Evaluation method for cementing quality in deepwater surface hydrate-bearing formations

Abstract

The present invention discloses a method for evaluating the cementing quality of a deepwater surface hydrate-containing formation, comprising: establishing a wellbore structural model of the hydrate-containing formation; dividing a grid and setting monitoring points in the cementing cement sheath and the formation; applying cementing engineering parameters to the wellbore structural model of the hydrate-containing formation; determining the volume of fluid migrating from the formation to the annulus, and simultaneously determining the volume of each component of the counter-invasion fluid counter-invading the annulus; correcting the migration direction of fluids in the cementing annulus and the wellbore structural model of the hydrate-containing formation; determining the crack distribution and crack ratio of the cementing cement sheath; establishing a partitioned PFC numerical model and calculating the compressive strength of the cementing cement sheath; and simultaneously calculating the permeability increment of each region and the entire cementing cement sheath using an improved KC equation. The present invention achieves a comprehensive evaluation of the cementing quality of the hydrate-containing formation.

Description

Technical Field

[0001] The present invention is aimed at the technical field of seabed cementing construction encountering hydrate-containing formations, 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 production plays a significant role in increasing global oil and gas reserves. However, due to the unique low-temperature, high-pressure environment, folds, and fault structures of the seafloor, hydrate-bearing layers often form above the oil and gas reserves, significantly hindering their extraction.

[0003] Cementing operations are essential for deepwater oil and gas production. Cementing quality directly determines the service life of the casing-cement sheath-formation complex and the production well, and is a crucial guarantee for long-term, stable production. When cementing in hydrate-bearing formations, the hydrates in the formation often decompose due to the heat transfer of the cement slurry during hydration. The resulting high-pressure free gas and water easily invade the annular cement sheath and form microcracks, thus affecting cementing quality. Currently, research both domestically and internationally focuses on exploring the basic physical properties of hydrate-bearing formations and developing new materials to improve cementing quality. However, there is a lack of comprehensive and systematic assessment of cement sheath quality.

[0004] Because field or laboratory tests are costly, difficult, time-consuming, and labor-intensive, numerical simulations are often used for research. The main difficulties in evaluating cementing quality in hydrate-bearing formations are: (1) the composition and content of the fluid that invades the annulus; (2) the increase in the fracture rate and its distribution in the invaded cement sheath; and (3) the changes 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 of 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 present invention adopts a technical solution: 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-bearing formation, wherein the wellbore structure model includes a submarine hydrate-bearing formation and a casing, and the annulus between the submarine hydrate-bearing 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-bearing formation;

[0011] Step 4: Determine the volume of fluid that migrates from the formation to the annulus by setting monitoring points at the interface between the formation and the cement sheath, and simultaneously determine the volume of each component of the anti-invasion fluid that invades the annulus using a flow rate formula;

[0012] Step 5: Correct the migration direction of the fluid in the cementing annulus and the wellbore structure model of the hydrate-bearing formation, convert the input cementing slurry into the output anti-invasion fluid, and refine the migration of the fluid in the wellbore structure model of the hydrate-bearing formation;

[0013] Step 6: Determine the crack distribution and crack ratio of the cement sheath;

[0014] Step 7: Establish a zoned PFC numerical model to calculate the compressive strength of the cement sheath. Simultaneously, use the improved Közeny-Kármán equation (KC equation) to calculate the permeability increment of each zone and the entire cement sheath, thereby achieving a comprehensive evaluation of the cementing quality of the hydrate-bearing formation.

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

[0016] Based on accuracy requirements and calculation needs, the cement slurry in the cementing annulus is divided into n equal parts, and monitoring points are set up in each part 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. A monitoring surface is set up at the cementing interface to monitor the fluid and heat transfer between the cement sheath and the formation. The distribution of fractures formed by the anti-invasion fluid is quantified through a zoning method, and the fracture ratio at different locations in different areas is determined. The fracture ratio at the interface is obtained by gridding the cement sheath.

[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 properties, 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 using 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 stone.

[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 were established, and different crack rates were assigned to each region. Then, a parallel bonding model was used to bond the regions together to form an overall invaded cement paste model. A fixed-rate pressure was applied to the invaded cement paste model to simulate a uniaxial compressive test and obtain the cement paste stress-strain curve.

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

[0033]

[0034] Where 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 the cement stone area. When φ takes the average value of the 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, realizing the evaluation of the sealing and bonding performance of the cement sheath.

