CN115952747A - Offshore pressure control cementing injection displacement design method - Google Patents
Offshore pressure control cementing injection displacement design method Download PDFInfo
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Abstract
The invention relates to the technical field of oil and gas exploitation, and discloses a design method for replacing displacement of injection of offshore pressure control cementing, which comprises the following steps: s1, dividing well cementation construction stages, S2, under the condition that rheological parameters of all well cementation liquids are known, determining the flow state of all the well cementation liquids in each construction stage of cement injection, S3, respectively calculating the critical flow rate and the discharge capacity of all the well cementation liquids in the annular space of the construction stage, S4, respectively calculating the critical flow rate and the discharge capacity of all the well cementation liquids in a sleeve pipe of the construction stage, S5, calculating the friction resistance pressure drop generated by the whole flow passage in the pumping stop and pressure cementing plug stage, S6, calculating the well bore pressure in the well cementation replacement process, and S7, comparing the well bore pressure with the stratum leakage pressure by taking the stratum leakage pressure as a design reference, and judging whether a leakage risk exists. The design method for the displacement of the offshore pressure-control cementing well injection ensures pressure balance in the well cementing construction process based on the rheological property of the fluid, and meanwhile, the effective displacement of the drilling fluid is achieved.
Description
Technical Field
The invention relates to the technical field of oil and gas exploitation, in particular to a design method of offshore pressure control cementing injection displacement.
Background
With the progress of exploration technology, the ocean exploration and development gradually and continuously develop from conventional water depth to deep water and ultra-deep water, and deep water oil and gas fields become the main growth points of world oil production. The well cementation quality of the ocean narrow pressure window well is an important bottleneck restricting the ocean oil and gas development under the influence of the displacement of the injection. The pressure control well cementation technology has unique advantages for improving the displacement of injection and displacement and becomes an effective technical means for improving the well cementation quality on the premise of meeting the safety well cementation requirement due to the fact that the pressure of an annular hydrostatic column is reduced.
Different from pressure-control drilling, in the pressure-control well cementation construction process, a shaft is composed of working fluids such as original drilling fluid, low-density drilling fluid, flushing fluid, isolation fluid and cement slurry, the density and rheological property of each working fluid are large in difference, the cyclic-lost motion equivalent density in the displacement process has a time-varying characteristic, displacement in different stages needs to be designed finely, and the purposes of pressure stabilization and leakage prevention in the injection process and displacement efficiency improvement are achieved. A large number of research results and field construction experience show that the optimal flow state for replacing cement paste is turbulent flow and plug flow. In conventional well cementation, turbulent flow displacement is usually adopted to improve cement slurry displacement efficiency, but for a narrow density window, a composite displacement technology of turbulent flow-plug flow variable displacement is used more. The adoption of turbulent flow displacement is beneficial to improving the displacement efficiency and ensuring the well cementation quality; the plug flow displacement is very little to the annular space friction resistance, can reduce the danger that the leakage took place, and the plug flow displacement simultaneously can guarantee that grout is in the mobile state, transmits annular space fluid column pressure steady gas layer, and in case the displacement is ended, grout can gel rapidly, prevents the emergence of annular space gas channeling.
The injection displacement design method disclosed and provided by the prior art mainly takes the formation leakage pressure as a design reference, performs stage division on well cementation construction, and respectively controls the annular equivalent circulating density of each stage by optimizing the injection displacement so as to reduce the leakage. However, the method does not consider the limitation of narrow safety density window of the shaft, and a complete and specific injection displacement design method aiming at the offshore pressure control well cementation is not formed.
Disclosure of Invention
The invention provides a design method for displacement of injection of offshore pressure-controlled cementing, which ensures pressure balance in the process of cementing construction based on rheological property of fluid and simultaneously achieves effective displacement of drilling fluid.
