Coal seam fracture parameter inversion prediction method based on equivalent medium theory model

Abstract

The present application relates to a kind of coal seam fracture parameter inversion prediction method based on equivalent medium theory model, the method includes: by prestack three-dimensional seismic trace set data extraction azimuth angle trace set, gradient term is calculated by linear formula fitting, the azimuth angle trace set includes several 3 bin ×3 bin super trace set;Gradient term is calculated by elliptic equation fitting gradient term each anisotropy gradient term G ani And fracture azimuth angle β.The present application is to solve the calculation and evaluation of coal seam fracture parameter in actual production, fully consider the difference between coal seam and conventional oil and gas reservoir, use equivalent medium theory model to establish the analytical relationship between coal seam fracture parameter and P wave reflection coefficient, and the previous oil and gas reservoir fracture parameter inversion and evaluation technology process and process are improved;In addition, the present application proposes a new inversion technology process and process, using the present application can realize the large-scale multidimensional coal seam fracture parameter inversion calculation and evaluation based on prestack seismic data.

Description

Technical Field

[0001] This invention belongs to the field of seismic exploration technology, and in particular relates to a method for predicting coal seam fracture parameters based on an equivalent medium theory model. Background Technology

[0002] Coalbed methane is an important unconventional energy source in my country. However, in its exploration, existing 3D seismic exploration data acquisition for coalfields largely relies on traditional narrow-azimuth observation systems. Furthermore, traditional structural interpretation is based on post-stack seismic data, neglecting the azimuth information contained in pre-stack seismic data. Additionally, in the seismic inversion and evaluation of coal seam fracture parameters, techniques and methods for fracture prediction from oil and gas reservoirs are often copied or modified, ignoring the characteristics of fracture development and fluid characteristics within coal seams, and failing to consider the unique features of coal seam fracture development from a seismic petrophysical perspective. This invention constructs inversion equations based on a reasonable petrophysical model and develops a targeted seismic inversion technology process and technology for coal seam fracture parameters. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a method for predicting coal seam fracture parameters based on an equivalent medium theoretical model.

[0004] The technical solution adopted in this invention is:

[0005] A method for inverting and predicting coal seam fracture parameters based on an equivalent medium theory model, the method comprising:

[0006] Azimuth angle gathers are extracted from pre-stack 3D seismic gather data, and gradient terms are calculated by fitting a linear formula. The azimuth angle gather includes several 3-dimensional × 3-dimensional super gathers;

[0007] Fitting the gradient term using the elliptic equation Calculate the anisotropic gradient term G ani And the crack azimuth angle β.

[0008] As one embodiment of the invention, the method for obtaining the 3×3 facet hypergather includes:

[0009] Acquire pre-stack 3D seismic gather data within the seismic work area;

[0010] Nine adjacent facets are extracted to form the 3-facet × 3-facet supergear, and the half-width of each supergear is no greater than the radius of the first Fresnel zone.

[0011] As one embodiment of the invention, the gradient term is calculated by fitting a linear formula. include:

[0012] Linear fitting is performed using the following formula, and the gradient term is calculated.

[0013]

[0014] in, Here, A represents the longitudinal wave reflection coefficient, θ represents the intercept, and θ represents the angle of incidence. Represents the azimuth angle.

[0015] As one embodiment of the invention, the method of fitting gradient terms using the elliptic equation include:

[0016] The gradient term is expressed by the following formula.

[0017]

[0018] Among them, W 11 W 12 W 22 Both represent the fitting coefficients of the ellipse equation. The azimuth angle set for the observation system;

[0019] Substituting multiple points to be fitted into the above formula yields the fitting coefficients W for the ellipse equation. 11 W 12 W 22 Each of the points to be fitted corresponds to an azimuth angle value and a gradient term value.

[0020] As one embodiment of the invention, the anisotropic gradient term G is calculated using the following formula. ani And crack azimuth angle β:

[0021]

[0022] As one embodiment of the invention, the method further includes:

[0023] The anisotropic gradient term G ani Perform a t-distribution hypothesis test to obtain the hypothesis test value;

[0024] By comparing the hypothesis test value with the allowable critical value, the anisotropic gradient term G is determined. ani Whether the crack azimuth angle β is within the confidence interval is reliable;

[0025] When it is determined that both are credible within the confidence interval, the anisotropic gradient term G is output. ani and the crack azimuth angle β; when it is determined that both are not credible within the confidence interval, the anisotropic gradient term G is... ani The crack azimuth β value was changed to 0.

