A design method for critical seepage parameters of reservoir cement sheath

Abstract

The present invention discloses a method for designing critical seepage parameters of a reservoir cementing sheath. The method comprises: (S100) obtaining basic parameters of the reservoir cementing sheath, formation, wellbore operating conditions, and physical and chemical properties of underground gas; (S200) analyzing the seepage pattern of gas in the reservoir cementing sheath; (S300) assessing whether gas seepage will occur in the cement sheath, and analyzing the seepage length based on a calculation model for the length of gas seepage along the cement sheath; (S400) conducting a sensitivity analysis based on the established seepage length calculation model to obtain the main control parameters for gas seepage along the cement sheath; (S500) analyzing the influence of the main control parameters on gas seepage, obtaining the critical seepage parameters of the reservoir cementing sheath, and establishing a corresponding chart. Based on this method, the present invention establishes a critical parameter chart for when gas begins to seep along the cement sheath and based on the effective isolation section length of the cement sheath, providing guidance for reservoir cementing sheath parameter design and operating condition optimization.

Description

Technical Field

[0001] The invention belongs to the field of petroleum engineering cementing, and in particular relates to a method for designing critical seepage parameters of a reservoir cementing cement ring. Background Art

[0002] A reservoir is a gas reservoir where surface natural gas or other gases are re-injected underground and can be sealed for a long time. There are four main types of underground reservoirs: depleted oil and gas reservoirs, aquifers, rock caverns, and mine and cavern underground gas storage. They are mainly used to solve problems such as peak load regulation and emergency safety gas supply. They can optimize pipeline operation, improve economic efficiency, and be used for strategic reserves. Generally, a reservoir requires no gas leakage during its service life. The cement ring is located in the annular space between the casing and the formation and is a key barrier to ensure the long-term sealing of the reservoir. During the operation of the reservoir, the changing pressure load in the wellbore can easily cause damage and failure of the cement ring, and form potential gas seepage channels in the cementing annulus (the annular space between the casing and the formation), causing underground gas leakage.

[0003] In the early years, domestic and international researchers focused on the damage and failure of the cement sheath itself to address the long-term sealing issues of reservoir cementing. By conducting indoor cement sheath load tests simulating a downhole casing-cement sheath-formation structure, they evaluated whether the cement sheath itself could meet the requirements of actual reservoir operating conditions. They demonstrated that the cement sheath can fail under reservoir injection and production loads, including axial failure, tensile failure, and fatigue damage. Later, researchers discovered that due to the varying deformation parameters of the casing, cement sheath, and formation, the casing, cement sheath, and formation deform inconsistently under operating loads. During reservoir operation, delamination can occur at the casing-cement sheath or cement sheath-formation interface, generating microannular gaps during unloading. Other researchers have experimentally discovered that fluid loads acting at the cement sheath interface can also lead to the continued propagation of interfacial cracks and fissures, resulting in fluid channeling along the interface. To address this, researchers have proposed technical measures to improve the long-term sealing capability of the cement sheath in reservoir cementing by improving performance parameters such as compressive strength, tensile strength, and elastic modulus.

[0004] It can be found that the current research conducted by scholars focuses on the damage and failure of the reservoir cement sheath and its interface. However, as a typical porous medium, the cement sheath in the initial state (no mechanical damage) may also experience underground gas slowly seeping along the naturally existing interconnected micropores and microcracks inside the cement sheath during its service life, leading to reservoir failure. Although the existing standard (SY / T 7648-2021) has proposed a reservoir cement sheath permeability index, the cement sheath permeability is affected by multiple factors such as displacement pressure, confining pressure, and gas medium. Correspondingly, conditions such as reservoir well depth and underground pressure will also affect the threshold of reservoir gas seepage along the cement sheath. The current standard's single and constant cement sheath permeability index is difficult to meet the sealing requirements of underground reservoirs.

[0005] Therefore, to address the above difficulties, it is necessary to establish a targeted design method for the critical seepage parameters of the reservoir cementing sheath to determine the main controlling factors and critical indicators of gas seepage along the cement sheath during the service life of the reservoir, so as to reduce the risk of gas seepage along the cement sheath and causing reservoir sealing failure. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for designing critical seepage parameters of a reservoir cement sheath. Based on this method, a critical parameter chart can be established for when gas begins to seep along the cement sheath and based on the effective isolation section length of the cement sheath (caprock thickness), providing guidance for the design of reservoir cement sheath parameters and the optimization of operating conditions.

