CN113283069B - Method and system for predicting reliability of well drilling casing - Google Patents
Method and system for predicting reliability of well drilling casing Download PDFInfo
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- 238000005553 drilling Methods 0.000 title claims abstract description 31
- 238000000034 method Methods 0.000 title claims abstract description 26
- 238000001125 extrusion Methods 0.000 claims abstract description 29
- 239000011435 rock Substances 0.000 claims abstract description 25
- 239000012530 fluid Substances 0.000 claims abstract description 24
- 238000004458 analytical method Methods 0.000 claims abstract description 8
- 238000004519 manufacturing process Methods 0.000 claims description 20
- 238000004364 calculation method Methods 0.000 claims description 8
- 238000004088 simulation Methods 0.000 claims description 8
- 229910000831 Steel Inorganic materials 0.000 claims description 6
- 239000010959 steel Substances 0.000 claims description 6
- 239000004568 cement Substances 0.000 claims description 3
- 230000008859 change Effects 0.000 claims description 3
- 238000007906 compression Methods 0.000 claims description 3
- 230000006835 compression Effects 0.000 claims description 3
- 230000002572 peristaltic effect Effects 0.000 description 9
- 230000015572 biosynthetic process Effects 0.000 description 8
- 238000005755 formation reaction Methods 0.000 description 8
- 238000004590 computer program Methods 0.000 description 7
- 230000006399 behavior Effects 0.000 description 5
- 230000002706 hydrostatic effect Effects 0.000 description 5
- 230000008569 process Effects 0.000 description 5
- 238000010586 diagram Methods 0.000 description 4
- 230000009471 action Effects 0.000 description 2
- 229910052602 gypsum Inorganic materials 0.000 description 2
- 239000010440 gypsum Substances 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
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- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
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Abstract
The invention provides a method and a system for predicting the reliability of a well casing, wherein the method comprises the following steps: acquiring a stratum Poisson coefficient, an overburden pressure gradient, a planned drilling fluid density, a completion fluid density and a casing emptying state of a planned casing; under different stress environments, simulating the creep behavior of rock strata around a borehole based on numerical values, and calculating the actual load borne by the casing when the casing reaches a critical safety state by combining the stress analysis of the casing and the extrusion strength of the casing; and comparing the actual load borne by the sleeve with zero to judge whether the sleeve is safe. By the scheme, the accuracy of casing reliability prediction can be improved, and reliable guidance is provided for drilling operation.
Description
Technical Field
The invention relates to the field of oilfield drilling, in particular to a method and a system for predicting the reliability of a drilling casing.
Background
In many oil field underground rock formations, large sections of salt-gypsum-rock and soft-mudstone formations exist, and casing damage in the lithologic intervals accounts for a large proportion. The major cause of the casing damage of the plastic layer is considered by many researchers at home and abroad to be the shrinkage damage of the casing caused by the gradually increased external extrusion load of the outer wall of the casing due to the creep of rocks. In order to ensure that the casing is not crushed to cause the occurrence of complex conditions such as drill sticking and the like in the drilling process, the external load borne by the casing needs to be analyzed, particularly in a haloite stratum and a soft mudstone stratum with high fluidity.
In the process of drilling a plastic creep stratum, the safety problem of a sleeve is considered, and most of the cases suggest that the sleeve design of a salt gypsum rock stratum is carried out by improving the steel grade of the sleeve, increasing the wall thickness of the sleeve or adopting a double-layer combined sleeve, but the damage accidents of all plastic stratum sleeves are found at the same time. The factors that actually cause damage to the plastic formation casing are many. Therefore, the problem of determining the reliability of the plastic formation casing becomes a very important prerequisite for safe use of the casing. At present, the evaluation of the reliability of the plastic stratum drilling casing at home and abroad is mostly carried out under a single condition, and the conditions such as the ground stress environment where the casing is located are not considered, so that the accuracy of the casing reliability prediction result of the plastic stratum drilling is not high.
Disclosure of Invention
In view of this, embodiments of the present invention provide a method and a system for predicting the reliability of a drilling casing, so as to solve the problem that the existing casing reliability prediction result is not accurate.
