A method for designing a displacement capacity of offshore pressure control cementing

Abstract

This invention relates to the field of oil and gas extraction technology and discloses a method for designing injection and displacement rates in offshore pressure-controlled cementing. The method includes: S1, dividing the cementing operation into stages; S2, determining the flow regime of each cementing fluid in each cementing stage given the known rheological parameters of each fluid; S3, calculating the critical velocity and displacement rate of each cementing fluid in the annulus during each stage; S4, calculating the critical velocity and displacement rate of each cementing fluid in the casing during each stage; S5, calculating the frictional pressure drop across the entire flow path during the pump shutdown and plugging stage; S6, calculating the wellbore pressure during the cementing and displacement process; and S7, comparing the wellbore pressure with the formation leakage pressure as a design benchmark to determine if there is a risk of leakage. This offshore pressure-controlled cementing injection and displacement rate design method, based on the rheological properties of fluids, ensures pressure balance during cementing operations while also achieving effective displacement of the drilling fluid.

Description

Technical Field

[0001] This invention relates to the field of oil and gas extraction technology, and in particular to a method for designing injection displacement in offshore pressure controlled cementing. Background Technology

[0002] With advancements in exploration technology, offshore exploration and development has gradually expanded from conventional water depths to deep and ultra-deep water, with deepwater oil and gas fields becoming a major driver of global oil production growth. The cementing quality of narrow pressure window wells in the offshore region, influenced by injection, displacement, and drainage rates, is a significant bottleneck restricting offshore oil and gas development. Pressure-controlled cementing technology, while meeting safety requirements, offers unique advantages in increasing injection, displacement, and drainage rates by reducing annular hydrostatic pressure, making it an effective technical means to improve cementing quality.

[0003] Unlike controlled pressure drilling, controlled pressure cementing involves working fluids within the wellbore, including the original drilling fluid, low-density drilling fluid, flushing fluid, isolation fluid, and cement slurry. These working fluids exhibit significant differences in density and rheological properties, leading to time-varying dynamic equivalent density in the annulus during the displacement process. This necessitates meticulous design of displacement rates at different stages to achieve pressure stabilization, leakage prevention, and improved displacement efficiency. Extensive research and field experience indicate that turbulent flow is the optimal flow pattern for cement slurry displacement, followed by plug flow. While turbulent displacement is commonly used in conventional well cementing to enhance slurry displacement efficiency, a composite displacement technique combining turbulent and plug flow with variable displacement is frequently employed for narrow density windows. Turbulent displacement improves displacement efficiency and ensures cementing quality; plug flow displacement results in very low annular friction, reducing the risk of leakage. Furthermore, plug flow displacement maintains the cement slurry in a flowing state, transmitting annular fluid column pressure to stabilize the gas layer. Once displacement is complete, the cement slurry rapidly solidifies, preventing annular gas channeling.

[0004] The existing publicly available injection-displacement design methods mainly use formation leakage pressure as the design benchmark, divide cementing operations into stages, and control the annular equivalent circulation density of each stage by optimizing the injection-displacement rate in order to reduce leakage. However, the above methods do not consider the limitation of the narrow wellbore safety density window and have not formed a complete and specific injection-displacement design method for offshore pressure-controlled cementing. Summary of the Invention

[0005] This invention provides a method for designing displacement of drilling fluid in offshore controlled pressure cementing. Based on the rheological properties of the fluid, it ensures pressure balance during cementing operations while also achieving effective displacement of the drilling fluid.

[0006] This invention provides a method for designing injection displacement in offshore pressure-controlled cementing, comprising the following steps:

[0007] S1. Divide the cementing construction stages;

[0008] S2. Given the rheological parameters of each cementing fluid, determine the flow regime of each cementing fluid in each stage of cementing construction.

[0009] S3. Calculate the critical flow rate and discharge rate of each cementing fluid in the annulus during the construction stage.

[0010] S4. Calculate the critical flow rate and discharge rate of each cementing fluid in the casing during the construction stage.

[0011] S5. Calculate the frictional pressure drop generated in the entire flow channel during the pump stop and rubber plug pressing stage.

