, a viscosity calculation model for gaseous natural gas, a constant-pressure specific heat calculation model for gaseous natural gas, and a thermal conductivity calculation model for gaseous natural gas; and a natural gas physical property parameter calculation model for a gas-liquid mixing zone, a pressure model for the fluid in the wellbore, and a saturated vapor pressure model for natural gas.

[0024] According to some preferred embodiments of the present invention, the natural gas physical property parameter calculation model of the gas-liquid mixing zone is established as follows:

[0025] ρ eff =θρ g +(1-θ)ρ l (18)

[0026]

[0027] λ eff =θλ g +(1-θ)λ l (20)

[0028] Where ρ represents the density of the fluid, C represents the specific heat capacity of the fluid, λ represents the thermal conductivity of the fluid, the subscript eff represents the effective value, θ represents the vaporization rate, L represents the latent heat of vaporization, and ρ g represents the density of gaseous natural gas, ρ l Indicates the density of liquid natural gas, C p,g represents the constant pressure specific heat of natural gas, Cp,l represents the specific heat of liquid natural gas at constant pressure, T f represents the fluid temperature in the wellbore, L represents the latent heat of vaporization, λ g Denotes the thermal conductivity of gaseous natural gas, λ l Represents the thermal conductivity of liquid natural gas.

[0029] According to some preferred embodiments of the present invention, the solution is achieved by a finite volume method and a Gauss-Seidel iteration method.

[0030] According to some preferred embodiments of the present invention, the design method further comprises:

[0031] S6: verifying the economic benefits of the natural gas-U-shaped well gasification system according to the temperature and pressure model of the natural gas-U-shaped well gasification system and the design parameters of the U-shaped geothermal well.

[0032] According to some preferred embodiments of the present invention, the verification is achieved through the following NPV model:

[0033]

[0034] I income =VP LNG +QηP Price-h (31)

[0035] C cost =C f +C v (32)

[0036] C f =C c +(1-k)C fi (33)

[0037] C v =C tax +(1-k)C vi (34)

[0038] C tax =I income β (35)

[0039] C vi =C H0 ·S+r H S 2 (36)

[0040] Where V represents the volume flow rate; P LNG represents the sales price of natural gas; Q represents the heat extraction; η represents the heat extraction efficiency, %; P price-h Indicates the heat price; C f represents fixed cost; Cv represents variable cost; C c represents operating and management expenses; C fi represents the initial investment that is independent of the length of the horizontal section of the U-shaped geothermal well; k represents the one-time subsidy rate; C tax represents the business tax of the enterprise; β represents the comprehensive tax rate; C H0 represents the benchmark value of drilling cost per meter in the horizontal section; r H It represents the growth coefficient of drilling cost per meter in the horizontal section; S represents the well depth.

[0041] The present invention has the following beneficial effects:

[0042] Compared with the prior art, the present invention can transform abandoned wells into U-shaped geothermal wells, realize the reuse of abandoned wells, and introduce LNG into the U-shaped geothermal wells. By utilizing the low temperature characteristics of LNG, the thermal utilization rate of geothermal energy can be greatly improved, and the cost of LNG gasification can be reduced. The present invention has the advantages of simple application process, low equipment requirements, and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a structural schematic diagram of a system for realizing LNG gasification by utilizing geothermal energy according to the present invention.

[0044] Figure 2 It is a schematic diagram of the calculation method of the finite volume method used in the specific implementation manner.

[0045] Figure 3 Schematic diagram of the calculation process of the Gauss-Seidel iteration method used in the specific implementation manner.

[0046] Figure 4 This is the temperature distribution curve of the entire wellbore obtained in Example 1.

[0047] Figure 5 This is the full wellbore NPV curve diagram obtained in Example 1. DETAILED DESCRIPTION

[0048] The present invention is described in detail below in conjunction with the embodiments and drawings, but it should be understood that the embodiments and drawings are only used to exemplify the present invention and do not constitute any limitation on the protection scope of the present invention. All reasonable changes and combinations within the scope of the inventive concept of the present invention fall within the protection scope of the present invention.

[0049] In some specific embodiments, the design method of the system for realizing LNG gasification by utilizing geothermal energy of the present invention includes:

[0050] S1 establishes a physical model of a natural gas-U-well gasification system for LNG gasification using geothermal energy from a U-shaped geothermal well.