[0026] As one embodiment of the invention, the anisotropic gradient term G is expressed using the following formula. ani Perform a t-distribution hypothesis test to obtain the hypothesis test value:

[0027]

[0028]

[0029] in, t represents the standard deviation of the anisotropic gradient term, and t represents the hypothesis test value. and The fitting coefficients W for the elliptic equation are respectively 11 W 12 W 22 Standard deviation.

[0030] The beneficial effects of adopting the above technical solution are as follows:

[0031] The coal seam fracture parameter inversion prediction method based on the equivalent medium theory model provided by this invention has the following advantages:

[0032] (1) An inversion equation and technical process for coal seam fracture parameters were constructed based on the equivalent medium theory model. From a rock physics perspective, the correspondence between coal seam fracture parameters and anisotropy coefficients was characterized, and then the analytical relationship between coal seam fracture parameters and seismic P-wave reflection coefficients was directly established. Based on this, a two-step inversion technical process was designed. This improvement closely adheres to the theoretical foundation of rock physics, which not only controls the inversion accuracy and effect of coal seam fracture parameters, but also effectively simplifies the inversion technical process and technology.

[0033] (2) A new hypothesis testing technology for coal seam fracture parameter prediction results is introduced. Traditional coal seam fracture parameter inversion technology has difficulty controlling the reliability of prediction results. To address this issue, this invention introduces a t-distribution hypothesis test for the inversion results. By setting a confidence interval, the reliability of the prediction results is calculated and judged, thereby extracting coal seam fracture parameter prediction results with higher reliability for imaging and application.

[0034] (3) This invention aims to solve the calculation and evaluation of coal seam fracture parameters in actual production. It fully considers the differences between coal seams and conventional oil and gas reservoirs, establishes the analytical relationship between coal seam fracture parameters and P-wave reflection coefficient using the equivalent medium theory model, and improves the previous technology and process for inverting and evaluating oil and gas reservoir fracture parameters. By inventing a coal seam fracture parameter inversion method based on the equivalent medium theory model, a new inversion technology process and process are proposed. Using this invention, large-scale multi-dimensional coal seam fracture parameter inversion calculation and evaluation based on pre-stack seismic data can be realized. Attached Figure Description

[0035] Figure 1 This is a flowchart of a method for predicting coal seam fracture parameters based on an equivalent medium theory model, provided by an embodiment of the present invention.

[0036] Figure 2 This is a schematic diagram showing the offset distance and azimuth distribution of a single surface element.

[0037] Figure 3 This is a schematic diagram of the offset distance and azimuth distribution of a 3×3 super gather.

[0038] Figure 4 This is a schematic diagram of the elliptic fitting of the gradient term.

[0039] Figure 5 This is a schematic diagram of the location of the study area and the parameters of the observation system.

[0040] Figure 6 This is a schematic diagram of the lithology columnar structure of the study area.

[0041] Figure 7 This is a schematic diagram of a single CDP gather.

[0042] Figure 8 It is the anisotropic gradient term G ani A schematic diagram of the inversion results for the crack azimuth angle β.

[0043] Figure 9 This is a rose diagram showing the location of cracks.

[0044] Figure 10 This is a schematic diagram comparing the crack parameter results obtained by the method provided in this invention with the structural interpretation results.

[0045] Figure 11 It is a statistical rose diagram of fault strike. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of the present invention clearer, the invention will be described clearly and completely below in conjunction with specific embodiments.

[0047] This invention provides a method for inverting and predicting coal seam fracture parameters based on an equivalent medium theory model, such as... Figure 1 As shown, the method includes:

[0048] Step S1: Extract azimuth angle gathers from pre-stack 3D seismic gather data. The azimuth angle gathers include several 3-square-element × 3-square-element super gathers.

[0049] Each of the 3×3 facet hypergatherings is obtained by the following method:

[0050] Step S101: Obtain the data of the pre-stack 3D seismic CDP gather d in the seismic work area.

[0051] The seismic work area has hundreds of thousands of traces, each trace is a surface element, with 500 sampling points and a sampling interval of 1ms. The data set collected from each sampling point is extracted by angle gathers to obtain pre-stack 3D seismic gather data.

[0052] Step S102: Extract 9 adjacent face elements to form the 3-face × 3-face hyper-set d S The half-width of each of the supergathers is no greater than the radius of the first Fresnel zone.

[0053] Here, the half-width of the super set refers to the long half-width and the wide half-width of the super set. In the embodiments of the present invention, the long half-width and the wide half-width are both no greater than the radius of the first Fresnel zone.

[0054] Repeating step S102 will yield multiple super gathers, which together constitute the azimuth angle gather.

