CN111963164A - Borehole wall collapse pressure evaluation method for multi-fracture development reservoir - Google Patents

Borehole wall collapse pressure evaluation method for multi-fracture development reservoir Download PDF

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CN111963164A
CN111963164A CN202010983408.0A CN202010983408A CN111963164A CN 111963164 A CN111963164 A CN 111963164A CN 202010983408 A CN202010983408 A CN 202010983408A CN 111963164 A CN111963164 A CN 111963164A
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stress
well
borehole
coordinate system
bedding
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冉佳蔚
范翔宇
张明明
张千贵
帅竣天
刘金华
赵鹏斐
么勃卫
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Southwest Petroleum University
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    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
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    • 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 discloses a borehole wall collapse pressure evaluation method for a multi-fracture development reservoir, which comprises the following steps: obtaining the ground stress azimuth coordinate, the bedding attitude and the well track of the target well zone under the geodetic coordinate system; parameters such as the ground stress and the pore pressure of the target well region; converting the ground stress from a geodetic coordinate system to a borehole rectangular coordinate system, converting the stress in the borehole rectangular coordinate system to a borehole polar coordinate to obtain a well circumferential stress component of a transverse isotropic stratum in the borehole polar coordinate, and converting the obtained well circumferential stress component into a main stress form; gradually calculating the intensity of each group of weak planes based on the Jaeger single weak plane intensity criterion, and establishing a multi-weak plane intensity criterion; acquiring the attitude of a stratum bedding surface under a geodetic coordinate system, and establishing the relationship between the bedding surface coordinate system and the geodetic coordinate system; and determining the included angle between the maximum main stress around the well and the normal direction of the bedding surface, obtaining the critical bottom hole pressure of each point around the well, and taking the maximum value as the collapse pressure under the well track.

Description

Borehole wall collapse pressure evaluation method for multi-fracture development reservoir
Technical Field
The invention relates to the technical field of oil engineering, in particular to a borehole wall collapse pressure evaluation method for a multi-fracture development reservoir.
Background
Shale gas is an unconventional resource, is clean, efficient and abundant in reserves, is a main source for increasing the future energy yield, and receives more and more attention worldwide. However, the problem of borehole wall instability caused by the development of bedding and cracks in the shale stratum is over 70 percent, and the bedding and cracks in the rock are not only positions (slippage and peeling surfaces) which are easy to cause instability, but also main channels for the drilling fluid filtrate to enter the stratum during drilling. The drilling fluid invades into the fracture to weaken the shale strength, and the communication between the flowing pressure in the fracture and the drilling fluid pressure in a shaft ensures that the stratum is more sensitive to the drilling operation (the circulation is stopped, the drilling is started), so that the pressure in the fracture is changed violently, the well wall is extremely unstable, the engineering problems of block falling, well collapse and the like are caused, and huge economic loss is brought.
In order to identify the mechanism of borehole instability, a large number of model studies have been conducted at home and abroad. The weak surface criterion firstly proposed by Jaeger considers the anisotropic strength of the shale bedding direction; vahid, Lu, Liu, Setiawan et al propose that the prior isotropic borehole wall stability model does not consider the elastic anisotropy of the rock, but the elastic anisotropy, particularly the elastic modulus, also has a significant influence on the borehole wall stability; amadei proposes a stress distribution model of a transverse isotropic formation; li et al combine the transverse isotropy theory with anisotropic strength to suggest that when the local stress and the elastic parameter have different degrees of anisotropy, the safe drilling pressure window is reduced; liu et al suggested that the elastic anisotropy induced by the settling effect did help to keep the borehole wall stable on any borehole trajectory, indicating that differences in the earth's stress state or bedding surface may cause significant deviations; some researchers have also studied the destabilized regions of the near-wellbore zone in the shale laminate.
