CN114970069A - Method for determining well-region carbonate reservoir fracture connectivity - Google Patents
Method for determining well-region carbonate reservoir fracture connectivity Download PDFInfo
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Abstract
The invention provides a determination method for well-region carbonate reservoir fracture connectivity, which can accurately determine the connectivity of well-region carbonate reservoir fractures and the interlayer penetrability of the well-region carbonate reservoir fractures by considering the types of well-region carbonate rocks and the lithology of different types of well-region carbonate rocks, overcomes the bottleneck effects of modeling and predicting the existing low-density well-region carbonate reservoir fractures, and provides a basis for the exploration and development of low-permeability oil and gas reservoirs.
Description
Technical Field
The invention relates to a method for determining well carbonate reservoir fracture connectivity, and belongs to the technical field of oil drilling.
Background
With the gradual transition from eastern to western and from conventional to unconventional reservoirs of oil and gas resource exploration and development, finding fractured oil and gas reservoirs has become a hot point, and the leading-edge problem of petroleum geological research is how to predict the spatial distribution of fractures and quantitatively characterize fracture parameters. Compared with other reservoirs, the carbonate reservoir is often formed in a marine environment, the stratum is transversely distributed stably, space difference of mechanical properties is often caused by changes of a shale interlayer and lithology between layers, fracture distribution heterogeneity is strong under the influence of structural movement, drilling density is relatively low compared with other types of oil and gas fields, and great difficulty is brought to the development of the carbonate oil and gas reservoir.
The low-density fracture space prediction method based on the geomechanical theory ignores the complexity of mechanical properties and stress distribution at a lithologic interface, and causes a bottleneck in fracture modeling and space connectivity determination of a low-density well region carbonate reservoir.
Disclosure of Invention
The invention provides a determination method for well-region carbonate reservoir fracture connectivity, which selects a corresponding fracture criterion according to the type of carbonate rock and can accurately determine the space connectivity of the well-region carbonate reservoir fracture.
The invention provides a method for determining well carbonate reservoir fracture connectivity, which comprises the following steps:
1) according to the fracture-making period of the well-region carbonate rock reservoir fractures, obtaining stress parameters of the well-region carbonate rock in the fracture-making period;
2) establishing a layered geomechanical model of the well carbonate rock according to the stress parameters;
3) acquiring a corrected mechanical parameter of the well carbonate rock in the fracture making period according to the layered geomechanical model;
4) determining the fracture criterion of the well zone carbonate rock according to the stress parameter and the type of the well zone carbonate rock;
5) acquiring the fracture stress of the well carbonate rock according to the corrected mechanical parameters and the fracture criterion;
6) obtaining a theoretical inclination angle of the well region carbonate reservoir fracture according to the fracture stress;
7) and determining the theoretical connectivity of the well region carbonate reservoir fractures according to the theoretical inclination angle.
The determination method as described above, wherein step 7) is followed by:
establishing a three-dimensional paleo-stress field distribution model according to the layered geomechanical model and the correction mechanical parameters;
acquiring the actual density and the actual opening of the well carbonate reservoir fracture according to the three-dimensional paleo-stress field distribution model, the energy conservation law and the maximum strain energy density theory;
acquiring the actual length of the well region carbonate reservoir fracture according to the actual density and the actual opening;
determining the actual connectivity of the well region carbonate reservoir fractures according to the actual density, the actual opening and the actual length;
the theoretical connectivity meets a first requirement when a ratio of the theoretical connectivity to the actual connectivity is greater than 90%.
The determination method as described above, wherein the step 4) further includes:
and under the three-dimensional stress state, if the included angle between the horizontal tensile stress sigma 3 of the layered stratum in the carbonatite of the well region and the lithologic interface is 45-75 degrees, the fracture criterion of the layered stratum is a tension-shear composite fracture criterion.
The determination method comprises the step of determining the fracture criterion of the layered stratum in the carbonatite of the well zone as the Mohr-Coulomb shear criterion if the included angle between the horizontal compressive stress sigma 1 of the layered stratum in the carbonatite of the well zone and the lithologic interface is 45-75 degrees under the three-dimensional stress state.
The determination method as described above, wherein the step 4) further includes:
and under the three-dimensional stress state, if the state of the bedrock in the carbonate rock of the well region is a fracture state, the horizontal tensile stress sigma 3 of the bedrock is less than 0, and the included angle between the sigma 3 and the lithologic interface is 0-45 degrees, the fracture criterion of the brittle rock in the bedrock is the Graves fracture criterion.
The determination method as described above, wherein the step 4) further includes:
and under the three-dimensional stress state, if the state of the bedrock in the well zone carbonate rock is a fracture state, the horizontal tensile stress sigma 3 of the bedrock is less than 0, and the included angle between the sigma 3 and the lithologic interface is 0-45 degrees, the fracture criterion of the weak rock in the bedrock is the Grignard fracture criterion modified by Mirington and Wales.
