Method for evaluating well cementation quality of deepwater surface layer hydrate-containing stratum

Abstract

The invention discloses a method for evaluating the well cementation quality of a hydrate-containing stratum on a deepwater surface layer. The method comprises the following steps: establishing a hydrate-containing stratum wellbore structure model; dividing grids, and setting monitoring points in the well cementation cement sheath and the stratum; applying well reinforcement engineering parameters to the hydrate-containing stratum shaft structure model; determining the volume of the fluid transferred from the stratum to the annulus, and determining the volume of the anti-invasion fluid of each component in the annulus; correcting the migration direction of fluid in the well cementation annulus and hydrate-containing stratum wellbore structure model; determining fracture distribution and fracture rate of the well cementation cement sheath; establishing a partition PFC numerical model, and calculating the compressive strength of the well cementation cement sheath; and meanwhile, the permeability increment of each area and the whole of the well cementation cement sheath is calculated by utilizing an improved KC equation. The comprehensive evaluation of the well cementation quality of the hydrate-containing stratum is realized.

Description

Technical Field

[0001] The invention aims at the technical field of seabed cementing construction encountering hydrate-containing formations, and in particular to a method for evaluating the cementing quality of deepwater surface hydrate-containing formations. Background Art

[0002] As onshore oil and gas exploration technology matures, offshore oil and gas production plays an important role in the growth of the world's oil and gas reserves. However, due to the influence of the specific low temperature and high pressure environment and fold and fault structures of the seabed, hydrate layers are often present above the seabed oil and gas, which poses a great obstacle to the production of oil and gas.

[0003] Cementing operations are an essential part of deepwater oil and gas production. The quality of cementing directly determines the service life of the casing-cement sheath-formation complex and the production well, and is an important guarantee for long-term stable production. When cementing encounters hydrate-containing formations, the hydrates in the formation are often decomposed by the hydration heat transfer of cement slurry. The high-pressure free gas and water produced can easily invade the annular cement sheath and form microcracks, thus affecting the cementing quality. At present, related research at home and abroad mainly focuses on exploring the basic physical properties of hydrate-containing formations and developing new materials to improve cementing quality. There is a lack of comprehensive and systematic evaluation of the quality of cement sheaths.

[0004] Since field or indoor tests are costly, difficult, time-consuming and laborious, numerical simulation is usually used for research. The main difficulties in evaluating cementing quality in hydrate-bearing formations are as follows: (1) the composition and content of the fluid that invades the annulus; (2) the increment and distribution of the fracture rate of the invaded cement sheath; (3) the change in the compressive strength and permeability of the invaded cement sheath.

[0005] Due to the limitations of numerical simulation methods, there is currently no similar method to simulate the cementing process in hydrate-containing formations. Summary of the invention

[0006] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a method for evaluating the cementing quality of deep-water surface hydrate-containing formations. The present invention can comprehensively evaluate the cementing quality of hydrate-containing formations.

[0007] To achieve the above object, the technical solution adopted by the present invention is: a method for evaluating the cementing quality of a deepwater surface hydrate-containing formation, comprising the following steps:

[0008] Step 1: establishing a wellbore structure model of a hydrate-containing formation, wherein the wellbore structure model of the hydrate-containing formation includes a submarine hydrate-containing formation and a casing, and the annulus between the submarine hydrate-containing formation and the casing is filled with cement slurry to form a cementing cement ring;

[0009] Step 2: Divide the grid and set up monitoring points in the cement sheath and formation;

[0010] Step 3, applying cementing engineering parameters to the wellbore structure model of the hydrate-containing formation;

[0011] Step 4: determine the volume of fluid migrated from the formation to the annulus by setting monitoring points on the interface between the formation and the cement ring, and use the flow formula to determine the volume of each component of the anti-invasion fluid invading the annulus;

[0012] Step 5, correcting the migration direction of the fluid in the cementing annulus and the wellbore structure model of the hydrate-containing formation, converting the input cementing slurry into the output anti-invasion fluid, and refining the migration of the fluid in the wellbore structure model of the hydrate-containing formation;

[0013] Step 6, determining the crack distribution and crack ratio of the cement sheath;

[0014] Step 7: Establish a partitioned PFC numerical model to calculate the compressive strength of the cement sheath; at the same time, use the improved Közeney-Kármán equation, i.e., the KC equation, to calculate the permeability increment of each region and the entire cement sheath, thereby achieving a comprehensive evaluation of the cementing quality of the hydrate-containing formation.