[0007] In order to achieve the above object, the present invention provides a method for designing critical seepage parameters of a reservoir cement sheath, the method comprising:

[0008] (S100) obtaining reservoir cementing cement sheath, basic formation parameters, wellbore working conditions, and physical and chemical properties of underground gas;

[0009] (S200) analyzing the seepage pattern of gas in the cement sheath of the reservoir cementing well;

[0010] (S300) It is judged whether gas seepage occurs in the cement sheath, and whether there is a gas storage pressure p at a certain time point under the service condition of the reservoir cementing cement sheath. e Exceeds the capillary force p inside the cement sheath f and the water column pressure p in the connected pores g If it exists, it is judged that there is a risk of gas seepage along the cement sheath; in the case of seepage risk, the length of gas seepage along the cement sheath is analyzed based on the calculation model of the length of gas seepage along the cement sheath;

[0011] The calculation model of the length of gas seepage along the cement sheath is:

[0012]

[0013] In formula (13), L is the seepage length of gas along the cement sheath; K a is the apparent permeability; μ is the gas viscosity; T sc is the temperature of the gas under standard conditions; Z sc is the gas compression factor under standard conditions; T is temperature; Z is the gas compression factor; p sc is the standard pressure; p e is the gas storage pressure; p w is the outlet pressure; t is the gas storage time;

[0014] p w =p f +p g (14)

[0015] In formula (14), p f is the internal resistance or capillary force of the cement sheath; p g is the water column pressure in the connected pores;

[0016] (S400) performing a sensitivity analysis based on the established seepage length calculation model to obtain the main control parameters of gas seepage along the cement sheath. This is done by changing the parameters in the seepage length calculation model that may affect the results, analyzing the degree of influence of each parameter on the results, and then determining the weight of each parameter to obtain the main control parameters of gas seepage along the cement sheath.

[0017] (S500) After determining the degree of influence of each parameter on the seepage length, the value of one of the main control parameters is changed, while the values of other parameters remain unchanged, and the influence of the main control parameters on the gas seepage is analyzed to obtain a critical seepage parameter chart of the gas in the cement ring of the reservoir cementing based on the thickness of the cap rock; wherein, the critical seepage parameter of the gas in the cement ring of the reservoir cementing is the value of each main control parameter corresponding to when the gas begins to seep along the cement ring at a specified gas storage time node.

[0018] Preferably, in step (S100), the basic parameters of the reservoir cementing cement sheath include: cement sheath porosity, average pore radius, and internal pore pressure; wherein the average pore radius is the average pore radius of the connected pores of the cement sheath.

[0019] Preferably, in step (S100), the basic formation parameters include: confining pressure and initial formation pressure.

[0020] Preferably, in step (S100), the wellbore operating conditions include: downhole temperature, well depth, and gas storage pressure.

[0021] Preferably, in step (S100), the physical and chemical properties of the gas include: gas viscosity, gas molar mass, and gas molecular diameter.

[0022] Preferably, in step (S200), the seepage form of the gas is calculated based on a Knudsen number calculation model, the Knudsen number under different conditions is calculated, and the seepage form of the gas in the cement sheath is clarified;

[0023] The Knudsen number calculation model is:

[0024]

[0025] In formula (1), K n is the Knudsen number; λ is the molecular mean free path; r is the average pore radius of the connected pores in the cement sheath.

[0026] The λ is expressed as:

[0027]

[0028] In formula (2), k B is the Boltzmann constant; T is the temperature; P is the pore pressure inside the cement sheath; d is the diameter of the gas molecule.

[0029] Preferably, the different conditions are different gas molecule diameters, pore pressures inside cement sheaths, and average pore radii; and the seepage forms include Darcy flow, slip flow, transition flow, and Knudsen diffusion.

[0030] Preferably, in step (S300), the gas seepage length calculation model along the cement sheath is based on the Beskok-Karniadakis permeability calculation model, while taking into account the influence of external load conditions on the porosity and average pore radius of the cement sheath, and introducing the time dimension to derive the seepage length calculation model through the assumption of planar unidirectional flow;

[0031] The Beskok-Karniadakis permeability calculation model is:

[0032]

[0033] In formula (3), K a is the apparent permeability; α is the ideal gas rarefaction coefficient; K ∞ is the absolute permeability; b is the slip coefficient; K n is the Knudsen number;

[0034] Among them, α and K ∞ Expressed as:

[0035]

[0036] In formula (4), is the porosity of the cement sheath; τ is the tortuosity; r is the average pore radius of the connected pores of the cement sheath;

[0037] Where τ is expressed as:

[0038]

[0039] In formula (5), is the porosity of the cement sheath.