In a first aspect of embodiments of the present invention, there is provided a method for predicting well casing reliability, comprising:
acquiring a stratum Poisson coefficient, an overburden pressure gradient, a planned drilling fluid density, a completion fluid density and a casing emptying state of a planned casing;
under different stress environments, simulating the creep behavior of rock strata around a borehole based on numerical values, and calculating the actual load borne by the casing when the casing reaches a critical safety state by combining the stress analysis of the casing and the extrusion resistance strength of the casing;
and comparing the actual load borne by the sleeve with zero to judge whether the sleeve is safe.
In a second aspect of embodiments of the present invention, there is provided a well casing reliability prediction system comprising:
the data acquisition module is used for acquiring a stratum Poisson coefficient, an overburden pressure gradient, a planned drilling fluid density, a completion fluid density and a casing emptying state at the position where the casing is planned to be lowered;
the load calculation module is used for simulating the creep behavior of rock strata around the borehole on the basis of numerical values under different stress environments, and calculating the actual load borne by the casing when the casing reaches a critical safety state by combining the stress analysis of the casing and the extrusion resistance strength of the casing;
and the judging module is used for comparing the actual load borne by the sleeve with zero and judging whether the sleeve is safe or not.
In a third aspect of the embodiments of the present invention, there is provided an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, where the processor executes the computer program to implement the steps of the method according to the first aspect of the embodiments of the present invention.
In a fourth aspect of the embodiments of the present invention, a computer-readable storage medium is provided, in which a computer program is stored, and the computer program, when executed by a processor, implements the steps of the method provided in the first aspect of the embodiments of the present invention.
In the embodiment of the invention, the related parameters of the stratum, the drilling well and the casing are obtained, the creep deformation of the rock stratum around the borehole is numerically simulated in different stress environments, the actual load of the casing corresponding to the casing reaching the critical safety state is obtained by combining the stress analysis of the casing and the corresponding casing anti-extrusion strength under the conditions of steel grade, wall thickness and the like of the adopted casing, and the actual load of the casing is compared with zero to judge whether the casing is safe or not. Therefore, the reliability of the casing can be accurately judged, the conditions such as the ground stress environment, the casing hollowing state and the like are comprehensively considered, the reliability of the casing drilled in the plastic stratum is accurately judged, the accuracy and reasonability of casing type selection are guaranteed, further, the damage of the casing is avoided, and the drilling operation efficiency is improved.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a schematic flow diagram of a method for predicting the reliability of a well casing provided by an embodiment of the present invention;
FIG. 2 is another schematic flow diagram of a method for predicting wellbore casing reliability provided by an embodiment of the present invention;
fig. 3 is a schematic structural diagram of a system for predicting the reliability of a well casing according to an embodiment of the present invention.
Detailed Description
In order to make the objects, features and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the embodiments described below are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons skilled in the art without any inventive work shall fall within the protection scope of the present invention, and the principle and features of the present invention shall be described below with reference to the accompanying drawings.
The terms "comprises" and "comprising," when used in this specification and claims, and in the accompanying drawings and figures, are intended to cover non-exclusive inclusions, such that a process, method or system, or apparatus that comprises a list of steps or elements is not limited to the listed steps or elements.
Referring to fig. 1, fig. 1 is a schematic flow chart of a method for predicting the reliability of a casing of a well according to an embodiment of the present invention, including:
s101, obtaining a stratum Poisson coefficient, an overburden pressure gradient, a planned drilling fluid density, a completion fluid density and a casing emptying state of a casing to be lowered;
the formation poisson's coefficient is used to represent the lateral deformation of the formation and the state of the casing voiding is generally expressed as the ratio of the unfilled volume in the casing to the total volume in the casing at the calculated depth. The more the sleeve is hollowed, the smaller the hydrostatic column pressure in the sleeve is, the larger the peristaltic pressure/equivalent breaking load borne by the sleeve is, the difference between the external extrusion resistance strength of the sleeve and the peristaltic pressure/equivalent breaking load borne by the sleeve tends to be smaller than zero, and the safer the sleeve is, otherwise, the more the sleeve is not hollowed, the safer the sleeve is. The accuracy of the actual load calculation of the casing can be guaranteed based on the determination of the empty state of the casing.