[0015] As a further improvement of the present invention, the step 2 is specifically as follows:

[0016] According to the accuracy requirements and calculation needs, the cementing annulus cement slurry is divided into n parts, and monitoring points are set up respectively to ensure that the monitoring points in the formation cover the entire model; two grid division methods, equal spacing and geometric ratio, are used in the formation. Within the preset distance range of the wellbore structure model of the hydrate-bearing formation close to the wellbore wall, the monitoring points are evenly arranged at preset intervals, and the remaining monitoring points are non-uniformly arranged in a geometric ratio, extending to the end of the model; and a monitoring surface is set up at the two interfaces of cementing to monitor the fluid and heat transfer between the cementing cement ring and the formation; the distribution of fractures formed by the anti-invasion fluid is quantified by the zoning method, and the fracture rate at different locations in different areas is determined; the fracture rate at the two interfaces is obtained by gridding the cement ring.

[0017] As a further improvement of the present invention, in step 3, the cementing engineering parameters include temperature, pressure, cement slurry heat release rate, cement slurry density, cementing pressure difference and pressure holding time.

[0018] As a further improvement of the present invention, in step 4, the anti-invasion fluid includes methane gas, decomposed water, and anti-invasion cement slurry.

[0019] As a further improvement of the present invention, in step 4, the volume ratio of the anti-invasion fluid is obtained using the flow ratio, and then the volume of each component of the anti-invasion fluid is determined; the flow formula is specifically as follows:

[0020]

[0021] Where Q is the flow rate; μ is the viscosity; k is the coefficient related to the pressure difference, and i is the number of anti-invasion fluid components; n is an index related to the fluid property, T is the temperature, and T0 is the initial temperature.

[0022] As a further improvement of the present invention, the step 5 is specifically as follows:

[0023] The fracture ratio formula is used to convert the intruding gas volume into the newly added fracture ratio volume:

[0024]

[0025] Where P is the converted gas pressure; P0 is the intrusion gas pressure;

[0026] The crack rate of cement paste after setting is determined by the following formula:

[0027] V 裂隙 =ΔV 裂隙 +V 毛细孔隙

[0028]

[0029] Among them, φ c is the capillary porosity; d p is the average diameter of cement particles; d s It is the effective particle diameter of cement paste.

[0030] As a further improvement of the present invention, in step 7, a partitioned PFC numerical model is established to calculate the compressive strength of the cement sheath as follows:

[0031] Discrete element models of multiple regions are established, and different crack rates are assigned to each region. Then, the regions are bonded together using a parallel bonding model to form an overall invaded cement paste model. A fixed-speed pressure is applied above the invaded cement paste model to simulate a uniaxial compression test and obtain the stress-strain curve of the cement paste.

[0032] As a further improvement of the present invention, in step 7, the improved Kozeny-Karman equation is as follows:

[0033]

[0034] Among them, ΔK r is the crack rate growth rate, φ is the crack rate; when the crack rate φ takes a certain area value of the cement sheath, ΔK r is the permeability increment of cement stone area. When φ takes the average value of cement ring, ΔK r is the overall permeability increment of cement stone; after the cement stone area is divided, the permeability of cement stone in the near-formation area is equivalent to the permeability of the two interfaces, so as to realize the evaluation of the sealing and bonding performance of cement ring.

[0035] The present invention corrects the gas-water back-invasion annulus algorithm; refines the calculation of the back-invasion fluid, determines the components of the back-invasion fluid as methane gas, decomposed water and back-pushed cement slurry, uses the flow formula to determine the volume of each phase of the invading fluid, and clarifies the process in which the cement slurry first invades the formation and then retreats to the cementing annulus with the back-invasion gas and water. The fracture rate of cement stone is further quantified, and the PFC particle flow model is established in different zones to predict the compressive strength of the cement sheath. The permeability increment of each zone and the whole cement sheath is determined by the improved KC equation, and the cementing quality of the hydrate-containing formation is comprehensively evaluated.