[0051] For more details, please refer to the attached Figure 1 , the physical model of the natural gas-U-well gasification system includes:

[0052] LNG storage tank 1, booster pump 2, U-shaped geothermal well 3, insulation layer 4 and throttle valve 5, wherein one end of the U-shaped geothermal well 3 is connected to the LNG storage tank 1 through the booster pump 2, and the other end thereof is provided with insulation layer 4 and connected to the natural gas user through the throttle valve 5.

[0053] In a preferred embodiment, the U-shaped geothermal well 3 can be formed by connecting two abandoned gas wells through directional drilling technology.

[0054] This preferred method can save drilling costs to a great extent, and by applying it to the gasification process of LNG, it can also realize the geothermal reuse of abandoned oil and gas wells.

[0055] According to its U-shaped form, in the U-shaped geothermal well 3, the vertical part containing one end connected to the LNG storage tank 1 is the LNG descending section, the vertical part containing one end connected to the user is the natural gas ascending section, and the horizontal part between the two is the horizontal section. The process of gasifying LNG is as follows: the low-temperature LNG is pressurized by the booster pump 2 and enters the descending section of the U-shaped geothermal well 3. In the descending section and the horizontal section, it continuously absorbs heat from the formation to be vaporized, and overflows from the ascending section and is transported to the user end through the throttle valve 5. Taking into account that natural gas will generate a part of heat loss in the ascending section, the present invention provides an insulating layer 4 with heat insulation and heat preservation function in the ascending section to reduce heat loss in the ascending section.

[0056] S2 establishes its formation temperature model according to the physical model of the natural gas-U-shaped well gasification system, as follows:

[0057] T d =T0+aH (1)

[0058] Among them, T d represents the formation temperature, K; T0 represents the ground temperature, K; a represents the formation temperature gradient, K / m; H represents the depth, m.

[0059] S3: According to the physical model of the natural gas-U-shaped well gasification system, a heat transfer model and a pressure model of the fluid in the well are established.

[0060] Further, in a specific embodiment, the heat transfer model and pressure model of the fluid in the U-shaped geothermal well are established as follows:

[0061]

[0062] in:

[0063]

[0064] Where z represents the axial coordinate, m; ρ represents the density of the fluid, kg / m 3 ; u represents the flow rate of the fluid, m / s; C represents the specific heat capacity of the fluid, J / (kg·K); T f represents the temperature of the fluid in the wellbore, K; λ represents the thermal conductivity of the fluid, W / (m·K); R1 represents the sum of the thermal resistances per unit length between the fluid and the formation, (m·K) / W; λ1, λ2, λ3, and λ4 represent the thermal conductivity of the casing, cement sheath, formation, and insulation layer, respectively, W / (m·K); d1 and d2 represent the inner and outer diameters of the casing, m; d3 represents the outer diameter of the cement sheath, m; d4 represents the outer diameter of the insulation layer, m; h represents the convective heat transfer coefficient between the inner wall of the casing and the working fluid, W / (m·K); T D represents dimensionless temperature; t D represents dimensionless time; τ represents the cumulative contact time between the working fluid and the formation, s;

[0065] S4 establishes a calculation model for the physical property parameters of natural gas in liquid, gas and gas-liquid mixed states.

[0066] Furthermore, in a specific embodiment, the natural gas physical property parameter calculation model includes:

[0067] The physical parameter calculation model of liquefied natural gas (LNG) includes:

[0068] Density of LNG:

[0069]

[0070]

[0071] Among them, ρ l Indicates the density of LNG, kg / m 3 ;M m Indicates the molar mass of LNG, kg / kmol; V m Indicates the molar volume of LNG, cm 3 / mol; I m represents an intermediate variable, dimensionless; ζ m represents the LNG configuration factor, dimensionless; T c represents the critical temperature of LNG, K; R represents the molar gas constant, 8.314 J / (mol·K); P c Indicates the critical pressure of LNG, MPa.

[0072] LNG viscosity:

[0073]

[0074] Among them, η l Indicates the viscosity of LNG, Pa·s.

[0075] Specific heat of LNG at constant pressure:

[0076] C p,l =-0.018+1.1982T f -9.8722×10 -3 T f 2 +3.167×10 -5 T f 3 (10)

[0077] Among them, C p,l It represents the specific heat of LNG at constant pressure, J / (kg·K).