[0040] Considering the influence of external load conditions on the porosity and average pore radius of cement sheath, the calculation method of cement sheath dynamic porosity and dynamic average pore radius is used. The calculation method of cement sheath dynamic porosity and dynamic average pore radius is expressed as:

[0041]

[0042] In formula (6), is the dynamic porosity of cement sheath; r in is the dynamic radius of the cement sheath; is the initial porosity of the cement sheath; r ini is the initial average pore radius of the cement sheath; c int is the compressibility of the nanopore; σ ini is the initial ground stress; σ in is the ground stress; p ini is the initial pore pressure inside the cement sheath; p in is the pore pressure inside the cement sheath.

[0043] The plane unidirectional flow assumes that the gas flows along the axial direction of the cylindrical porous medium; the introduction of the time dimension is a seepage length model that can consider the time parameter and is established by referring to the relationship between length, velocity and time.

[0044] Preferably, in step (S400), under the premise that other parameters remain unchanged, the value of a certain parameter is increased or decreased by 20% from the original value. After all parameter values that may affect the results are increased or decreased by 20%, the seepage length values calculated for each parameter are compared to determine the degree of influence of each parameter on the seepage length value, and obtain the main control parameters for gas seepage along the cement sheath.

[0045] Preferably, in step (S500), the critical seepage parameter chart of the cement sheath of the reservoir cementing is based on the thickness of the cap rock, and clarifies the critical permeability and porosity of the cement sheath for ensuring gas sealing under different gas storage pressures. When the porosity and permeability of the cement sheath under different gas storage pressures are within a safe range, the gas sealing of the cement sheath of the reservoir cementing can be guaranteed; wherein, the cap rock is a protective layer located above the reservoir layer that can isolate the reservoir layer and prevent the gas therein from escaping upward; and the safe range is an area where the length of gas seepage is below the top of the cap rock, which can ensure that the gas does not overflow upward.

[0046] The critical seepage parameter design method of the reservoir cement sheath of the present invention has the following advantages:

[0047] (1) The present invention establishes a new method for designing critical seepage parameters of cement sheaths for reservoir cementing. Based on this method, a critical parameter chart can be established for the start of gas seepage along the cement sheath and the effective isolation section length of the cement sheath (caprock thickness), providing guidance for the design of cement sheath parameters and optimization of working conditions for reservoir cementing.

[0048] (2) The present invention takes into account the seepage of cement sheath. During the service life of the reservoir, the cement sheath may leak gas not only after the cement sheath body and interface are damaged and fail, but also in the initial state (no mechanical damage to the cement sheath), the formation gas may also seep slowly through the naturally connected micropores and microcracks in the cement sheath.

[0049] (3) The present invention takes the time dimension into consideration. This model can determine the length of gas seepage along the cement sheath at a certain time point under actual reservoir conditions, providing a theoretical reference for predicting the remaining service life of the cement sheath (assuming that the gas seepage length exceeds the caprock thickness to reach the ultimate service life).

[0050] (4) The present invention quantifies the critical seepage parameter chart based on the time when gas begins to seep along the cement sheath and the thickness of the caprock. The current standard (SY / T 7648-2021) requires the permeability of the cement sheath to be constant and single, without considering the impact of the complex downhole environment on the permeability of the cement sheath. The chart drawn by the present invention takes into account factors such as actual working conditions, permeability, and well depth. By quantifying the chart, the porosity, permeability, and gas storage pressure of the cement sheath can be scientifically controlled to reduce the risk of gas seepage along the cement sheath of the reservoir cementing. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 This is a schematic diagram of gas seepage along the cement sheath based on the assumption of planar unidirectional flow in an application example of the present invention.

[0052] Figure 2 This is a diagram showing the seepage pattern of helium in the cement ring of a well cementing plant in an application example of the present invention.

[0053] Figure 3 This is the result of sensitivity analysis of the seepage length of helium seepage along the cement sheath in the application example of the present invention.

