CN105386756B - A method of brittle formation porosity is calculated using dependent variable - Google Patents
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Abstract
The present invention provides a kind of method for calculating brittle formation porosity using dependent variable, structure interpretation is carried out to brittle formation, tomography first, Depth Domain geological model is established using explanation data, then the cumulative dependent variable of brittle formation is calculated by three-dimensional paleostructure restoration methods, the rock elastic parameter of brittle formation is sought using drilling extracting core test, the fracture porosity of brittle formation is calculated according to dependent variable-elasticity mechanics parameter of foundation-fracture porosity relational expression, and be corrected using core porosity, work out porosity flat distribution map.The present invention improves the precision of prediction of brittle formation porosity, and method is simple, has a good application prospect improving oil-gas reservoir probing success rate, avoiding construction risk and improve efficiency aspect.
Description
Technical Field
The invention relates to the field of oil and gas geological exploration processing methods, in particular to a method for calculating the porosity of a brittle stratum by applying a dependent variable.
Background
A large number of exploration practices show that holes and fractures in a brittle stratum are main channels and storage spaces for oil and gas seepage, so that the porosity of the stratum has a close relationship with the type of oil reservoirs. The method mainly comprises a porosity calculation method based on well logging, a calculation method based on earthquake and geological statistics and a porosity calculation method based on structural cause. There are two ways to calculate the fracture porosity based on well logging, one is to use the calculation of the dual lateral well logging information, such as li army, etc. to use the dual lateral numerical simulation based on the flat plate model to derive the calculation model of the fracture porosity (li army, 1996), but because the fracture porosity calculated by the dual lateral is greatly related to the resistivity value, there is still a big problem in the resistivity reduction section caused by non-fracture factors when the fracture porosity is calculated by the dual lateral (zhao, 2012). And the other method is to use micro-resistivity imaging logging information for calculation, the method can be used for precisely describing and quantitatively evaluating reservoir cracks and holes, but the imaging logging cost is high, so that the application range of the method is limited. In addition, the method for calculating the formation porosity by logging can only obtain the formation porosity at a well point, and the formation porosity of formations other than the well point, especially formations with large mean lateral variation, cannot be accurately presumed. The method for calculating the formation porosity based on the seismic data is influenced by the resolution of the seismic data, and the prediction precision and accuracy are generally low.
As the porosity of the stratum is influenced by deposition, diagenesis and tectonic action, for the brittle stratum, stratum deformation caused by the tectonic action causes a large amount of secondary cracks and holes in the rock, and becomes a main factor influencing the porosity of the brittle stratum. And researches prove that the rock deformation has a close relation with the behavior of the rock to generate secondary cracks and holes (fracture) and the elastic mechanical property of the rock. Since the 70 s of the 20 th century, Brady and Duvall (1973), Jeager and cook (1976), Brace and Kohlstedt (1980) and He et al (1990) and the like successively carried out experiments, discussion and summarization on the whole process, development stage, curve change and related mechanical physical phenomena of rock fracture behavior, and domestic scholars of Kummer and Hei (2004), Zhang years (2011) and the like also carried out researches on rock fracture behavior and the relationship between the rock fracture behavior and rock elastic mechanical parameters, and found that in uniaxial and triaxial failure experiments of rocks, the rocks generally undergo four stages of fracture closure compaction deformation, pore contraction elastic deformation, micro-cleavage expansion and macro-cleavage development. Based on the knowledge of rock deformation and fracture cause, the scholars establish a mathematical model between stress-rock elastic mechanics parameter-fracture parameter on the basis of numerical simulation of structural stress field, and describe the pore permeability (porosity and permeability) of the brittle stratum by calculating fracture related parameters (fracture density, opening degree and the like). For example, Deng Pan et al (2006) simulates a three-dimensional structural stress field through numerical values, and establishes a relation between the stress field and the fracture rate according to a Griffis criterion and a coulomb-mol fracture criterion; the Quzongzhewn and the like (2010) derive relational expressions between stress, fracture surface energy, rock elastic mechanical parameters and fracture parameters (fracture opening, fracture porosity and permeability) from the angle of energy conservation before and after rock deformation. However, the process of numerically simulating the tectonic stress field by the finite element method needs to construct a reasonable geological model and endow reasonable boundary acting force, and due to strong subjectivity in recognizing the ancient tectonic pattern and the model boundary load, the problems of contradiction between the simplified model and the complex geological body and the like, the reliability of the predicted crack related parameters is influenced.