[0012] S6. Calculate the wellbore pressure during the cementing and displacement process;

[0013] S7. Using formation leakage pressure as the design benchmark, compare the wellbore pressure with the formation leakage pressure to determine whether there is a risk of leakage.

[0014] Optionally, step S1 is as follows: based on the formation leakage pressure, the cementing operation is divided into the first stage before the cement slurry exits the casing shoe and the second stage after the cement slurry exits the casing shoe. In the first and second stages, attention should be paid to the injection and displacement design during the pump shutdown and rubber plug pressing stage.

[0015] Optionally, the cementing fluids in step S2 include: drilling fluid, low-density drilling fluid, flushing fluid, isolation fluid, and cement slurry; the flow state of each cementing fluid in each stage of cementing is determined as follows: turbulent-plug flow composite displacement technology is adopted, with turbulent displacement in the first stage and plug flow displacement in the second stage.

[0016] Optionally, the critical velocity of each cementing fluid in the annulus at different construction stages in step S3 is calculated using the critical velocity formulas for annular plug flow and annular turbulent flow, as follows:

[0017]

[0018]

[0019] In the formula, V tf V represents the critical velocity for turbulent flow in the first stage, in m / s. pf The critical velocity for the second-stage slug flow is τ0 (m / s); the dynamic shear force is η0 (Pa); the plastic viscosity is η0 (Pa·s); and the slug force is η0 (Pa·s). w D is the wellbore diameter, in cm. o Re is the outer diameter of the casing, in cm; c ρ is the critical Reynolds number; ρ is the cement fluid density, g / cm³. 3 ;

[0020] The critical discharge rates of various cementing fluids within the annulus at different construction stages are calculated using the following formula:

[0021]

[0022]

[0023] In the formula, Q tf Q represents the critical displacement for the first stage of turbulent flow, in L / s; pf The critical discharge rate for the second-stage slug flow is L / s;

[0024] The critical Reynolds number is calculated by the following formula:

[0025] Re c =3470-1370n

[0026] In the formula, n is the flowability index of cement slurry.

[0027] Optionally, the critical flow rate of each cementing fluid inside the casing at different construction stages in step S4 is determined by the following formula:

[0028]

[0029]

[0030] In the formula, V s The first stage flow velocity in the pipe is in m / s; D i V is the inner diameter of the casing, in cm; w The flow velocity in the pipe during the second stage is m / s;

[0031] The critical discharge rate of each cementing fluid inside the casing at different construction stages is determined by the following formula:

[0032]

[0033]

[0034] In the formula, Q s Q represents the critical discharge rate in the first stage of the pipeline, in L / s; w The critical discharge rate in the second stage is L / s.

[0035] Optionally, in step S5, during the pump shutdown and rubber plug application stage, a combined blowout preventer (BOP) and choke manifold construction method can be used for pressure-controlled cementing. That is, the BOP is shut down before the pump shutdown and rubber plug application. After the pump shutdown and rubber plug application, the cementing fluid enters the choke manifold through the casing annulus and returns. The specific process is as follows: First, the BOP is shut down within 1 minute before the pump shutdown and rubber plug application. The cementing fluid return channel changes from the conventional riser annulus to the choke manifold. Then, during the pump shutdown and rubber plug application, the choke valve is closed. This process of reducing the discharge rate should be completed within 30 to 40 seconds to allow sufficient time for adjusting the wellhead back pressure.

[0036] Optionally, during the pump shutdown and plugging stage in step S5, the frictional pressure drop generated by the choke manifold during the process of the cementing fluid returning from the mudline to sea level is calculated using the following formula:

[0037]

[0038] In the formula, P jl The frictional pressure drop generated by the throttling manifold, MPa; V i ρ is the flow velocity of cementing fluid in the choke manifold, m / s; f is the friction coefficient; L is the water depth, m; D ji The manifold inner diameter is in cm.