[0054] Figure 4 The following are the effects of the main control parameters of the application examples of the present invention on the seepage length of helium in the cement ring: (a) the effect of gas storage pressure on the seepage length of helium in the cement ring of cementing well; (b) the effect of different pore sizes on the seepage length of helium in the cement ring of cementing well; (c) the effect of different porosities on the seepage length of helium in the cement ring of cementing well; (d) the effect of cement ring permeability on the seepage length of helium in the cement ring of cementing well.

[0055] Figure 5 This is a chart showing the critical seepage parameters of helium in the cement ring of a reservoir cementing system according to an application example of the present invention. DETAILED DESCRIPTION

[0056] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0057] A method for designing critical seepage parameters of a reservoir cement sheath is developed to determine the critical parameters of gas seepage along the cement sheath without mechanical damage, providing a theoretical reference for extending the service life of the reservoir. The method includes the following steps:

[0058] (S100) obtaining reservoir cementing cement sheath, basic formation parameters, wellbore working conditions, and physical and chemical properties of underground gas;

[0059] (S200) analyzing the seepage pattern of gas in the cement sheath of the reservoir cementing well;

[0060] (S300) determining whether gas seepage occurs in the cement sheath, and analyzing the gas seepage length along the cement sheath based on a calculation model for the length of gas seepage along the cement sheath;

[0061] (S400) performing a sensitivity analysis based on the established seepage length calculation model to obtain the main control parameters of gas seepage along the cement sheath;

[0062] (S500) Analyze the influence of the main control parameters on the gas seepage, and obtain a critical gas seepage parameter plate of the reservoir cement sheath based on the caprock thickness.

[0063] In step (S100), basic reservoir cement sheath parameters include: cement sheath porosity, average pore radius, and pore pressure within the cement sheath. The average pore radius refers to the average pore radius of the connected pores within the cement sheath. Basic formation parameters include: confining pressure, initial formation pressure, etc. Wellbore operating conditions include: downhole temperature, well depth, and gas storage pressure. Gas physical and chemical properties include: gas viscosity, gas molar mass, gas molecular diameter, and other parameters.

[0064] In step (S200), the gas flow pattern in the cement sheath is determined by calculating the Knudsen number under different conditions based on a Knudsen number calculation model. These conditions include differences in gas molecular diameter, pore pressure within the cement sheath, and average pore radius. Flow patterns include Darcy flow, slip flow, transitional flow, and Knudsen diffusion.

[0065] The above Knudsen number calculation model is:

[0066]

[0067] In formula (1), K n is the Knudsen number, dimensionless; λ is the molecular mean free path, unit is m; r is the average pore radius of the connected pores of the cement sheath, unit is m.

[0068] The above λ can be expressed as:

[0069]

[0070] In formula (2), k B is the Boltzmann constant, in J / K, which is 1.3805×10 -23 J / K; T is temperature, unit K, which is 293.15K; P is the pore pressure inside the cement sheath, unit Pa; d is the diameter of the gas molecule, unit m.

[0071] In step (S300), the calculation model for the length of gas seepage along the cement sheath is based on the Beskok-Karniadakis permeability calculation model, while taking into account the influence of external load conditions such as pore pressure on the porosity and average pore radius of the cement sheath. The seepage length calculation model is derived by introducing the time dimension through the assumption of planar unidirectional flow.

[0072] The above Beskok-Karniadakis permeability calculation model is:

[0073]

[0074] In formula (3), K a is the apparent permeability, unit is m 2 ; α is the ideal gas rarefaction coefficient, dimensionless; K ∞ is the absolute permeability, unit is m 2 ; b is the slip coefficient, dimensionless, and is taken as -1; K n is the Knudsen number.

[0075] The above α and K ∞ Expressed as:

[0076]

[0077] In formula (4), is the porosity of the cement sheath, dimensionless; τ is the tortuosity, dimensionless; r is the average pore radius of the connected pores of the cement sheath;

[0078] The above τ is expressed as:

[0079]

[0080] In formula (5), is the porosity of the cement sheath, dimensionless.

[0081] The above consideration of the influence of external load conditions such as the pore pressure inside the cement sheath on the porosity and average pore radius of the cement sheath is based on the calculation method of the dynamic porosity and dynamic average pore radius of the cement sheath. The calculation method of the dynamic porosity and dynamic average pore radius of the cement sheath can be expressed as:

[0082]

[0083] In formula (6), is the dynamic porosity of the cement sheath, dimensionless; r in is the dynamic radius of the cement sheath, in m; is the initial porosity of the cement sheath; r ini is the initial average pore radius of the cement sheath; c int is the compressibility of the nanopore, in Pa -1 , take 30000Pa -1 ; σ ini is the initial ground stress, unit is Pa; σ in is the ground stress, unit is Pa; p ini is the initial pore pressure inside the cement sheath, unit: Pa; p in is the pore pressure inside the cement sheath, unit: Pa.