S102, under different stress environments, simulating the creep behavior of rock strata around a borehole based on numerical values, and calculating the actual load borne by the casing when the casing reaches a critical safety state by combining the stress analysis of the casing and the extrusion strength of the casing;
it can be understood that the numerical simulation method is used for researching the change rule of the external extrusion load of the casing when the plastic stratum-casing-cement sheath interact under different stress environments, the geotechnical engineering and the underground structure analysis software FLAC are used for carrying out numerical simulation on the creep of rock strata around a borehole, and the creep pressure P is used for simulating the creep of the rock strata around the borehole 1 External load and equivalent failure load P to which the casing is subjected under uniformly stressed conditions 2 The time course curve of the external load borne by the casing under the condition of non-uniform ground stress is combined with the corresponding extrusion resistance P of the casing under the conditions of steel grade, wall thickness and the like of the adopted casing 3 Obtaining the actual load on the casing when the casing reaches the critical safety state, wherein P i ＝P 3 -P 1 /P 2 。
The different stress environments include a uniformly stressed environment and a non-uniformly stressed environment.
Specifically, under uniform stress conditions, the actual load to which the casing is subjected:
peristaltic pressure P 1 Comprises the following steps:
the actual load P to which the surface casing and the technical casing are subjected under uniform stress conditions is then i ：
The actual load P to which the production casing and production liner are subjected under evenly stressed conditions is then i ：
Actual loads to which the casing is subjected under non-uniform ground stress conditions:
equivalent breaking load to which the surface casing and the technical casing are subjected:
equivalent damage loads to the production casing and production liner:
in the formula:
the extrusion strength of the sleeve under the action of the non-uniform load is changed, and the calculation formula is as follows:
the actual load P to which the surface casing and the technical casing are subjected under non-uniformly stressed conditions is then i ：
Actual load P to which the production casing and production liner are subjected under non-uniform ground stress conditions i ：
In the above formula, P 3 Expresses the compression strength of the casing, v expresses the poisson coefficient of stratum rock, G ν Representing the overburden pressure gradient, k m Represents the casing hollowing coefficient (k) m 1,1 denotes full cut), h denotes calculation point well depth, ρ =0 min Represents the minimum drilling fluid density, rho, of the next drilling w The density of the well completion fluid is shown, s is the area enclosed by the non-uniform external load curve, pi is the circumferential ratio, K P Representing the ratio of two non-uniform external extrusion loads, K, to which the casing is subjected under non-uniform ground stress conditions p ＝q 1 /q 2 ，q 1 ，q 2 Respectively two non-uniform external extrusion loads, P, to which the casing is subjected under non-uniform conditions CU API crushing strength, sigma, for casing Y For the yield strength of the casing, a, b are the intercepts of the Casini ellipse on the x, y axes, representing the long and short axes of the ellipse for the distributed load, b/a represents the non-uniformity of the ellipse, A 1 、B 1 、C 1 、D 1 All represent intermediate variables (for simplified formulation), K r ＝r 1 /r 2 ，r 1 、r 2 Respectively the inner diameter and the outer diameter of the casing.
S103, comparing the actual load borne by the casing with zero, and judging whether the casing is safe.
And comparing the actual load value of the sleeve obtained by calculation with zero to judge whether the sleeve is safe.
In one embodiment, as shown in fig. 2, if the actual load on the casing is greater than zero, the casing is determined to be safe; if the actual load borne by the sleeve is less than zero, judging that the sleeve is unsafe; and if the actual load borne by the casing is equal to zero, judging that the casing reaches a critical safety state when the plastic stratum drills.
