A method for establishing a three-dimensional velocity model of a discordant formation

Abstract

The application discloses a method for establishing a three-dimensional velocity model of an unconformable formation, which realizes full coverage of velocity data by constructing a three-dimensional velocity surface model, realizes strong-coherence prediction through formations by establishing a grid distribution model of the unconformable formation, separately calculates a model of a grid in a special lithology area, and realizes three-dimensional velocity prediction by sequentially interpolating from top to bottom. The application provides an effective technical means and field implementation method for velocity prediction of the unconformable formation, and can solve the technical problem of inaccuracy of a velocity model in the unconformable formation, and provides important technical information for researches on reservoir distribution and pressure prediction.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas exploitation, and in particular to a method for establishing a three-dimensional velocity model of an unconformity stratum. Background Art

[0002] An unconformity refers to a stratigraphic contact relationship characterized by a sedimentary hiatus or missing formation between two distinct strata. Velocity is a key parameter in oil and gas exploration and development, providing crucial information for structural characterization, reservoir distribution, and pressure prediction. However, due to missing formations and abrupt lithologic changes within unconformable formations, velocities vary significantly. Conventional horizontal interpolation of wellbore velocities often distorts unconformable formation velocities and fails to account for unconformities and lithologic variations.

[0003] The establishment of the unconformity velocity model currently requires overcoming the following technical difficulties:

[0004] (1) The problem of how to establish an unconformity stratigraphic framework model.

[0005] (2) How to use lithologic changes to constrain the establishment of velocity models and improve the accuracy of velocity models. Summary of the Invention

[0006] In response to the above technical problems, the present invention provides a method for establishing a three-dimensional velocity model of unconformity strata, which achieves full coverage of velocity data by constructing a three-dimensional velocity surface model, establishes a grid distribution model for unconformity strata to achieve strong coherence prediction throughout the strata, uses a separate calculation mode for grids in special lithologic areas, and implements three-dimensional interpolation from top to bottom in a sequential interpolation mode to achieve velocity prediction in three-dimensional space.

[0007] In order to solve the above technical problems, the present invention adopts a technical solution: a method for establishing a three-dimensional velocity model of an unconformity stratum, comprising the following steps:

[0008] (1) Using the calibration results of synthetic seismic records, the layered data of each well is converted into time and depth, calibrated in the seismic data volume, and each layer is tracked in the seismic data volume. The location of the pinch-out point must be accurate, and the stratigraphic unconformity model of each layer is established;

[0009] (2) Count the number of wells that encountered special lithology in the formation integration area and establish the distribution range of special lithology. The boundary of the special lithology area is half the distance between the wells that did not encounter special lithology and the wells that encountered special lithology;

[0010] (3) Using the velocity data of synthetic seismic records, the time-depth conversion is performed on each layer data of seismic interpretation to establish a depth domain structural map. The initial depth domain structural map is depth-corrected using the layered data of each well.

[0011] (4) Compensate for missing data in time-domain seismic interpretation layer data and depth-domain structural maps to achieve full regional data coverage;

[0012] (5) using the time domain and depth domain data of the corresponding layers to obtain velocity data of each layer, the method of obtaining the velocity data of the corresponding layer is to divide the depth domain structural map of the corresponding layer by the time domain seismic interpretation layer data to obtain the velocity data of the corresponding layer;

[0013] (6) The study area is processed into a three-dimensional grid. The three-dimensional grid of the conformable stratum is established in a proportional manner, and the three-dimensional grid of the unconformable stratum is established by interpolation according to the stratum trend, and the three-dimensional grid range of the special lithologic area is determined;

[0014] (7) Based on the velocity data of each layer of the drilled well, the linear variable velocity formula is used to predict the change of the velocity of the well with depth, and the velocity data on the well is assigned to the three-dimensional grid passed by the well;

[0015] (8) Using the Kriging interpolation method, the three-dimensional grid cells are assigned values ​​along the layers from top to bottom and from the well point to the far side. The predicted results are used as the known values ​​in the next prediction process and participate in the next grid calculation. The velocity values ​​of each grid are predicted in sequence.