The determination method as described above, wherein the step 4) further includes:
and under the three-dimensional stress state, if the state of the bedrock in the well area carbonate rock is a fracture state, and the included angle between the horizontal compressive stress sigma 1 of the bedrock and the lithologic interface is 0-45 degrees and 75-90 degrees, the fracture criterion of the brittle rock in the bedrock is a two-stage Moire-Coulomb fracture criterion.
The determination method as described above, wherein the step 4) further includes:
determining the state of the bedrock using the Deluker-prager yield criterion.
The determination method as described above, wherein step 1) further includes:
and collecting seismic interpretation sections of the research area, recovering the fracture distance and the two-dimensional fold form by adopting a structural balance section method, and determining the fracture-making period of the well area carbonate reservoir fractures.
The determination method as described above, wherein the obtaining stress parameters of the carbonate rock of the well zone in the fracture making period includes the following steps:
statistically calculating the orientation of a bisector of a conjugate fracture included angle by adopting a structural tracing method, and determining the maximum principal stress direction of the carbonate rock of the well region at different positions in the carbonate rock reservoir of the well region in the fracture-making period;
and preparing the well zone carbonate rocks at different positions in the well zone carbonate rock reservoir stratum into various standard plunger samples, and testing Kessel points of the standard plunger samples by a rock cracking acoustic emission method to obtain the paleo-stress values of the well zone carbonate rocks at different positions in the fracture-making period.
According to the method for determining the well zone carbonate reservoir fracture connectivity, the well zone carbonate reservoir fracture connectivity can be accurately determined and the interlayer penetrability of the well zone carbonate reservoir fracture can be determined by considering the types of the well zone carbonate and the lithology of different types of well zone carbonate, the bottleneck effects of modeling and prediction of the existing low-density well zone carbonate reservoir fracture are overcome, and a basis is provided for exploration and development of low-permeability oil and gas reservoirs.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the related art, the drawings used in the description of the embodiments of the present invention or the related art are briefly introduced below. It is obvious that the drawings in the following description are only some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.
FIG. 1 is a flow chart of a well carbonate reservoir fracture connectivity determination method according to some embodiments of the invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Currently, the predictive research on reservoir fractures focuses mainly on the following aspects:
(1) natural fracture parameters are estimated based on geological analysis, previous people mostly describe and count the fracture parameters through field outcrop, well drilling rock core and slice analysis, and a qualitative calculation method of fracture porosity, permeability, fracture bulk density and fracture strength is provided. The current industrial CT scanning technology is successfully introduced into the field of petroleum engineering, can realize high-resolution crack three-dimensional imaging and accurate crack parameter characterization, can visually display the damage process from the initiation and the expansion of microcracks to the penetration, and is a good supplement to conventional crack identification and characterization means.
(2) The method is characterized in that the relation between the main curvature of the structure and the development of the crack is discussed from the structural characteristics, a mechanical model of the fractured rock body is established, and the relation between the fold and the crack parameters is discussed, but the curvature method is only applicable to the open crack of the brittle rock, the plastic deformation of the weak rock is not considered, and the method has certain limitations.
(3) Starting from a tectonic stress field, a rock fracture criterion and a numerical simulation method are applied to establish a quantitative prediction model among strain energy, fracture rate, fracture frequency and fracture bulk density. The method starts in the 90 s of the 20 th century, and shows relatively good application practice effects by taking a binary method for quantitative modeling of brittle sandstone structural cracks, which is proposed by Dingzhongyi, Qianxiang and the like, as a representative; a fracture parameter quantitative calculation model of brittle sandstone unidirectional and three-directional extrusion stress states is established by Zhongxingui, Dajunsheng and the like, and on the premise that strain energy released when a rock is fractured is equal to surface energy of a newly-added fracture, three-dimensional space prediction of parameters such as single-stage fracture density and openness is realized, friction energy consumption and plastic deformation energy of fracture surfaces are simplified, but the adopted geological model and mechanical model are established based on a homogeneous body, and the influence of early fracture and rock strength heterogeneity is not excessively considered. In the past, wave connection, child hengmao, zhao tengtao and the like also respectively propose a reservoir comprehensive characterization method based on fracture cause analysis, and discuss comprehensive factors influencing the fracture development of the low-permeability compact sandstone and the heterogeneity of mechanical strength.
(4) A discrete fracture network model (DFN) is established by applying a multivariate statistical method and a random interpolation method, the model can fully integrate seismic, well logging, geology, well drilling, production and other data, and vividly and finely describe the fracture system from the geometric form to the seepage behavior, but the accuracy of the model depends on the reliability of basic data and fracture master control factor analysis.
(5) The method is a common means for identifying and predicting the fracture at present, but the method is still in a qualitative (or semi-quantitative) research stage, and the identification precision is often influenced by various factors.