The invention provides a design method for displacement of injection of offshore pressure control cementing, which comprises the following steps:
s1, dividing a well cementation construction stage;
s2, determining the flow state of each well cementation liquid in each construction stage of cement injection under the condition that the rheological parameters of each well cementation liquid are known;
s3, respectively calculating the critical flow rate and the displacement of each well cementation liquid in the annular space in the construction stage;
s4, respectively calculating the critical flow rate and the displacement of each cementing fluid in the casing pipe in the construction stage;
s5, calculating friction resistance pressure drop generated by the whole flow channel in the pump stopping and rubber plug pressing stage;
s6, calculating the pressure of a shaft in the well cementation displacement process;
and S7, comparing the wellbore pressure with the formation leakage pressure by taking the formation leakage pressure as a design reference, and judging whether the leakage risk exists.
Optionally, step S1 specifically includes: the method is characterized in that stratum leakage pressure is taken as a design reference, well cementation construction is divided into a first stage before cement slurry is discharged out of a casing shoe, a second stage after the cement slurry is discharged out of the casing shoe, and injection displacement design of a pumping stop rubber plug stage needs to be concerned in the first stage and the second stage.
Optionally, each of the cementing fluids in step S2 includes: drilling fluid, low-density drilling fluid, flushing fluid, spacer fluid and cement slurry; the concrete steps for determining the flow state of each well cementing liquid in each construction stage of cementing are as follows: the turbulent flow-plug flow composite displacement technology is adopted, the turbulent flow displacement is adopted in the first stage, and the plug flow displacement is adopted in the second stage.
Optionally, in step S3, the critical flow rate of each cementing fluid in the annulus at different construction stages is calculated by an annulus plug flow critical flow rate formula and an annulus turbulent flow critical flow rate formula, where the formula is as follows:
in the formula, V tf Is the critical flow velocity of the turbulent flow in the first stage, m/s; v pf The second stage plug flow critical flow rate, m/s; tau is 0 Is dynamic shear force, pa; eta 0 Is plastic viscosity, pas; d w Is the borehole diameter, cm; d o Is the outer diameter of the sleeve, cm; re c Is the critical Reynolds number; rho is the density of the well cementing liquid, g/cm 3 ；
The critical discharge capacity of each well cementation fluid in the annular space at different construction stages is calculated by the following formula:
in the formula, Q tf Is the critical displacement of the turbulent flow in the first stage, L/s; q pf The second stage is the plug flow critical displacement, L/s;
the critical reynolds number is calculated by the following equation:
Re c ＝3470-1370n
in the formula, n is a cement paste fluidity index.
Optionally, 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, V s The flow velocity in the first stage pipe is m/s; d i Is the inner diameter of the sleeve, cm; v w The flow velocity in the second stage pipe is m/s;
the critical discharge capacity of each cementing fluid in the casing at different construction stages is determined by the following formula:
in the formula, Q s The critical displacement in the first stage pipe is L/s; q w The critical displacement in the second stage pipe is L/s.
Optionally, in the step S5, in the step of stopping pumping and pressing the rubber plug, pressure-controlled cementing is performed by using a construction method combining the blowout preventer and the choke manifold, that is, the operation of closing the blowout preventer is completed before stopping pumping and pressing the rubber plug, and after stopping pumping and pressing the rubber plug, cementing fluid enters the choke manifold from the casing annulus and returns out, and the specific process flow is as follows: firstly, the blowout preventer is closed within 1min before the pumping stop of the rubber plug, the well cementation fluid return flow channel is changed from the riser annulus of the conventional scheme to a choke manifold, then the choke valve is closed in the pumping stop and the rubber plug pressing process, and the process of reducing the discharge capacity is completed within 30-40 s, so that sufficient time is provided for adjusting the wellhead back pressure value.