On the other hand, a rock is deformed by stress, and the rate of change from its original size after deformation is called strain. It is generally accepted that for brittle formations, the greater the strain, the more developed secondary formation fractures and pores in the formation, the better the pore permeability of the formation. For example, the upper disc strata generates wrinkle deformation after passing through fault turning in the structural deformation process, and the more times of passing through fault turning, the larger the obtained accumulated strain, the more the crack is developed, such as the front wing, the dorsal oblique axial plane and other parts of the wrinkle (what happens, 2005). With the improvement of the structure recovery theory and the improvement of the computer performance, the form of the stratum before deformation can be recovered through three-dimensional structure recovery software such as Geosec-3D, 3DMove and the like, the strain is obtained according to the change rate of the area or the volume before and after the deformation of the stratum unit, and the fracture development condition is qualitatively evaluated according to the strain distribution rule. For example, 3DMove applied to Wavensheng and the like (2009) performs structure recovery and strain quantity calculation on carbonate rock underground mountains in south of the wheel, and the position with large strain quantity is found to correspond to a crack growth area; a pipe tree Wei and the like (2010) establish a surface model and a body model on a Gocad platform, and utilize a three-dimensional recovery plug-in unit to carry out structural recovery so as to obtain a strain distribution map of a stratum; korea et al (2013) have predicted the degree of crack development by finding the amount of strain of the invaded rock mass based on the three-dimensional structure restoration technique. However, the method for obtaining the formation strain through three-dimensional structure recovery can only qualitatively describe the development of the fracture and the permeability of the formation pore, but cannot quantitatively calculate fracture parameters such as the porosity of the fracture, and the method for obtaining the porosity of the fracture through the strain is not reported.
The invention provides a method for calculating the porosity of a brittle stratum by using a strain, which aims at the problems, and the method comprises the steps of obtaining the strain of the stratum by using a three-dimensional structure recovery method, establishing a relational expression of the strain, rock elastic mechanical parameters and the porosity of a crack according to a rock elastic deformation theory, calculating the porosity of the crack, correcting the porosity of the crack by using the porosity of a rock core, and finally obtaining a porosity plane distribution diagram of the brittle stratum. The method is high in prediction accuracy, simple and easy to implement, and has good application prospects in the aspects of improving the drilling success rate of the oil-gas reservoir, avoiding construction risks and improving efficiency.
Disclosure of Invention
The invention provides a method for calculating porosity of a brittle formation by applying dependent variable, which is characterized by establishing a fracture model of the brittle formation rock according to a rock physics theory, deducing a dependent variable-porosity relational expression, calculating the formation dependent variable by applying three-dimensional structure recovery software 3Dmove, and simply and quickly calculating the fracture porosity of the brittle formation by combining elastic mechanical parameters of the brittle formation rock so as to achieve the purposes of improving the accuracy and precision of predicting the porosity of the brittle formation and reducing the investment cost.
In order to achieve the purpose, the invention adopts the following technical measures to realize the purpose:
a method of calculating porosity of a brittle formation using a strain quantity, comprising the steps of:
step 1, determining a main geological period of brittle stratum fracture development by analyzing a geological background of an area where a brittle stratum is located, applying three-dimensional seismic data and well drilling and logging data to make an artificially synthesized seismic record so as to determine a stratum corresponding to the main geological period of brittle stratum fracture development and a corresponding homodromous axis of the brittle stratum on a seismic data body, tracking and interpreting the stratum to obtain a time domain stratum, and interpreting fault data intersected with the time domain stratum;
step 2, establishing a depth domain geological model by using the interpreted time domain stratum and fault data;
step 3, calculating the strain epsilon of the brittle stratum, namely if the grid intervals in the X direction and the Y direction are defined as a and b respectively in the step 2, the area A of each triangular grid after the stratum is deformediThe area B of each triangular mesh of the initial stratum corresponding to the restored structure is (a multiplied by B)/2i(a '× b')/2, then correspondingly to groundStrain epsilon of each triangular mesh after layer deformationi＝|Ai-Bi|/Bi；
Step 4, measuring the elastic mechanical parameters of the brittle stratum rock at a certain well hole, namely the elastic modulus (E) and the internal friction angle by using a triaxial stress tester
And 5, calculating the fracture porosity phi of the brittle stratum by using the following formula:
wherein,
epsilon is the formation strain, E is the elastic modulus,the internal friction angle is defined as rho, the average paleodensity of the overlying stratum, g is the acceleration of gravity (9.8Kg/N), and h' is the paleoburial depth of the brittle stratum;
and 6, correcting the calculated fracture porosity phi by using the well drilling and logging data at the well hole to obtain a porosity plane distribution map of the brittle stratum.