[0036] The present invention clarifies the three-phase ratio and content of the fluid produced by hydrate decomposition to generate gas and water invading the annulus; clarifies the crack rate, crack distribution and compressive strength of the invaded cement sheath; and clarifies the horizontal permeability and change of the cement sheath. In addition, the present invention achieves: (1) numerical simulation and quantification of the entire process of hydrate decomposition to generate high-pressure gas and water and invading the annulus; (2) accurate evaluation of cementing quality, with evaluation indicators including cement stone crack rate, compressive strength, cement stone and two-interface permeability.

[0037] The beneficial effects of the present invention are:

[0038] (1) The content of each component of the three-phase fluid in the reverse invasion is accurately calculated through a self-derived formula; (2) The fluid migration mechanism between the cement sheath and the formation is corrected, and the invasion of cement slurry into the formation in a single direction is corrected to a dynamic process in which the cement slurry first invades and then retreats; (3) The volume of the invading gas is quantified into the volume of the fracture, and the distribution of fractures after the cement stone solidifies is determined by subdividing the area; (4) The PFC numerical model is established by adopting the method of first partitioning modeling and then adhesion to accurately simulate the fracture rate and fracture distribution of the invaded cement sheath; (5) By improving the KC equation, the ambiguous constant value is avoided and the cement sheath permeability increment is accurately calculated. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a flow chart of an embodiment of the present invention;

[0040] Figure 2 Schematic diagram of a wellbore model of a hydrate-containing formation model in an embodiment of the present invention;

[0041] Figure 3 A schematic diagram of a cement stone model according to an embodiment of the present invention;

[0042] Figure 4 Schematic diagram of the volume of three-phase intrusive fluid under different cementing parameters in an embodiment of the present invention;

[0043] Figure 5 Schematic diagram of fracture ratio under different hydrate saturation conditions in an embodiment of the present invention;

[0044] Figure 6 Schematic diagram of uniaxial compression test of invaded cement sheath in an embodiment of the present invention;

[0045] Figure 7 Schematic diagram of the permeability of each area of ​​the cement sheath in an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0047] Example 1

[0048] like Figure 1 As shown, a numerical simulation evaluation method for cementing quality during cementing of a natural gas hydrate-containing formation comprises:

[0049] (1) Establish Figure 2 The cementing model of the hydrate-bearing formation shown includes: the seabed hydrate-bearing formation (formation rock, hydrate, pores filled with water), the annulus between the formation and the casing (filled with cement slurry), the casing, and the space inside the casing (filled with post-filling fluid).

[0050] (2) Divide the grid and set up monitoring points. The inner space of the casing is set as cell A0000, the casing is set as cell A0001, and the annulus between the casing and the formation is A0002. Two grid division methods, equal spacing and geometric ratio, are used in the formation to make the grid dense in the area near the wellbore wall and gradually sparse in the area far from the wellbore wall, with a total of 113 cells A0003-A0115. Within the 2% distance range near the wellbore wall of the model, the monitoring points are evenly arranged at intervals of 5 mm; the remaining monitoring points are non-uniformly arranged in a geometric ratio and extend to the end of the model. A monitoring surface is set up at the second interface to track the flow and heat exchange between the cementing annulus and the formation.

[0051] The conservation of mass and energy within the model can be achieved by means of finite volume integral differences:

[0052]

[0053] Where: V, V n ——Volume, the volume of subdomain n [L 3 ]; M κ ——Mass accumulation term of component κ [kg m -3 ]; A,Γ n ——Surface area, surface area of ​​subdomain n [L 2 ]; F κ ——Darcy flux vector of component κ [kg m -2 s -1 ]; n——inward unit normal vector; q κ ——Source / sink term of component κ [kg m -3 s -1]; t——time [s];

[0054] β includes H (hydrate phase), A (liquid phase), G (gas phase), and I (ice phase);

[0055] κ includes w (water), m (natural gas), h (hydrate), i (water-soluble inhibitor), and θ (heat);