[0016] (9) The calculation reaches the grid area of ​​the unconformity formation. Wells that have not encountered the missing part of the unconformity are not included in the calculation until all grid velocity assignments are completed and a three-dimensional velocity model of the unconformity formation is established.

[0017] In step (1), the wells in the study area that have completed synthetic seismic records are used to perform time-depth conversion on each well layer according to the principle of closest distance, and each layer is calibrated in the time domain seismic data volume; according to the position of each well layer in the seismic data volume, the tracking of each layer is completed, especially for unconformable oil and gas reservoirs, where there are generally missing strata. The position tracking of the pinch-out point must be accurate, and finally the stratigraphic contact model of the study area is established to prepare for the establishment of the velocity framework.

[0018] In step (3), the velocity data of the wells of the completed synthetic seismic records are used to perform time-depth conversion on the data of each layer. The wells of the synthetic seismic records should be selected by region to establish the initial depth domain structural map of each layer; the initial depth domain structural map is depth-corrected using the layered data of each well to overcome the inaccuracy of the initial depth domain structural map caused by local stratum missing and large lithologic changes, and to ensure that the corrected depth domain structural map is completely matched with the layered data on the well.

[0019] In step (4), for areas where some strata are missing, the time domain seismic interpretation layer data and the depth domain structural map data are compensated to achieve layer overlap.

[0020] In step (5), the velocity surface of each layer is obtained using the time domain and depth domain data of each corresponding layer, that is, the velocity data of each layer is obtained by dividing the time data by the depth data, in preparation for obtaining the velocity data of the entire spatial model.

[0021] In step (6), the special lithologic areas must also be accurately located in the three-dimensional grid.

[0022] In step (7), the linear variable velocity formula is used to predict the change of the velocity of the well with depth based on the velocity of each well passing through the layer site, and the complete velocity data of the well is obtained; the number of velocity data on each grid is analyzed, the average value of several data is calculated, and the value is assigned to the grid, and finally the grid assignment of all wells passing through is completed.

[0023] In step (8), when the velocity difference between the special lithology area and the ordinary lithology area reaches more than 100%, the velocity of the three-dimensional grid range where the special lithology distribution range exists is calculated separately; the wells that have not encountered special lithology are not involved in the calculation of the grid velocity within the special lithology distribution range; the grid velocity outside the special lithology distribution range is calculated for the entire well according to the Kriging interpolation formula.

[0024] The beneficial effects of the present invention are: providing effective technical means and field implementation methods for velocity prediction of unconformity formations, solving the technical problem of inaccurate velocity models in unconformity formations, and providing important technical information for research on reservoir distribution and pressure prediction. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The method of the present invention is to track the location map of pinch-out points of unconformities in seismic data volumes.

[0026] Figure 2 It is a diagram of the formation contact pattern of the method of the present invention.

[0027] Figure 3 This is a distribution range map of special lithology of the present invention.

[0028] Figure 4 This is a diagram of the method level speed establishment method of the present invention.

[0029] Figure 5 This is a diagram of a model for establishing an unconformity stratigraphic grid using the method of the present invention.

[0030] Figure 6 It is a schematic diagram of assigning uphole velocity data to a grid according to the method of the present invention. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention; it is obvious that the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0032] The method for establishing a three-dimensional velocity model of an unconformity stratum according to the present invention comprises the following steps:

[0033] (1) Using the calibration results of synthetic seismic records, the layered data of each well is converted into time and depth, calibrated in the seismic data volume, and each layer is tracked in the seismic data volume. The location of the pinch-out point must be accurate, and the stratigraphic unconformity model of each layer is established;

[0034] (2) Count the number of wells that encountered special lithology in the formation integration area and establish the distribution range of special lithology. The boundary of the special lithology area is half the distance between the wells that did not encounter special lithology and the wells that encountered special lithology;

[0035] (3) Using the velocity data of synthetic seismic records, the time-depth conversion is performed on each layer data of seismic interpretation to establish a depth domain structural map. The initial depth domain structural map is depth-corrected using the layered data of each well.