The above scheme further comprises:
the three-dimensional seismic data in the step 1 are seismic data subjected to pre-stack time migration processing or post-stack time migration processing; the tracing interpretation of the brittle stratum is usually a transverse tracing interpretation after the existing well drilling and logging data are utilized to carry out fine calibration and identify the corresponding coaxial axis of the brittle stratum in a Geoframe or Landmark interpretation system;
the step 2 of establishing the depth domain model by using the interpreted time domain stratum and fault data refers to applying three-dimensional structure recovery software, selecting a data format, selecting time domain stratum and fault data to be imported, defining grid intervals in the X direction and the Y direction in an interval, selecting a time-depth conversion module, inputting time-depth conversion parameters, and establishing the depth domain geological model.
Calculating the strain epsilon of the brittle formation in the step 3 refers to selecting brittle formation data by applying three-dimensional structure recovery software 3Dmove and calculating the area A of each triangular mesh after the formation is deformedi(ii) a Selecting fault data, selecting upper plate stratum data and lower plate stratum data to obtain ancient burial depth h' of brittle stratum fracture development period, and calculating area B of each triangular mesh of initial stratum after structure recoveryiWhile being based on epsiloni＝|Ai-Bi|/BiCalculating the strain epsilon of each triangular mesh after the corresponding stratum is deformedi。
Step 4, determining the elasto-mechanical parameters of the brittle stratum rock at a certain well hole by using the triaxial stress tester, namely, taking the core of the non-development well section of the brittle stratum fracture, estimating the stratum average density of the brittle stratum fracture in the development period by combining the geological background of the brittle stratum, applying the paleoburial depth h' of the brittle stratum fracture in the development period obtained in the step 3, and calculating the paleostatic rock pressure (P) range of the brittle stratum according to the following formula
P＝ρgh′
Rho is the average paleo-density of the overburden stratum, g is the acceleration of gravity (9.8Kg/N), and h' is the paleoburial depth of the brittle stratum
Taking the median value of the pressure of the paleontological lithology as the test confining pressure, and loading sigma by using a triaxial stress tester1、σ2、σ3To test confining pressure, holding sigma1、σ2Constant, gradual unloading σ3Until the rock is cracked, drawing the stress-strain relation curve of the core of the brittle stratumCalculating to obtain the elastic modulus of the brittle stratum rock according to the E ═ sigma/epsilon, namely the elastic modulus E is the slope of the stress-strain curve; changing the test confining pressure (the test confining pressure is in the range of the pressure of the ancient lithostatic rocks), and loading sigma by using a triaxial stress tester1、σ2、σ3To test confining pressure, holding sigma1、σ2Constant, gradual unloading σ3Until rock fracture occurred, test for different sigma1Value σ at rock fracture3Drawing a Mohr circle and a Mohr envelope, wherein the included angle between the Mohr envelope and a horizontal line is the internal friction angle of the core of the brittle stratum
And 6, correcting the calculated fracture porosity phi by using the well drilling and logging data at the well hole, namely calculating a correlation coefficient between the actually measured porosity of the well drilling and logging and the fracture porosity calculated in the step 5 by using a correlation coefficient formula, repeating the steps 1 to 5 if the correlation coefficient is smaller than a certain set threshold, and correcting the fracture porosity calculated in the step 5 by using an inverse distance weighting correction method if the correlation coefficient is larger than the certain set threshold.
The more optimized technical scheme of the invention comprises the following steps:
step 1, determining a main geological period of brittle stratum fracture development by analyzing a geological background of an area where a brittle stratum is located, applying three-dimensional seismic data and well drilling and logging data to make an artificially synthesized seismic record so as to determine a stratum corresponding to the main geological period of brittle stratum fracture development and a corresponding homodromous axis of the brittle stratum on a seismic data body, tracking and interpreting the stratum to obtain a time domain stratum, and interpreting fault data intersected with the time domain stratum;
the three-dimensional seismic data are seismic data subjected to pre-stack time migration processing or post-stack time migration processing;
the tracing interpretation of the brittle stratum is usually a transverse tracing interpretation after a logging interpretation result of an existing well is utilized to perform fine calibration and identify a coaxial corresponding to the brittle stratum in a Geoframe or Landmark interpretation system;
step 2, establishing a depth domain geological model by using the interpreted time domain stratum and fault data;
the method for establishing the depth domain model by using the explained time domain stratum and fault data is characterized by applying three-dimensional structure recovery software 3Dmove, clicking File in a menu bar of the 3Dmove software, clicking Import, selecting a data format Ascii, selecting time domain stratum and fault data to be imported, clicking Create Surfaces and clicking OK. Clicking Edit in the menu bar, clicking repeat, selecting Grid, and defining Grid spacing in the X direction and the Y direction in interval. Clicking on the Operation on a menu bar, selecting a Depth Conversion module Depth Conversion, clicking Time- > Depth, clicking Depth Time function, selecting Exponential, inputting a Depth Conversion parameter, clicking Apply, and establishing a Depth domain geological model.