Optionally, in the step S5, in the step of stopping pumping and pressing the rubber plug, when the cementing fluid returns to the sea level from the mud line position, the friction pressure drop generated by the choke manifold is calculated by the following formula:
in the formula, P jl The friction pressure drop is MPa generated by a throttle manifold; v i The flow speed of the well cementation fluid in the choke manifold is m/s; f is the friction coefficient; l is water depth m; d ji Is the inner diameter of the manifold, cm;
the flow rate of the cementing fluid in the choke manifold is calculated by the following formula:
the coefficient of friction resistance is calculated from the following formula:
where Re is the Reynolds number, which is calculated by the following equation:
in the stage of stopping pumping and pressing the rubber plug in the step S5, in the process that the well cementing fluid returns to the sea level from the position of the mud line, the friction pressure drop generated by the choke manifold is calculated by the following formula:
in the formula, P jl The friction pressure drop is MPa generated by a throttle manifold; v i The flow speed of the well cementation fluid in the choke manifold is m/s; f is the friction coefficient; l is water depth m; d ji Is the inner diameter of the manifold, cm;
the flow rate of the cementing fluid in the choke manifold is calculated by the following formula:
the coefficient of friction is calculated from the formula:
where Re is the Reynolds number, which is calculated by the following equation:
optionally, in the step S6, the wellbore pressure in the well cementation substitution process is composed of annulus friction pressure drop and annulus well cementation hydrostatic pressure, wherein the wellbore pressures in the first stage and the second stage are calculated by the following formulas:
ΔP p ′＝ΔP f ′+ΔP h ′
ΔP p ″＝ΔP f ″+ΔP h ″+P jl
in the formula,. DELTA.P p ' is the first stage wellbore pressure, MPa;ΔP f ' is the friction pressure drop in the first stage, MPa; delta P h ' is the static pressure of the cementing fluid in the first stage, MPa; delta P p "is second stage wellbore pressure, MPa; delta P f "is the second stage friction drag pressure drop, MPa; delta P h "is the second stage cementing hydrostatic pressure, MPa;
the first stage and the second stage of the well cementation hydrostatic pressure are respectively calculated by the following formula
In the formula, ρ i ' is the density of the ith fluid in the first stage annulus in g/cm 3 ；H i ' is the depth, m, corresponding to the ith fluid in the first stage annulus; rho j ' is the depth, m, corresponding to the jth fluid in the first stage tube; h j ' is the depth, m, corresponding to the jth fluid in the first stage tube; rho i "is the density of the ith fluid in the second stage annulus in g/cm 3 ；H i "is the depth corresponding to the ith fluid in the second stage annulus, m; rho j "is the depth, m, corresponding to the jth fluid in the second stage pipe; h j "is the depth, m, corresponding to the jth fluid in the second stage pipe; i, j are fluid types; n, m is the total type of the well cementing fluid;
the friction resistance pressure drop of the well cementation fluid annulus in the first stage and the second stage is respectively calculated by the following formula:
in the formula (f) i Is a first stage ringFriction coefficient of each well cementation liquid inside; l is i ' is the length of each well cementation liquid section in the annulus of the first stage, m; f. of i "is the friction coefficient of each cementing fluid in the second stage annulus; l is a radical of an alcohol i "is the length of each well cementation liquid section m in the second stage annulus.
Optionally, the criterion for determining whether there is a risk of loss is:
ΔP p ″＜P l
in the formula, P l The formation leakage pressure is MPa.
Compared with the prior art, the invention has the beneficial effects that: by dividing pressure control well cementation construction stages, designing the critical flow rate and the critical discharge capacity of the well cementation liquid aiming at different stages, forming a construction method combining a blowout preventer and a throttle manifold at the stage of stopping pumping a rubber plug, calculating the final displacement pumping pressure and the formation leakage pressure by the offshore pressure control well cementation displacement obtained by design, and verifying the reliability of the design method. The method can give consideration to leakage prevention and displacement efficiency improvement, innovatively considers the injection displacement design in the pump-stopping and rubber plug-pressing stage, and guarantees the safety of well cementation construction and the quality of well cementation.
Drawings
FIG. 1 is a flow chart of a design method for displacement by injection of a marine pressure control cementing well according to an embodiment of the present invention;
FIG. 2 is a flow chart of the offshore pressure control cementing and pumping stop pump plug stage displacement design provided by the embodiment of the invention.
Detailed Description
An embodiment of the present invention will be described in detail below with reference to the accompanying drawings, but it should be understood that the scope of the present invention is not limited to the embodiment.
In the description of the present invention, it is to 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", "circumferential", etc. indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of describing technical solutions of the present invention and simplifying the description, but do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention.
