Seismic exploration method based on drill hole information structure constraint

Abstract

The invention discloses a seismic exploration method based on drilling information structure constraint, which comprises the following steps of: stopping drilling at any time to excite seismic waves in a drilling process by adopting a seismic source while drilling, and continuously changing the position of the seismic source in the drilling process so as to acquire seismic wave data in a plurality of time periods; carrying out independent inversion imaging by using conventional seismic inversion in a first serial number acquisition time period, and carrying out time-shifting seismic source data inversion imaging of borehole information structure constraint in a subsequent moment, namely introducing an inversion result of a previous time period and the structure constraint of borehole information up to a current acquisition time period into a traditional seismic inversion objective function; according to the method, the difference of the next time period is effectively regulated and controlled, so that the resolution of structural constraint inversion is higher, the change condition of abnormal body exposure in the drilling process is accurately captured, the abnormal body in the detection range can be effectively distinguished finally, mutual verification of drilling and geophysical prospecting data is realized, and accurate detection of a drilling detection area is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of seismic exploration, and in particular to a seismic exploration method based on borehole information structure constraints. Background Art

[0002] Seismic exploration is a key technology used in geology and geophysics to detect underground structures. It infers the properties and structure of underground rock formations by generating seismic waves on the surface or underground and recording the propagation of these waves underground. Traditional seismic exploration methods usually rely on seismic data actively excited by artificial seismic sources such as tamping and explosives. However, this method has some limitations, such as the influence of underground medium properties, aliasing of characteristic signals of abnormal bodies, and low exploration accuracy. In order to overcome these limitations, borehole seismic exploration technology came into being. Borehole seismic exploration can be excited deep underground or far away from the exploration surface, so as to obtain more accurate reflection signals of abnormal media and provide more accurate local underground medium information.

[0003] Traditional time-lapse seismic is to conduct repeated three-dimensional (two-dimensional or other) seismic exploration of underground media at different times. After subtracting the time-delayed seismic imaging, the static properties of the underground media are eliminated, thereby directly obtaining the imaging results of the dynamic changes of the medium. However, this method is only applicable to the situation where the underground medium will change rapidly within a certain period of time. However, most underground media currently have a long change time without human intervention, and the underground medium information is known, which belongs to the monitoring category. Therefore, the acquisition cycle is long, and the time interval between each two acquisitions is often more than half a year, and the observation system is fixed. Therefore, the above-mentioned traditional time-lapse seismic interpretation method is not suitable for the situation where the detection area information is unknown in borehole seismic exploration, and the spatiotemporal position of the earthquake source changes as the drill bit drills. It is urgent to study the fast imaging algorithm for this kind of borehole time-lapse earthquake source.

[0004] Full Waveform Inversion (FWI) is an advanced seismic data processing technology that optimizes the physical properties of underground media by minimizing the difference between observed data and model predicted data, and can provide a high-resolution underground velocity model. However, the traditional FWI method requires a high-precision initial model, and there are more noise and incomplete acquisition problems in the actual seismic data obtained later, and there is a lack of effective constraints on the results during the inversion process, which ultimately limits the accuracy and resolution of the inversion.

[0005] Therefore, how to provide an improved seismic exploration method that not only makes up for the shortcoming of the full waveform inversion algorithm that requires a better initial model, but also takes into account the continuity of the temporal and spatial excitation acquisition information of the seismic source in the hole, and constrains the inversion process based on the borehole information structure, and ultimately effectively improves the accuracy and resolution of seismic exploration, is the research direction required by the present invention. Summary of the Invention

[0006] In view of the problems existing in the above-mentioned prior art, the present invention provides a seismic exploration method based on the structural constraint of borehole information, which not only makes up for the shortcoming that the full waveform inversion algorithm requires a better initial model, but also considers the continuity of the excitation acquisition information of the in-hole source in terms of time and space, and finally effectively improves the accuracy and resolution of seismic exploration based on the inversion process of the borehole information structure constraint.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is: a seismic exploration method based on the structural constraint of borehole information, comprising the following steps:

[0008] Step 1: Determine the drilling exploration distance and the acquisition time period: Obtain the required detection range according to the prior geological data, determine the number of acquisition time periods during the drilling process and number them, and the number of acquisition moments in each acquisition time period is the same;

[0009] Step 2: Obtain borehole information: Construct a first borehole in the detection area, and arrange a plurality of geophones in the borehole. Then connect the downhole source to the drill pipe and the drill bit and construct a second borehole in the detection area. During the drilling process of the second borehole, according to the acquisition moments set in Step 1, every time a moment is reached, the drill bit stops drilling and the downhole source is used to generate seismic waves, and each geophone receives waveform data to obtain the borehole information for this time until the acquisition process for all moments is completed;