[0055] The technology disclosed in step S1 mainly solves the problem that, due to the limitation of the number of stacking operations, the distribution of some azimuth angles and offsets of a single surface element in a traditional pre-stack angle gather is limited. Figure 2 (As shown); The method of forming a super gather by combining multiple facets improves the problem of the limited number of individual facet offsets and shot-receiver distances. Theoretically, the half-width of the extracted super gather is no greater than the radius of the first Fresnel zone. Figure 3 The diagram shows a 3×3 supergathering composed of 9 adjacent facets, which satisfies the above requirements and significantly improves the distribution of offset and azimuth.

[0056] Step S2: Calculate the gradient term by fitting a linear formula.

[0057] In one possible implementation, after obtaining the azimuth angle gather, multiple P-wave reflection coefficients from the azimuth angle gather are used... and the azimuth values ​​corresponding to each longitudinal wave reflection coefficient. Linear fitting is performed using the following formula, and the gradient term is calculated.

[0058]

[0059] in, Here, A represents the longitudinal wave reflection coefficient, θ represents the intercept, and θ represents the angle of incidence. Represents the azimuth angle.

[0060] Step S3: Fit the gradient term using the elliptic equation Calculate the anisotropic gradient term G ani And the crack azimuth angle β.

[0061] Multiple gradient terms are obtained through step S2. Then, the azimuth angle value corresponding to each gradient term can be obtained from the azimuth angle gather. This yields multiple points to be fitted, from which the anisotropic gradient term G can be obtained using the elliptic equation. ani And the crack azimuth angle β.

[0062] It includes the following steps:

[0063] Step S301: Express the gradient term using the following formula.

[0064]

[0065] Among them, W 11 W 12 W 22 Both represent the fitting coefficients of the ellipse equation. The azimuth angle set for the observation system;

[0066] Step S302: Substitute multiple points to be fitted into the formula to obtain the fitting coefficients W of the ellipse equation. 11 W 12 W 22 Each point to be fitted corresponds to an azimuth angle value and a gradient term value. The image of the ellipse fitting can be obtained as follows: Figure 4 As shown, the aforementioned fitting coefficients are obtained based on this image.

[0067] Step S303: Calculate the anisotropic gradient term G using the following formula. ani And crack azimuth angle β:

[0068]

[0069] Step S4: The anisotropic gradient term G... ani Perform a t-distribution hypothesis test to obtain the hypothesis test value.

[0070] The anisotropic gradient term G is expressed by the following formula. ani Perform a t-distribution hypothesis test to obtain the hypothesis test value:

[0071]

[0072]

[0073] in, and The fitting coefficients W for the elliptic equation are respectively 11 W 12 W 22 Standard deviation t represents the standard deviation of the anisotropic gradient term, and t represents the hypothesis test value.

[0074] Step S5: Compare the hypothesis test value with the allowable critical value to determine the anisotropic gradient term G. ani Whether the crack azimuth angle β is within the confidence interval is reliable.

[0075] Step S6: When the anisotropic gradient term G is determined... ani When the crack azimuth angle β is within the confidence interval, the anisotropic gradient term G is output. ani and the crack azimuth angle β; when the anisotropic gradient term G is determined ani When the crack azimuth angle β is not within the confidence interval, the anisotropic gradient term G is... ani The crack azimuth β value was changed to 0.

[0076] Thus, the anisotropic gradient term G is obtained through the aforementioned steps. ani And the crack azimuth angle β.

[0077] Based on the relationship between the Schoenberg linear slip model and the Hudson model, the relationship between the coal seam anisotropy coefficient and the fracture density is established, as shown in equation (1):

[0078]

[0079] In the formula, ε (V) γ (V) and δ (V) C is the anisotropy coefficient; ij Represents the elastic constant. V P and V S denoted as longitudinal wave velocity and transverse wave velocity, respectively, and e represents crack density.

[0080] Substituting equation (1) into the approximate equation for the reflection coefficient of PP waves in HTI-type media, and discarding higher-order terms, we obtain equation (2):

[0081]

[0082] In the formula, Represents the longitudinal wave reflection coefficient, θ and These are the incident angle and the azimuth angle, respectively; Z = ρV P For vertical longitudinal wave impedance, The vertical transverse wave shear modulus; △Z=Z2-Z1, △G=G2-G1, △V P =V P2 -V P1 , △e=e2-e1.

[0083] Further simplified to equation (3):

[0084]

[0085] In the formula, A represents the intercept. G represents the gradient. iso and G ani These are the isotropic and anisotropic gradient phases, respectively.