With the progress of exploration technology, the ocean exploration and development gradually and continuously develop from conventional water depth to deep water and ultra-deep water, and deep water oil and gas fields become the main growth points of world oil production. The well cementation quality of the ocean narrow pressure window well is an important bottleneck restricting the ocean oil and gas development under the influence of the displacement of the injection. The pressure control well cementation technology has unique advantages for improving the displacement of injection and displacement and becomes an effective technical means for improving the well cementation quality on the premise of meeting the safety well cementation requirement due to the fact that the pressure of an annular hydrostatic column is reduced.
Different from pressure control drilling, in the pressure control well cementation construction process, a shaft is internally composed of working fluids such as original drilling fluid, low-density drilling fluid, flushing fluid, isolation fluid, cement slurry and the like, the density and rheological property of each working fluid are different greatly, the cyclic-motion equivalent density in the displacement process has time-varying characteristics, displacement in different stages needs to be designed finely, and the purposes of pressure stabilization and leakage prevention in the injection displacement process and displacement efficiency improvement are achieved. A large number of research results and field construction experience show that the optimal flow state for replacing cement paste is turbulent flow and plug flow. In conventional well cementing, turbulent flow displacement is generally adopted to improve the cement slurry displacement efficiency, but for a narrow density window, a composite displacement technology of turbulent flow-plug flow variable displacement is more adopted. The adoption of turbulent flow displacement is beneficial to improving the displacement efficiency and ensuring the well cementation quality; the plug flow displacement is very little to the annular space friction resistance, can reduce the danger that the leakage took place, and the plug flow displacement simultaneously can guarantee that grout is in the mobile state, transmits annular space fluid column pressure steady gas layer, and in case the displacement is ended, grout can gel rapidly, prevents the emergence of annular space gas channeling.
The injection displacement design method disclosed and provided by the prior art mainly takes the formation leakage pressure as a design reference, performs stage division on well cementation construction, and respectively controls the annular equivalent circulating density of each stage by optimizing the injection displacement so as to reduce the leakage. However, the method does not consider the limitation of narrow safety density window of the shaft, and a complete and specific injection displacement design method aiming at the offshore pressure control well cementation is not formed.
In order to solve the above technical problems, an embodiment of the present invention provides a method for designing an injection displacement of an offshore pressure-controlled cementing well, which is based on rheological properties of a fluid to ensure pressure balance during a cementing construction process and simultaneously achieve effective displacement of a drilling fluid, and a specific embodiment of the present invention will be described in detail below with reference to the accompanying drawings, wherein fig. 1 is a flowchart of the method for designing the injection displacement of the offshore pressure-controlled cementing well according to the embodiment of the present invention, and fig. 2 is a flowchart of a design of the injection displacement at a plug stage of a pressure-controlled cementing pump stop and pump stop according to the embodiment of the present invention.
As shown in fig. 1-2, an embodiment of the present invention provides a method for designing displacement for injection displacement of an offshore pressure control cementing well, including the following steps:
s1, dividing well cementation construction stages, wherein stratum leakage pressure is taken as a design reference, the well cementation construction is divided into a first stage before cement slurry is discharged out of a casing shoe, a second stage after the cement slurry is discharged out of the casing shoe, and the injection displacement design of a pump stopping and rubber plug pressing stage needs to be concerned in the first stage and the second stage;
s2, determining the flow state of each well cementing liquid in each construction stage of cementing under the condition that the rheological parameters of each well cementing liquid are known, wherein each well cementing liquid comprises: the method comprises the following steps of drilling fluid, low-density drilling fluid, flushing fluid, spacer fluid and cement slurry, wherein the flow state of each well cementation fluid in each construction stage of cement injection is determined as follows: adopting a turbulent flow-plug flow composite displacement technology, wherein the turbulent flow displacement is adopted in the first stage, and the plug flow displacement is adopted in the second stage;
s3, respectively calculating the critical flow rate and the displacement of each cementing fluid in the annular space in the construction stage;
calculating by an annular plug flow critical flow velocity formula and an annular turbulence critical flow velocity formula, wherein the formula is as follows:
in the formula, V tf Is the critical flow velocity of the turbulent flow in the first stage, m/s; v pf The second stage plug flow critical flow rate, m/s; tau is 0 Is dynamic shear force, pa; eta 0 Is plastic viscosity, pa · s; d w Is the borehole diameter, cm; d o Is the outer diameter of the sleeve, cm; re c Is the critical Reynolds number; rho is the density of the cementing fluid, g/cm 3 ；
The critical discharge capacity of various kinds of well cementation fluids in the annular space at different construction stages is calculated by the following formula:
in the formula, Q tf Is the critical displacement of the turbulent flow in the first stage, L/s; q pf The second stage is the plug flow critical displacement, L/s;
the critical reynolds number is calculated by the following equation:
Re c ＝3470-1370n
in the formula, n is a cement slurry fluidity index.