[0056] in:

[0057] ① Mass accumulation term Mκ

[0058]

[0059] Where: φ——porosity; S β ——saturation of phase β; ρβ——density of phase β [kg / m 3 ];X β κ ——mass fraction of component κ in phase β;

[0060] ②Heat accumulation term M θ

[0061]

[0062]

[0063] Where: R —— rock density [kg m -3 ]; CR——heat capacity of dry rock [J kg -1 K -1 ]; U β ——Specific internal energy of phase β [J kg -1 ]; Δ()——the change of the quantity in the brackets at the current time step; ΔU H ——Specific enthalpy of hydrate dissociation / formation [J kg -1 ]

[0064] ③Flow term Fκ

[0065] The mass flow rates of water, methane and inhibitors depend on the changes in the liquid and gas phases. Since the two solid phases (β≡H, I) do not affect the fluid flow rate, when the kinetic model is used, the mass flow rate of the hydrate component passing through all unit boundaries is 0;

[0066] Liquid phase:

[0067]

[0068] Where: k is the absolute permeability of rock and soil [m2]; k rA ——relative permeability of liquid phase; μ A——Liquid phase viscosity [Pa·s]; P A ——Liquid phase pressure [Pa]; g——Gravity acceleration vector [m·s -2 ];

[0069] Gas phase:

[0070]

[0071] Where: k0——absolute permeability under high pressure conditions [m 2 ]; b——gas slip factor [Pa]; k rG ——Gas phase permeability; μ G ——Gas phase viscosity [Pa·s]; J κG ——Diffusion mass flow rate of component κ in the gas phase;

[0072] Heat flow:

[0073]

[0074] Where: K R ——Thermal conductivity of rock and soil [W·m -1 ·K -1 ]; K β ——Thermal conductivity of phase β [W·m -1 ·K -1 ];h β ——Specific enthalpy of phase β [J·kg -1 ]; fσ——thermal radiation factor; σ0——Stefan-Boltzmann constant, whose value is 5.6687×10 -8 J.m -2 ·K -4 ;

[0075] ④ Source / sink term qκ

[0076] Formation fluid production or injection of inhibitors:

[0077]

[0078] Where: q β ——Production or injection rate of phase β [kg·m -3 ];

[0079] Heat injection or extraction from the formation:

[0080]

[0081] For the spatial discretization of equation (1), the finite volume integral difference method can be used to achieve it:

[0082] Volume integral

[0083]

[0084] Where: M is the volume normalized extension; Mn is the average value in volume Vn;

[0085] Surface integral

[0086]

[0087] Where: Fnm is the normal direction of F through the interface Anm between volume elements Vn and Vm (the average value of the inward component);

[0088] In actual solution, numerical calculation is performed through Newton-Raphson iteration method.

[0089] (3) Cementing engineering parameters including temperature, pressure, cement slurry heat release rate, cement slurry density, cementing pressure difference, and pressure holding time are applied to the model. Among them, the initial setting time of common deepwater oil and gas cement is 3-4 hours, and the cement slurry is mixed on the drilling platform, pumped to the target formation, and reserved for a safe time of about 2 hours. Based on this, the impact of cement slurry on the formation before the initial setting of 3600 seconds is investigated.

[0090] (4) According to the method of patent CN112855075B, the start of back-invasion is judged, and the components of back-invasion fluid are determined through theoretical analysis: methane gas, decomposed water, and back-invasion cement slurry. The volume of fluid migrated from the formation to the annulus is determined by setting monitoring points at the interface between the formation and the cement ring.

[0091] Furthermore, since the volume of fluid passing through a certain section per unit time can be expressed by flow rate, the volume ratio of the three-phase fluid can be obtained by using the ratio of flow rates, and then the volume of the annular space of each component can be determined. The calculation results are as follows: Figure 4 Specifically, the formula used is as follows:

[0092]

[0093] Where Q is the flow rate, m 3 / s; μ is viscosity, Pa s -1 ; k is a coefficient related to the pressure difference, in the pressure difference of 0.2-7×10 5 When Pa, k is 0.86-0.96. n is an exponent related to the properties of the fluid.