[0086] In equation (3), only the anisotropic gradient term G ani Related to the relative fracture density Δe of the reservoir, it is generally believed that the fracture density of various rocks within a certain range of the study area is related to the relative fracture density Δe of the reservoir. The value changes relatively little and can be obtained through statistical analysis of well logging curves; therefore, G ani It has a linear positive correlation with Δe, while It can be directly used as an indicator of azimuth. The invention will be further illustrated below with specific implementation examples.

[0087] The study area is located in the Ordos Basin, covering an area of ​​approximately 1.6 km². 2 ,like Figure 5 As shown. Figure 6 The image shows a typical borehole column in this area. The coal-bearing strata are the Carboniferous Taiyuan Formation (C3t) and the Permian Shanxi Formation (P1s). Coal seam #6 is the target reservoir, located at the top of the Taiyuan Formation, with a thickness of 7.04m to 20.77m, averaging 12.70m. The roof is mostly coarse-grained sandstone and fine-grained sandstone, with mudstone in some areas; the floor is mostly mudstone and sandy claystone, with coarse-grained sandstone in some areas.

[0088] 3D seismic data acquisition employed a 16-line, 6-shot high-density, wide-azimuth bundled observation system. Specific observation parameters are as follows: Figure 5 As shown, the size of a single CDP cell is 5m × 5m, and the number of full coverage operations is 8 times (vertical) × 8 times (horizontal); the single CDP gather after pre-stack offset processing is as follows. Figure 7 As shown, the two-way travel time of the reflected wave from coal seam #6 is approximately 400 ms, the phase axis is clear, and the signal-to-noise ratio is high.

[0089] The g-values ​​for the coalbed methane reservoir and mudstone roof in the statistical study area are 0.25 and 0.33, respectively. The value is 0.29, based on the anisotropic gradient term G. ani The relative fracture density Δe of the coalbed methane reservoir was calculated, and the fracture development orientation β was also calculated, such as... Figure 8 and Figure 9 As shown.

[0090] Furthermore, to verify the accuracy of this invention, fault information from the structural interpretation of the study area was projected onto fracture parameter slices obtained using the novel technique proposed in this invention. The results are as follows: Figure 10 As shown in the figure, except for the DF3 and DF5 faults where fractures are not well developed, the other faults are located near areas with well-developed or moderately developed fractures. Figure 11 A statistical rose diagram of the fault strike in this area is presented. Most faults strike near 0°, which is consistent with... Figure 8 The indicated fracture orientations are largely consistent. Overall, the fracture density and orientation results obtained using the technology of this invention show significant consistency with the location and strike of most faults.

[0091] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for predicting coal seam fracture parameters based on an equivalent medium theory model, characterized in that, The method includes: Acquire pre-stack 3D seismic gather data within the seismic work area; Nine adjacent facets are extracted to form the 3×3 facet supergather, and the half-width of each supergather is no greater than the radius of the first Fresnel zone; azimuth angle gathers are extracted from pre-stack 3D seismic gather data, and gradient terms are calculated by fitting a linear formula. The azimuth angle gather includes several 3-square × 3-square super gathers; Linear fitting is performed using the following formula, and the gradient term is calculated. : ； in, A represents the longitudinal wave reflection coefficient, and A represents the intercept. Represents the angle of incidence. Represents azimuth; Fitting the gradient term using the elliptic equation Calculate the anisotropic gradient term and crack azimuth ; The gradient term is expressed by the following formula. : ； in, Both represent the fitting coefficients of the ellipse equation. The azimuth angle set for the observation system; Substituting multiple points to be fitted into the above formula yields the fitting coefficients for the ellipse equation. Each of the points to be fitted corresponds to an azimuth angle value and a gradient term value; The anisotropic gradient term is calculated using the following formula. and crack azimuth : ； The anisotropic gradient term Perform a t-distribution hypothesis test to obtain the hypothesis test value; By comparing the hypothesis test value with the allowable critical value, the anisotropic gradient term is determined. and crack azimuth Is it reliable within the confidence interval? 2. The method for inverting and predicting coal seam fracture parameters based on an equivalent medium theory model as described in claim 1, characterized in that, The method further includes: When it is determined that both are credible within the confidence interval, the anisotropic gradient term is output. and crack azimuth When it is determined that both are unreliable within the confidence interval, the anisotropic gradient term is... and crack azimuth The value was changed to 0.

3. The method for inverting and predicting coal seam fracture parameters based on an equivalent medium theory model according to claim 2, characterized in that, The anisotropic gradient term is expressed using the following formula. Perform a t-distribution hypothesis test to obtain the hypothesis test value: ； ； in, represents the standard deviation of the anisotropic gradient term, and t represents the hypothesis test value; , and These are the fitting coefficients of the elliptic equation. Standard deviation.

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