Based on the above, the invention provides a determination method for well carbonate reservoir fracture connectivity. FIG. 1 is a flow chart of a well carbonate reservoir fracture connectivity determination method according to some embodiments of the invention. As shown in fig. 1, the determination method of the present invention includes the steps of:
s1: according to the well region carbonate rock reservoir fracture setting period, acquiring stress parameters of the well region carbonate rock in the fracture setting period;
s2: establishing a layered geomechanical model of the well carbonate rock according to the stress parameters;
s3: acquiring a corrected mechanical parameter of the well carbonate rock in the seam making period according to the layered geomechanical model;
s4: determining the fracture criterion of the well area carbonate rock according to the stress parameters and the type of the well area carbonate rock;
s5: acquiring the fracture stress of the well carbonate rock according to the corrected mechanical parameters and the fracture criterion;
s6: obtaining a theoretical inclination angle of a well region carbonate reservoir fracture according to the fracture stress;
s7: and determining the theoretical connectivity of the well region carbonate reservoir fractures according to the theoretical inclination angle.
In the present invention, prior to S1, the method further includes: and collecting the seismic interpretation section of the research area, recovering the fracture distance and the two-dimensional fold form by adopting a structural balance section method, and determining the fracture-making period of the carbonate reservoir fracture of the well area.
In particular, the section plane, which contains the layer of interest and the fracture interpretation, runs as perpendicular as possible to the body structure. The sewing period is a strong activity period or a key sewing period of the structure.
The stress parameters of the well carbonate rock in the crack making period obtained in the step S1 comprise the following steps:
statistically calculating the positions of bisectors of included angles of the conjugated fractures by adopting a structural shape trace method, and determining the maximum principal stress directions of the carbonate rocks of the well regions at different layers in the carbonate rock reservoir of the well regions in the fracture-making period;
the well area carbonate rocks at different positions in the well area carbonate rock reservoir stratum are made into various standard plunger samples, the standard plunger samples are used for testing Kessel points through a rock cracking acoustic emission method, and the paleo-stress values of the well area carbonate rocks at different positions in the crack making period are obtained.
The obtaining of the value of the paleo-stress specifically includes: sampling the carbonate rock reservoirs of the well region according to different positions, respectively taking 4 carbonate rock samples at each position, wherein the long axis direction of the carbonate rock samples is basically the same as that of the carbonate rock reservoirs of the well region, and respectively processing the carbonate rock samples of different positions into standard plunger samples of 50 multiplied by 25mm in a laboratory;
testing Kessel points of the standard plunger samples by a rock cracking acoustic emission method, calculating corresponding paleo-stress values according to the Kessel points, selecting the maximum main stress value of which the maximum value represents the fracture period, and calculating the stress average value of the carbonate rock samples at each layer.
S2 specifically includes: and establishing a geological model of the target interval structure of the research area according to the collected depth domain seismic interpretation results.
S3 specifically includes: a. sampling carbonate rock reservoirs of wells at horizontal intervals according to different lithologies, wherein the interval is 45 degrees, the carbonate rock reservoirs of the wells comprise sandstone and mudstone according to the lithologies, generally 6 carbonate rock samples of each lithology of the wells are taken, the long axis direction of the carbonate rock samples of the wells is vertical to the long axis direction of the carbonate rock reservoirs of the wells, and the carbonate rock samples of different layers are processed into standard plunger samples of 50mm multiplied by 25mm in a laboratory;
b. selecting 1 block of each lithologic well area carbonate rock sample, carrying out a uniaxial loading experiment on an MTS (maximum temperature stress) true triaxial rock mechanics apparatus, and testing to obtain uniaxial compressive strength, yield strength, Poisson ratio and elastic modulus of the well area carbonate rock sample;
c. selecting 1 well carbonate rock sample of each lithology, performing a direct shearing experiment on the well carbonate rock sample on a direct shear rock mechanical instrument, and testing to obtain the uniaxial shear strength of the well carbonate rock sample;
d. 1 well area carbonate rock sample of each lithology is selected for carrying out Brazilian splitting experiment, and uniaxial tensile strength or tensile strength of the obtained rock is tested;
e. according to a key crack making period, combining a stratum burying history map of a research area to obtain the ancient buried depth of a well area carbonate rock sample, calculating the confining pressure range of an overlying stratum of the well area carbonate rock sample, setting confining pressure grades at intervals of 5-10Mpa, selecting 3 well area carbonate rock samples of each lithology to perform a true triaxial loading experiment on an MTS true triaxial rock mechanics instrument, and testing to obtain the triaxial compression strength, yield strength, Poisson's ratio, elastic modulus, internal friction angle and cohesion of the well area carbonate rock sample;
f. selecting a logging interpretation model, vertically and continuously interpreting dynamic rock mechanical parameters such as Young modulus, Poisson ratio, density and the like, and performing dynamic-static correction with a rock mechanical experiment result to obtain corrected mechanical parameters, wherein the corrected mechanical parameters are more real stratum static mechanical strength parameters;
g. according to the step a, drilling a well area carbonate rock sample near a lithologic interface of a well area carbonate rock reservoir, processing the well area carbonate rock sample into a standard plunger sample, and performing a Brazilian splitting experiment and a direct shearing experiment along the direction of the lithologic interface to obtain the tensile strength, the friction coefficient and the shearing strength of the well area carbonate rock sample at the lithologic interface.