[0035] This invention refines the annular gas-water counter-invasion algorithm and refines the calculation of the counter-invasion fluid, defining its components as methane gas, decomposed water, and back-pushed cement slurry. Using flow formulas, the volumes of each phase of the counter-invasion fluid are determined, clarifying the process in which the cement slurry first invades the formation and then retreats into the cementing annulus along with the counter-invasion gas and water. Furthermore, the fracture rate of the cement paste is quantified, and a PFC particle flow model is established for each zone to predict the compressive strength of the cement sheath. Using the improved KC equation, the permeability increment of each zone and the entire cement sheath is determined, enabling a comprehensive assessment of the cementing quality in hydrate-bearing formations.

[0036] The present invention clarifies the three-phase ratios and contents of the fluids that form the annular space during hydrate decomposition, including gas and water intrusion; the fracture ratio, fracture distribution, and compressive strength of the invaded cement sheath; and the horizontal permeability and variation of the cement sheath. Furthermore, the present invention achieves: (1) numerical simulation and quantification of the entire process of hydrate decomposition, generation of high-pressure gas and water, and intrusion into the annular space; and (2) accurate evaluation of cementing quality, using evaluation indicators including cement stone fracture ratio, compressive strength, and cement stone and two-interface permeability.

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

[0038] (1) Accurately calculate the content of each component of the three-phase fluid in the reverse invasion through a self-derived formula; (2) Modify the fluid migration mechanism between the cement sheath and the formation, and modify the cement slurry invasion into the formation in a single direction to a dynamic process in which the cement slurry first invades and then retreats; (3) Quantify the volume of invading gas into the volume of cracks, and determine the distribution of cracks after the cement stone solidifies by subdividing the area; (4) Use the method of first partitioning modeling and then adhesion to establish a PFC numerical model to accurately simulate the crack rate and crack distribution of the invaded cement sheath; (5) By improving the KC equation, avoid the ambiguous constant value and accurately calculate the cement sheath permeability increment. 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 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 the 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 various areas 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-bearing formation comprises:

[0049] (1) Establish Figure 2 The cementing model of the hydrate-bearing formation shown includes: the submarine 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-flush fluid).

[0050] (2) Grid division and monitoring points were established. The internal space of the casing was designated as cell A0000, the casing as cell A0001, and the annulus between the casing and the formation as cell A0002. Two grid division methods, equal spacing and geometric ratio, were used within the formation, resulting in a dense grid near the wellbore wall and a gradually sparse grid far from the wellbore wall. A total of 113 cells, A0003-A0115, were used. Within a 2% distance from the wellbore wall, monitoring points were evenly spaced at 5mm intervals; the remaining monitoring points were non-uniformly spaced in a geometric ratio, extending to the end of the model. A monitoring surface was established at the interface between the two to track the flow and heat exchange between the cementing annulus and the formation.

[0051] 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, the 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 item 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: ρ 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 using the kinetic model, the mass flow rate of the hydrate components when 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 ——is the relative permeability of the 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 ——The extraction 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), it can be achieved by using the finite volume integral difference method:

[0082] Volume integral

[0083]

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

[0085] Surface integral

[0086]

[0087] Where: Fnm is the normal 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 using the 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 were applied to the model. The initial setting time of commonly used deepwater oil and gas cement is 3-4 hours, and the time between mixing the cement slurry on the drilling platform, pumping it into the target formation, and leaving a safety period of approximately 2 hours was used. Based on this, the impact of the cement slurry on the formation during the first 3600 seconds of initial setting was investigated.

[0090] (4) Based on the method of patent CN112855075B, the initiation of back-invasion is determined by theoretical analysis, and the components of the back-invasion fluid are determined as follows: methane gas, decomposed water, and back-invasion cement slurry. The volume of fluid migrating from the formation to the annulus is determined by monitoring points set at the interface between the formation and the cement sheath.

[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 reverse-invasion fluid can be obtained by using the ratio of flow rates, and then the volume of the reverse-invasion annulus 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, and 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 counter-invasion gas is difficult to leave the cement sheath and will eventually transform into cracks. The conversion volume is calculated 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. These parts are named as regions I-X in order of their distance from the two interfaces from small to large. Monitoring cells are set in each of them. The above steps are repeated to obtain the three-phase fluid intrusion volume and crack rate in each region 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] ②Use 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 and obtain its stress-strain curve, such as Figure 6 shown.