It should be noted that, from a mechanical point of view, under uniformly stressed conditions (i.e. with uniform loading of the casing), the creep pressure generated by the creep of the plastic formation is uniformly distributed and gradually increases with time, and becomes stable after a while. Under non-uniform stress conditions, when the sleeve is subjected to non-uniform loads, the load carrying capacity of the sleeve is reduced considerably from that of the uniform case. The larger the difference between the maximum extrusion force (force in the direction of 0 °) and the minimum extrusion force (force in the direction of 90 °) acting on the sleeve, the more the strength of the sleeve is decreased, and when the external load exceeds the strength of the sleeve, breakage occurs. It can be seen that under non-uniform stress, the external loads generated by rock creep are more harmful to the casing.
The method for establishing the reliability of the casing for drilling the plastic stratum based on the numerical simulation of the creep behavior of the rock stratum around the borehole has the following theoretical basis: drilling a borehole in the salt-gypsum layer section, and when a casing is put into the borehole, due to creep of a plastic stratum, the rock can form an external squeezing force from outside to inside on the outer wall of the casing, namely creep pressure/equivalent breaking load; on the other hand, due to hydrostatic column pressure formed by the liquid in the casing, inside-out pressure is formed on the inner wall of the casing, and the peristaltic pressure/equivalent breaking load generated by creep of the plastic stratum is resisted together with the extrusion resistance strength of the casing. Wherein the external extrusion resistance of the sleeve is a certain value, and the actual load P borne by the sleeve i Whether it is greater than zero depends on the hydrostatic column pressure, i.e., on the undermining condition in the casing, i.e., the undermining coefficient k m . (1) When hollowing out coefficient k m The larger the sleeve, i.e. the more the sleeve is hollowed, the smaller the hydrostatic column pressure in the sleeve, the greater the peristaltic pressure/equivalent breaking load to which the sleeve is subjected, the greater the anti-extrusion strength of the sleeve and the sleeveThe difference between the applied peristaltic pressure and the equivalent breaking load tends to be less than zero, the more unsafe the bushing is; (2) When hollowing out coefficient k m The smaller, i.e. the less empty the sleeve, the greater the hydrostatic column pressure within the sleeve, the smaller the peristaltic pressure/equivalent breaking load to which the sleeve is subjected, the greater the difference between the outward extrusion strength of the sleeve and the peristaltic pressure/equivalent breaking load to which the sleeve is subjected, the safer the sleeve.
In the embodiment, on the basis of numerically simulating the creep of rock strata around a borehole, the actual load of the casing is calculated by combining the conditions of the steel grade, the wall thickness and the like of the used casing, and the actual condition can be accurately checked on site, so that the accuracy of the reliability prediction result of the casing can be guaranteed, and reliable guidance is provided for drilling operation.
It should be understood that, the sequence numbers of the steps in the foregoing embodiments do not imply an execution sequence, and the execution sequence of each process should be determined by its function and inherent logic, and should not constitute any limitation to the implementation process of the embodiments of the present invention.
Fig. 3 is a schematic structural diagram of a system for predicting the reliability of a casing of a well, according to an embodiment of the present invention, where the system includes:
the data acquisition module 310 is used for acquiring a stratum Poisson coefficient, an overburden pressure gradient, a planned drilling fluid density, a completion fluid density and a casing emptying state at a planned casing running position;
the load calculation module 320 is used for simulating the creep behavior of rock strata around the borehole based on numerical values under different stress environments, and calculating the actual load borne by the casing when the casing reaches a critical safety state by combining the stress analysis of the casing and the extrusion strength of the casing;
wherein the ground stress environment comprises a uniformly stressed environment and a non-uniformly stressed environment.
Specifically, the actual load to which the casing is subjected when it reaches a critical safety state in a uniformly stressed environment:
peristaltic pressure:
actual load P to which surface casing and technical casing are subjected under uniform stress i ：
Actual load P to which the production casing and production liner are subjected under uniform stress i ：
Wherein v is the poisson coefficient of stratum rock G ν Is overburden pressure gradient, k m The casing hollowing coefficient is calculated, and h is the calculated point well depth; rho min Minimum drilling fluid density, ρ, for the next drilling w To completion fluid density.