[0039] The flow velocity of the cementing fluid in the choke manifold is calculated using the following formula:

[0040]

[0041] The coefficient of friction is calculated by the following formula:

[0042]

[0043] In the formula, Re is the Reynolds number, which is calculated by the following formula:

[0044]

[0045] In step S5, during the pump shutdown and plugging stage, the frictional pressure drop generated by the choke manifold during the return of the cementing fluid from the mudline to sea level is calculated using the following formula:

[0046]

[0047] In the formula, P jl The frictional pressure drop generated by the throttling manifold, MPa; V i ρ is the flow velocity of cementing fluid in the choke manifold, m / s; f is the friction coefficient; L is the water depth, m; D ji The manifold inner diameter is in cm.

[0048] The flow velocity of the cementing fluid in the choke manifold is calculated using the following formula:

[0049]

[0050] The coefficient of friction is calculated by the following formula:

[0051]

[0052] In the formula, Re is the Reynolds number, which is calculated by the following formula:

[0053]

[0054] Optionally, in step S6, the wellbore pressure during the cementing and displacement process consists of the annular friction pressure drop and the annular cementing fluid static pressure, wherein the wellbore pressures in the first and second stages are calculated by the following formulas:

[0055] ΔP p ′=ΔP f +ΔP h ′

[0056] ΔP p "=ΔP f "+ΔP" h "+P jl

[0057] In the formula, ΔP p ′ represents the first-stage wellbore pressure, in MPa; ΔP f ′ represents the first-stage frictional pressure drop, in MPa; ΔP h ′ represents the static pressure of the cementing fluid in the first stage, in MPa; ΔP p "The second stage wellbore pressure is in MPa; ΔP" f "This represents the frictional pressure drop in the second stage, in MPa; ΔP" h "This is the static pressure of the cementing fluid in the second stage, in MPa."

[0058] The hydrostatic pressures of the cementing fluid in the first and second stages are calculated using the following formulas, respectively.

[0059]

[0060]

[0061] In the formula, ρ i ′ represents the density of the i-th fluid in the first stage annulus, in g / cm³. 3 H i ′ represents the depth corresponding to the i-th type of fluid in the first stage annulus, in meters; ρ j ′ represents the depth corresponding to the j-th type of fluid in the pipe during the first stage, in meters; H j ′ represents the depth corresponding to the j-th type of fluid in the pipe during the first stage, in meters; ρ i "The density of the i-th fluid in the second-stage annulus is given in g / cm³." 3 H i " is the depth corresponding to the i-th type of fluid in the second-stage annulus, in meters; ρ" j "H represents the depth corresponding to the j-th type of fluid in the pipe during the second stage, in meters." j "" represents the depth (m) corresponding to the j-th type of fluid in the pipe during the second stage; i,j represent the fluid type; n,m represent the total type of cementing fluid.

[0062] The pressure drop of cementing fluid annular friction in the first and second stages is calculated using the following formulas:

[0063]

[0064]

[0065] In the formula, f i ′ represents the friction coefficient of each cementing fluid within the annulus in the first stage; L i ′ represents the length of each cementing fluid section within the annulus in the first stage, in meters; f i "L" represents the friction coefficient of each cementing fluid within the annulus during the second stage. i "" represents the length of each cementing fluid section within the annulus in the second stage, in meters.

[0066] Optionally, the criteria for determining whether there is a risk of leakage are:

[0067] ΔP p "<P l

[0068] In the formula, P l The pressure is the formation leakage pressure, in MPa.

[0069] Compared with existing technologies, the advantages of this invention are as follows: By dividing the controlled-pressure cementing construction into stages and designing the critical flow rate and critical discharge rate of the cementing fluid for different stages, a construction method combining blowout preventers and choke manifolds is formed in the pump shutdown and rubber plug stage. The final displacement pump pressure is calculated from the designed offshore controlled-pressure cementing injection and discharge rate and compared with the formation leakage pressure, verifying the reliability of the design method. This method can balance leakage prevention and improved displacement efficiency, and innovatively considers the injection and discharge rate design in the pump shutdown and rubber plug stage, ensuring cementing construction safety and cementing quality. Attached Figure Description

[0070] Figure 1 A flowchart of a method for designing displacement of cementing injection in offshore controlled pressure wells, provided as an embodiment of the present invention;

[0071] Figure 2 This is a flowchart illustrating the design of the injection and displacement volume during the pump shutdown and plug press stage in offshore controlled pressure cementing, as provided in an embodiment of the present invention. Detailed Implementation

[0072] The following detailed description of a specific embodiment of the present invention is provided in conjunction with the accompanying drawings. However, it should be understood that the scope of protection of the present invention is not limited to the specific embodiment.