Step 3, calculating the dependent variable e of the brittle stratum;
the strain epsilon of the brittle stratum is calculated by defining the grid intervals in the X direction and the Y direction as a and b respectively in the step 2, and then the area A of each triangular grid after the stratum is deformediThe area B of each triangular mesh of the initial stratum corresponding to the restored structure is (a multiplied by B)/2i(a '× b')/2, the strain amount ∈ of each triangular mesh after deformation of the corresponding formationi＝|Ai-Bi|/Bi。
The Strain epsilon of the brittle stratum is calculated by applying three-dimensional structure recovery software 3Dmove, clicking Analysis on a menu bar, clicking string, selecting brittle stratum data, clicking Apply on a string Analysis window, and calculating the area A of each triangular grid after the stratum is deformedi. Clicking restore On a menu bar, selecting Move On Fault, selecting an incorporated Shear algorithm, selecting Fault data On the Fault bar, and selecting a Hanging wall layer On a Hanging wall barAnd (4) data. Selecting Variable heel, clicking heel band on a heel Editor window, clicking Create, selecting upper disc stratum data and lower disc stratum data, clicking Apply, and clicking start on an incorporated Shear window to carry out construction recovery, thereby obtaining the paleoburial depth h' of the brittle stratum fracture development period. Clicking Apply in a string Analysis window, and calculating the area B of each triangular mesh of the initial stratum corresponding to the restored structureiWhile being based on epsiloni＝|Ai-Bi|/BiCalculating the strain epsilon of each triangular mesh after the corresponding stratum is deformedi。
Step 4, measuring the elastic mechanical parameters of the brittle stratum rock at a certain well hole by using a triaxial stress tester;
the method for measuring the elastic mechanical parameters of the brittle stratum rock at a certain well hole by using the triaxial stress tester comprises the steps of taking the core of the non-development well section of the brittle stratum fracture, estimating the stratum average density of the brittle stratum fracture in the development period by combining the geological background of the brittle stratum, calculating the paleo-burial depth h 'of the brittle stratum fracture in the development period by applying the paleo-burial depth h' of the brittle stratum fracture in the step 3, and calculating the paleo-statis pressure (P) range of the brittle stratum according to the following formula
P＝ρgh′
Rho is the average paleo-density of the overburden stratum, g is the acceleration of gravity (9.8Kg/N), and h' is the paleoburial depth of the brittle stratum
Taking the median value of the pressure of the paleontological lithology as the test confining pressure, and loading sigma by using a triaxial stress tester1、σ2、σ3To test confining pressure, holding sigma1、σ2Constant, gradual unloading σ3When the rock is cracked, drawing a stress-strain relation curve of the core of the brittle stratum, and calculating to obtain the elastic modulus of the rock of the brittle stratum according to the condition that E is sigma/epsilon, namely the elastic modulus E is the slope of the stress-strain curve; changing the test confining pressure (the test confining pressure is in the range of the pressure of the ancient lithostatic rocks), and loading sigma by using a triaxial stress tester1、σ2、σ3To test confining pressure, holding sigma1、σ2Invariable, gradually unloadedCarrying sigma3Until rock fracture occurred, test for different sigma1Value σ at rock fracture3Drawing a Mohr circle and a Mohr envelope, wherein the included angle between the Mohr envelope and a horizontal line is the internal friction angle of the core of the brittle stratum
And 5, calculating the fracture porosity phi of the brittle stratum by using the following formula:
wherein,
epsilon is the formation strain, E is the elastic modulus,and the internal friction angle is defined as rho, the average paleodensity of the overlying stratum, g, the gravity acceleration (9.8Kg/N) and h', the paleoburial depth of the brittle stratum.