[0084] The above-mentioned planar unidirectional flow assumes that the gas flows along the axial direction of the cylindrical porous medium; the above-mentioned introduction of the time dimension is a seepage length model that can consider the time parameter based on the relationship between reference length, velocity and time.

[0085] The calculation model for the seepage length of the above-mentioned gas seepage along the cement sheath is derived as follows:

[0086] 1) The seepage velocity v can be expressed as:

[0087]

[0088] In formula (7), x is the vertical distance from a point inside the cylinder to the bottom, in meters; p is the gas pressure at a point inside the cylinder, in MPa; μ is the gas viscosity, in Pa·s.

[0089] 2) Volume flow rate q is:

[0090]

[0091] In formula (8), A is the cross-sectional area of the cylinder, unit is m 2 .

[0092] 3) Mass flow rate q m for:

[0093]

[0094] In formula (9), ρ g is the gas density, kg / m 3 , T sc is the temperature of the gas under standard conditions, in K, which is 273.15K; Z sc is the gas compressibility factor under standard conditions, dimensionless, and is taken as 1; ρ gsc is the gas density under standard conditions, kg / m 3 ;p sc is the standard pressure, unit Pa, take 10 5 Pa; T is temperature, unit K, which is 293.15K; Z is the gas compressibility factor, dimensionless, and is taken as 1.

[0095] 4) Separate variables and integrate:

[0096]

[0097] In formula (10), p e is the gas storage pressure, unit is Pa; p w is the outlet pressure, unit is Pa; L is the seepage length of gas along the cement sheath, unit is m.

[0098] 5) The volume flow rate under standard conditions is:

[0099]

[0100] 6) The seepage velocity can be expressed as:

[0101]

[0102] 7) The seepage length can be expressed as:

[0103]

[0104] in,

[0105] In formula (13), Z is the gas compression factor, dimensionless, and is set to 1; t is the gas storage time, in months.

[0106] The above p w Expressed as:

[0107] p w =p f +p g (14)

[0108] In formula (14), p f is the internal resistance of the cement sheath (capillary force), unit is Pa; p g It is the water column pressure in the connected pores, unit is Pa.

[0109] The above p f and p g It can be expressed as:

[0110]

[0111] Where ρ is the density of water in the connected pores, unit: kg / m 3 , take 1kg / m 3 ; H is the well depth, unit is m; L is the gas seepage length, unit is m.

[0112] In step (S300), it is judged whether gas seepage will occur, and the seepage length calculation model is used to analyze whether there is a gas storage pressure p at a certain time node under the service condition of the reservoir cement sheath. e Exceeds the capillary force p inside the cement sheath f The sum of the pressure of the water column in the connected pores. If it exists, it is judged that there is a risk of gas seepage along the cement sheath.

[0113] In step (S400), a sensitivity analysis is conducted to obtain the main controlling parameters for gas seepage along the cement sheath. This is done by varying parameters in the seepage length calculation model that may affect the results, such as the average pore radius of the cement sheath connected pores, the cement sheath porosity, the permeability, and the gas storage pressure. The degree of influence of each parameter on the results is analyzed, and then the weight of each parameter is determined to obtain the main controlling parameters. The specific method is as follows: while other parameters remain unchanged, the value of a parameter is increased or decreased by 20% from the original value. After increasing or decreasing the values of all parameters that may affect the results by 20%, the calculated seepage length values for each parameter are compared. This determines the degree of influence of each parameter on the seepage length value and obtains the main controlling parameters for gas seepage along the cement sheath.

[0114] In step (S500), the influence of the main control parameters on the gas seepage is analyzed after determining the degree of influence of each parameter on the seepage length. Specifically, the values of other parameters remain unchanged, the value of one of the main control parameters is changed, and the law of change of the seepage length value with the main control parameter value is analyzed.

[0115] The above-mentioned changing rules may be that the seepage length value and the main control parameter value are positively correlated, negatively correlated, first positively correlated and then negatively correlated, first negatively correlated and then positively correlated, first rapidly changed and then slowly changed, and first slowly changed and then rapidly changed.