However, the above researches only consider the influence of the bedding surface elasticity and strength anisotropy on the borehole wall collapse pressure, a shale reservoir often develops multiple groups of microcracks, the influence of neglected natural fractures can bring larger errors to borehole wall stability analysis, and the collapse pressure is obviously increased along with the increase of the natural fractures. Furthermore, bedding and cracks are two completely different concepts that should be studied separately. Therefore, the method establishes a well wall stability model considering bedding and fractures, establishes a multi-weak-surface criterion based on a single weak-surface criterion, analyzes the well wall collapse pressure of the shale reservoir developed by the bedding fractures, determines the optimal drilling track according to the direction of the minimum collapse pressure with the minimum risk of well wall instability, and is beneficial to improving the actual drilling efficiency on site.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a borehole wall collapse pressure evaluation method for a multi-fracture development reservoir, which comprises the following steps:
the method comprises the following steps: obtaining the ground stress azimuth coordinate, the bedding attitude and the well track of the target well zone under the geodetic coordinate system; the ground stress and pore pressure of the target well region are measured; and parameters such as cohesion, internal friction angle and the like of the rock body and the fracture surface of the mining area of the target well region;
step two: converting the ground stress from a geodetic coordinate system to a borehole rectangular coordinate system, converting the stress in the borehole rectangular coordinate system to a borehole polar coordinate to obtain a well circumferential stress component of a transverse isotropic stratum in the borehole polar coordinate, and converting the obtained well circumferential stress component into a principal stress form;
step three: gradually calculating the intensity of each group of weak planes based on the Jaeger single weak plane intensity criterion, and establishing a multi-weak plane intensity criterion;
step four: acquiring the attitude of a stratum bedding surface under a geodetic coordinate system, and establishing the relationship between the bedding surface coordinate system and the geodetic coordinate system; determining the included angle between the maximum principal stress around the well and the normal direction of the bedding surface, firstly, calculating the maximum principal stress vector of each circumferential angle around the well and the normal vector of the weak surface in the geodetic coordinate system, and then calculating the cosine of the relative angle.
Step five: given the bottom hole pressure P after determining the inclination and heading of the boreholewAnd calculating the maximum and minimum principal stresses at different positions anticlockwise along the axial direction of the borehole, substituting the maximum and minimum principal stresses into a multi-weak-surface criterion, judging the stable condition of the bottom hole pressure value, gradually increasing the bottom hole pressure, repeating the steps until the bottom hole pressure when the borehole wall is stable is the critical bottom hole pressure at the point, obtaining the critical bottom hole pressure at each point around the borehole, and taking the maximum value of the critical bottom hole pressure as the collapse pressure at the borehole track.
Further, the method for converting the ground stress from the earth coordinate system to the borehole rectangular coordinate system, and converting the stress in the borehole rectangular coordinate system to the borehole polar coordinate system to obtain the well circumferential stress component of the transverse isotropic formation in the borehole polar coordinate system, and converting the obtained well circumferential stress component to the form of the principal stress includes the following steps:
the downhole circumferential stress component in the rectangular coordinate system is shown as the following formula:
Figure BDA0002688319920000021
in the formula (I), the compound is shown in the specification,
Figure BDA0002688319920000022
representing taking only the real part of the result in parentheses, σx,σy,σz,τxy,τxz,τyzFor the total stress component, subscript i denotes the component due to ground stress, subscript a denotes the component due to elastic anisotropy;
converting the well circumferential stress component under the borehole rectangular coordinate system into a polar coordinate, wherein the polar coordinate is represented by the following formula:
Figure BDA0002688319920000023
converting the well circumferential stress component in the polar coordinate system into a principal stress form, which is expressed by the following formula:
Figure BDA0002688319920000031
further, the Jaeger single weak plane criterion is shown as follows:
Figure BDA0002688319920000032
in the formula, coAnd cbpRespectively the cohesive force of the shale body and the shale bedding surface; phi is aoAnd phibpShale bedding and bedding surface internal friction angles respectively; beta is an included angle between the loading stress and the normal direction of the bedding surface;
representing the Jaeger single weak plane rule formula in a maximum shear stress plane and a maximum positive stress plane by using a Mohr circle; obtaining the function of the upper and lower boundaries of the loading stress and the normal included angle beta of the bedding surface, wherein the included angle beta1And beta2As shown in the following formula:
Figure BDA0002688319920000033
in the formula, τmAnd σmMaximum shear stress and maximum normal stress on the bedding surface respectively;
based on the single weak surface criterion, establishing a multi-weak surface intensity criterion as shown in the following formula:
Figure BDA0002688319920000034
in the formula, cbp,i,φbp,i,βiRespectively representing the cohesion, internal friction angle and included angle between loading stress and normal direction of the bedding surface of the ith weak surface;
wherein the range of the bedding inclination angle of the fracture along the ith crack surface is shown as the following formula:
Figure BDA0002688319920000035
in the formula, σm,iAnd τm,iIs the average normal stress and the maximum shear stress on the ith plane of weakness.