The actual load to which the casing is subjected when it reaches a critical safety state in a non-uniformly stressed environment:
the surface casing and the technical casing are subjected to equivalent breaking load:
equivalent damage loads to the production casing and production liner:
wherein,
the extrusion strength of the sleeve under the action of the non-uniform load is changed, and the calculation formula is as follows:
the actual load P to which the surface casing and the technical casing are subjected under non-uniform stress conditions is then i ：
Actual load P to which the production casing and production liner are subjected under non-uniform geostress conditions i ：
Wherein ν is the Poisson's coefficient, G, of the formation rock ν Is the overburden pressure gradient, k m Hollowing out coefficient (k) for casing m 1,1 denotes full cut), h is the calculated point well depth, ρ =0 min Minimum drilling fluid density, ρ, for the next drilling w For completion fluid density, s is the area enclosed by the non-uniform out-loading curve (MP) a 2 ) Where π is the circumference ratio, it is generally taken that 3.14, a, b are the intercepts of the Casini ellipse on the x, y axes, which represent the major and minor axes, respectively, of the distributed load of the ellipse, b/a represents the non-uniformity of the ellipse, P CU API crush strength, K, for casing r ＝r 1 /r 2 ，K p ＝q 1 /q 2 ，r 1 、r 2 Respectively the inner and outer diameters of the casing, q 1 、q 2 Respectively two non-uniform external extrusion loads, sigma, to which the casing is subjected under non-uniform conditions Y Is the yield strength of the bushing.
And the judging module 330 is configured to compare the actual load borne by the casing with zero, and judge whether the casing is safe.
Specifically, if the actual load borne by the casing is greater than zero, the casing is judged to be safe; if the actual load borne by the sleeve is less than zero, the sleeve is judged to be unsafe; and if the actual load borne by the casing is equal to zero, judging that the casing reaches a critical safety state when the plastic stratum is drilled.
It is understood that in one embodiment, the electronic device includes a memory, a processor, and a computer program stored in the memory and executable on the processor, the computer program performs all or part of the steps S101 to S103 in the first embodiment, and the processor implements casing reliability prediction when executing the computer program.
Those skilled in the art will appreciate that all or part of the steps in the method according to the above embodiments may be implemented by hardware that is related to instructions of a program, and the program may be stored in a computer-readable storage medium, such as ROM/RAM.
In the above embodiments, the description of each embodiment has its own emphasis, and reference may be made to the related description of other embodiments for parts that are not described or recited in any embodiment.
The above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (4)
1. A method of predicting wellbore casing reliability, comprising:
acquiring a stratum Poisson coefficient, an overburden pressure gradient, a planned drilling fluid density, a completion fluid density and a casing emptying state of a planned casing;
under different stress environments, simulating the creep behavior of rock strata around a borehole based on numerical values, and calculating the actual load borne by the casing when the casing reaches a critical safety state by combining the stress analysis of the casing and the extrusion resistance strength of the casing;
specifically, a numerical simulation method is used for researching the change rule of the external extrusion load of the casing when the plastic stratum-casing-cement sheath interact under different stress environments, geotechnical engineering and underground structure analysis software FLAC are used for carrying out numerical simulation on the creep of rock strata around a borehole, and the creep pressure P is used 1 External load and equivalent failure load P to which the casing is subjected under uniform ground stress 2 The time-course curve of the external load borne by the casing under the condition of non-uniform ground stress is combined with the steel grade of the adopted casing and the corresponding extrusion resistance P of the casing under the condition of wall thickness 3 Obtaining the actual load borne by the casing when the casing reaches the critical safety state;
wherein the ground stress environment comprises a uniformly stressed environment and a non-uniformly stressed environment;
under the condition of uniform stress, the actual load borne by the surface casing and the technical casing is as follows:
the actual loads to which the production casing and production liner are subjected under uniform stress conditions are:
under the condition of non-uniform ground stress, the actual load of the surface casing and the technical casing is as follows:
under non-uniform ground stress conditions, the actual loads experienced by the production casing and production liner are:
in the formula, P 3 Expressing the compression strength of the casing, v expressing the Poisson coefficient of stratum rock, G ν Representing the overburden pressure gradient, k m Represents the casing hollowing coefficient, h represents the calculated point well depth, rho min Represents the minimum drilling fluid density, rho, of the next drilling w The density of the well completion fluid is shown, s is the area enclosed by the non-uniform external load curve, pi is the circumferential ratio, K P Representing the ratio of two non-uniform external extrusion loads, P, to which the casing is subjected under non-uniform ground stress conditions CU API crushing strength, sigma, for casing Y As yield strength of the bushing, A 1 、B 1 、C 1 、D 1 All of them represent the intermediate variable(s),
r 1 、r 2 the inner diameter and the outer diameter of the sleeve are respectively;
and comparing the actual load borne by the sleeve with zero to judge whether the sleeve is safe.