[0104] The force-displacement basic equation of the PFC linear parallel bond 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 key 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 (the line passing through 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 torsion-rotation increment of Δθ

[0122]

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

[0124] (7) The Körzeny-Kármán equation (hereinafter referred to as the KC equation) is used to calculate the incremental permeability of each zone and the overall cement sheath. The permeability of each zone is calculated using the crack ratio of each zone, and the incremental permeability of the overall cement sheath is calculated using the average crack ratio. In order to avoid the value problem of the Körzeny 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 deepwater surface hydrate-bearing formations includes the following:

[0129] Content 1: Establish a wellbore structural model of the natural gas hydrate-bearing formation and monitor the fluid migration and physical property changes in the formation and cementing annulus;

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

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

[0132] Content 4: Establish a PFC particle flow model for cement paste by partitioning 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 Part 1, the cementing annulus was divided into n equal parts based on accuracy and calculation requirements. Monitoring points were then established in each part to ensure full coverage of the model. Monitoring points were spaced 5 mm apart in the first 2% of the model, while those in the last 98% were distributed evenly and in a geometrically distributed manner. A detection surface was also set at the cementing interface to monitor fluid and heat transfer between the cement sheath and the formation. Using a zoning method, the distribution of fractures created by the back-invasion fluid was quantified, and the fracture rate at different locations in different regions was 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, and 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 was divided into 10 equal sections, and the steps in Section 2 were followed for each section to determine the volume and position distribution of the three-phase intruding fluid within the annulus. Specifically, the gas saturation information fed back from each monitoring point within the cement sheath was used to determine the position and volume of the intruding gas, thereby obtaining the position information of the three-phase fluid. Furthermore, the fracture ratio formula (2) was used to convert the intruding gas volume into the newly added fracture ratio volume, and the fracture ratio of the cement paste after setting was 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 The effective particle diameter of cement paste, usually calculated from the fineness or specific surface area of ​​the cement. The porosity of ordinary Portland cement after one day of consolidation is usually 20%.

[0144] In Section 4, the established PFC particle flow model for cement paste was divided into ten equal regions, each assigned a different crack ratio. Specifically, each of the ten regions with different crack ratios was established, and then the regions were bonded together using a parallel bonding model to form an accurate model of the impacted cement paste. A constant pressure rate was applied to the model to simulate a uniaxial compression test, resulting in a stress-strain curve for the 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] Where 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 the cement stone area. When φ takes the average value of the cement ring, ΔK r is the overall permeability increment of cement stone.

[0148] Furthermore, when the cement stone area is divided into sufficiently fine areas, 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-described embodiments merely represent specific implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A method for evaluating the cementing quality of deepwater surface hydrate-containing formations, characterized in that: The following steps are involved: Step 1: establishing a wellbore structure model of a hydrate-bearing formation, wherein the wellbore structure model includes a submarine hydrate-bearing formation and a casing, and the annulus between the submarine hydrate-bearing 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-bearing formation; Step 4: Determine the volume of fluid that migrates from the formation to the annulus by setting monitoring points at the interface between the formation and the cement sheath, and simultaneously determine the volume of each component of the anti-invasion fluid that invades the annulus using a flow rate formula; Step 5: Correct the migration direction of the fluid in the cementing annulus and the wellbore structure model of the hydrate-bearing formation, convert the input cementing slurry into the output anti-invasion fluid, and refine the migration of the fluid in the wellbore structure model of the hydrate-bearing formation; 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 using the following formula: ； ； in, is the capillary porosity; is the average diameter of cement particles; is the effective particle diameter of cement paste; Step 6: Determine the crack distribution and crack ratio of the cement sheath; Step 7: Establish a zoned PFC numerical model to calculate the compressive strength of the cement sheath. Simultaneously, use the improved Közeny-Kármán equation (KC equation) to calculate the permeability increment of each zone and the entire cement sheath, thereby achieving a comprehensive evaluation of the cementing quality of the hydrate-bearing formation. In step 7, a partitioned PFC numerical model is established to calculate the compressive strength of the cement sheath as follows: A discrete element model of multiple regions was established, assigning different crack rates to each region. A parallel bonding model was then used to bond the regions together to form an integrated model of the invaded cement paste. A constant rate of pressure was applied to the invaded cement paste model to simulate a uniaxial compression test and obtain the cement paste stress-strain curve. In step 7, the improved Korzeny-Karman equation is as follows: ； Where ΔK r is the crack rate growth rate, is the crack rate; when the crack rate When taking the value of a certain area of ​​cement sheath, ΔK r is the permeability increment of cement stone area, when When taking the average value of cement sheath, 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, realizing the evaluation of the sealing and bonding performance of the cement sheath.

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: Based on accuracy requirements and calculation needs, the cement slurry in the cementing annulus is divided into n equal parts, and monitoring points are set up in each part 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. A monitoring surface is set up at the cementing interface to monitor the fluid and heat transfer between the cement sheath and the formation. The distribution of fractures formed by the anti-invasion fluid is quantified through a zoning method, and the fracture ratio at different locations in different areas is determined. The fracture ratio at the interface is obtained by gridding the cement sheath.

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 reverse invasion fluid includes methane gas, decomposed water, and reverse 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: ； in, It is traffic; is viscosity; is a coefficient related to the pressure difference, is the number of anti-invasion fluid components; ; is an index related to fluid properties, T is temperature, and T0 is the initial temperature.

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