In the step 5, a mathematical model established based on rock physics is adopted, and a relation between the strain quantity and the porosity of the brittle formation is deduced according to a rock mechanics basic theory, wherein the concrete derivation process is as follows:
(1) from the law of stress-strain for brittle rock fracture (according to ge peace, 2004), the following mathematical model is established: assuming that the initial volume before rock deformation is V1Stress reaches the rock strength limit (σ)c) The volume of time is V2The corresponding strain is the rock fracture tensile strain (. epsilon.) in this casec) I.e. the deformation of the rock causes fractures in the rock but no macroscopic cracking, assuming that the rock is only brittle and that after macroscopic cracking the rock strain is caused by the volume enlargement of the crack, the final body after crackingProduct is V3The corresponding dependent variable is the total dependent variable (epsilon) of the rock. When the total rock strain (epsilon) is larger than the rock fracture strain (epsilon)c) When it is, then there are
The strain-porosity equation can thus be established as:
εcachieving ultimate strength σ for rockcThe amount of strain at time, i.e., the amount of fracture strain; ε is the amount of strain in the rock.
(2) According to generalized hooke's law, the stress-strain relationship under triaxial stress is:
assuming that constructive distortion occurs only at σ1-σ3In-plane, along the minimum principal stress (σ)3) Direction is subjected to stretching deformation, and the direction of intermediate principal stress (sigma)2) Does not produce strain (epsilon)20), then derived from equation (2):
(3) according to the coulomb-moire breakup criterion:
is an internal friction angle
(4) According to the research of the institute of geology and geophysical tension (2011), the Poisson's ratio (mu) and the internal friction angle of the brittle rockThe relationship of (1) is:
then from the above relationship, the maximum principal stress (σ) can be derived1) With maximum tensile strain (epsilon)3) The relation of (1):
wherein
Therefore, the maximum principal stress (σ) can be derived1) Reach rock fracture limit (sigma)c) When the corresponding tensile strain of fracture (epsilon) of the rockc) The size of (2):
according to the Anderson fault classical mode, the direction of the maximum principal stress of an extended basin is the gravity direction, and if the pore fluid pressure of the stratum is not considered, the maximum principal stress generates vertical pressure, namely the palustite pressure (P):
P＝ρgh′
rho is the average paleo-density of the overburden stratum, g is the acceleration of gravity (9.8Kg/N), and h' is the paleoburial depth of the brittle stratum
The process of generating the normal fault when the rock is cracked is that when the maximum principal stress, namely the vertical pressure (P) of the overlying strata is kept constant, the minimum principal stress is continuously reduced, and when the stress Morel circle is tangent to the shear fracture line, the rock is cracked, so that the normal fault is generated
σc≈ρgh′
Then derive
The formula is synthesized to obtain
Wherein,
epsilon is the formation strain, E is the elastic modulus,and the internal friction angle is defined as rho, the average paleodensity of the overlying stratum, g, the gravity acceleration (9.8Kg/N) and h', the paleoburial depth of the brittle stratum.
And 6, correcting the calculated fracture porosity by using the well drilling and logging data of the well hole to obtain a porosity plane distribution map of the brittle stratum.
And (5) calculating the correlation coefficient between the actually measured porosity of the well drilling and well logging and the porosity of the fracture calculated in the step (5) by using a correlation coefficient formula, wherein the formula for calculating the correlation coefficient is as follows:
r is the correlation coefficient between the measured porosity and the calculated porosity, assuming n wells participating in the calculation, phiiIs the measured porosity value of the ith well,is the average value of the measured porosity of n wells, phi' is the calculated porosity of the ith well,the average value of porosity was calculated for n wells.
If the correlation coefficient r is smaller than a certain set threshold value, repeating the steps 1-5, if the correlation coefficient r is larger than the certain set threshold value, correcting the calculated fracture porosity value by using the actually measured porosity value such as actually measured drilling and well logging, and correcting by using an inverse distance weighting method, wherein the calculation formula is as follows:
phi (x, y) is the porosity value at the point with coordinates (x, y), phiiMeasured porosity value of the ith well, diIs the distance from the (x, y) point to the ith well.
The invention has the beneficial effects that:
the method overcomes the defects of limited range of the porosity of the tested stratum, high testing price and the like of well logging and well drilling coring, is less influenced by the resolution of seismic data, combines the physical parameters of the rock of the brittle stratum, calculates the porosity of the stratum simply, conveniently and quickly by applying dependent variables, improves the accuracy and precision of predicting the porosity of the brittle stratum and reduces the investment cost.
Drawings
FIG. 1 is a flow chart of an embodiment of a method of calculating porosity of a brittle formation using a strain gauge according to the present invention;
FIG. 2 is a built depth domain geological model;
FIG. 3 is a strain profile of a brittle formation calculated using three-dimensional formation recovery software;
FIG. 4 is a graph of fracture porosity of a brittle formation calculated using a strain-porosity relationship;
FIG. 5 is a cross-plot of measured porosity versus calculated porosity;
FIG. 6 is a plot of porosity in a brittle formation corrected using inverse distance weighting.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the present invention will be further described in detail with reference to the accompanying drawings 1-6. The exemplary embodiments and descriptions of the present invention are provided to explain the present invention, but not to limit the present invention.