[0036] (4) Compensate for missing data in time-domain seismic interpretation layer data and depth-domain structural maps to achieve full regional data coverage;

[0037] (5) using the time domain and depth domain data of the corresponding layers to obtain velocity data of each layer, the method of obtaining the velocity data of the corresponding layer is to divide the depth domain structural map of the corresponding layer by the time domain seismic interpretation layer data to obtain the velocity data of the corresponding layer;

[0038] (6) The study area is processed into a three-dimensional grid. The three-dimensional grid of the conformable stratum is established in a proportional manner, and the three-dimensional grid of the unconformable stratum is established by interpolation according to the stratum trend, and the three-dimensional grid range of the special lithologic area is determined;

[0039] (7) Based on the velocity data of each layer of the drilled well, the linear variable velocity formula is used to predict the change of the velocity of the well with depth, and the velocity data on the well is assigned to the three-dimensional grid passed by the well;

[0040] (8) Using the Kriging interpolation method, the three-dimensional grid cells are assigned values ​​along the layers from top to bottom and from the well point to the far side. The predicted results are used as the known values ​​in the next prediction process and participate in the next grid calculation. The velocity values ​​of each grid are predicted in sequence.

[0041] (9) The calculation reaches the grid area of ​​the unconformity formation. Wells that have not encountered the missing part of the unconformity are not included in the calculation until all grid velocity assignments are completed and a three-dimensional velocity model of the unconformity formation is established.

[0042] In step (1), the wells in the study area that have completed synthetic seismic records are used to perform time-depth conversion on each well layer according to the principle of closest distance, and each layer is calibrated in the time domain seismic data volume; according to the position of each well layer in the seismic data volume, the tracking of each layer is completed, especially for unconformable oil and gas reservoirs, where there are generally missing strata. The position tracking of the pinch-out point must be accurate, and finally the stratigraphic contact model of the study area is established to prepare for the establishment of the velocity framework.

[0043] In step (3), the velocity data of the wells of the completed synthetic seismic records are used to perform time-depth conversion on the data of each layer. The wells of the synthetic seismic records should be selected by region to establish the initial depth domain structural map of each layer; the initial depth domain structural map is depth-corrected using the layered data of each well to overcome the inaccuracy of the initial depth domain structural map caused by local stratum missing and large lithologic changes, and to ensure that the corrected depth domain structural map is completely matched with the layered data on the well.

[0044] In step (4), for areas where some strata are missing, the time domain seismic interpretation layer data and the depth domain structural map data are compensated to achieve layer overlap.

[0045] In step (5), the velocity surface of each layer is obtained using the time domain and depth domain data of each corresponding layer, that is, the velocity data of each layer is obtained by dividing the time data by the depth data, in preparation for obtaining the velocity data of the entire spatial model.

[0046] In step (6), the special lithologic areas must also be accurately located in the three-dimensional grid.

[0047] In step (7), the linear variable velocity formula is used to predict the change of the velocity of the well with depth based on the velocity of each well passing through the layer site, and the complete velocity data of the well is obtained; the number of velocity data on each grid is analyzed, the average value of several data is calculated, and the value is assigned to the grid, and finally the grid assignment of all wells passing through is completed.

[0048] In step (8), when the velocity difference between the special lithology area and the ordinary lithology area reaches more than 100%, the velocity of the three-dimensional grid range where the special lithology distribution range exists is calculated separately; the wells that have not encountered special lithology are not involved in the calculation of the grid velocity within the special lithology distribution range; the grid velocity outside the special lithology distribution range is calculated for the entire well according to the Kriging interpolation formula.

[0049] The calculation formula involved in the present invention is as follows.

[0050] Formula 1 Linear speed change formula

[0051]

[0052] Where: Z is the depth of the target layer, Z0 is the initial depth, V0 is the average velocity, V is the velocity of the layer, t is the target layer time, t0 is the initial time, and K is the coefficient of linear velocity;

[0053] Formula 2 Kriging interpolation formula

[0054] Let x1,...,x n is a series of observation points on the region, z(x1),...,z(x n ) is the corresponding observation value. The value z of the regionalized variable at x0 * (x0) can be estimated using a linear combination:

[0055]

[0056] where λ i is the weight coefficient.