[0078] Thermal conductivity of LNG:

[0079]

[0080] Among them, λ l Indicates the thermal conductivity of LNG, W / (m 2 ·K).

[0081] The physical parameter calculation model of natural gas (NG) includes:

[0082] Density of NG:

[0083] P g =Zρ g RT (12)

[0084]

[0085] Among them, P g Indicates the pressure of NG, Pa; ρ g Indicates the density of NG, kg / m 3 ; Z represents the compression factor, dimensionless; ρ r Represents the relative density, dimensionless; P r Represents the relative pressure, dimensionless; T r Represents the comparative temperature, dimensionless; A 1～8 Represents a constant, dimensionless.

[0086] Viscosity of NG:

[0087] η g =3.844+0.4011T f -1.4303×10 -4 T f 2 (15)

[0088] Among them, η g Indicates the viscosity of NG, Pa·s.

[0089] Specific heat of NG at constant pressure:

[0090] C p,g =4.598+1.245×10 -2 T+2.860×10 -6 T 2 -2.703×10 -9 T 3 (16)

[0091] Among them, C p,g Indicates the specific heat of NG at constant pressure, J / (kg·K).

[0092] Thermal conductivity of NG:

[0093]

[0094] Among them, λ g Indicates the thermal conductivity of NG, W / (m 2 ·K); r represents the specific heat ratio, which is 1.67.

[0095] Calculation model of natural gas physical property parameters in gas-liquid mixing zone:

[0096] ρ eff =θρ g +(1-θ)ρ l (18)

[0097]

[0098] λ eff =θλ g +(1-θ)λ l (20)

[0099] Wherein, θ represents the vaporization rate, %; L represents the latent heat of vaporization, J / kg; and the subscript eff represents the effective value.

[0100] Pressure model of fluid in the wellbore:

[0101]

[0102] Saturated vapor pressure model of natural gas:

[0103]

[0104] Where p is the pressure of the fluid, Pa; u is the flow rate of the fluid, m / s; g is the acceleration due to gravity, 9.81 m / s 2; f represents the flow friction coefficient, which is dimensionless.

[0105] S5 couples the formation temperature model, the heat transfer model and pressure model of the fluid in the well, and the natural gas physical property parameter calculation model to obtain the temperature and pressure model of the natural gas-U-shaped well gasification system, solves it, and obtains the design parameters of the U-shaped geothermal well.

[0106] In a specific embodiment, by Figure 2 The finite volume method shown is used to solve the temperature and pressure model of the natural gas-U-shaped geothermal well, including:

[0107] Simplify formula (2) to:

[0108]

[0109] in: q m represents the mass flow rate of the working fluid, kg / s; A represents the cross-sectional area of ​​the wellbore, m 2 ; δz represents the grid length, m; subscript W represents the adjacent control unit on the left; w represents the left boundary; subscript P represents the selected control unit; subscript E represents the control unit on the right; e represents the right boundary;

[0110] Furthermore, formula (23) is rewritten as Gaussian iteration formula:

[0111]

[0112] in:

[0113]

[0114] a E =λ e (26)

[0115]

[0116] The iterative equations are:

[0117]

[0118] Through the attached Figure 3 The Gauss-Seidel iteration method shown in the figure is used for numerical solution, and the process is as follows:

[0119] (1) Assume the initial value T i (0) ;

[0120] (2) Calculate the saturated vapor pressure at the temperature by using equation (22), and compare the current pressure with the saturated vapor pressure to determine the phase state at the location;

[0121] (3) According to the phase state, substitute the physical property parameter equation of gas / mixed state / liquid state to obtain its physical property parameters;

[0122] (4) Substitute the physical property parameters into equations (25) to (28) to solve the coefficients of the GS iteration equation;

[0123] (5) Substitute the coefficients of the GS iteration formula into formula (29) to obtain T i (1) ;

[0124] (6) Calculation If the maximum value of spx does not meet the accuracy requirement, then use T i (1) As the initial value, repeat steps (2) to (6) until the maximum value of spx meets the accuracy requirements.