The carbonate reservoir of the well area generally comprises limestone, dolomite, argillaceous limestone and mudstone, and is characterized by layered composite rock, and the mudstone limestone and the mudstone have certain plastic deformation characteristics under deep conditions, so that the overall damage of the carbonate reservoir of the well area has strong heterogeneity.
S4 specifically includes: a. judging the failure condition of the lithologic interface under different inclination angles, and judging the horizontal tensile stress sigma of the layered stratum in the carbonate rock of the well region in a three-dimensional stress state 3 The included angle between the layered stratum and the lithologic interface is 45-75 degrees, and the fracture criterion of the layered stratum is a tension-shear composite fracture criterion.
In this case, the layered formation is within the range of bedding-in tensile, tensile-shear, or shear fracture possibilities, so it is desirable to employ a tensile-shear composite fracture criterion. The tension-shear composite fracture criterion can be understood as that whether the laminated stratum is subjected to tension fracture is judged by using the tension fracture criterion on the laminated stratum, if the tension fracture condition of the laminated stratum cannot be met, whether the laminated stratum is subjected to shear fracture is judged by using the Mohr-Coulomb criterion on the laminated stratum, and the formula (1) is the tension-fracture criterion:
in the formula: f t Tensile fracture stress (MPa) for a layered formation; f s Shear fracture stress (MPa) for a layered formation;
internal angle of friction (°) for bedding; sigma t Is a layer ofTensile strength (MPa), and numerous experiments prove that the tensile strength of the laminated stratum increases along with the increase of confining pressure P, sigma t And P obey the following linear relationship: sigma tc ＝σ t0 +2 μ P, where σ tc Tensile Strength under confining pressure (MPa), σ t0 The tensile strength (MPa) when the confining pressure is zero, mu is the Poisson's ratio of the rock, and P is the confining pressure (MPa);
horizontal compressive stress sigma of layered stratum in carbonatite of well area under three-dimensional stress state 1 The included angle between the layered stratum and the lithologic interface is 45-75 degrees, and the fracture criterion of the layered stratum is the Mohr-Coulomb shear criterion.
In this case, the layered stratum is located in the range of possible shear fracture of the interface, and the moire-coulomb shear criterion is adopted to judge whether the interface is subjected to shear fracture due to slippage, wherein the moire-coulomb shear criterion is shown as formula (2):
in the formula: c. C j ，
The cohesion and the internal friction angle (GPa, °) of the interface respectively; f s Shear fracture stress (MPa) of the layered formation, and theta is horizontal compressive stress sigma 1 Angle (°) to lithologic interface.
b. Well carbonatites include layered formations and bedrocks outside of lithological boundaries. Bedrocks include weak rocks and brittle rocks. The invention adopts the Deluker-prager yield criterion to determine the state of the bedrock.
The deluke-prager yield criterion is that when the maximum shape deformation energy (also called distortion energy) of a material reaches a certain constant, the material yields and enters a plastic state, and the deluke-prager yield criterion is shown as formula (3):
wherein I 1 ＝σ 1 +σ 2 +σ 3 Is a first invariant of stress;
is a second invariant of stress; a. k is an experimental constant related only to the internal friction angle and cohesion of the bedrock,
is the internal friction angle (°) of the bedrock.
In the invention, if the bedrock is in a yield state, the bedrock is not cracked and is not cracked, and if the bedrock is in a cracked state, the cracking criterion is further determined. Further, if the weak rock does not reach the critical yield state, the weak rock is considered to be cracked, and can be regarded as brittle rock, and the cracking criterion of the weak rock is determined.
If the state of the bedrock in the carbonatite of the well zone is a fracture state, a three-dimensional stress state and the horizontal tensile stress sigma of the bedrock 3 ＜0，σ 3 And the included angle between the fracture standard and the lithologic interface is 0-45 degrees, the fracture standard of the brittle rock in the bedrock is the Gravefield fracture standard.
In the three-dimensional stress state, when σ is 3 < 0, and σ 3 When the included angle with the lithologic interface is 0-45 degrees, the bedrock is positioned in the possible range of tension fracture and tension-shear fracture, and the griffis fracture criterion is adopted for brittle rocks such as limestone and dolomite (sigma is 1 +3σ 3 ) At > 0, the rupture criterion is as shown in equation (4):
when (sigma) 1 +3σ 3 ) At 0 or less, the criterion of rupture is as shown in formula (5):
θ＝0，σ 3 ＝-σ T (5)
in the formula: sigma T Tensile strength (MPa) of bedrock; θ is the tensile failure angle (°), and for weak rocks undergoing brittle failure, such as mudstones and argillaceous limestone, the "critical fracture stress value" can be obtained according to the mirlintoke and wales modified griffy brittle failure criterion, which is shown in equation (6):
in the formula: sigma III Is the triaxial compressive strength (MPa) of the bedrock; f is a parameter of the strength of the bedrock,
k is the soft coefficient of bedrock under triaxial compression, wherein the K of limestone and dolomite is between 1 and 4.5, and the K of mudstone is between 4.5 and 7.