2. The method of claim 1, wherein comparing the actual load experienced by the casing to zero to determine whether the casing is safe comprises:
if the actual load borne by the casing is greater than zero, the safety of the casing is judged;
if the actual load borne by the sleeve is less than zero, the sleeve is judged to be unsafe;
and if the actual load borne by the casing is equal to zero, judging that the casing reaches a critical safety state when the plastic stratum is drilled.
3. A system for predicting well casing reliability, comprising:
the data acquisition module is used for acquiring a stratum Poisson coefficient, an overburden pressure gradient, a planned drilling fluid density, a completion fluid density and a casing emptying state at a planned casing running position;
the load calculation module is used for simulating the creep behavior of rock strata around the borehole on the basis of numerical values under different stress environments, and calculating the actual load borne by the casing when the casing reaches a critical safety state by combining the stress analysis of the casing and the extrusion strength of the casing;
the method comprises the steps of researching the change rule of the external extrusion load of the casing pipe when the plastic stratum-casing pipe-cement sheath interact in different ground stress environments by using a numerical simulation method, carrying out numerical simulation on the creep of rock strata around a borehole by using geotechnical engineering and underground structure analysis software FLAC, and carrying out numerical simulation according to the creep pressure P 1 External load and equivalent failure load P to which the casing is subjected under uniformly stressed conditions 2 The time course curve of the external load borne by the casing under the condition of non-uniform ground stress is combined with the steel grade of the adopted casing and the extrusion resistance of the corresponding casing under the condition of wall thicknessDegree P 3 Obtaining the actual load borne by the casing when the casing reaches the critical safety state;
the ground stress environment comprises a uniformly stressed environment and a non-uniformly stressed environment;
under the condition of uniform stress, the actual load of the surface casing and the technical casing is as follows:
the actual loads to which the production casing and production liner are subjected under uniform stress conditions are:
under the condition of non-uniform ground stress, the actual load of the surface casing and the technical casing is as follows:
under non-uniform ground stress conditions, the actual loads experienced by the production casing and production liner are:
in the formula, P 3 Expressing the compression strength of the casing, v expressing the Poisson coefficient of stratum rock, G ν Representing the overburden pressure gradient, k m Represents the casing hollowing coefficient, h represents the calculated point well depth, rho min Represents the minimum drilling fluid density, rho, of the next drilling w The density of the well completion fluid is shown, s is the area enclosed by the non-uniform external load curve, pi is the circumferential ratio, K P Representing the ratio of two non-uniform external extrusion loads, P, to which the casing is subjected under non-uniform ground stress conditions CU API crushing strength, sigma, for casing Y Yield strength of the bushing, A 1 、B 1 、C 1 、D 1 All of them represent the intermediate variable(s),
r 1 、r 2 the inner diameter and the outer diameter of the sleeve are respectively;
and the judging module is used for comparing the actual load borne by the sleeve with zero and judging whether the sleeve is safe or not.
4. The system of claim 3, wherein comparing the actual load experienced by the casing to zero to determine whether the casing is safe comprises:
if the actual load borne by the casing is greater than zero, the safety of the casing is judged;
if the actual load borne by the sleeve is less than zero, judging that the sleeve is unsafe;
and if the actual load borne by the casing is equal to zero, judging that the casing reaches a critical safety state when the plastic stratum is drilled.
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| US9646115B2 (en) * | 2012-04-17 | 2017-05-09 | Schlumberger Technology Corporation | Determining a limit of failure in a wellbore wall |
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