[0057] From the above formula, we can know that z * The key to (x0) is to use statistical models to determine λ i The value of λ is chosen as the unbiasedness and the minimum estimated variance. i Standards:

[0058] E[Z*(x0)-Z(x0)]=0

[0059]

[0060] From these two relations, we can deduce that λ i The Kriging equations.

[0061] First, starting from the second-order stationary assumption, we know that E[Z(x)] is a constant, so

[0062]

[0063] The relationship can be obtained:

[0064]

[0065] Starting from the minimum estimated variance, the Lagrange multiplier method can be used to obtain:

[0066]

[0067] Further deduction can obtain the n+1 order linear equations, namely the Kriging equations:

[0068]

[0069] When the random function does not satisfy the second-order stationary condition but satisfies the intrinsic assumption, the kriging equations can be expressed as follows using the variogram r(h):

[0070]

[0071] By solving the above equations, we can obtain a series of λ i (i=1,...,n), based on which the Kriging estimate of the estimated point can be solved. The minimum estimated variance, that is, the Kriging variance, can be solved using the following formula:

[0072]

[0073] Or expressed as a variogram:

[0074]

[0075] The present invention establishes an unconformity stratigraphic framework model based on seismic interpretation layer data, adopts stratigraphic deposition model to establish the model in stratigraphic grids, and then interpolates the velocity along the stratigraphic model through the uphole velocity. Meanwhile, the lithologic changes in the stratigraphic layer are referred to, thereby improving the rationality of the interpolated velocity along the interpreted horizon.

[0076] Example

[0077] A method for establishing a three-dimensional velocity model for unconformity formations is provided. Establishing a velocity model for unconformity formations can be considered an effective method for using the technology. The specific implementation method is as follows:

[0078] (1) Complete the tracking of each layer in the seismic data volume, especially ensure the accuracy of the location of the pinch-out point, and establish the stratigraphic unconformity model of each layer

[0079] Using the wells in the study area that have completed synthetic seismic records, we perform time-depth conversion on each well layer according to the principle of closest distance, and mark each layer in the time domain seismic data volume. According to the position of each well layer in the seismic data volume, we complete the tracking of each layer. Especially for unconformable oil and gas reservoirs, there are generally missing strata, and the position tracking of the pinch-out point must be accurate ( Figure 1 ), and finally established the stratigraphic contact model of the study area ( Figure 2 ), in preparation for building the speed grid next.

[0080] (2) Count the number of wells that encountered special lithology in the formation integration area and establish the distribution range of special lithology. The boundary of the special lithology area is half the distance between the wells that did not encounter special lithology and the wells that encountered special lithology ( Figure 3 ).

[0081] (3) Using the velocity data of the synthetic seismogram to convert the time-depth of each layer data of the seismic interpretation, and establish the depth domain structure map

[0082] Using the velocity data of the synthetic seismogram on the well to convert the time-depth of each layer data, the well of the synthetic seismogram is selected by region, and all the synthetic seismograms are not used for operation. Since the velocity difference between adjacent wells is too large, the depth domain structure map after conversion may be distorted. The initial depth domain structure map of each layer is established.

[0083] Using the layered data on each well to correct the initial depth domain structure map, to overcome the problem of inaccurate initial depth domain structure map caused by local stratigraphic loss and large lithology change, and to ensure that the corrected depth domain structure map is completely matched with the layered data on the well.

[0084] (4) Compensate the missing area data of the time domain seismic interpretation layer data and the depth domain structure map to realize full area coverage of the data

[0085] For the area where part of the stratum is missing, compensate the time domain seismic interpretation layer data and the depth domain structure map data, such as Figure 1 In the middle, there is a problem of partial layer loss in the eastern region of T2 and T3 layers, which needs to be compensated according to the position of T1 layer to realize layer coincidence.