[0094] Furthermore, the migration direction of the fluid in the cementing annulus and the formation model is changed, the input cementing slurry is converted into the output three-phase intrusive fluid, and the migration of the fluid in the model is refined.

[0095] (5) The gas that invades the well is difficult to leave the cement sheath and will eventually transform into fractures. The calculation method for the transformed volume is as follows:

[0096]

[0097] V 裂隙 =ΔV 裂隙 +V 毛细孔隙

[0098] Where P is the converted gas pressure, MPa; P0 is the intrusion gas pressure, MPa. 毛细孔隙 It is formed by the physical and chemical reaction when the cement slurry changes from liquid phase to solid phase.

[0099] Furthermore, according to the accuracy requirements and calculation needs, the cement sheath is divided into n equal parts in the radial direction (generally set to 10 parts) according to the volume, and named as areas I-X in order of distance from the two interfaces from small to large. Monitoring cells are set in each of them, and the above steps are repeated to obtain the three-phase fluid intrusion amount and crack rate in each area before the initial setting of the cement slurry.

[0100] (6) Use PFC particle flow numerical simulation software to establish a cement stone model. The specific steps are as follows:

[0101] ① Establish a discrete element model of ten regions and assign each region the crack rate calculated in step (4), such as Figure 5 shown.

[0102] ② Use parallel bonding model to bond each area to form an overall cement stone model, such as Figure 3 shown.

[0103] ③ Apply a fixed speed axial pressure to the specimen to obtain its stress-strain curve, such as Figure 6 shown.

[0104] The basic force-displacement equation of the PFC linear parallel bonding model is:

[0105] Contact force and moment

[0106]

[0107] F l is the linear force, F d is the damping force, is the parallel bond force, is the parallel bond torque.

[0108] Decompose the parallel bond force into normal force and shear force, and decompose the parallel bond moment into torsional bending moment:

[0109]

[0110] in is tension. The parallel key shear force and bending moment are located on the contact plane and are expressed using the contact plane coordinate system:

[0111]

[0112] Cross-sectional properties of the key:

[0113]

[0114] is the cross-sectional area, is the moment of inertia of the parallel bond cross section (through the line x c and direction ), is the polar moment of inertia of the parallel key cross section (the line passing through x c and direction ). The key cross section is rectangular in 2D and circular in 3D.

[0115] Bond strain energy

[0116]

[0117] Δδ n The relative normal displacement increment of this equation

[0118]

[0119] Δδ s Relative shear displacement increment

[0120]

[0121] Δθ t is the relative twist-rotation increment of Δθ

[0122]

[0123] Δθ b is the relative bend rotation increment.

[0124] (7) The Kozeny-Karman equation (hereinafter referred to as the KC equation) is used to calculate the increment of cement sheath permeability in each zone and the whole. The permeability of each zone is calculated using the crack ratio of each zone, and the increment of cement sheath permeability is calculated using the average crack ratio. In order to avoid the problem of the value of the Kozeny constant c and the specific surface area constant S in the equation, the equation is deformed, and the results are as follows:

[0125]

[0126] Furthermore, when the cement stone area is dense enough, that is, when area I is close enough to the two interfaces, its permeability increment can be approximately equivalent to the permeability increment of the two interfaces, which reflects the bonding quality between the cement sheath and the formation, such as Figure 7 shown.

[0127] Example 2

[0128] A method for evaluating cementing quality in a deepwater surface hydrate-containing formation comprises the following contents:

[0129] Content 1: Establish a wellbore structure model for gas hydrate-containing formations and monitor fluid migration and physical property changes in the formation and cementing annulus;

[0130] Content 2: Determine the composition of three-phase intrusive fluids by fluid formula;

[0131] Content 3: Convert and calculate the distribution and crack ratio of cement sheath cracks;

[0132] Content 4: Establish the cement paste PFC particle flow model by partition to predict the compressive strength of cement sheath;

[0133] Content 5: Improve the KC equation to evaluate the permeability of each zone and the overall cement sheath.