Further comprising: in the three-dimensional stress state, if the state of the bedrock in the carbonate rock of the well zone is a fracture state and the horizontal compressive stress sigma of the bedrock is 1 The included angle between the base rock and the lithologic interface is 0-45 degrees and 75-90 degrees, and the fracture criterion of the brittle rock in the bedrock is a two-stage Moire-Coulomb fracture criterion.
The two-stage Moire-coulomb rupture criterion is shown in equation (7):
in the formula, with confining pressure σ 0 Dividing Mohr-Coulomb rupture envelope into left and right segments under 5MPa, and when confining pressure is less than the value, internal friction angle is increased
Larger, reduced fracture angle, mainly tensile fracture, and internal friction angle when confining pressure is greater than a certain value
The fracture angle is increased, and the fracture of the bedrock is gradually changed from tensile fracture to tensile-shear fracture;
internal friction angle, k, of the left and right segments of the envelope broken respectively 1 、k 2 The slopes of the straight line segments of the left segment and the right segment of the fracture envelope line are respectively called as the internal friction coefficient.
In S5, the fracture stress of the well carbonatite can be obtained by substituting the corrected mechanical parameters obtained in S3 into the fracture criterion determined in S4.
S6 specifically includes: calculating, vertically expanding or extending the crack occurrence, determining the dip angle and the trend of the crack in a three-dimensional space by adopting a projection calculation method based on the multi-stage composite fracture criterion, wherein the X axis of an Ansys coordinate system is superposed with the X axis (east) of the geodetic coordinate, the Z axis is superposed with the negative direction (south) of the Y axis, and the Y axis is superposed with the Z axis, so that if the direction residual of the normal direction vector of the crack surface under the integral coordinate system is determined as
Will be provided with
Projected to an XOZ plane, and the included angle between the projection line and the Z-axis negative direction is alpha z Then there is alpha z The strike angle α can be determined from the equations (8) and (9),
if 0 is less than or equal to alpha z ＜90°，α＝90°-α z (8)
If-90 DEG < alpha z ＜0，α＝(-90°-α z )+360° (9)
From the geological point of view, the fracture dip angle is the angle between the fracture surface and the XOZ plane, namely the angle alpha between the plane lx + my + nz-0 and the plane y-0 dip (0°≤α dip 90 deg.) and in the Ansys three-dimensional coordinate system the crack angle is the angle between the crack surface and the XY plane, i.e. the angle α between the plane lx + my + nz 0 and the plane y 0 dip (0°≤α dip Is less than or equal to 90 degrees), the calculation formula is shown as the formula (10):
s7 concretely, a, calculating the theoretical trend and the theoretical inclination angle of the well carbonate reservoir fracture from S6, and when alpha is calculated dip >At 15 deg.c, it is considered that the well region carbonate rock reservoir crack is not horizontal crack, so that the crack may extend upwards and downwards and penetrate through the stratum surface and enter the matrix, according to the fracture mechanics theory, the crack formed by expansion in underground layered stratum is mainly expressed as I-type (open type), II-type (slide type) or I-II composite type, after the form change specific energy density criterion is corrected, in the cylindrical surface coordinate system or polar coordinate system the expansion or penetration condition of crack tip can be effectively judged, and S is used d Intensity of field, S, representing the change in shape of the fracture tip d Obtained according to equation (11):
S d ＝C 11 K I 2 +2C 12 K I K II +C 22 K II 2 (11)
wherein the content of the first and second substances,
in the formula, K I 、K II Is a stress intensity factor of
) The larger the value is, the greater the tendency of crack instability is; theta is the crack tip polar angle (rad); beta is the included angle between the existing crack and the main stress; g is the shear elastic modulus (GPa); mu is Poisson' S ratio and the fracture changes the specific energy density factor S along the shape d Minimum local cracking, theta ═ theta 0 Where S is dmin Reaches a critical value S dc Then, the crack begins to propagate, the criterion being formula (12):
wherein a is the half length (m) of the crack, calculated by the Ansys software, S dc Fracture toughness parameter, which can be regarded as the resistance of the crack to propagation, is determined by the fracture toughness K IC Is calculated to obtain θ 0 Angle of fracture (°) at which crack starts to propagate;
b. on the basis of judging whether the vertical cracks continue to expand and penetrate through the lithologic interface or not and simulating the stress field in the step a, in an Ansys finite element simulation platform, firstly, a node N close to the lithologic interface is selected 1 Judging whether the points are fractured or not by adopting a multi-stage composite fracture criterion, and if the points are fractured, determining the fracture property and the inclination angle alpha of the well carbonate fracture dip Searching nearby adjacent nodes N simultaneously 2 、N 3 、N 4 、N 5 、N 6 And judging the fracture property and the inclination angle of the well carbonate crack if the well carbonate crack is connected with the node N 1 The conditions are similar and are considered to belong to the same crack, the length of the crack is simplified and set as the distance a between two nodes, and then the node N under the polar coordinates in the Ansys stress field is read out 1 Calculating the stress intensity factor K of the crack tip I 、K II And the shape change specific energy density factor S d And determining the extended cracking angle theta of the point 0 Finding the next node N in the lithologic interface of the crack deflection direction or the rock on the other side 7 And extracting the stress component again and judging whether the crack is generated, so that a node stress extrapolation method is used for solving and determining continuous nodes of the same crack until the node is not cracked any more, and finally finishing quantitative characterization of connectivity in a crack space.