Step 1, determining a main geological period of brittle stratum fracture development by analyzing geological background of an area where a brittle stratum is located, and making an artificial synthetic seismic record by using three-dimensional seismic data and well drilling and logging data to determine brittle stratum fractureThe stratums corresponding to the main geological period of development and the stratums corresponding to the brittle stratums on the seismic data volume are tracked and interpreted to obtain time domain horizon data (the time domain horizon corresponding to the main geological period of development of the brittle stratums is assumed to be T1The time domain horizon corresponding to the brittle stratum is T2) And interpreting fault data intersected with the time domain stratum;
step 2, establishing a depth domain geological model by using the interpreted time domain horizon and fault data;
the method comprises the steps of applying three-dimensional structure recovery software 3Dmove, clicking File in the 3Dmove software, clicking Import, selecting a data format Ascii, and selecting a time domain horizon (T) to be imported1、T2) And fault data, click Creat Surfaces, click OK. Clicking Edit in the menu bar, clicking repeat, selecting Grid, and defining the Grid spacing in the X direction and the Y direction as 300 and 300 respectively in interval. Click on Operation, select Depth Conversion module Depth Conversion, click Time->Depth, click on the DepthTime Function, select Exponential, and convert formula D to a × ebTIn + c, time-depth conversion parameters a 3846, b 0.000244, and c 3846 are given, respectively, and then Apply is clicked to create a depth domain geological model (see fig. 2).
Step 3, calculating the strain epsilon of the brittle stratum;
using three-dimensional structure recovery software 3Dmove, clicking Analysis and string in menu bar, selecting brittle stratum (T)2) Clicking Apply on the string Analysis window to calculate the area A of each triangular mesh after deformation of the brittle formation (T2)i300 × 300/2, the area a of any two grids Grid1 and Grid21＝300，A2300. Clicking restore On a menu bar, selecting Move On Fault, selecting incorporated Shear algorithm, selecting Fault data in Fault bar, and selecting T in Hanging wall bar1、T2And (4) hanging the disk formation data. Selecting Variable Heave, clicking Heave band on Heave Editor window, clicking Create, selecting T1Upper disk formation data and T1The data of the lower wall stratum is clicked, Aply is clicked, start is clicked in an incorporated Shear window to carry out structure recovery, the structure recovery process is completed, and the brittle stratum (T) is obtained2) During crack development, i.e. T1The paleoburial depth h' during the deposition period. Clicking Apply on a string Analysis window to calculate the brittle formation (T)2) Initial area of each triangular mesh BiAccording to epsiloni＝|Ai-Bi|/BiCalculating a brittle formation (T)2) Strain epsilon of each triangular mesh after deformationi(refer to fig. 3). Suppose a corresponding brittle formation (T) after recovery of the formation2) Any two triangular grids Grid1, Grid2 initial area B1＝250，B2350, then
ε1＝|300-250|/250＝0.2
ε2＝|300-350|/350＝0.142
Step 4, measuring the elastic mechanical parameters of the brittle stratum rock at a certain well hole by using a triaxial stress tester: modulus of elasticity (E) and internal angle of friction
Taking a rock core of a non-developing well section of a brittle stratum fracture, and estimating the average stratum density rho of the brittle stratum fracture during the development period to be 2.3 multiplied by 10 by combining the geological background of the brittle stratum3Kg/m3And applying the ancient burial depth h ' of the brittle stratum fracture development period obtained in the step 3 to obtain h ' more than or equal to 1350m and less than or equal to 2200m, and then obtaining the depth h ' more than or equal to 1350m according to a formula
P＝ρgh′
Rho is the average paleo-density of the overburden stratum, g is the acceleration of gravity (9.8Kg/N), and h' is the paleoburial depth of the brittle stratum
Calculating to obtain the ancient dead rock pressure P: p is more than or equal to 30.4MPa and less than or equal to 49.6 MPa.
Taking the median value of the pressure of the paleontological lithology, namely 40MPa, and loading sigma by using a triaxial stress tester1、σ2、σ3To 40MPa, holding sigma1、σ2Constant, gradual unloading σ3When the rock is cracked, drawing a stress-strain relation curve of the core of the brittle stratum, and calculating to obtain the elastic modulus of the rock of the brittle stratum to be 17.6GPa according to the condition that E is sigma/epsilon and the elastic modulus E is the slope of the stress-strain curve; varying the magnitude of confining pressure, measuring sigma1Sigma of core fracture at 30MPa, 50MPa respectively3Drawing a Mohr circle and a Mohr envelope line, measuring an included angle between the Mohr envelope line and a horizontal line, namely an internal friction angle of the core of the brittle stratum, and calculating to obtain the internal friction angle of the brittle stratumIs 36 deg..