[0073] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the technical solution of this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0074] With advancements in exploration technology, offshore exploration and development has gradually expanded from conventional water depths to deep and ultra-deep water, with deepwater oil and gas fields becoming a major driver of global oil production growth. The cementing quality of narrow pressure window wells in the offshore region, influenced by injection, displacement, and drainage rates, is a significant bottleneck restricting offshore oil and gas development. Pressure-controlled cementing technology, while meeting safety requirements, offers unique advantages in increasing injection, displacement, and drainage rates by reducing annular hydrostatic pressure, making it an effective technical means to improve cementing quality.

[0075] Unlike controlled pressure drilling, controlled pressure cementing involves working fluids within the wellbore, including the original drilling fluid, low-density drilling fluid, flushing fluid, isolation fluid, and cement slurry. These working fluids exhibit significant differences in density and rheological properties, leading to time-varying dynamic equivalent density in the annulus during the displacement process. This necessitates meticulous design of displacement rates at different stages to achieve pressure stabilization, leakage prevention, and improved displacement efficiency. Extensive research and field experience indicate that turbulent flow is the optimal flow pattern for cement slurry displacement, followed by plug flow. While turbulent displacement is commonly used in conventional well cementing to enhance slurry displacement efficiency, a composite displacement technique combining turbulent and plug flow with variable displacement is frequently employed for narrow density windows. Turbulent displacement improves displacement efficiency and ensures cementing quality; plug flow displacement results in very low annular friction, reducing the risk of leakage. Furthermore, plug flow displacement maintains the cement slurry in a flowing state, transmitting annular fluid column pressure to stabilize the gas layer. Once displacement is complete, the cement slurry rapidly solidifies, preventing annular gas channeling.

[0076] The existing publicly available injection-displacement design methods mainly use formation leakage pressure as the design benchmark, divide cementing operations into stages, and control the annular equivalent circulation density of each stage by optimizing the injection-displacement rate in order to reduce leakage. However, the above methods do not consider the limitation of the narrow wellbore safety density window and have not formed a complete and specific injection-displacement design method for offshore pressure-controlled cementing.

[0077] To address the aforementioned technical problems, embodiments of the present invention provide a method for designing displacement rates in offshore pressure-controlled cementing. Based on the rheological properties of the fluid, this method ensures pressure balance during cementing operations while also achieving effective displacement of the drilling fluid. The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Figure 1 A flowchart illustrating a method for designing displacement rates in offshore pressure-controlled cementing operations, as provided in this embodiment of the invention. Figure 2 This is a flowchart illustrating the design of the injection and displacement volume during the pump shutdown and plug press stage in offshore controlled pressure cementing, as provided in an embodiment of the present invention.

[0078] like Figure 1-2 As shown in the figure, an embodiment of the present invention provides a method for designing the injection displacement rate in offshore pressure-controlled cementing, which includes the following steps:

[0079] S1. The cementing construction is divided into stages. Based on the formation leakage pressure, the cementing construction is divided into the first stage before the cement slurry exits the casing shoe and the second stage after the cement slurry exits the casing shoe. In the first and second stages, attention should be paid to the injection and displacement design during the pump stoppage and rubber plug pressing stage.

[0080] S2. Given the rheological parameters of each cementing fluid, determine the flow regime of each cementing fluid in each stage of cementing construction. Each cementing fluid includes: drilling fluid, low-density drilling fluid, flushing fluid, isolation fluid, and cement slurry. Specifically, the flow regime of each cementing fluid in each stage of cementing construction is determined as follows: turbulent flow-plug flow composite displacement technology is adopted. Turbulent flow displacement is adopted in the first stage, and plug flow displacement is adopted in the second stage.

[0081] S3. Calculate the critical flow rate and discharge rate of each cementing fluid in the annulus during the construction stage.