Further, in the fourth step, the attitude of the stratum bedding plane is obtained under the geodetic coordinate system, and the relationship between the bedding plane coordinate system and the geodetic coordinate system is established; determining an included angle between the maximum principal stress around the well and the normal direction of a bedding surface, firstly, calculating the maximum principal stress vector of each circumferential angle around the well and the normal vector of a weak surface in a geodetic coordinate system, and then calculating the cosine of a relative angle; the method comprises the following steps:
in the geodetic coordinate system, the maximum principal stress vector is given by:
Figure BDA0002688319920000041
wherein the content of the first and second substances,
γ＝0.5arctan[2τθz/(σθ-σz)] (i＝1,2,…)
in the formula, alphabAnd betabRespectively, wellbore inclination and dip angle, °; gamma is the maximum principal stress and borehole axis ZeAngle of (d);
the normal vector of the ith group of weak planes is shown as follows:
Figure BDA0002688319920000042
in the formula, alphabpAnd betabpRespectively, bedding tendency and inclination angle;
finally, the cosine of the relative angle of the ith group of weak planes is obtained as shown in the formula (11):
βi＝arccos(Nni/|N||ni|)。
further, said determining the inclination and inclination of the borehole gives the bottom hole pressure PwCalculating the maximum and minimum principal stresses at different positions counterclockwise along the axial direction of the boreholeSubstituting the value into a multi-weak-surface criterion, judging the stable condition under the bottom hole pressure value, gradually increasing the bottom hole pressure, repeating the steps until the bottom hole pressure when the well wall is stable is the critical bottom hole pressure at the point, obtaining the critical bottom hole pressure at each point around the well, and taking the maximum value of the critical bottom hole pressure as the collapse pressure under the well track, wherein the method comprises the following steps:
1. setting the circumferential angle of the well from 0 to 360 degrees at intervals of 2 degrees;
2. setting the bottom hole pressure to increase from 0 to x at an interval of 0.01MPa, substituting the bottom hole pressure into formula (2), respectively calculating the well circumferential stress at each point around the well, and respectively converting the well circumferential stress into three major stresses around the well;
3. substituting the major stress of the well periphery into a multi-weak-surface judgment criterion formula, and repeating the step 2 until a well wall stable critical condition is reached, wherein the bottom hole pressure is a critical value for maintaining the stability of the well wall;
4. and 3, obtaining bottom hole pressure critical values for maintaining the stability of the well wall at each point of 0-360 degrees around the well according to the step 3, wherein the maximum value is the well wall collapse pressure under the condition of the well track.
The invention has the beneficial effects that: the method solves the problem that errors brought by neglecting natural cracks to well wall stability analysis are solved, a well wall instability region model considering bedding and cracks is established, the multi-weak-surface criterion and a transverse isotropic well circumferential stress model are adopted, the borehole collapse pressure of the stratum shale after development is predicted, and further suggestions are provided for the design of the borehole trajectory.
Drawings
FIG. 1 is a flow chart of a calculation of a borehole wall collapse pressure of a layered reservoir;
FIG. 2 is a spatial distribution diagram of the geostress orientation and borehole trajectory of a target well zone in a geodetic coordinate system;
FIG. 3 is a strength envelope of a single weak plane criterion at a maximum shear-normal stress plane;
FIG. 4 is a weak face rock strength envelope on the shear-normal plane;
FIG. 5 is a representation of the attitude of the target well zone reservoir rock bedding surface in the geodetic coordinate system;
FIG. 6 is a cloud of borehole wall collapse pressures with isotropic strength.
FIG. 7 is a borehole wall collapse pressure cloud with only bedding considerations.
FIG. 8 is a borehole wall collapse pressure cloud when considering bedding and a set of fractures.
FIG. 9 is a borehole wall collapse pressure cloud with bedding and two sets of fractures taken into account.
FIG. 10 is a borehole wall collapse pressure cloud with bedding and three sets of fractures taken into account.
Detailed Description
The technical solutions of the present invention are further described in detail below with reference to the accompanying drawings, but the scope of the present invention is not limited to the following.