[0010] Step 3: First inversion imaging: For the data of the first numbered acquisition time period, set an initial model using the data within its time period, and perform a separate full waveform inversion on this data using the conjugate gradient optimization algorithm to obtain the inversion imaging result of this acquisition time period;

[0011] Step 4: Second inversion imaging: For the data of the second numbered acquisition time period, use the inversion imaging result of the first numbered acquisition time period as the initial model for the second inversion, and establish an objective function for the second inversion based on the structural constraint of the borehole information by combining the borehole information of the first numbered and second numbered acquisition time periods. Then perform a full waveform inversion on the data of the second numbered acquisition time period using the conjugate gradient optimization algorithm to obtain the inversion imaging result of this acquisition time period;

[0012] Step 5: Third inversion imaging: For the data of the third numbered acquisition time period, use the inversion imaging result of the second numbered acquisition time period as the initial model for the third inversion, and establish an objective function for the third inversion based on the structural constraint of the borehole information by combining the borehole information of the first numbered to third numbered acquisition time periods. Then perform a full waveform inversion on the data of the third numbered acquisition time period using the conjugate gradient optimization algorithm to obtain the inversion imaging result of this acquisition time period;

[0013] Step 6: Seismic exploration imaging: Repeat Step 5 for each subsequent time period until the inversion is complete for all numbered acquisition time periods, thereby forming the seismic exploration imaging of the required detection area.

[0014] Furthermore, the number of acquisition time periods in Step 1 is at least 3. Setting at least 3 acquisition time periods can ensure that the method of the present invention achieves the required accuracy.

[0015] Furthermore, the interval time between each acquisition moment within each acquisition time period in Step 1 is the same. By setting the same sampling interval, the geological changes in the detection area at different moments within equal time periods can be further observed, thereby more accurately revealing the geological information and the development of abnormal bodies during the drilling process of the drill bit.

[0016] Furthermore, the objective function used in the first inversion in Step 3 is specifically:

[0017]

[0018] where m T1 is the inversion imaging result of the first numbered acquisition time period, W d is the covariance matrix of the data fitting term, where d is the forward operator, and dbos,1 is the seismic data of the first numbered acquisition time period.

[0019] Furthermore, the objective function used in the second inversion in Step 4 is specifically:

[0020]

[0021] where m T2 is the inversion imaging result of the second numbered acquisition time period, W d is the covariance matrix of the data fitting term, generally defined as the identity matrix, d is the forward operator, dbos,2 is the seismic data of the second numbered acquisition time period, β is the regularization parameter of the gradient structure constraint, W m is the covariance matrix of the borehole information structure constraint, m T1+T2 represents the model constraint information of the borehole information for the first and second numbered acquisition time periods; one of the purposes of this model constraint information is to construct the initial model for the next inversion based on the model results of the previous inversion (this step is equivalent to constraining the spatial position and shape of the abnormal body), and the other is to optimize the attributes of this constraint model based on the parameters of the first inversion and the borehole information; the purpose of the model constraint information is one to constrain the shape and size of the abnormal body, that is, the geometric information, and the other is to constrain the wave velocity, density, etc. of the abnormal body, that is, the elastic parameters.

[0022] Furthermore, the objective function used in the third inversion in Step 5 is specifically:

[0023]

[0024] where m T3 is the inversion imaging result of the third numbered acquisition period, W d is the covariance matrix of the data fitting term, d is the forward operator, dbos,3 is the seismic data of the third numbered acquisition period, β is the regularization parameter of the gradient structure constraint, and W m is the covariance matrix of the borehole information structure constraint, and m T1+T2+T3 represents the model constraint information of the borehole information for the three acquisition periods numbered from the first to the third.

[0025] Furthermore, the borehole information is the lithological properties and elastic parameters obtained during the drilling process.