Step 5, calculating the strain quantity epsilon and the paleoburial depth h' of the brittle formation by combining the step 3 and the relational expression of the strain quantity and the porosity of the brittle formation deduced according to the rock mechanics basic theory, and testing the elastic modulus E and the internal friction angle of the brittle formation obtained in the step 4And calculating the fracture porosity phi of the brittle stratum.
Wherein,
epsilon is the formation strain, E is the elastic modulus,the internal friction angle rho is the average paleo-density of the overburden stratum, g is the acceleration of gravity (9.8Kg/N), and h' is the paleo-burial depth of the brittle stratum.
Suppose that the above-mentioned brittle formation (T) is obtained according to step 32) Any two triangular meshes Grid1, Grid2Of ancient buried depth h'1＝1350m，h′21600m, according to
εc1＝2.3×103Kg/m3×9.8N/Kg×1350m×0.083/17.6GPa＝0.146
εc2＝2.3×103Kg/m3×9.8N/Kg×1650m×0.083/17.6GPa＝0.179
Due to epsilon1＝0.2，ε2＝0.142
ε1＞εc1Phi of1＝(ε1-εc1)/(ε1+1)＝0.045＝4.5％
ε2＜εc2Phi of2＝0
For brittle formation (T)2) And (3) calculating each triangular mesh according to the formula to obtain the crack porosity value of each triangular mesh, wherein the calculation result is shown in figure 4.
And 6, correcting the calculated fracture porosity by using actually measured porosity data of well drilling and well logging at the well hole to obtain a porosity plane distribution map of the brittle stratum.
And (5) calculating the correlation coefficient between the measured porosity of the well drilling and well logging and the porosity calculated in the step (5) by using a correlation coefficient formula, wherein the formula for calculating the correlation coefficient is as follows:
r is the correlation coefficient between the measured porosity and the calculated porosity, assuming n wells participating in the calculation, phiiIs the measured porosity value of the ith well,is the average value of the measured porosity of n wells, phi' is the calculated porosity of the ith well,the average value of porosity was calculated for n wells.
Setting the threshold value of the correlation coefficient r to be 0.55, repeating the steps 1-5 when r is less than 0.55, and correcting the calculated fracture porosity value by using the actually measured porosity values of actually measured drilling, logging and the like if r is more than or equal to 0.55. Fig. 5 is an intersection of the measured porosity and the calculated porosity, and 38 wells are involved in the calculation to obtain a correlation coefficient r between the measured porosity and the calculated porosity, which is 0.62, and since r is greater than 0.55, the calculated fracture porosity is corrected by applying an inverse distance weighting method, and the calculation formula is as follows:
phi (x, y) is the porosity value at the point with coordinates (x, y), phiiMeasured porosity value of the ith well, diIs the distance from the (x, y) point to the ith well.
FIG. 6 is a plot of porosity in a brittle formation corrected using inverse distance weighting, where the porosity values at well points are consistent with actual values and the porosity values outside the well points have greater reliability.
Therefore, the method is effectively supplemented by the existing complex mathematical calculation and software simulation, improves the prediction precision of the porosity of the brittle stratum, is simple and easy to implement, and has good application prospects in the aspects of improving the drilling success rate of the oil-gas reservoir, avoiding construction risks and improving efficiency.
Claims (6)
1. A method for calculating the porosity of a brittle formation by applying a dependent variable is characterized by comprising the following steps:
step 1, determining a stratum corresponding to a main geological period of brittle stratum fracture development by analyzing a geological background of an area where a brittle stratum is located, applying three-dimensional seismic data and well drilling and logging data to make an artificially synthesized seismic record so as to determine a corresponding homodromous axis of the brittle stratum on a seismic data body, tracking and interpreting the homodromous axis, wherein the result of tracking and interpreting the homodromous axis is time domain stratum data of the brittle stratum, and interpreting fault data intersected with the time domain stratum data;
step 2, establishing a depth domain geological model by using the explained time domain stratum data and fault data;
step 3, calculating the strain epsilon of the brittle stratum, namely if the grid intervals in the X direction and the Y direction are defined as a and b respectively in the step 2, the area A of each triangular grid after the stratum is deformediThe area B of each triangular mesh of the initial stratum corresponding to the restored structure is (a multiplied by B)/2i(a '× b')/2, the strain amount ∈ of each triangular mesh after deformation of the corresponding formationi＝|Ai-Bi|/Bi；
Step 4, measuring the elastic mechanical parameters of the brittle stratum rock at a certain well hole, namely the elastic modulus E and the internal friction angle by using a triaxial stress tester
And 5, calculating the fracture porosity phi of the brittle stratum by using the following formula:
wherein,
epsilon is the formation strain, E is the elastic modulus,the internal friction angle is defined as rho, the average paleodensity of the overlying stratum, g is the acceleration of gravity, and h' is the paleoburial depth of the brittle stratum;
and 6, correcting the calculated fracture porosity phi of the brittle stratum by using the actually measured porosity of the rock core to obtain a porosity plane distribution map of the brittle stratum.