For better understanding of the objects, technical solutions and advantages of the present invention, the following detailed description of the present invention is provided in conjunction with the accompanying drawings and embodiments, wherein the method is illustrated in fig. 1, and it should be understood that the embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.
The purpose of the invention is realized by the following technical scheme: a method for predicting the collapse pressure of a bedding fracture shale reservoir by adopting a multi-weak-face criterion comprises the following steps:
firstly, investigating field drilling and logging information, and obtaining space parameters such as a target well area ground stress azimuth, a bedding attitude, a well track and the like under a geodetic coordinate system; in the geodetic coordinate system, two variables of dip angle and inclination are used for describing the spatial distribution of the ground stress and the borehole trajectory, as shown in FIG. 2;
secondly, obtaining the ground stress and pore pressure of the target well region through investigating and researching data such as on-site drilling, well logging and the like, and obtaining strength parameters such as cohesive force, internal friction angle and the like of the rock body and the fracture surface of the mining area through indoor experimental tests;
converting the ground stress from a geodetic coordinate system to a borehole rectangular coordinate system, converting the stress in the borehole rectangular coordinate system to a borehole polar coordinate, obtaining a well circumferential stress component of a transverse isotropic stratum in the borehole polar coordinate, and converting the well circumferential stress component into a main stress form;
wherein, the downhole circumferential stress component in the rectangular coordinate system is as shown in formula (1):
Figure BDA0002688319920000051
in the formula (I), the compound is shown in the specification,
Figure BDA0002688319920000052
representing taking only the real part of the result in parentheses, σx,σy,σz,τxy,τxz,τyzFor the total stress component, the index i indicates the component caused by the ground stress, and the index a indicates the component caused by the elastic anisotropy.
Converting the well circumferential stress component in the borehole rectangular coordinate system into a polar coordinate, as shown in formula (2):
Figure BDA0002688319920000061
converting the well circumferential stress component in the polar coordinate system into a principal stress form, as expressed by the formula (3):
Figure BDA0002688319920000062
step four, gradually calculating the intensity of each group of weak planes based on the Jaeger single weak plane intensity criterion, and establishing a multi-weak plane intensity criterion;
the Jaeger single weak criterion is shown in equation (4):
Figure BDA0002688319920000063
in the formula, coAnd cbpThe cohesive force of the shale body and the shale bedding surface is MPa; phi is aoAnd phibpShale bedding and bedding plane internal friction angle degree; beta is the angle between the loading stress and the normal direction of the bedding plane.
Jaeger single weak planeCriterion (4) is represented by a Mohr circle at the maximum shear stress and maximum normal stress plane, as shown in fig. 3; according to the physical meaning and geometric characteristics of the Mohr circle, obtaining the function of the upper and lower boundaries of the included angle beta between the loading stress and the normal direction of the bedding surface, wherein the included angle beta1And beta2As shown in formula (5):
Figure BDA0002688319920000064
in the formula, τmAnd σmMaximum shear stress and maximum normal stress on the bedding surface respectively;
based on a single weak face criterion, a multi-weak face strength criterion is established as shown in formula (6), and in a shear stress and normal stress plane, the strength envelope curves of a plurality of groups of fractured rock masses are shown in figure 4:
Figure BDA0002688319920000071
in the formula, cbp,i,φbp,i,βiRespectively representing the cohesion, internal friction angle and included angle between loading stress and normal direction of the bedding surface of the ith weak surface;
wherein, the range of the bedding inclination angle damaged along the ith crack surface is shown as the formula (7):
Figure BDA0002688319920000072
in the formula, σm,iAnd τm,iIs the average normal stress and the maximum shear stress on the ith weak plane, MPa.
Fifthly, acquiring the attitude of the stratum bedding surface of the stratum under the geodetic coordinate system, and establishing the relationship between the bedding surface coordinate system and the geodetic coordinate system, wherein the attitude of the stratum rock bedding surface of the reservoir under the geodetic coordinate system is shown in figure 5; determining the included angle between the maximum principal stress around the well and the normal direction of the bedding surface, firstly, calculating the maximum principal stress vector of each circumferential angle around the well and the normal vector of the weak surface in the geodetic coordinate system, and then calculating the cosine of the relative angle.