[0082] The critical velocity is calculated using the formulas for critical flow in annular slug flow and critical flow in annular turbulence, as follows:

[0083]

[0084]

[0085] In the formula, V tf V represents the critical velocity for turbulent flow in the first stage, in m / s. pf The critical velocity for the second-stage slug flow is τ0 (m / s); the dynamic shear force is η0 (Pa); the plastic viscosity is η0 (Pa·s); and the slug force is η0 (Pa·s). w D is the wellbore diameter, in cm. o Re is the outer diameter of the casing, in cm; c ρ is the critical Reynolds number; ρ is the cement fluid density, g / cm³. 3 ;

[0086] The critical discharge rates of various cementing fluids in the annulus at different construction stages are calculated using the following formula:

[0087]

[0088]

[0089] In the formula, Q tf Q represents the critical displacement for the first stage of turbulent flow, in L / s; pf The critical discharge rate for the second-stage slug flow is L / s;

[0090] The critical Reynolds number is calculated by the following formula:

[0091] Re c =3470-1370n

[0092] In the formula, n is the flowability index of cement slurry.

[0093] Table 1 shows the slurry performance parameters of this embodiment; Table 2 shows the critical displacement rates for turbulent and slug flows of this embodiment.

[0094] Table 1. Slurry Performance Parameters

[0095]

[0096] Table 2 Critical Displacement Rates for Turbulent and Clogged Flow

[0097]

[0098] S4. Calculate the critical flow rate and displacement of each cementing fluid in the casing during the construction stage:

[0099]

[0100]

[0101] In the formula, V s The first stage flow velocity in the pipe is in m / s; D i V is the inner diameter of the casing, in cm; w The flow velocity in the pipe during the second stage is m / s;

[0102] The critical discharge rate of each cementing fluid inside the casing at different construction stages is determined by the following formula:

[0103]

[0104]

[0105] In the formula, Q s Q represents the critical discharge rate in the first stage of the pipeline, in L / s; w The critical discharge rate in the second stage is L / s.

[0106] S5. Calculate the frictional pressure drop generated in the entire flow channel during the pump shutdown and rubber plug pressing stage. During the pump shutdown and rubber plug pressing stage, consider using a construction method combining blowout preventer (BOP) and choke manifold for pressure control cementing. That is, the BOP is closed before the pump shutdown and rubber plug pressing. After the pump shutdown and rubber plug pressing, the cementing fluid enters the choke manifold from the casing annulus and returns. The specific process is as follows: First, the BOP is closed within 1 minute before the pump shutdown and rubber plug pressing. The cementing fluid return flow channel changes from the conventional riser annulus to the choke manifold. Then, during the pump shutdown and rubber plug pressing, the choke valve is closed. This process of reducing the discharge rate should be completed within 30-40 seconds to allow sufficient time for adjusting the wellhead back pressure value.

[0107] Optionally, during the pump shutdown and plugging stage in step S5, the frictional pressure drop generated by the choke manifold during the process of the cementing fluid returning from the mudline to sea level is calculated using the following formula:

[0108]

[0109] In the formula, P jl The frictional pressure drop generated by the throttling manifold, MPa; V i ρ is the flow velocity of cementing fluid in the choke manifold, m / s; f is the friction coefficient; L is the water depth, m; D ji The manifold inner diameter is in cm.

[0110] The flow velocity of the cementing fluid in the choke manifold is calculated using the following formula:

[0111]

[0112] The coefficient of friction is calculated by the following formula:

[0113]

[0114] In the formula, Re is the Reynolds number, which is calculated by the following formula:

[0115]

[0116] S6. Calculate the wellbore pressure during the cementing and displacement process. The wellbore pressure consists of the annular friction pressure drop and the annular cementing fluid static pressure. The wellbore pressures in the first and second stages are calculated using the following formulas:

[0117] ΔP p ′=ΔP f +ΔP h ′

[0118] ΔP p "=ΔP f "+ΔP" h "+P jl

[0119] In the formula, ΔPp ′ represents the first-stage wellbore pressure, in MPa; ΔP f ′ represents the first-stage frictional pressure drop, in MPa; ΔP h ′ represents the static pressure of the cementing fluid in the first stage, in MPa; ΔP p "The second stage wellbore pressure is in MPa; ΔP" f "This represents the frictional pressure drop in the second stage, in MPa; ΔP" h "This is the static pressure of the cementing fluid in the second stage, in MPa."