As shown in Table 1, the slurry performance parameters of this example are shown; as shown in table 2, the displacement is critical displacement for turbulent flow and plug flow in the present embodiment;
TABLE 1 slurry Performance parameters
TABLE 2 Critical displacement of turbulent flow and plug flow
S4, respectively calculating the critical flow rate and the discharge capacity of each well cementation liquid in the casing in the construction stage:
in the formula, V s The flow velocity in the first stage pipe is m/s; d i Is the inner diameter of the sleeve, cm; v w The flow velocity in the second stage pipe is m/s;
the critical discharge capacity of each cementing fluid in the casing at different construction stages is determined by the following formula:
in the formula, Q s Is the critical displacement in the first stage pipe, L/s; q w The critical displacement in the second stage pipe is L/s.
S5, calculating friction drag pressure drop pumping stop rubber plug generated by the whole flow channel in the pumping stop rubber plug pressing stage, and considering that pressure control well cementation is carried out by using a construction method combining a blowout preventer and a throttle manifold, namely, the blowout preventer is closed before the pumping stop rubber plug pressing, well cementation fluid enters the throttle manifold from a sleeve annulus and returns out after the pumping stop rubber plug pressing, and the specific process flow is as follows: firstly, the blowout preventer is closed within 1min before the pumping stop of the rubber plug, the well cementation fluid return flow channel is changed from the riser annulus of the conventional scheme to a choke manifold, then the choke valve is closed in the pumping stop and the rubber plug pressing process, and the process of reducing the discharge capacity is completed within 30-40 s, so that sufficient time is provided for adjusting the wellhead back pressure value.
Optionally, in the step S5, in the step of stopping pumping and pressing the rubber plug, when the cementing fluid returns to the sea level from the mud line position, the friction pressure drop generated by the choke manifold is calculated by the following formula:
in the formula, P jl The friction pressure drop is MPa generated by a throttle manifold; v i The flow speed of the well cementation fluid in the choke manifold is m/s; f is the friction coefficient; l is water depth m; d ji Is the inner diameter of the manifold, cm;
the flow rate of the cementing fluid in the choke manifold is calculated by the following formula:
the coefficient of friction resistance is calculated from the following formula:
where Re is the Reynolds number, which is calculated by the following equation:
s6, calculating the wellbore pressure in the well cementation displacement process, wherein the wellbore pressure consists of annulus friction pressure drop and annulus well cementation hydrostatic pressure, and the wellbore pressure in the first stage and the wellbore pressure in the second stage are respectively calculated by the following formula:
ΔP p ′＝ΔP f ′+ΔP h ′
ΔP p ″＝ΔP f ″+ΔP h ″+P jl
in the formula,. DELTA.P p ' is the first stage wellbore pressure, MPa; delta P f ' is the friction pressure drop in the first stage, MPa; delta P h ' is the static pressure of the cementing fluid in the first stage, MPa; delta P p "is second stage wellbore pressure, MPa; delta P f "is the second stage friction drag pressure drop, MPa; delta P h "is the second stage cementing hydrostatic pressure, MPa;
the first stage and the second stage of the well cementation hydrostatic pressure are respectively calculated by the following formula
In the formula, ρ i ' is the density of the ith fluid in the first stage annulus in g/cm 3 ；H i ' is the depth, m, corresponding to the ith fluid in the first stage annulus; rho j ' is the depth, m, corresponding to the jth fluid in the first stage tube; h j ' is the depth, m, corresponding to the jth fluid in the first stage tube; rho i "is the density of the ith fluid in the second stage annulus in g/cm 3 ；H i "is the depth, m, corresponding to the ith fluid in the second stage annulus; rho j "is the depth, m, corresponding to the jth fluid in the second stage pipe; h j "is the depth, m, corresponding to the jth fluid in the second stage pipe; i, j are fluid types; n, m is the total type of the well cementing fluid;
the friction resistance pressure drop of the well cementation fluid annulus in the first stage and the second stage is respectively calculated by the following formula:
in the formula, f i The friction coefficient of each cementing fluid in the annulus at the first stage is' the friction coefficient of each cementing fluid in the annulus at the first stage; l is a radical of an alcohol i ' is the length of each cementing liquid section in the annulus of the first stage, m; f. of i Is "toFriction drag coefficient of each cementing fluid in the annulus at the second stage; l is a radical of an alcohol i "is the length of each well cementation liquid section m in the second stage annulus.