In the geodetic coordinate system, the maximum principal stress vector is as shown in equation (8):
Figure BDA0002688319920000073
wherein the content of the first and second substances,
γ＝0.5arctan[2τθz/(σθ-σz)] (i＝1,2,…) (9)
in the formula, alphabAnd betabRespectively, wellbore inclination and dip angle, °; gamma is the maximum principal stress and borehole axis ZeAngle of (c) is greater than (d).
The normal vector of the ith group of weak planes is shown as formula (10):
Figure BDA0002688319920000074
in the formula, alphabpAnd betabpRespectively, the bedding tendency and the inclination angle.
Finally, the cosine of the relative angle of the ith group of weak planes is obtained as shown in the formula (11):
βi＝arccos(Nni/|N||ni|) (11)
step six, after the inclination angle and the inclination direction of the well hole are determined, the bottom hole pressure P is givenwCalculating the maximum and minimum principal stresses at different positions anticlockwise along the axial direction of the well bore, substituting the maximum and minimum principal stresses into a multi-weak-surface criterion, judging the stable condition of the bottom hole pressure value, gradually increasing the bottom hole pressure, repeating the steps until the bottom hole pressure when the well wall is stable is the critical bottom hole pressure at the point, and taking the maximum value as the collapse pressure under the well bore track after the critical bottom hole pressure of each point around the well is obtained;
and step seven, drawing the collapse pressures of different well tracks into a pie cloud chart, and determining the most stable well track and the safe drilling fluid density of the well during drilling along different tracks according to the pie cloud chart.
The sixth step in the scheme is specifically as follows:
6.1 setting the circumferential angle of the well to be changed from 0 to 360 degrees at intervals of 2 degrees;
6.2 setting the bottom hole pressure to be increased from 0 to x at intervals of 0.01MPa, replacing the bottom hole pressure into the formula (2), calculating the well circumference stress at each point around the well respectively, and converting the well circumference stress into three major stresses around the well respectively;
6.3 substituting the major stress of the Wednesday into the multi-weak-surface criterion formula, and repeating the step 6.2 until reaching the well wall stability critical condition, wherein the bottom hole pressure is a critical value for maintaining the well wall stability
6.4, obtaining a bottom hole pressure critical value for maintaining the stability of the well wall at each point of 0-360 degrees around the well in the step 6.3, and taking the maximum value of the bottom hole pressure critical value as the well wall collapse pressure under the condition of the well track.
Examples
Taking a typical sliding shale reservoir as an example:
(1) calculation process
S1, investigating and researching on-site drilling, logging and formation testing data, and acquiring a drilled formation borehole depth of 3937.29m to obtain bedding with a horizontal maximum ground stress azimuth of 85 degrees and a due north direction, a reservoir development tendency of 85 degrees and a bedding with an inclination angle of 23.5 degrees, a development tendency of 69 degrees and a crack 1 with an inclination angle of 80 degrees, a development tendency of 60 degrees and a crack 2 with an inclination angle of 90 degrees, a development tendency of 159 degrees and a crack 3 with an inclination angle of 80 degrees;
s2, acquiring the horizontal maximum stress of the drilled stratum to be 81.11MPa, the horizontal minimum stress of the drilled stratum to be 58.66MPa, the vertical ground stress to be 74.09MPa and the formation pore pressure to be 39.47MPa by using field drilling and logging information;
s3, obtaining Jaeger weak surface criterion material parameters of the rock of the target well region by utilizing an indoor single triaxial experiment and a minimum root mean square error principle, wherein the cohesion of a body, a bedding surface, a crack 1, a crack 2 and a crack 3 in the weak surface criterion is respectively 21.4MPa, 18.1MPa, 15MPa, 16.2MPa and 12MPa, and the internal friction angles are respectively 28.3 degrees, 23.8 degrees, 31.1 degrees, 32.4 degrees and 25.6 degrees;
s4, setting the variation range of the well hole orientation to be 0-360 degrees and the interval to be 2 degrees according to the reservoir geomechanical parameters and the rock strength parameters obtained in the steps S1-S3; the inclination angle change range of the well is 0-90 degrees, and the interval is 5 degrees; substituting the block into the borehole wall stability analysis model considering the multiple weak surfaces for calculation to obtain the change rule of the collapse pressure along with the inclination angle and the azimuth angle, and obtaining a borehole wall collapse pressure cloud chart with the isotropic block strength as shown in fig. 6, a borehole wall collapse pressure cloud chart only considering the bedding course as shown in fig. 7, and borehole wall collapse pressure cloud charts respectively considering the bedding course and one or three groups of cracks as shown in fig. 8-10.