[0125] Preferably, the design method further comprises:

[0126] S6: verifying the economic benefits of the natural gas-U-shaped well gasification system according to the temperature and pressure model of the natural gas-U-shaped well gasification system and the design parameters of the U-shaped geothermal well.

[0127] Furthermore, S6 includes: To determine the economic benefits of the system, the following NPV model is established:

[0128]

[0129] I income =VP LNG +QηP Price-h (31)

[0130] C cost =C f +C v (32)

[0131] C f =C c +(1-k)C fi (33)

[0132] C v =C tax +(1-k)C vi (34)

[0133] C tax =I income β (35)

[0134] C vi =C H0 ·S+r H S 2 (36)

[0135] Where V is the volume flow rate, m 3 / s;P LNG represents the sales price of natural gas, ￥ / m3; Q represents the heat extraction, kWh; η represents the heat extraction efficiency, %; P price-h Indicates the heat price, ￥ / kWh; C f represents fixed cost, ￥; C v represents variable cost, ￥; C c represents operating and management expenses, ￥; C fi represents the initial investment that is independent of the length of the horizontal segment, ￥; k—one-time subsidy rate; C tax represents the business tax of the enterprise, ￥; β represents the comprehensive tax rate; C H0 Indicates the benchmark value of drilling cost per meter in horizontal profile, ￥ / m; r H It represents the growth coefficient of drilling cost per meter in the horizontal profile, RMB / m; S represents the well depth, m.

[0136] Example 1

[0137] A U-shaped well with a vertical depth of 4000m and a horizontal section length of 2000m is used as the U-shaped geothermal well 3 in the system for realizing LNG gasification by geothermal energy in the present invention. Its basic structural parameters are shown in Table 1, and its economic parameters are shown in Table 2.

[0138] Table 1

[0139] parameter value <![CDATA[Inner diameter d1 of the casing, m]]> 0.1784 <![CDATA[Outer diameter of the casing d2, m]]> 0.1937 <![CDATA[Outer diameter d3 of the cement sheath, m]]> 0.245 <![CDATA[Outer diameter of the thermal insulation layer d4, m]]> 0.265 Thermal conductivity of casing, W / (m·K) 40 Thermal conductivity of insulation layer, W / (m·K) 0.05 Thermal conductivity of cement sheath, W / (m·K) 0.5 Formation thermal conductivity, W / (m·K) 2.294 Ground temperature, K 294.15 Geothermal gradient, K / m 0.033 Inlet pressure, MPa 0.1 Inlet temperature, K 90 Mass flow, kg / s 20

[0140] Table 2

[0141]

[0142]

[0143] The simulation calculation of the system for realizing LNG gasification by geothermal energy can obtain the following results: Figure 4 The temperature distribution curve of the whole wellbore and Figure 5 As shown in the NPV curve of the whole wellbore, it can be seen from the temperature distribution curve of the whole wellbore that there is an obvious phase change area of ​​LNG in the wellbore, and the outlet temperature is higher than the normal temperature, which means that the system realizes the gasification of LNG through geothermal energy. It can be seen from the NPV curve of the whole wellbore that the NPV values ​​are all positive after the depth of the U-shaped well exceeds 20m, which means that the LNG gasification method using the system of the present invention is sustainable and feasible.

[0144] The above embodiments are only preferred implementations of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A method for designing a system for realizing LNG gasification using geothermal energy, characterized in that: It includes: S1 Establish a physical model of a natural gas-U-well gasification system for LNG gasification using geothermal energy from a U-shaped geothermal well; S2, establishing a formation temperature model according to the physical model of the natural gas-U-shaped well gasification system; S3, establishing a heat transfer model and a pressure model of the fluid in the well according to the physical model of the natural gas-U-shaped well gasification system; S4 establishes a calculation model for the physical property parameters of natural gas in liquid, gas and gas-liquid mixed states; S5 couples the formation temperature model, the heat transfer model and pressure model of the fluid in the well, and the natural gas physical property parameter calculation model to obtain the temperature and pressure model of the natural gas-U-shaped well gasification system, solves it, and obtains the design parameters of the U-shaped geothermal well.

2. The design method according to claim 1, characterized in that: The physical model of the natural gas-U-shaped well gasification system is established as follows: it includes: an LNG storage tank, a booster pump, a U-shaped geothermal well, an insulation layer and a throttle valve, wherein one end of the U-shaped geothermal well is connected to the LNG storage tank through the booster pump, and the other end is provided with the insulation layer and connected to the natural gas user through the throttle valve.