[0116] In step (S500), the critical seepage parameters of the reservoir cement sheath are the values of the main control parameters corresponding to when the gas begins to seep along the cement sheath at a specified gas storage time node, providing certain data support for reducing reservoir sealing failure.

[0117] The above-mentioned gas storage time node is in units of weeks, months, years, etc.

[0118] The above-mentioned chart of critical gas seepage parameters of the reservoir cementing cement sheath is based on the thickness of the caprock, and clarifies the critical permeability and porosity of the cement sheath to ensure gas sealing under different gas storage pressures. When the porosity and permeability of the cement sheath under different gas storage pressures are within the safe range, the gas sealing of the reservoir cementing cement sheath can be guaranteed.

[0119] The cap rock is a protective layer located above the reservoir that can seal the reservoir and prevent the gas therein from escaping upward. The safety zone is the area where the length of gas seepage is below the top of the cap rock, ensuring that gas does not escape upward.

[0120] Application Examples

[0121] The method of the present invention is used to design the critical seepage parameters of underground gas along the cement sheath of a reservoir cementing well based on the actual working conditions of a reservoir in operation and the basic performance of the cement sheath in use. The specific steps are as follows:

[0122] A salt cavity reservoir with a well depth of 1000m. The operating pressure range during the injection and production process is 17-7MPa, the emergency gas production pressure is 6MPa, the salt cavity temperature is 53℃ at the end of the injection period, and the temperature at the end of the gas production period is 35℃. The gas storage time is 240 months, and the cap rock thickness is 50m.

[0123] In the application, the maximum gas storage pressure of 23 MPa and the maximum salt chamber temperature of 53°C are selected as the basic parameter values.

[0124] The confining pressure of the formation in the application is 7-11 MPa, and the initial formation pressure is 7.4 MPa.

[0125] The compressive strength of the cement sheath in use is 32.8 MPa, the tensile strength is 2.65 MPa, the elastic modulus is 5.55 GPa, the Poisson's ratio is 0.13, the cohesion is 6.02 MPa, the internal friction angle is 34.5°, the porosity is 8.5%, the average radius of the connected pores is 15 nm, and the pore pressure inside the cement sheath is 4.2 MPa.

[0126] In this application, the underground gas is helium, which has a viscosity of 2.01×10 -5 Pa·s, molar mass is 0.004 kg / mol, and molecular diameter is 2.6×10 -10 m.

[0127] It should be noted that the subsequent sensitivity analysis of gas seepage along the cement sheath of the reservoir cementing well is conducted based on the above-mentioned parameter values. The sensitivity analysis is only used to determine the degree of influence of each parameter on the gas seepage length along the cement sheath of the reservoir cementing well, and is not limited to the operating conditions of the operating reservoir and the basic parameters of the cement sheath in use. Further explanation: using the maximum gas storage pressure of the reservoir as the basic parameter value in the calculation method of the present invention, when conducting a sensitivity analysis, increasing the gas storage pressure by 20% will result in a gas storage pressure higher than the maximum gas storage condition at the reservoir site. Using this as an example, the same meaning applies to other parameters.

[0128] like Figure 1 As shown in FIG, a schematic diagram of gas seepage along the cement sheath in an application example is shown, assuming that gas seepage occurs only along the axial direction of the cement sheath.

[0129] like Figure 2 As shown in the figure, the helium seepage form in the cement sheath of the cement well in the application example is shown. There are three possible seepage forms of helium in the cement sheath: Knudsen diffusion, transitional flow and slip flow. Since the average pore radius of the connecting pores of the cement sheath is generally above 10 nm, it can be judged that the helium seepage in the cement sheath may be transitional flow, slip flow or transitional flow and slip flow. In the application example, the average pore radius of the connecting pores of the cement sheath is 15 nm, and the pore pressure inside the cement sheath is 4.2 MPa. Therefore, it is judged that the seepage form of helium in the cement sheath of the cement well is transitional flow.

[0130] like Figure 3 Figure 2 shows the sensitivity analysis results of helium seepage length along the cement sheath in the application example. The main controlling parameters of helium seepage length along the cement sheath of reservoir cementing are helium storage pressure, average pore radius of connected pores in the cement sheath, cement sheath porosity, helium storage time, and permeability. The degree of influence of each main controlling parameter on helium seepage is helium storage pressure > average pore radius of connected pores in the cement sheath > cement sheath porosity > helium storage time > permeability.