In some embodiments of the present invention, step 7) is followed by: establishing a three-dimensional paleo-stress field distribution model according to the layered geomechanical model and the correction mechanical parameters;
acquiring actual density and actual opening of the well carbonate reservoir fractures according to a three-dimensional paleo-stress field distribution model, an energy conservation law and a maximum strain energy density theory;
acquiring the actual length of the well carbonate reservoir fracture according to the actual density and the actual opening;
determining the actual connectivity of the well region carbonate reservoir fractures according to the actual density, the actual opening and the actual length;
the theoretical connectivity meets the first requirement when the actual connectivity is more than 90% consistent with the theoretical connectivity.
Specifically, the establishing of the three-dimensional paleo-stress field distribution model according to the layered geomechanical model and the correction mechanical parameters comprises the following steps:
a. based on a finite element simulation platform, assigning the corrected mechanical parameters of the seam making period obtained in the step 3) to the layered geomechanical model established in the step 2), wherein the corrected mechanical parameters are respectively assigned to limestone or dolomite, mudstone or argillaceous limestone, a lithologic interface and a fault, and the corrected mechanical parameters comprise: density, elastic modulus, Poisson ratio and fault mechanics parameters can refer to a rock mechanics course, and the elastic modulus is 0.65 times of that of limestone or dolomite generally;
b. carrying out grid division on the geomechanical model after assignment, and aiming at the actual situation, the grid density of the general stratum is higher, and the grid encryption processing is carried out at the fault and lithologic interface;
c. setting mechanical boundary conditions according to the structural evolution analysis result, applying force to the model according to the ancient stress state test analysis result,the boundary conditions and force ranges are iteratively adjusted until the stress difference (σ) 1 -σ 3 ) Or the stress intensity is in accordance with the structural distribution characteristics of the research area, namely the forming mechanism of the fracture and the wrinkle can be reasonably explained.
Acquiring actual density and actual openness of well carbonate reservoir fractures according to a three-dimensional paleo-stress field distribution model, an energy conservation law and a maximum strain energy density theory, and specifically comprising the following steps of:
a. on the basis of finite element grids and unit division of a well carbonate reservoir, a node is taken as a center, a micro-parallelepiped is selected as a representation unit body (REV), and the maximum principal stress sigma is followed 1 The length of the directional unit body is L1, along the median principal stress sigma 2 The side length of the direction unit body is L 2 Along the minimum principal stress σ 3 The side length of the directional unit body is L 3 And has L 1 >L 2 >L 3 With normal to the fracture plane at σ 1 -σ 3 In the principal plane, the principal direction of the crack (long axis direction) makes an angle theta with the maximum principal stress, where
Is the internal friction angle, and the median principal stress sigma 2 Parallel connection;
b. a large number of rock mechanics experiment results prove that the elastic strain energy is the root cause for promoting the rock damage, and no matter whether the rock is brittle or elastic-plastic, when the loading stress reaches the peak stress sigma c 0.85 times of the maximum strain energy, the strain energy at this moment is called the maximum elastic strain energy which must be overcome to generate cracks, and according to the law of conservation of energy and the theory of maximum strain energy density, the energy required for newly increasing the surface area of cracks is equal to the difference between the strain energy in the current cell body and the maximum elastic strain energy which must be overcome to generate cracks, namely, the formula (13):
in the formula, D vf Defined as the bulk density of the fractures in the REV unit cell, i.e., the ratio of the total surface area of the fracture cell to the volume of the unit cell, m 2 /m 3 ；σ c The uniaxial peak strength of the carbonate rock in the well region is MPa; e is 0.85 sigma c The unloading average elastic modulus of the stress point is approximately equal to the elastic modulus of the elastic stage in loading, GPa and mu are Poisson's ratio,
is a coefficient; sigma p The fracture stress is obtained when the carbonate rock in the well region meets the Mohr-Coulomb fracture condition, and is MPa; w is the stored releasable elastic strain energy density, J/m 3 ；w e Elastic strain energy density, J/m, that must be overcome to generate cracks 3 ；w f Strain energy density, J/m, for newly added fracture surface area 3 ，S f Surface area of newly added crack, m 2 (ii) a J is the energy required to generate a crack per unit area, i.e., the crack surface energy, J/m 2 ；
c. Calculating the actual density and the actual opening of the well area carbonate reservoir fractures in different stress states, judging according to nodes through the multi-stage composite fracture criterion in the three-dimensional compressive stress state without tensile stress, wherein when the well area carbonate rock does not reach the fracture condition, the fracture parameters are all 0, and when the well area carbonate rock reaches the fracture condition, the actual density, the actual linear density and the actual opening are calculated according to the formula (14):