S7, comparing the shaft pressure with the stratum leakage pressure by taking the stratum leakage pressure as a design reference, judging whether a leakage risk exists or not, wherein the criterion for judging whether the leakage risk exists is as follows:
ΔP p ″＜P l
in the formula, P l The formation leakage pressure is MPa. .
At present, the construction method of combining the blowout preventer and the throttle manifold is considered to carry out pressure control cementing. The blowout preventer is closed before the pump is stopped and the rubber plug is pressed. And (4) entering a stage of stopping pumping and pressing the cement plug along with the completion of cement injection, and enabling the well cementation fluid to enter the throttle manifold from the annular space of the casing and return out. During this process, the frictional pressure drop increases, which may cause the wellbore to become too large and fracture the formation. Therefore, the circulating friction equivalent density generated in the process of passing the well cementation fluid through the choke manifold and the corresponding construction measures need to be optimized.
Firstly, the blowout preventer needs to be closed within 1min before the pumping is stopped and the rubber plug is pressed, and a well cementing fluid return flow channel is changed into a choke manifold from a riser annulus in the conventional scheme. Then, during the process of stopping pumping and pressing the rubber plug, the operation of closing the throttle valve is carried out, and the process of reducing the displacement is finished within 30-40 s, so that sufficient time is provided for adjusting the wellhead back pressure value. In the process that the well cementation fluid returns to the sea level from the position of a mud line, the friction pressure drop generated by the choke manifold can be calculated by the following formula:
in the formula,. DELTA.P f Generating friction pressure drop, MPa, for the choke manifold; v i Is the average flow velocity of the well cementing fluid, m/s; rho i Is the density of the well cementing fluid, g/cm3; l is water depth m; d i Is the manifold inner diameter, m.
Determining the construction displacement of the injection replacement parameters of the well needs attention:
in the first stage, before the cement paste is discharged out of the casing shoe, large-displacement construction is adopted as far as possible, but the construction displacement is controlled not to exceed the critical displacement of the lost circulation, and the phenomenon that drilling fluid (the density of the drilling fluid is less than that of the cement paste) generates in-pipe channeling under the action of buoyancy can be avoided. After cement paste is discharged from a casing shoe at the second stage, the annular static fluid column pressure and the annular friction resistance are gradually increased, the discharge capacity is reduced, the Reynolds number is controlled to be less than 100, parameters calculated according to theory are used as reference, turbulent flow displacement is adopted after the pilot slurry and the isolation liquid are discharged from the casing, namely the discharge capacity of the pilot slurry is 14.2L/s, the discharge capacity of the isolation liquid is 10.8L/s, and plug flow displacement is adopted after the cement paste is discharged from the casing, namely the discharge capacity is 1.83L/s. And the friction resistance pressure drop generated by the throttle manifold in the stage of stopping pumping the rubber plug is 0.54MPa, and plug flow displacement is carried out, so that the pressure fluctuation of the annulus can be reduced, the well leakage is avoided, and the effect of improving the annulus displacement efficiency can be achieved. And judging whether the leakage risk exists or not by comparing the bottom hole pressure under the current designed discharge capacity with the formation leakage pressure. And simulating the displacement efficiency corresponding to the displacement capacity by using displacement efficiency displacement software, and judging whether the engineering requirements are met.