[0131] like Figure 4 As shown in the figure, the influence of various main control parameters on the helium seepage in the cement sheath in the application example is shown. The influence of the main control parameters on the gas seepage is as follows: the seepage length of the gas along the cement sheath of the reservoir cementing increases with the increase of the helium storage pressure; the larger the average pore radius and porosity of the cement sheath of the reservoir cementing, the longer the gas seepage length; the seepage length of the gas along the cement sheath of the reservoir cementing increases with time in a power function, first increasing rapidly and then increasing slowly; the greater the permeability of the gas in the cement sheath, the greater the degree of gas seepage along the cement sheath.

[0132] According to the application example Figure 4The influence of the main control parameters described above on the gas seepage along the cement sheath is used to determine the critical parameters for gas to begin to seep along the cement sheath of the reservoir cementing well, taking the gas storage time of 240 months as the node. The critical parameters can be shown in Table 1:

[0133] Table 1 Critical seepage parameters of reservoir cementing sheath

[0134]

[0135] To further explain the critical parameter values calculated in this embodiment, under the current cement sheath performance of a certain reservoir, the value of one of the main control parameters is changed, while the values of the other main control parameters remain unchanged. With the gas storage time of 240 months as the node, if the seepage length is greater than 0, gas seepage begins. Furthermore, if the helium storage pressure is lower than 17.8 MPa, it will be lower than the sum of the capillary force in the cement sheath and the pressure of the saturated water column, preventing gas from seeping along the cement sheath. Furthermore, under the maximum helium storage pressure of 23 MPa, if the average pore radius of the cement sheath reaches 9 nm or the permeability reaches 1×10 -5 mD, the gas will not seep.

[0136] like Figure 5 As shown in the figure, the critical helium seepage parameters of the reservoir cement ring in the application example are different. Different gas storage pressures correspond to different critical permeabilities and porosities of the cement ring. Based on the caprock thickness, there is a safety interval at the time point when the reservoir cement ring has been in service for 240 months. When the porosity and permeability of the cement ring at different gas storage pressures are within the safety interval, the long-term gas sealing of the reservoir cement ring can be guaranteed.

[0137] Although the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description is not intended to limit the present invention. After reading the above description, various modifications and substitutions of the present invention will become apparent to those skilled in the art. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method for designing critical seepage parameters of reservoir cementing sheath, characterized in that: The method includes: (S100) obtaining reservoir cementing cement sheath, basic formation parameters, wellbore working conditions, and physical and chemical properties of underground gas; (S200) analyzing the seepage pattern of gas in the cement sheath of the reservoir cementing well; (S300) It is judged whether gas seepage occurs in the cement sheath, and whether there is a gas storage pressure p at a certain time point under the service condition of the reservoir cementing cement sheath. e Exceeds the capillary force p inside the cement sheath f and the water column pressure p in the connected pores g If it exists, it is judged that there is a risk of gas seepage along the cement sheath; in the case of seepage risk, the length of gas seepage along the cement sheath is analyzed based on the calculation model of the length of gas seepage along the cement sheath; The calculation model of the length of gas seepage along the cement sheath is: In formula (13), L is the seepage length of gas along the cement sheath; K a is the apparent permeability; μ is the gas viscosity; T sc is the temperature of the gas under standard conditions; Z sc is the gas compression factor under standard conditions; T is temperature; Z is the gas compression factor; p sc is the standard pressure; p e is the gas storage pressure; p w is the outlet pressure; t is the gas storage time; p w =p f +p g (14) In formula (14), p f is the internal resistance or capillary force of the cement sheath; p g The water column pressure in the pores connected by the cement sheath; (S400) performing a sensitivity analysis based on the established seepage length calculation model to obtain the main control parameters of gas seepage along the cement sheath. This is done by changing the parameters in the seepage length calculation model that may affect the results, analyzing the degree of influence of each parameter on the results, and then determining the weight of each parameter to obtain the main control parameters of gas seepage along the cement sheath. (S500) After determining the degree of influence of each parameter on the seepage length, changing the value of one of the main control parameters while keeping the values of other parameters unchanged, analyzing the influence of the main control parameters on the gas seepage, and obtaining a critical seepage parameter plate of the gas in the cement sheath of the reservoir cementing based on the caprock thickness; Among them, the critical seepage parameters of gas in the cement sheath of the reservoir cementing are the values of the main control parameters corresponding to when the gas begins to seep along the cement sheath at a specified gas storage time node.