[0086] (5) Use the corresponding time domain and depth domain data of each layer to calculate the velocity surface of each layer, that is, divide the time data by the depth data to obtain the velocity data of each layer Figure 4 ), which is prepared for obtaining the velocity data of the entire space model.

[0087] (6) Three-dimensional gridding processing is performed on the study area, and the three-dimensional grid of the integrated stratum is established in the equal proportion type, and the three-dimensional grid of the non-integrated stratum is established according to the stratum trend interpolation, and the three-dimensional grid range of the special lithology area is implemented.

[0088] Three-dimensional gridding processing is performed on the study area to prepare for the final three-dimensional velocity model. The three-dimensional grid of the integrated stratum is established in the equal proportion type, and the three-dimensional grid of the non-integrated stratum is established according to the stratum trend interpolation, and the special lithology area is also determined in the three-dimensional grid Figure 5 ).

[0089] (7) According to the velocity data of each layer of the drilled well, the velocity changes with depth are predicted through the linear variable velocity formula, and the velocity data on the well is assigned to the three-dimensional grid through which the well passes

[0090] Using the linear variable velocity formula (Formula 1), the velocity variation of each well with depth is predicted based on the velocity of each well passing through the layer, and the complete velocity data of the well is obtained.

[0091] Analyze the number of speed data on each grid, find the average value of several data, and assign it to the grid ( Figure 6 ), and finally complete the grid assignment for all wells passing through.

[0092] (8) Using the Kriging interpolation method, the three-dimensional grid cells are assigned values ​​along the layers from top to bottom and from the well points from near to far.

[0093] A. Assign values ​​to the three-dimensional grid cells from top to bottom and from the well point to the farthest point along the grid layer. The assignment method is Kriging interpolation (Formula 2). The prediction results are used as known values ​​for subsequent predictions and participate in subsequent grid calculations. The velocity values ​​of each grid are predicted in sequence.

[0094] B. The 3D grid area encountered in the special lithology area is calculated separately, and the wells that do not encounter special lithology are not included in the velocity calculation;

[0095] When the velocity difference between the special lithology area and the common lithology area exceeds 100%, a separate velocity calculation is required for the 3D grid area containing the special lithology distribution. For example, at a depth of 2000 meters, if the vertical average velocity of common lithology is 2500 m / s and the vertical evaluation velocity of the special lithology is above 5000 m / s, wells that have not encountered the special lithology are not included in the grid velocity calculation within the special lithology distribution area. Grid velocity outside the special lithology distribution area is calculated for the entire well using the Kriging interpolation formula.

[0096] C. Calculate to the unconformity formation grid area. Wells that have not encountered this formation will not be included in the calculation until all grid velocity assignments are completed.

[0097] The calculation reaches the unconformity formation grid area. Wells that have not encountered the unconformity missing part of the formation are not included in the calculation to avoid interference between different formation velocities until all grid velocity assignments are completed.

[0098] In summary, the content of the present invention is not limited to the above-mentioned embodiments. People with insight in the same field can easily propose other embodiments within the technical guidance of the present invention, but such embodiments are all included in the scope of the present invention.