[0120] The hydrostatic pressures of the cementing fluid in the first and second stages are calculated using the following formulas, respectively.

[0121]

[0122]

[0123] In the formula, ρ i ′ represents the density of the i-th fluid in the first stage annulus, in g / cm³. 3 H i ′ represents the depth corresponding to the i-th type of fluid in the first stage annulus, in meters; ρ j ′ represents the depth corresponding to the j-th type of fluid in the pipe during the first stage, in meters; H j ′ represents the depth corresponding to the j-th type of fluid in the pipe during the first stage, in meters; ρ i "The density of the i-th fluid in the second-stage annulus is given in g / cm³." 3 H i " is the depth corresponding to the i-th type of fluid in the second-stage annulus, in meters; ρ" j "H represents the depth corresponding to the j-th type of fluid in the pipe during the second stage, in meters." j "" represents the depth (m) corresponding to the j-th type of fluid in the pipe during the second stage; i,j represent the fluid type; n,m represent the total type of cementing fluid.

[0124] The pressure drop of cementing fluid annular friction in the first and second stages is calculated using the following formulas:

[0125]

[0126]

[0127] In the formula, f i ′ represents the friction coefficient of each cementing fluid within the annulus in the first stage; L i ′ represents the length of each cementing fluid section within the annulus in the first stage, in meters; f i "L" represents the friction coefficient of each cementing fluid within the annulus during the second stage. i "" represents the length of each cementing fluid section within the annulus in the second stage, in meters.

[0128] S7. Using formation leakage pressure as the design benchmark, compare the wellbore pressure with the formation leakage pressure to determine if there is a risk of leakage. The criteria for determining whether there is a risk of leakage are as follows:

[0129] ΔP p "<P l

[0130] In the formula, P l This represents the formation leakage pressure, in MPa.

[0131] Currently, a combined approach using blowout preventers (BOPs) and choke manifolds is being considered for pressure-controlled cementing. The BOP is shut off before the pump is stopped and the rubber plug is applied. With cementing complete, the pump is stopped and the rubber plug is applied, and cementing fluid enters from the casing annulus and exits through the choke manifold. During this process, the frictional pressure drop increases, potentially leading to excessive wellbore movement and formation fracturing. Therefore, the equivalent density of circulating friction generated during the cementing fluid's passage through the choke manifold and the corresponding construction measures need to be optimized.

[0132] First, the blowout preventer (BOP) must be shut off within 1 minute before the pump is stopped and the rubber plug is applied. The cementing fluid return path changes from the conventional riser annulus to the choke manifold. Then, during the pump stop and rubber plug application process, the choke valve is closed. This reduction in flow rate should be completed within 30-40 seconds to allow sufficient time for adjusting the wellhead back pressure. The frictional pressure drop generated by the choke manifold during the cementing fluid's return from the mudline to sea level can be calculated using the following formula:

[0133]

[0134] In the formula, ΔP f The frictional pressure drop generated by the throttling manifold is measured in MPa and V. i ρ is the average flow velocity of the cementing fluid, in m / s; i Density of cementing fluid, g / cm³; L is water depth, m; D i The manifold inner diameter is in meters (m).

[0135] When determining the injection and displacement parameters and construction flow rate for this well, the following should be noted:

[0136] Before the cement slurry exits the casing shoe in the first stage, a large displacement should be used as much as possible, but the displacement should be controlled to not exceed the critical displacement for well leakage. This also helps to prevent drilling fluid (whose density is less than that of cement slurry) from channeling through the casing under the influence of buoyancy. In the second stage, after the cement slurry exits the casing shoe, the annular hydrostatic pressure and annular friction gradually increase. The displacement should be reduced, and the Reynolds number should be controlled to be less than 100. Based on theoretically calculated parameters, turbulent displacement is used after the pilot slurry and separator fluid exit the casing, i.e., the pilot slurry displacement is 14.2 L / s, the separator fluid displacement is 10.8 L / s, and after the cement slurry exits the casing, plug displacement is used, i.e., the displacement is 1.83 L / s. During the pump shutdown and plugging stage, the frictional pressure drop generated by the choke manifold is 0.54 MPa. Plug displacement is then used to reduce annular pressure fluctuations, prevent well leakage, and improve annular displacement efficiency. By comparing the bottom hole pressure with the formation leakage pressure at the current design displacement, a risk of leakage is determined. Displacement efficiency software is used to simulate the displacement efficiency corresponding to the injection displacement rate to determine if it meets engineering requirements.