[0134] In content 1, the cement slurry in the cementing annulus is divided into n parts according to the accuracy requirements and calculation needs, and monitoring points are set up respectively to ensure that the monitoring points in the formation cover the entire model. The monitoring points in the first 2% area are spaced 5mm apart, and the monitoring points in the last 98% area are set up unevenly in a geometric form. A detection surface is set at the cementing interface to monitor the fluid and heat transfer between the cementing annulus and the formation. Through the zoning method, the distribution of cracks formed by the anti-invasion fluid is quantified, and the crack rate at different locations in different areas is determined.

[0135] In content 2, the principle of determining the composition of the anti-invasion three-phase fluid is as follows through the fluid formula (1):

[0136]

[0137] Where Q is the flow rate, m 3 / s; μ is viscosity, Pa s -1 ; k is a coefficient related to the pressure difference, in the pressure difference of 0.2-7×10 5 When Pa, k is 0.86-0.96. n is an exponent related to the properties of the fluid.

[0138] In the specific calculation, the proportion of the anti-invasion fluid is determined by the real-time temperature and pressure feedback from the monitoring point in content 1, and the total volume of the anti-invasion fluid is determined by the flow rate feedback from the monitoring surface, and then the volume of each component is determined.

[0139] The cement sheath is divided into 10 equal parts, and each area is operated according to the steps in content 2 to obtain the volume and position distribution of the three-phase invading fluid in the annulus. Specifically, the position and volume information of the invading gas are determined through the gas saturation information fed back by each monitoring point in the cement sheath, thereby obtaining the position information of the three-phase fluid. Furthermore, the volume of the invading gas is converted into the volume of the newly added fracture ratio using the fracture ratio formula (2), and the fracture ratio of the cement stone after solidification is determined using formulas (3) and (4).

[0140]

[0141] V 裂隙 =ΔV 裂隙 +V 毛细孔隙 (3)

[0142]

[0143] Among them, φ c is the capillary porosity (dimensionless, usually expressed as a percentage); d p is the average diameter of cement particles, m. d s It is the effective particle diameter of cement paste, usually calculated by the fineness or specific surface area of ​​cement. The empirical value of porosity of ordinary Portland cement after one day of consolidation is usually 20%.

[0144] In content 4, the established cement paste PFC particle flow model is divided into ten regions, and different crack rates are given to each region. Specifically, ten regions with different crack rates are established respectively, and then the regions are bonded using a parallel bonding model to form an accurate model of invaded cement paste. A fixed speed pressure is applied on the model to simulate a uniaxial compression test and obtain the stress-strain curve of cement paste.

[0145] In content 5, the KC equation is improved to avoid the value problem of Kozeny constant c and specific surface area constant S in the equation, and the accurate permeability increment is obtained. The specific equation after improvement is:

[0146]

[0147] Among them, ΔK r is the crack rate growth rate, φ is the crack rate. When the crack rate φ takes a certain area value of the cement sheath, ΔK r is the permeability increment of cement stone area. When φ takes the average value of cement ring, ΔK r is the overall permeability increment of cement paste.

[0148] Furthermore, when the cement stone area is divided finely enough, the permeability of the cement stone in the near-formation area can be equivalent to the permeability of the two interfaces, thereby realizing the evaluation of the sealing and bonding performance of the cement sheath.

[0149] The above-mentioned embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for evaluating the cementing quality of a deepwater surface hydrate-containing formation, characterized in that: The following steps are involved: Step 1: establishing a wellbore structure model of a hydrate-containing formation, wherein the wellbore structure model of the hydrate-containing formation includes a submarine hydrate-containing formation and a casing, and the annulus between the submarine hydrate-containing formation and the casing is filled with cement slurry to form a cementing cement ring; Step 2: Divide the grid and set up monitoring points in the cement sheath and formation; Step 3, applying cementing engineering parameters to the wellbore structure model of the hydrate-containing formation; Step 4: determine the volume of fluid migrated from the formation to the annulus by setting monitoring points on the interface between the formation and the cement ring, and use the flow formula to determine the volume of each component of the anti-invasion fluid invading the annulus; Step 5, correcting the migration direction of the fluid in the cementing annulus and the wellbore structure model of the hydrate-containing formation, converting the input cementing slurry into the output anti-invasion fluid, and refining the migration of the fluid in the wellbore structure model of the hydrate-containing formation; Step 6, determining the crack distribution and crack ratio of the cement sheath; Step 7: Establish a partitioned PFC numerical model to calculate the compressive strength of the cement sheath; at the same time, use the improved Közeney-Kármán equation, i.e., the KC equation, to calculate the permeability increment of each region and the entire cement sheath, thereby achieving a comprehensive evaluation of the cementing quality of the hydrate-containing formation.