3. The design method according to claim 1, characterized in that: The formation temperature model is established as follows: T d =T0+aH(1) Among them, T d represents the formation temperature; T0 represents the ground temperature; a represents the formation temperature gradient; H represents the depth.

4. The design method according to claim 1, characterized in that: The well fluid heat transfer model and pressure model include: in: Where z represents the axial coordinate; ρ represents the density of the fluid; u represents the flow rate of the fluid; C represents the specific heat capacity of the fluid; T f represents the temperature of the fluid in the wellbore; λ represents the thermal conductivity of the fluid; R1 represents the sum of the thermal resistances per unit length between the fluid and the formation; λ1, λ2, λ3, and λ4 represent the thermal conductivity of the casing, cement sheath, formation, and insulation layer, respectively; d1 and d2 represent the inner and outer diameters of the casing; d3 represents the outer diameter of the cement sheath; d4 represents the outer diameter of the insulation layer; h represents the convective heat transfer coefficient between the inner wall of the casing and the working fluid; T D represents dimensionless temperature; t D represents dimensionless time; τ represents the cumulative contact time between the working fluid and the formation.

5. The design method according to claim 1, characterized in that: The natural gas physical property parameter calculation model includes: a density calculation model for liquefied natural gas, a viscosity calculation model for liquefied natural gas, a constant pressure specific heat calculation model for liquefied natural gas, and a thermal conductivity calculation model for liquefied natural gas; a density calculation model for gaseous natural gas, a viscosity calculation model for gaseous natural gas, a constant pressure specific heat calculation model for gaseous natural gas, and a thermal conductivity calculation model for gaseous natural gas; and a natural gas physical property parameter calculation model for a gas-liquid mixing zone, a pressure model for fluids in a wellbore, and a saturated vapor pressure model for natural gas.

6. The design method according to claim 5, characterized in that: The calculation model of natural gas physical property parameters in the gas-liquid mixing zone is established as follows: r eff =th g +(1-θ)ρ l (18) l eff =θλ g +(1-θ)λ l (20) Where ρ represents the density of the fluid, C represents the specific heat capacity of the fluid, λ represents the thermal conductivity of the fluid, the subscript eff represents the effective value, θ represents the vaporization rate, L represents the latent heat of vaporization, and ρ g represents the density of gaseous natural gas, ρ l Indicates the density of liquid natural gas, C p,g represents the constant pressure specific heat of natural gas, C p,l represents the specific heat of liquid natural gas at constant pressure, T f represents the fluid temperature in the wellbore, L represents the latent heat of vaporization, λ g Denotes the thermal conductivity of gaseous natural gas, λ l Represents the thermal conductivity of liquid natural gas.

7. The design method according to claim 1, characterized in that: The solution is achieved by finite volume method and Gauss-Seidel iteration method.

8. The design method according to claim 1, characterized in that: It also includes: S6: verifying the economic benefits of the natural gas-U-shaped well gasification system according to the temperature and pressure model of the natural gas-U-shaped well gasification system and the design parameters of the U-shaped geothermal well.

9. The design method according to claim 8, characterized in that: The verification is achieved through the following NPV model: I income =VP LNG +QηP Price-h (31) C cost =C f +C v (32) C f =C c +(1-k)C fi (33) C v =C tax +(1-k)C vi (34) C tax =I income ·b(35) C vi =C H0 ·S+r H S 2 (36) Where V represents the volume flow rate; P LNG represents the sales price of natural gas; Q represents the heat extraction; η represents the heat extraction efficiency, %; P price-h Indicates the heat price; C f represents fixed cost; C v represents variable cost; C c represents operating and management expenses; C fi represents the initial investment that is independent of the length of the horizontal section of the U-shaped geothermal well; k represents the one-time subsidy rate; C tax represents the business tax of the enterprise; β represents the comprehensive tax rate; C H0 represents the benchmark value of drilling cost per meter in the horizontal section; r H It represents the growth coefficient of drilling cost per meter in the horizontal section; S represents the well depth.

10. The natural gas-U-shaped well gasification system obtained according to the design method of any one of claims 1-9.

Images