[0026] Due to the current situation of the non-uniqueness existing in seismic full waveform inversion, it is impossible to accurately obtain the spatial position and elastic parameters of the abnormal body in front through single seismic inversion alone, and there may be artifacts. Based on this, the present invention adopts a downhole source that can stop drilling at any time during the drilling process to generate seismic wave excitation. By continuously changing the source position during a drilling process and continuously receiving data with a geophone in another borehole, seismic wave data for multiple periods can be collected. First, conventional seismic inversion is used for single inversion imaging of the first numbered acquisition period, and time-lapse source data inversion imaging with borehole information structure constraint is carried out at subsequent times, that is, the inversion results of the previous period and the structure constraints of the borehole information up to the current acquisition period are introduced into the traditional seismic inversion objective function, effectively regulating the differences in the subsequent period, making the resolution of the structure constraint inversion higher, thus accurately capturing the changes in the exposure of the abnormal body during the drilling process, and finally being able to effectively distinguish the abnormal bodies within the detection range, realizing the mutual verification of drilling and geophysical exploration data, and achieving precise detection of the drilled detection area. Description of the Drawings

[0027] Figure 1 is the schematic diagram of borehole exploration of the present invention;

[0028] Figure 2 is the flow chart of the time-lapse full waveform inversion imaging method of the present invention;

[0029] Figure 3 is the geological model diagram to be revealed in the experimental verification;

[0030] Figure 4 is the data acquisition and inversion result diagram of the T1 acquisition period in the experimental verification

[0031] where (a) is the schematic diagram of data acquisition; (b) is the single inversion result;

[0032] Figure 5 is the data acquisition and inversion result diagram of the T2 acquisition period in the experimental verification;

[0033] Among them, (a) is a schematic diagram of data acquisition; (b) is the result of individual inversion; (c) is the inversion result of the present invention with the constraint of borehole information of T1 and T2 added.

[0034] Figure 6 It is a diagram of data acquisition and inversion result during the T3 acquisition period in the experimental verification.

[0035] Among them, (a) is a schematic diagram of data acquisition; (b) is the result of individual inversion; (c) is the inversion result of the present invention with the constraint of borehole information of T1 to T3 added.

[0036] Figure 7 It is a diagram of data acquisition and inversion result during the T4 acquisition period in the experimental verification.

[0037] Among them, (a) is a schematic diagram of data acquisition; (b) is the result of individual inversion; (c) is the inversion result of the present invention with the constraint of borehole information of T1 to T4 added. Detailed implementation manners

[0038] The present invention will be further described below.

[0039] As Figure 2 shown, the present invention includes the following steps:

[0040] Step 1: Determine the drilling exploration distance and acquisition time period: Obtain the required exploration range according to prior geological data, and determine that the number of acquisition time periods during the drilling process is n, numbered T1, T2, T3... Tn respectively. The number of acquisition moments within each acquisition time period is the same and the time interval between each acquisition moment is the same. By setting the same sampling interval, the geological changes in the area to be detected at different moments within equal time periods can be further observed, so as to more accurately reveal the geological information and the development of abnormal bodies during the drilling process of the drill bit.

[0041] Step 2: Obtain borehole information: Use drill pipe 1 to construct the first borehole in the exploration area, and arrange a plurality of geophones in the borehole. Then connect the seismic source while drilling to drill pipe 2 and the drill bit, and construct the second borehole in the exploration area. During the drilling process of the second borehole, according to the acquisition moments set in Step 1, every time a moment is reached, the drill bit stops drilling and the seismic source while drilling is used to generate seismic waves, and each geophone receives waveform data to obtain the borehole information for this time until the acquisition process for all moments is completed; the seismic source while drilling is an existing device and can be directly purchased from the market. The present invention only utilizes its function of generating seismic waves while drilling.

[0042] Step 3: First inversion imaging: For the data in the T1 acquisition period, set an initial model using the data within this time period, and perform individual full waveform inversion on this data using the conjugate gradient optimization algorithm to obtain the inversion imaging result of this acquisition period. The specific objective function used for inversion is:

[0043]

[0044] where m T1 is the inversion imaging result of the T1 acquisition period, and W d is the covariance matrix of the data fitting term, where d is the forward operator, and dbos,1 is the seismic data of the T1 acquisition period.

[0045] Step 4: Second inversion imaging: For the data of the T2 acquisition period, use the inversion imaging result of the T1 acquisition period as the initial model for the second inversion, and establish an objective function for the second inversion based on the borehole information structure constraint by combining the borehole information of the T1 and T2 acquisition periods. Then, use the conjugate gradient optimization algorithm to perform full waveform inversion on the data of the T2 acquisition period to obtain the inversion imaging result of this acquisition period; the specific objective function used in the inversion is:

[0046]