Claims

1. A method for establishing a three-dimensional velocity model of an unconformity stratum, characterized in that: The following steps are involved: (1) Using the calibration results of synthetic seismic records, the layered data of each well is converted into time and depth, calibrated in the seismic data volume, and each layer is tracked in the seismic data volume. The location of the pinch-out point must be accurate, and the stratigraphic unconformity model of each layer is established; (2) Count the number of wells that encountered special lithology in the formation integration area and establish the distribution range of special lithology. The boundary of the special lithology area is half the distance between the wells that did not encounter special lithology and the wells that encountered special lithology; (3) Using the velocity data of synthetic seismic records, the time-depth conversion is performed on each layer data of seismic interpretation to establish a depth domain structural map. The initial depth domain structural map is depth-corrected using the layered data of each well. (4) Compensate for missing data in time-domain seismic interpretation layer data and depth-domain structural maps to achieve full regional data coverage; (5) using the time domain and depth domain data of the corresponding layers to obtain velocity data of each layer, the method of obtaining the velocity data of the corresponding layer is to divide the depth domain structural map of the corresponding layer by the time domain seismic interpretation layer data to obtain the velocity data of the corresponding layer; (6) The study area is processed into a three-dimensional grid. The three-dimensional grid of the conformable stratum is established in a proportional manner, and the three-dimensional grid of the unconformable stratum is established by interpolation according to the stratum trend, and the three-dimensional grid range of the special lithologic area is determined; (7) Based on the velocity data of each layer of the drilled well, the linear variable velocity formula is used to predict the change of the velocity of the well with depth, and the velocity data on the well is assigned to the three-dimensional grid passed by the well; (8) Using the Kriging interpolation method, the three-dimensional grid cells are assigned values ​​along the layers from top to bottom and from the well point to the far side. The predicted results are used as the known values ​​in the next prediction process and participate in the next grid calculation. The velocity values ​​of each grid are predicted in sequence. (9) The calculation reaches the grid area of ​​the unconformity formation. Wells that have not encountered the missing part of the unconformity are not included in the calculation until all grid velocity assignments are completed and a three-dimensional velocity model of the unconformity formation is established.

2. The method for establishing a three-dimensional velocity model of an unconformity stratum according to claim 1, characterized in that: In step (1), using the wells that have completed synthetic seismic records in the study area, time-depth conversion is performed on each well layer according to the principle of closest distance, and each layer is marked in the time domain seismic data volume; According to the position of each well layer in the seismic data volume, the tracking of each layer is completed. In particular, unconformable oil and gas reservoirs generally have missing strata. The position tracking of the pinch-out point must be accurate. Finally, the stratigraphic contact model of the study area is established to prepare for the establishment of the velocity framework.

3. The method for establishing a three-dimensional velocity model of an unconformity stratum according to claim 1, characterized in that: In step (3), the velocity data of the wells of the completed synthetic seismic records are used to perform time-depth conversion on the data of each layer. The wells of the synthetic seismic records should be selected by region to establish the initial depth domain structural map of each layer; the initial depth domain structural map is depth-corrected using the layered data of each well to overcome the inaccuracy of the initial depth domain structural map caused by local stratum missing and large lithologic changes, and to ensure that the corrected depth domain structural map is completely matched with the layered data on the well.

4. The method for establishing a three-dimensional velocity model of an unconformity stratum according to claim 1, characterized in that: In step (4), for areas where some strata are missing, the time domain seismic interpretation layer data and the depth domain structural map data are compensated to achieve layer overlap.

5. The method for establishing a three-dimensional velocity model of an unconformity stratum according to claim 1, characterized in that: In step (5), the velocity surface of each layer is obtained using the time domain and depth domain data of each corresponding layer, that is, the velocity data of each layer is obtained by dividing the time data by the depth data, in preparation for obtaining the velocity data of the entire spatial model.

6. The method for establishing a three-dimensional velocity model of an unconformity stratum according to claim 1, characterized in that: In step (6), the special lithologic areas must also be accurately located in the three-dimensional grid.

7. The method for establishing a three-dimensional velocity model of an unconformity stratum according to claim 1, characterized in that: In step (7), the linear variable velocity formula is used to predict the change of the velocity of the well with depth based on the velocity of each well passing through the layer site, and the complete velocity data of the well is obtained; the number of velocity data on each grid is analyzed, the average value of several data is calculated, and the value is assigned to the grid, and finally the grid assignment of all wells passing through is completed.

8. The method for establishing a three-dimensional velocity model of an unconformity stratum according to claim 1, characterized in that: In step (8), when the velocity difference between the special lithology area and the ordinary lithology area reaches more than 100%, the velocity of the three-dimensional grid range where the special lithology distribution range exists is calculated separately; the wells that have not encountered special lithology are not involved in the calculation of the grid velocity within the special lithology distribution range; the grid velocity outside the special lithology distribution range is calculated for the entire well according to the Kriging interpolation formula.

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