By dividing pressure control well cementation construction stages, designing the critical flow rate and the critical discharge capacity of the well cementation liquid aiming at different stages, forming a construction method combining a blowout preventer and a throttle manifold at the stage of stopping pumping a rubber plug, calculating the final displacement pumping pressure and the formation leakage pressure by the offshore pressure control well cementation displacement obtained by design, and verifying the reliability of the design method. The method can give consideration to leakage prevention and displacement efficiency improvement, innovatively considers the injection displacement design in the pump-stopping and rubber plug-pressing stage, and guarantees the safety of well cementation construction and the quality of well cementation.
Finally, it should be noted that: although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art will understand that various changes, modifications and substitutions can be made without departing from the spirit and scope of the invention as defined by the appended claims. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. A design method for displacement by injection of offshore pressure control cementing is characterized by comprising the following steps:
s1, dividing a well cementation construction stage;
s2, determining the flow state of each well cementation liquid in each construction stage by cementing under the condition that the rheological parameters of each well cementation liquid are known;
s3, respectively calculating the critical flow rate and the displacement of each well cementation liquid in the annular space of the construction stage;
s4, respectively calculating the critical flow rate and the displacement of each well cementation liquid in the casing in the construction stage;
s5, calculating friction resistance pressure drop generated by the whole flow passage in the stage of stopping pumping the rubber plug;
s6, calculating the wellbore pressure in the well cementation displacement process;
and S7, comparing the wellbore pressure with the formation leakage pressure by taking the formation leakage pressure as a design reference, and judging whether a leakage risk exists.
2. The offshore pressure control cementing displacement design method according to claim 1, wherein the step S1 is specifically as follows: the method is characterized in that stratum leakage pressure is taken as a design reference, well cementation construction is divided into a first stage before cement slurry is discharged out of a casing shoe, a second stage after the cement slurry is discharged out of the casing shoe, and the first stage and the second stage need to pay attention to injection displacement design in a pumping stop rubber plug stage.
3. The offshore pressure control cementing displacement design method according to claim 2, wherein each cementing fluid in the step S2 comprises: drilling fluid, low-density drilling fluid, flushing fluid, spacer fluid and cement slurry; the concrete steps for determining the flow state of each cementing fluid in each construction stage are as follows: and (2) adopting a turbulent flow-plug flow composite displacement technology, wherein the turbulent flow displacement is adopted in the first stage, and the plug flow displacement is adopted in the second stage.
4. The offshore pressure-control cementing displacement-by-injection design method according to claim 3, wherein the critical flow rate of each cementing fluid in the annulus at different construction stages in the step S3 is calculated by an annulus plug flow critical flow rate formula and an annulus turbulence critical flow rate formula, wherein the formula is as follows:
in the formula, V tf Is the critical flow velocity of the turbulent flow in the first stage, m/s; v pf The second stage plug flow critical flow rate, m/s; tau is 0 Is dynamic shear force, pa; eta 0 Is plastic viscosity, pas; d w Is the borehole diameter, cm; d o Is the outer diameter of the sleeve, cm; re c Is the critical Reynolds number; rho is the density of the cementing fluid, g/cm 3 ；
The critical discharge capacity of each well cementation fluid in the annular space of different construction stages is calculated by the following formula:
in the formula, Q tf Is the critical displacement of the turbulent flow of the first stage, L/s; q pf The second stage is the plug flow critical displacement, L/s;
the critical reynolds number is calculated by the following equation:
Re c ＝3470-1370n
in the formula, n is a cement paste fluidity index.
5. The offshore pressure control cementing displacement volume design method according to 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, V s The flow velocity in the first stage tube, m/s; d i Is the inner diameter of the sleeve, cm; v w The flow velocity in the second stage pipe is m/s;
the critical discharge capacity of each cementing fluid in the casing at different construction stages is determined by the following formula:
in the formula, Q s Is the critical displacement in the first stage pipe, L/s; q w And the critical displacement in the second stage pipe is L/s.