[0047] where m T2 is the inversion imaging result of the T2 acquisition period, and W d is the covariance matrix of the data fitting term, generally defined as the identity matrix, d is the forward operator, dbos,2 is the seismic data of the T2 acquisition period, β is the regularization parameter of the gradient structure constraint, and W m is the covariance matrix of the borehole information structure constraint, and m T1+T2 represents the model constraint information of the borehole information in the T1 and T2 acquisition periods. The purpose of this model constraint information is, first, to construct the initial model for the next inversion based on the model result of the previous inversion (this step is equivalent to constraining the spatial position and shape of the anomaly), and second, to optimize the attributes of this constraint model according to the parameters of the first inversion and the borehole information; the purpose of the model constraint information is, first, to constrain the shape and size of the anomaly, that is, the geometric information, and second, to constrain the wave velocity, density, etc. of the anomaly, that is, the elastic parameters. For example, if the real anomaly is 5*5 in size and has a wave velocity of 1500 m / s, and the result of the first inversion shows that the anomaly is 7*7 in size and has a wave velocity of 1300 m / s, the second inversion introduces a structure constraint, and the constraint model is also set according to the result of the first inversion and the borehole information, for example, set to 6*6 and a wave velocity of 1400 m / s, and the inversion obtains an anomaly size of 5*5 and a wave velocity of 1450 m / s; in this way, a more accurate inversion result can be obtained.

[0048] Step 5. Third inversion imaging: For the data in the T3 acquisition period, use the inversion imaging result in the T2 acquisition period as the initial model for the third inversion, and establish an objective function for the third inversion based on the borehole information structure constraint by combining the borehole information in the T1 to T3 acquisition periods. Then, use the conjugate gradient optimization algorithm to perform full waveform inversion on the data in the T3 acquisition period to obtain the inversion imaging result for this acquisition period; the specific objective function used in the inversion is as follows:

[0049]

[0050] where m T3 is the inversion imaging result in the T3 acquisition period, W d is the covariance matrix of the data fitting term, d is the forward operator, dbos,3 is the seismic data in the T3 acquisition period, β is the regularization parameter of the gradient structure constraint, W m is the covariance matrix of the borehole information structure constraint, and m T1+T2+T3 represents the model constraint information of the borehole information in the first three acquisition periods from T1 to T3.

[0051] Step 6: Seismic exploration imaging: Repeat Step 5 for each subsequent time period until the inversion is completed for all numbered acquisition period data, thereby forming the seismic exploration imaging of the required detection area.

[0052] Experimental verification:

[0053] To verify the effect of the inversion imaging of the present invention, a numerical simulation experiment was carried out with the area to be drilled in front of the roadway heading face as the experimental scheme. To simplify the calculation, the model size was set to 100m×50m, the background wave velocity was set to 1000m / s, the wave velocities of the abnormal bodies were all set to 1500m / s, dx = dz = 1m, dt = 0.0001s, T = 0.17s, and a total of three abnormal bodies were set, namely Abnormal Body 1 to 3. The specific geological model is as Figure 3 shown; then, an observation system was arranged. The geophones covered the entire model space along the borehole axis, the trace interval was 5m, and there were 20 in total; 20 acquisition times were set during the drilling process of the drill bit. The drill bit collected the seismic waves generated by the source excitation every 5m of advancement. The 20 acquisition times were evenly divided into 4 acquisition periods, that is, each acquisition period included the acquisition times of 5 excitations. The inversion parameters were the same as those in the forward process. In particular, to quickly test the effect of the method of the present invention, only 10 iterations were set for each inversion, and the conjugate gradient optimization algorithm was used for inversion iteration calculation, and the adaptive step size calculation method was used to calculate the step size of each iteration.

[0054] First, perform a separate inversion on the data in the T1 acquisition period. The inversion result is as Figure 4 (b) shown, and compare it with Figure 4(a) By comparing the actual geological models during the T1 acquisition period, it can be seen that since most of the seismic waves generated by the drill-bit source excitation during this period are reflected waves on the left side of the abnormal body and the signal energy of the relatively distant abnormal body 3 is weak, only the approximate positions of abnormal body 1 and abnormal body 2 are inverted, and the difference between the central velocity and the true model is large, with many false anomalies.

[0055] Then the drill bit continues to drill, and the data during the T2 acquisition period are separately inverted and inverted with borehole information structure constraints. The inversion results are as Figure 5 shown. First, compare Figure 5 (b) and Figure 5 (c) with Figure 4 (b). It can be seen that due to the change in the spatial position of the drill bit, the acquired signals include both reflected wave signals from the right side of abnormal body 1, as well as information supplementation for abnormal body 2 and reflected signals from the left side of abnormal body 3. Therefore, the positions of the three abnormal bodies can be inverted by both methods, and the overall inversion effect of each abnormal body is better than the separate inversion during the T1 acquisition period; comparing Figure 5 (b) and Figure 5 (c), it can be seen that the inversion effect is better after adding borehole information structure constraints, the positions of the abnormal bodies are more accurately delineated, and the central velocity is closer to the true model. However, at this time, due to the model structure constraints during the T1 acquisition period, only the approximate position of abnormal body 3 can be seen.