[0137] By dividing the controlled-pressure cementing construction into stages, and designing the critical flow rate and critical displacement of the cementing fluid for each stage, a construction method combining blowout preventers and choke manifolds was developed for the pump shutdown and rubber plug application stage. The final displacement pump pressure was calculated using the designed injection and displacement rates for offshore controlled-pressure cementing, and compared with the formation loss pressure to verify the reliability of the design method. This method can balance leakage prevention and improved displacement efficiency, and innovatively considers the injection and displacement rate design for the pump shutdown and rubber plug application stage, ensuring cementing construction safety and cementing quality.

[0138] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for designing injection displacement in offshore pressure-controlled cementing, characterized in that, Includes the following steps: S1. Divide the cementing construction stages: Based on the formation leakage pressure, the cementing construction is divided into the first stage before the cement slurry exits the casing shoe and the second stage after the cement slurry exits the casing shoe. The first and second stages need to pay attention to the injection and displacement design of the pump stoppage and rubber plug stage. S2. Given the rheological parameters of each cementing fluid, determine the flow regime of each cementing fluid in each of the aforementioned construction stages of cementing. S3. Calculate the critical flow rate and discharge rate of each cementing fluid in the annulus during the construction stage. S4. Calculate the critical flow rate and discharge rate of each cementing fluid in the casing during the construction stage. S5. Calculate the frictional pressure drop generated in the entire flow channel during the pump shutdown and rubber plugging stage. During the pump shutdown and rubber plugging stage, consider using a construction method combining blowout preventer and choke manifold for controlled cementing. That is, the blowout preventer is closed before the pump shutdown and rubber plugging is completed. After the pump shutdown and rubber plugging, the cementing fluid enters the choke manifold from the casing annulus and returns. The specific process is as follows: First, the blowout preventer is closed within 1 minute before the pump shutdown and rubber plugging, and the cementing fluid return flow channel changes from the conventional riser annulus to the choke manifold. Then, during the pump shutdown and rubber plugging process, the choke valve is closed. This process of reducing the discharge rate should be completed within 30-40 seconds to allow sufficient time for adjusting the wellhead back pressure value. S6. Calculate the wellbore pressure during the cementing and displacement process; S7. Using the formation leakage pressure as the design benchmark, compare the wellbore pressure with the formation leakage pressure to determine whether there is a risk of leakage.

2. The offshore managed pressure drilling displacement design method of claim 1, wherein, The cementing fluids in step S2 include: drilling fluid, low-density drilling fluid, flushing fluid, isolation fluid, and cement slurry; the flow state of each cementing fluid in each of the construction stages of cementing is determined as follows: turbulent-plug flow composite displacement technology is adopted, with turbulent displacement in the first stage and plug flow displacement in the second stage.

3. The offshore managed pressure cementing displacement design method of claim 2, wherein, The critical flow velocities of various cementing fluids in the annulus at different construction stages in step S3 are calculated using the critical flow velocity formulas for annular plug flow and annular turbulent flow, as follows: In the formula, The critical velocity for turbulent flow in the first stage is given in m / s. The critical velocity for the second-stage slug flow is given in m / s. Let Pa be the dynamic shear force. The viscosity is plastic, Pa·s; The diameter of the wellbore is in centimeters. The outer diameter of the casing is in centimeters. The critical Reynolds number; Density of cementing fluid, g / cm³ 3 ; The critical discharge rates of various cementing fluids within the annulus at different construction stages are calculated using the following formula: In the formula, The critical displacement for the first stage of turbulent flow is given in L / s. The critical discharge rate for the second-stage slug flow is L / s; The critical Reynolds number is calculated by the following formula: In the formula, is the fluidity index of the cement paste.