(2) Comparative analysis
Comparing fig. 6 with fig. 7, it can be seen that the collapse pressure cloud of a homogeneous isotropic formation has biaxial symmetry, and after taking into account bedding, the collapse pressure cloud no longer has symmetry, but also has uniaxial symmetry in the least horizontally stressed direction due to bedding tendency to coincide with the direction of the greatest principal stress, and bedding has the most significant effect on high dip borehole collapse pressure in the horizontally least stressed direction; comparing fig. 8-10, it can be found that, as the number of fractures increases, the borehole collapse pressure gradually increases near the direction of the horizontal maximum principal stress, the selection range of the optimal borehole trajectory gradually decreases, and for simplifying the borehole wall stability analysis model and process, the influence of the weakest surface of strength can be considered, and the area is designed as the optimal borehole trajectory. Therefore, if the influence of the weak surface is ignored, a larger error is caused, and the engineering requirement cannot be met.
The foregoing is illustrative of the preferred embodiments of this invention, and it is to be understood that the invention is not limited to the precise form disclosed herein and that various other combinations, modifications, and environments may be resorted to, falling within the scope of the concept as disclosed herein, either as described above or as apparent to those skilled in the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (5)

1. A borehole wall collapse pressure evaluation method for a multi-fracture development reservoir is characterized by comprising the following steps:
the method comprises the following steps: obtaining the ground stress azimuth coordinate, the bedding attitude and the well track of the target well zone under the geodetic coordinate system; the ground stress and pore pressure of the target well region are measured; and parameters such as cohesion, internal friction angle and the like of the rock body and the fracture surface of the mining area of the target well region;
step two: converting the ground stress from a geodetic coordinate system to a borehole rectangular coordinate system, converting the stress in the borehole rectangular coordinate system to a borehole polar coordinate to obtain a well circumferential stress component of a transverse isotropic stratum in the borehole polar coordinate, and converting the obtained well circumferential stress component into a principal stress form;
step three: gradually calculating the intensity of each group of weak planes based on the Jaeger single weak plane intensity criterion, and establishing a multi-weak plane intensity criterion;
step four: acquiring the attitude of a stratum bedding surface under a geodetic coordinate system, and establishing the relationship between the bedding surface coordinate system and the geodetic coordinate system; determining the included angle between the maximum principal stress around the well and the normal direction of the bedding surface, firstly, calculating the maximum principal stress vector of each circumferential angle around the well and the normal vector of the weak surface in the geodetic coordinate system, and then calculating the cosine of the relative angle.
Step five: given the bottom hole pressure P after determining the inclination and heading of the boreholewAnd calculating the maximum and minimum principal stresses at different positions anticlockwise along the axial direction of the borehole, substituting the maximum and minimum principal stresses into a multi-weak-surface criterion, judging the stable condition of the bottom hole pressure value, gradually increasing the bottom hole pressure, repeating the steps until the bottom hole pressure when the borehole wall is stable is the critical bottom hole pressure at the point, obtaining the critical bottom hole pressure at each point around the borehole, and taking the maximum value of the critical bottom hole pressure as the collapse pressure at the borehole track.