2. The method of calculating porosity of a brittle formation using a strain according to claim 1, wherein:
the three-dimensional seismic data in the step 1 are seismic data subjected to pre-stack time migration processing or post-stack time migration processing; the tracking interpretation of the same directional axis is a transverse tracking interpretation after the same directional axis corresponding to the brittle stratum is identified by using the existing well drilling and logging data to perform fine calibration in a Geoframe or Landmark interpretation system.
3. The method of calculating porosity of a brittle formation using a dependent variable as claimed in claim 1 or 2, wherein:
the step 2 of establishing the depth domain model by using the interpreted time domain stratum and fault data refers to applying three-dimensional structure recovery software, selecting a data format, selecting time domain stratum and fault data to be imported, defining grid intervals in the X direction and the Y direction in an interval, selecting a time-depth conversion module, inputting time-depth conversion parameters, and establishing the depth domain geological model.
4. The method of calculating porosity in a brittle formation as claimed in claim 3, wherein the method further comprises the steps of: calculating the strain epsilon of the brittle formation in the step 3 refers to selecting brittle formation data by applying three-dimensional structure recovery software 3Dmove and calculating the area A of each triangular mesh after the formation is deformedi(ii) a Selecting fault data, selecting upper plate stratum data and lower plate stratum data to obtain ancient burial depth h' of brittle stratum fracture development period, and calculating area B of each triangular mesh of initial stratum after structure recoveryiWhile being based on epsiloni＝|Ai-Bi|/BiCalculating the strain epsilon of each triangular mesh after the corresponding stratum is deformedi。
5. The method of calculating porosity in a brittle formation as claimed in claim 4, wherein the method further comprises the steps of: step 4, determining the elasto-mechanical parameters of the brittle stratum rock at a certain well hole by using the triaxial stress tester, namely, taking the core of the non-development well section of the brittle stratum fracture, estimating the stratum average density of the brittle stratum fracture in the development period by combining the geological background of the brittle stratum, applying the paleoburial depth h' of the brittle stratum fracture in the development period obtained in the step 3, and calculating the paleostatic rock pressure P of the brittle stratum according to the following formula
P＝ρgh′
Rho is the average paleo-density of the overburden stratum, g is the acceleration of gravity, and h' is the paleo-burial depth of the brittle stratum
Taking the median value of the pressure of the paleontological lithology as the test confining pressure, and loading sigma by using a triaxial stress tester1、σ2、σ3To test confining pressure, holding sigma1、σ2Constant, gradual unloading σ3When the rock is cracked, drawing a stress-strain relation curve of the core of the brittle stratum, and calculating to obtain the elastic modulus of the rock of the brittle stratum according to the condition that E is sigma/epsilon, namely the elastic modulus E is the slope of the stress-strain curve; increasing or decreasing the test confining pressure, loading sigma with a triaxial stress tester1、σ2、σ3To test confining pressure, holding sigma1、σ2Constant, gradual unloading σ3Until rock fracture occurred, test for different sigma1Value σ at rock fracture3Drawing a Mohr circle and a Mohr envelope curve, wherein the included angle between the Mohr envelope curve and a horizontal line is the internal friction angle of the brittle stratum rock
6. The method of calculating porosity of a brittle formation using a strain according to claim 5, wherein: and 6, correcting the calculated fracture porosity phi by using the drilling and logging data at the well hole, namely calculating a correlation coefficient of the actually measured porosity of the drilling and logging and the porosity calculated in the step 5 by using a correlation coefficient formula, repeating the steps 1 to 5 if the correlation coefficient is smaller than a certain set threshold, and correcting the porosity calculated in the step 5 by using an inverse distance weighting correction method if the correlation coefficient is larger than the certain set threshold.
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