4. The offshore managed pressure cementing displacement design method of claim 1, wherein, The critical flow rate of each cementing fluid in the casing at different construction stages in step S4 is determined by the following formula: In the formula, The flow velocity inside the pipe during the first stage is in m / s; The inner diameter of the casing is in cm; The flow velocity inside the pipe during the second stage is in m / s; The critical displacement for the first stage of turbulent flow is given in L / s. The critical discharge rate for the second-stage slug flow is L / s; Density of cementing fluid, g / cm³ 3 ; The critical discharge rate of each cementing fluid inside the casing at different construction stages is determined by the following formula: In the formula, The critical discharge rate in the pipe during the first stage is expressed in L / s. The critical discharge rate in the pipe during the second stage is expressed in L / s.

5. The offshore managed pressure cementing displacement design method of claim 1, wherein, In step S5, during the pump shutdown and plugging stage, the frictional pressure drop generated by the choke manifold during the process of the cementing fluid returning from the mudline to sea level is calculated using the following formula: In the formula, The frictional pressure drop generated by the throttling manifold, in MPa; The flow velocity of the cementing fluid in the choke manifold is expressed in m / s. The coefficient of friction; The water depth is in meters (m). The manifold inner diameter is in cm. Density of cementing fluid, g / cm³ 3 ; The flow rate of the cementing fluid in the choke manifold is calculated using the following formula: In the formula, Qc2is the second stage plug flow critical discharge, L / s; The friction coefficient is calculated using the following formula: In the formula, Let be the Reynolds number, which is calculated by the following formula: wherein Pa is the dynamic shear stress, Pa; Pa.s is the plastic viscosity, Pa.s.

6. The offshore pressure-controlled cementing displacement design method as described in claim 1, characterized in that, In step S6, the wellbore pressure during the cementing and displacement process consists of the annular friction pressure drop and the annular cementing fluid static pressure. The wellbore pressures in the first and second stages are calculated using the following formulas: + P jl In the formula, The first stage wellbore pressure is in MPa. The first stage frictional pressure drop is measured in MPa. The static pressure of the cementing fluid in the first stage is MPa; The second stage wellbore pressure is in MPa. The second stage frictional pressure drop is measured in MPa. The static pressure of the cementing fluid in the second stage is MPa; P jl The frictional pressure drop generated by the throttling manifold, in MPa; The hydrostatic pressure of the cementing fluid in the first and second stages are calculated by the following formulas. In the formula, g The acceleration due to gravity is expressed in m / s². For the first phase of the ring air The density of the fluid, g / cm³ 3 ; For the first phase of the ring air The depth corresponding to a certain fluid, in meters (m); For the first phase within the jurisdiction The depth corresponding to a certain fluid, in meters (m); For the first phase within the jurisdiction The depth corresponding to a certain fluid, in meters (m); For the second phase of the ring air The density of the fluid, g / cm³ 3 ; For the second phase of the ring air The depth corresponding to a certain fluid, in meters (m); For the second phase within the jurisdiction The depth corresponding to a certain fluid, in meters (m); For the second phase within the jurisdiction The depth corresponding to a certain fluid, in meters (m); Fluid type; This refers to the general types of cementing fluids; The pressure drop of the cementing fluid annulus friction in the first and second stages is calculated by the following formulas: In the formula, The first stage is the pressure drop due to frictional resistance in the cementing fluid annulus. The second stage is the pressure drop due to frictional resistance in the cementing fluid annulus. The friction coefficient of each cementing fluid in the annulus during the first stage; The length of each cementing fluid section within the annulus in the first stage is in meters. The friction coefficient of each cementing fluid in the annulus during the second stage; The length of each cementing fluid section within the annulus in the second stage, in meters; The critical velocity for turbulent flow in the first stage is given in m / s. The diameter of the wellbore is in centimeters. The outer diameter of the casing is in centimeters. The coefficient of friction; The water depth is in meters (m).

7. The offshore managed pressure cementing displacement design method of claim 1, wherein, The criteria for determining whether there is a risk of leakage are as follows: wherein is the second stage friction pressure drop, MPa; is the formation leakage pressure, MPa.

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