6. The offshore pressure-control cementing displacement design method according to claim 1, wherein in the step S5, in the step of stopping pumping and cementing the rubber plug, the pressure-control cementing is performed by using a construction method combining a blowout preventer and a choke manifold, that is, the blowout preventer is closed before the rubber plug is stopped pumping, after the rubber plug is stopped pumping, cementing fluid enters the choke manifold from the annular space of a casing and returns out, and the specific process flow is as follows: firstly, the blowout preventer is closed within 1min before the pumping stop of the rubber plug, the well cementation fluid return flow channel is changed from the riser annulus of the conventional scheme to a choke manifold, then the choke valve is closed in the pumping stop and the rubber plug pressing process, and the process of reducing the discharge capacity is completed within 30-40 s, so that sufficient time is provided for adjusting the wellhead back pressure value.
7. The method for designing the displacement of injection control cementing in the offshore well according to the claim 6, wherein in the stage of stopping pumping and cementing in the step S5, the cementing fluid returns to the sea level from the mud line position, and the friction pressure drop generated by the choke manifold is calculated by the following formula:
in the formula, P jl The friction pressure drop is MPa generated by a throttle manifold; v i The flow speed of the well cementation fluid in the choke manifold is m/s; f is the friction coefficient; l is water depth m; d ji Is the manifold inner diameter, cm;
the flow rate of the well cementing fluid in the choke manifold is calculated by the following formula:
the friction coefficient is calculated by the following formula:
where Re is the Reynolds number, which is calculated by the following equation:
8. the offshore pressure control cementing displacement volume design method according to claim 1, wherein the cementing displacement process wellbore pressure in the step S6 is composed of annulus friction pressure drop and annulus cementing hydrostatic pressure, and the first-stage wellbore pressure and the second-stage wellbore pressure are respectively calculated by the following formula:
ΔP p ′＝ΔP f ′+ΔP h ′
ΔP p ″＝ΔP f ″+ΔP h ″+P jl
in the formula,. DELTA.P p ' is the first stage wellbore pressure, MPa; delta P f ' is the friction pressure drop in the first stage, MPa; delta P h ' is the static pressure of the cementing fluid in the first stage, MPa; delta P p "is second stage wellbore pressure, MPa; delta P f "is the friction pressure drop in the second stage, MPa; delta P h "is the second stage cementing hydrostatic pressure, MPa;
the first stage and the second stage of the well cementation hydrostatic pressure are respectively calculated by the following formula
In the formula, ρ i ' is the density of the ith fluid in the first stage annulus in g/cm 3 ；H i ' is the depth, m, corresponding to the ith fluid in the first stage annulus; ρ is a unit of a gradient j ' is the depth, m, corresponding to the jth fluid in the first stage tube; h j ' is the depth, m, corresponding to the jth fluid in the first stage tube; rho i "is the density of the ith fluid in the second stage annulus in g/cm 3 ；H i "is the depth, m, corresponding to the ith fluid in the second stage annulus; ρ is a unit of a gradient j "is the depth, m, corresponding to the jth fluid in the second stage pipe; h j "is the depth, m, corresponding to the jth fluid in the second stage pipe; i, j are fluid types; n, m is the total type of the well cementing fluid;
the annular friction resistance pressure drop of the first-stage and second-stage well cementation liquid is calculated by the following formula:
in the formula (f) i The friction coefficient of each cementing fluid in the annulus at the first stage is' the friction coefficient of each cementing fluid in the annulus at the first stage; l is i ' is the length of each cementing liquid section in the annulus of the first stage, m; f. of i "is the friction coefficient of each cementing fluid in the second stage annulus; l is i "is the length of each well cementation liquid section m in the second stage annulus.
9. The offshore pressure control cementing displacement by injection design method of claim 1, wherein the criterion for judging whether the risk of loss exists is as follows:
ΔP p ″＜P l
in the formula, P l Is the formation leakage pressure, MPa.
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