wherein, J 0 The surface energy of the crack is J/m when the confining pressure is zero 2 ；ε 0 The maximum elastic tensile strain of the carbonate rock of the well region; e 0 Proportional coefficient, lithology-related, limestone, dolomite J 0 ＝1087.35J/m 2 ，E 0 112.6, mudstone, argillaceous limestone J 0 ＝1391.44J/m 2 ，E 0 101.4; the other parameters w are the strain energy density, w f Is the strain energy density, w, of the crack e Strain energy, sigma, necessary to overcome to increase unit fracture surface area p Fracture stress of carbonate rock for well region, D vf Is the fracture bulk density, D lf Minimum tensile strain, b crack opening, E elastic modulus, C 0 The cohesive force is,
Is the internal friction angle, mu is the Poisson's ratio, J is the fracture surface energy, theta is the fracture angle;
d. under the three-dimensional compressive stress state that has tensile stress, judge according to the node through above multistage compound fracture criterion, when the well zone carbonatite does not reach the rupture condition, the crack parameter is 0, adopts equation (15) to calculate crack bulk density, linear density, aperture when the well zone carbonatite reaches the rupture condition:
wherein epsilon 0 Maximum elastic strain of carbonate rock in well zone, limestone, dolomite J 0 ＝1087.35J/m 2 ，ε 0 ＝-4.9×10 -4 Mudstone, argillaceous limestone J 0 ＝1391.44J/m 2 ，ε 0 ＝-7.2×10 -4 ；σ t Uniaxial tensile strength at zero confining pressure, in MPa; other parameters w, w f 、w e 、D vf 、D lf 、ε 3 B, J and theta are the same as those of the three-dimensional compressive stress;
e. based on the parameters such as actual density and actual opening, the average length of the cracks in the unit body can be obtained after simplification, according to the theory of structural geology, a group of cracks with approximately consistent directions and scales are often generated under the same stress action in the same period, wherein the group of cracks in the unit body are equivalent to a plurality of cuboids, the lengths, the widths and the heights of the cuboids are respectively L, b and H, the cracks are equivalent to be arranged in parallel at equal intervals, the cracks do not exceed the boundaries of the unit body, and according to the knowledge, theta is a crack fracture angle and is defined as the trend of a crack surface and sigma 1 So as to lie along and L 3 The number of cracks in the direction of the included angle theta can be expressed as:
the bulk density of the cleft in the unit cell is expressed as:
a large number of experiments prove that: length, height parameter of single crack and maximum principal stress sigma at node 1 And σ 3 In connection with, i.e.
The average length of the resulting set of fractures was:
determining the actual connectivity of the well region carbonate reservoir fractures according to the actual density, the actual opening and the actual length comprises the following steps:
based on the ancient stress field simulation result, a secondary development interface in a finite element simulation platform is adopted, quantitative expressions of crack body density, linear density, length, opening degree, inclination angle and trend parameters are compiled into an APDL program, the spatial distribution characteristics of crack parameters are predicted, and actual crack connectivity is obtained.
The theoretical connectivity meets the first requirement when the ratio of the theoretical connectivity to the actual connectivity is greater than 90%.
In the present invention, it can be considered that the first requirement is a requirement within an error range, and it can be confirmed that the obtained theoretical connectivity is high in reliability.
In particular embodiments, the geomechanical model and the fracture parameter calculation model may be revised if the ratio of theoretical connectivity to actual connectivity is less than or equal to 90%.
According to the method, the space longitudinal connectivity condition of the crack and the effectiveness condition of the crack can be predicted by combining the theoretical density of the crack, the theoretical length of the crack and the theoretical inclination angle parameter of the crack. If the theoretical length of the crack is generally smaller than the thickness of the interlayer under normal distribution, the crack is of a non-penetrating type, namely the effectiveness of the interlayer is good, if the theoretical length of the crack is generally larger than the thickness of the interlayer, the crack is of a penetrating type, namely the effectiveness of the interlayer is poor, if the theoretical length of the crack is partially larger than the thickness of the interlayer, the crack is of a semi-penetrating type, the crack is evaluated by combining with the theoretical inclination angle range of the crack, if the theoretical inclination angle of the crack is mainly in the range of 0-45 degrees and is a low-angle crack, the effectiveness of the interlayer is good, if the theoretical inclination angle of the crack is mainly in the range of 45-75 degrees and is an oblique intersection crack, the effectiveness of the interlayer is general, and if the theoretical inclination angle of the crack is mainly in the range of 75-90 degrees and is a high-angle crack, the effectiveness of the interlayer is the worst.