2. The method for evaluating cementing quality of deepwater surface hydrate-containing formations according to claim 1, characterized in that: The step 2 is specifically as follows: According to the accuracy requirements and calculation needs, the cementing annulus cement slurry is divided into n parts, and monitoring points are set up respectively to ensure that the monitoring points in the formation cover the entire model; two grid division methods, equal spacing and geometric ratio, are used in the formation. Within the preset distance range of the wellbore structure model of the hydrate-bearing formation close to the wellbore wall, the monitoring points are evenly arranged at preset intervals, and the remaining monitoring points are non-uniformly arranged in a geometric ratio, extending to the end of the model; and a monitoring surface is set up at the two interfaces of cementing to monitor the fluid and heat transfer between the cementing cement ring and the formation; the distribution of fractures formed by the anti-invasion fluid is quantified by the zoning method, and the fracture rate at different locations in different areas is determined; the fracture rate at the two interfaces is obtained by gridding the cement ring.

3. The method for evaluating cementing quality of deepwater surface hydrate-containing formations according to claim 1, characterized in that: In step 3, the cementing engineering parameters include temperature, pressure, cement slurry heat release rate, cement slurry density, cementing pressure difference and pressure holding time.

4. The method for evaluating cementing quality of deepwater surface hydrate-containing formations according to claim 1, characterized in that: In step 4, the anti-invasion fluid includes methane gas, decomposed water, and anti-invasion cement slurry.

5. The method for evaluating cementing quality of deepwater surface hydrate-containing formations according to claim 1 or 4, characterized in that: In step 4, the volume ratio of the anti-invasion fluid is obtained using the flow ratio, and then the volume of each component of the anti-invasion fluid is determined; the flow formula is as follows: Where Q is the flow rate; μ is the viscosity; k is the coefficient related to the pressure difference, and i is the number of anti-invasion fluid components; n is an index related to the fluid property, T is the temperature, and T0 is the initial temperature.

6. The method for evaluating cementing quality of deepwater surface hydrate-containing formations according to claim 1, characterized in that: The step 5 is specifically as follows: The fracture ratio formula is used to convert the intruding gas volume into the newly added fracture ratio volume: Where P is the converted gas pressure; P0 is the intrusion gas pressure; The crack rate of cement paste after setting is determined by the following formula: V 裂隙 =ΔV 裂隙 +V 毛细孔隙 Among them, φ c is the capillary porosity; d p is the average diameter of cement particles; d s It is the effective particle diameter of cement paste.

7. The method for evaluating cementing quality of deepwater surface hydrate-containing formations according to claim 1, characterized in that: In step 7, a partitioned PFC numerical model is established to calculate the compressive strength of cement sheath as follows: Discrete element models of multiple regions are established, and different crack rates are assigned to each region. Then, the regions are bonded together using a parallel bonding model to form an overall invaded cement paste model. A fixed-speed pressure is applied above the invaded cement paste model to simulate a uniaxial compression test and obtain the stress-strain curve of the cement paste.

8. The method for evaluating cementing quality of deepwater surface hydrate-containing formations according to claim 1, characterized in that: In step 7, the improved Kozeny-Karman equation is as follows: Among them, ΔK r is the crack rate growth rate, φ is the crack rate; when the crack rate φ takes a certain area value of the cement sheath, ΔK r is the permeability increment of cement stone area. When φ takes the average value of cement ring, ΔK r is the overall permeability increment of cement stone; after the cement stone area is divided, the permeability of cement stone in the near-formation area is equivalent to the permeability of the two interfaces, so as to realize the evaluation of the sealing and bonding performance of cement ring.

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