2. The method for evaluating borehole wall collapse pressure for a multi-fracture development reservoir according to claim 1, wherein the method comprises the following steps of converting the ground stress from a geodetic coordinate system to a borehole rectangular coordinate system, converting the stress in the borehole rectangular coordinate system to a borehole polar coordinate system, obtaining a borehole circumferential stress component of a transverse isotropic formation in the borehole polar coordinate system, and converting the obtained borehole circumferential stress component into a form of a principal stress, wherein the method comprises the following steps:
the downhole circumferential stress component in the rectangular coordinate system is shown as the following formula:
Figure FDA0002688319910000011
in the formula (I), the compound is shown in the specification,
Figure FDA0002688319910000012
representing taking only the real part of the result in parentheses, σx,σy,σz,τxy,τxz,τyzFor the total stress component, subscript i denotes the component due to ground stress, subscript a denotes the component due to elastic anisotropy;
converting the well circumferential stress component under the borehole rectangular coordinate system into a polar coordinate, wherein the polar coordinate is represented by the following formula:
Figure FDA0002688319910000021
converting the well circumferential stress component in the polar coordinate system into a principal stress form, which is expressed by the following formula:
Figure FDA0002688319910000022
3. the method for evaluating borehole wall collapse pressure for a multi-fracture developmental reservoir according to claim 1, wherein the Jaeger single weak plane criterion is as follows:
Figure FDA0002688319910000023
in the formula, coAnd cbpRespectively the cohesive force of the shale body and the shale bedding surface; phi is aoAnd phibpAre shale layers respectivelyInner friction angles of the physical and bedding surfaces; beta is an included angle between the loading stress and the normal direction of the bedding surface;
representing the Jaeger single weak plane rule formula in a maximum shear stress plane and a maximum positive stress plane by using a Mohr circle; obtaining the function of the upper and lower boundaries of the loading stress and the normal included angle beta of the bedding surface, wherein the included angle beta1And beta2As shown in the following formula:
Figure FDA0002688319910000024
in the formula, τmAnd σmMaximum shear stress and maximum normal stress on the bedding surface respectively;
based on the single weak surface criterion, establishing a multi-weak surface intensity criterion as shown in the following formula:
Figure FDA0002688319910000025
in the formula, cbp,i,φbp,i,βiRespectively representing the cohesion, internal friction angle and included angle between loading stress and normal direction of the bedding surface of the ith weak surface;
wherein the range of the bedding inclination angle of the fracture along the ith crack surface is shown as the following formula:
Figure FDA0002688319910000031
in the formula, σm,iAnd τm,iIs the average normal stress and the maximum shear stress on the ith plane of weakness.
4. The method for evaluating the borehole wall collapse pressure of the multi-fracture development reservoir according to claim 1, wherein the attitude of the formation bedding plane is obtained under the geodetic coordinate system in the fourth step, and the relationship between the bedding plane coordinate system and the geodetic coordinate system is established; determining an included angle between the maximum principal stress around the well and the normal direction of a bedding surface, firstly, calculating the maximum principal stress vector of each circumferential angle around the well and the normal vector of a weak surface in a geodetic coordinate system, and then calculating the cosine of a relative angle; the method comprises the following steps:
in the geodetic coordinate system, the maximum principal stress vector is given by:
Figure FDA0002688319910000032
wherein the content of the first and second substances,
γ＝0.5arctan[2τθz/(σθ-σz)](i＝1,2,…)
in the formula, alphabAnd betabRespectively, wellbore inclination and dip angle, °; gamma is the maximum principal stress and borehole axis ZeAngle of (d);
the normal vector of the ith group of weak planes is shown as follows:
Figure FDA0002688319910000033
in the formula, alphabpAnd betabpRespectively, bedding tendency and inclination angle;
finally, the cosine of the relative angle of the ith group of weak planes is obtained as shown in the formula (11):
βi＝arccos(Nni/|N||ni|)。
5. the method for evaluating borehole wall collapse pressure of a multi-fracture developmental reservoir as claimed in claim 1, wherein said determining the borehole inclination and dip is followed by a given bottom hole pressure PwThe method comprises the following steps of calculating maximum and minimum principal stresses at different positions anticlockwise along the axial direction of a well bore, substituting the maximum and minimum principal stresses into a multi-weak-surface criterion, judging the stable condition of the bottom hole pressure value, gradually increasing the bottom hole pressure, repeating the steps until the bottom hole pressure when the well wall is stable is the critical bottom hole pressure at the point, obtaining the critical bottom hole pressure at each point around the well, and taking the maximum value as the collapse pressure at the well track, wherein the method comprises the following steps:
1. setting the circumferential angle of the well from 0 to 360 degrees at intervals of 2 degrees;
2. setting the bottom hole pressure to increase from 0 to x at an interval of 0.01MPa, substituting the bottom hole pressure into formula (2), respectively calculating the well circumferential stress at each point around the well, and respectively converting the well circumferential stress into three major stresses around the well;
3. substituting the major stress of the well periphery into a multi-weak-surface judgment criterion formula, and repeating the step 2 until a well wall stable critical condition is reached, wherein the bottom hole pressure is a critical value for maintaining the stability of the well wall;
4. and 3, obtaining bottom hole pressure critical values for maintaining the stability of the well wall at each point of 0-360 degrees around the well according to the step 3, wherein the maximum value is the well wall collapse pressure under the condition of the well track.
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