The method can also be used for evaluating the spatial fracture transverse connectivity by combining the simulation calculation results of the theoretical density, the theoretical opening degree and the trend, if the theoretical density and the theoretical opening degree of the fractures are large and the trend is stable under normal distribution, the fracture transverse connectivity is good, if the theoretical density and the theoretical opening degree of the fractures are large but the trend distribution is disordered or mainly low-angle fractures, the fracture transverse connectivity is general, if the theoretical density and the theoretical opening degree of the fractures are low and the trend is stable, the fracture transverse connectivity is general, if the theoretical density and the theoretical opening degree of the fractures are low and the trend is disordered, the fracture transverse connectivity is worst.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and these modifications or substitutions do not depart from the spirit of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A method for determining well zone carbonate reservoir fracture connectivity is characterized by comprising the following steps:
1) according to the fracture-making period of the well-region carbonate rock reservoir fractures, obtaining stress parameters of the well-region carbonate rock in the fracture-making period;
2) establishing a layered geomechanical model of the well area carbonate rock according to the stress parameters;
3) acquiring a corrected mechanical parameter of the well carbonate rock in the fracture making period according to the layered geomechanical model;
4) determining the fracture criterion of the well zone carbonate rock according to the stress parameter and the type of the well zone carbonate rock;
5) acquiring the fracture stress of the well carbonate rock according to the corrected mechanical parameters and the fracture criterion;
6) obtaining a theoretical inclination angle of the well region carbonate rock reservoir fracture according to the fracture stress;
7) and determining the theoretical connectivity of the well region carbonate reservoir fractures according to the theoretical inclination angle.
2. The method of claim 1, further comprising, after step 7):
establishing a three-dimensional paleo-stress field distribution model according to the layered geomechanical model and the correction mechanical parameters;
acquiring the actual density and the actual opening of the well carbonate reservoir fracture according to the three-dimensional paleo-stress field distribution model, the energy conservation law and the maximum strain energy density theory;
acquiring the actual length of the well region carbonate reservoir fracture according to the actual density and the actual opening;
determining the actual connectivity of the well region carbonate reservoir fractures according to the actual density, the actual opening and the actual length;
the theoretical connectivity satisfies a first requirement when a ratio of the theoretical connectivity to the actual connectivity is greater than 90%.
3. The determination method according to claim 1 or 2, characterized in that step 4) further comprises:
and under the three-dimensional stress state, if the included angle between the horizontal tensile stress sigma 3 of the layered stratum in the carbonatite of the well region and the lithologic interface is 45-75 degrees, the fracture criterion of the layered stratum is a tension-shear composite fracture criterion.
4. The method for determining the stratum fracture parameters in the carbonatite well zone according to the claim 1 or 2, wherein in the triaxial stress state, if the included angle between the horizontal compressive stress sigma 1 of the layered stratum in the carbonatite well zone and a lithological interface is 45-75 degrees, the fracture criterion of the layered stratum is the Mohr-Coulomb shear criterion.
5. The determination method according to any one of claims 1 to 4, wherein step 4) further comprises:
and under the three-dimensional stress state, if the state of the bedrock in the carbonate rock of the well region is a fracture state, the horizontal tensile stress sigma 3 of the bedrock is less than 0, and the included angle between the sigma 3 and the lithologic interface is 0-45 degrees, the fracture criterion of the brittle rock in the bedrock is the Graves fracture criterion.
6. The determination method according to any one of claims 1 to 4, wherein step 4) further comprises:
and under the three-dimensional stress state, if the state of the bedrock in the well zone carbonate rock is a fracture state, the horizontal tensile stress sigma 3 of the bedrock is less than 0, and the included angle between the sigma 3 and the lithologic interface is 0-45 degrees, the fracture criterion of the weak rock in the bedrock is the Grignard fracture criterion modified by Mirington and Wales.
7. The determination method according to any one of claims 1 to 4, wherein step 4) further comprises:
and under the three-dimensional stress state, if the state of the bedrock in the well area carbonate rock is a fracture state, and the included angle between the horizontal compressive stress sigma 1 of the bedrock and the lithologic interface is 0-45 degrees and 75-90 degrees, the fracture criterion of the brittle rock in the bedrock is a two-stage Moire-Coulomb fracture criterion.
8. The method according to any one of claims 5 to 7, wherein step 4) further comprises:
determining the state of the bedrock using the Deluker-prager yield criterion.
9. The determination method according to any one of claims 1 to 8, characterized in that step 1) is preceded by:
and collecting the seismic interpretation section of the research area, recovering the fracture distance and the two-dimensional fold form by adopting a structural balance section method, and determining the fracture-making period of the carbonate reservoir fracture of the well area.
10. A determination method according to any one of claims 1 to 9, wherein the obtaining stress parameters of the carbonate rock of the well zone during the fracture making period comprises the following:
statistically calculating the orientation of a bisector of a conjugate fracture included angle by adopting a structural tracing method, and determining the maximum principal stress direction of the carbonate rock of the well region at different positions in the carbonate rock reservoir of the well region in the fracture-making period;
and preparing the well zone carbonate rocks at different positions in the well zone carbonate rock reservoir stratum into various standard plunger samples, and testing Kessel points of the standard plunger samples by a rock cracking acoustic emission method to obtain the paleo-stress values of the well zone carbonate rocks at different positions in the fracture-making period.
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