2. The design method according to claim 1, characterized in that: In step (S100), the basic parameters of the reservoir cementing cement sheath include: cement sheath porosity, average pore radius, and internal pore pressure; wherein the average pore radius is the average pore radius of the connected pores of the cement sheath.

3. The design method according to claim 1, characterized in that: In step (S100), the basic formation parameters include: confining pressure and initial formation pressure.

4. The design method according to claim 1, characterized in that: In step (S100), the wellbore operating conditions include: downhole temperature, well depth, and gas storage pressure.

5. The design method according to claim 1, characterized in that: In step (S100), the physical and chemical properties of the gas include: gas viscosity, gas molar mass, and gas molecular diameter.

6. The design method according to claim 1, characterized in that: In step (S200), the gas seepage form is determined based on the Knudsen number calculation model, and the Knudsen number under different conditions is calculated to clarify the gas seepage form in the cement sheath; The Knudsen number calculation model is: In formula (1), K n is the Knudsen number; λ is the molecular mean free path; r is the average pore radius of the cement sheath connected pores; The λ is expressed as: In formula (2), k B is the Boltzmann constant; T is the temperature; P is the pore pressure inside the cement sheath; d is the diameter of the gas molecule.

7. The design method according to claim 6, characterized in that: The different conditions are different gas molecule diameters, pore pressures inside cement sheaths, and average pore radii; and the seepage forms include Darcy flow, slip flow, transition flow, and Knudsen diffusion.

8. The design method according to claim 1, characterized in that: In step (S300), the gas seepage length calculation model along the cement sheath is based on the Beskok-Karniadakis permeability calculation model, while taking into account the influence of external load conditions on the porosity and average pore radius of the cement sheath, and introducing the time dimension through the assumption of planar unidirectional flow to derive the seepage length calculation model; The Beskok-Karniadakis permeability calculation model is: In formula (3), K a is the apparent permeability; α is the ideal gas rarefaction coefficient; K ∞ is the absolute permeability; b is the slip coefficient; K n is the Knudsen number; Among them, α and K ∞ Expressed as: In formula (4), is the porosity of the cement sheath; τ is the tortuosity; r is the average pore radius of the connected pores of the cement sheath; Where τ is expressed as: In formula (5), is the porosity of the cement sheath; Considering the influence of external load conditions on the porosity and average pore radius of cement sheath, the calculation method of cement sheath dynamic porosity and dynamic average pore radius is referenced. The calculation method of cement sheath dynamic porosity and dynamic average pore radius is expressed as: In formula (6), is the dynamic porosity of cement sheath; r in is the dynamic average pore radius of the cement sheath; is the initial porosity of the cement sheath; r ini is the initial average pore radius of the cement sheath; c int is the compressibility of the nanopore; σ ini is the initial ground stress; σ in is the ground stress; p ini is the initial pore pressure inside the cement sheath; p in is the pore pressure inside the cement sheath; The plane unidirectional flow assumes that the gas flows along the axial direction of the cylindrical porous medium; the introduction of the time dimension is a seepage length model that can consider the time parameter and is established by referring to the relationship between length, velocity and time.

9. The design method according to claim 1, characterized in that: In step (S400), under the premise that other parameters remain unchanged, the value of a certain parameter is increased or decreased by 20% from the original value. After all the parameter values that may affect the results are increased or decreased by 20%, the seepage length values calculated by each parameter are compared to determine the degree of influence of each parameter on the seepage length value, and obtain the main control parameters of gas seepage along the cement sheath.

10. The design method according to claim 1, characterized in that: In step (S500), the critical seepage parameter chart of the gas in the cement sheath of the reservoir cementing is based on the thickness of the cap rock, and clarifies the critical permeability and porosity of the cement sheath for ensuring gas sealing under different gas storage pressures. When the porosity and permeability of the cement sheath under different gas storage pressures are within a safe range, the gas sealing of the cement sheath of the reservoir cementing can be guaranteed; wherein, the cap rock is a protective layer located above the reservoir layer that can isolate the reservoir layer and prevent the gas therein from escaping upward; the safe range is an area where the length of gas seepage is below the top of the cap rock, which can ensure that the gas does not overflow upward.

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