[0056] Subsequently, the borehole information of the drill bit during the T3 acquisition period is separately inverted and inverted with borehole information structure constraints. The inversion results are as Figure 6 shown. First, compare Figure 6 (b) with Figure 5 (b). It can be seen that since the drill bit is located between abnormal body 2 and abnormal body 3 at this time, the acquired signals mainly cover the right side area of the model. Therefore, the inverted position of abnormal body 3 is relatively clear, and the inversion effect on the right sides of abnormal body 1 and abnormal body 2 is better than that during the T1 and T2 acquisition periods; comparing Figure 6 (b) and Figure 6 (c), it can be seen that the inversion effects of the positions and central velocities of the three abnormal bodies are better than the separate inversion results after adding borehole information structure constraints; comparing Figure 6 (c) and Figure 5 (c), it can be seen that abnormal body 3 is clearly revealed without affecting the inversion effects of abnormal body 1 and abnormal body 2.

[0057] Finally, the seismic data generated by the drill bit during the T4 acquisition period are separately inverted and inverted with the addition of borehole information structure constraints. The inversion results are as Figure 7 shown. Comparing Figure 7 (b) with Figure 6As can be seen from (b), since the drill bit further penetrates into the right region of the detection model at this time, the inversion effect on the right sides of the anomaly 3 and the anomaly 2 is better, while the inversion effect of the anomaly 1 is not as good as that in the T3 acquisition period; comparing Figure 7 (b) and Figure 7 (c), it can be seen that after adding the structural constraint of the borehole information in the T3 acquisition period, most of the inversion false anomalies are eliminated, and the central velocity value is closer to the true model; comparing Figure 7 (c) and Figure 6 (c), it can be seen that the positions of the inversion anomalies are more convergent, more in line with the sizes of the anomalies in the true model, and the central velocity values obtained by inversion are further optimized.

[0058] The above experiments prove that the present invention can effectively combine borehole seismic information for full-waveform inversion, accurately reveal the positions of unknown anomalies during the drilling process with the passage of time and the change of the drill bit position, and finally realize the geological interpretation of the integration of drilling and geophysical prospecting.

[0059] The above are only the preferred embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A seismic exploration method based on the structural constraint of drilling information, characterized in that, It includes the following steps: Step 1: Determine the drilling penetration detection distance and the acquisition time periods: Obtain the required detection range based on prior geological data, determine the number of acquisition time periods during the drilling process and number them, and the number of acquisition moments within each acquisition time period is the same; Step 2: Acquisition of borehole information: Construct the first borehole in the detection area, deploy multiple geophones in the borehole, then connect the seismic source while drilling to the drill pipe and the drill bit and construct the second borehole in the detection area. During the drilling process of the second borehole, at each moment set in Step 1, when a moment is reached, the drill bit stops drilling and the seismic source while drilling is used to generate seismic waves, and each geophone receives waveform data to obtain the borehole information for this time, until the acquisition process for all moments is completed; Step 3: First inversion imaging: For the data of the first numbered acquisition time period, set an initial model using the data within its time period, and perform a separate full waveform inversion on this data using the conjugate gradient optimization algorithm to obtain the inversion imaging result for this acquisition time period; Step 4: Second inversion imaging: For the data of the second numbered acquisition time period, use the inversion imaging result of the first numbered acquisition time period as the initial model for the second inversion, and establish an objective function for the second inversion based on the borehole information structure constraint by combining the borehole information of the first numbered and the second numbered acquisition time periods. Then, perform a full waveform inversion on the data of the second numbered acquisition time period using the conjugate gradient optimization algorithm to obtain the inversion imaging result for this acquisition time period; Step 5: Third inversion imaging: For the data of the third numbered acquisition time period, use the inversion imaging result of the second numbered acquisition time period as the initial model for the third inversion, and establish an objective function for the third inversion based on the borehole information structure constraint by combining the borehole information of the first numbered to the third numbered acquisition time periods. Then, perform a full waveform inversion on the data of the third numbered acquisition time period using the conjugate gradient optimization algorithm to obtain the inversion imaging result for this acquisition time period; Step 6: Seismic exploration imaging: Repeat Step 5 for each subsequent time period until the inversion is completed for all numbered acquisition time period data, thereby forming the seismic exploration imaging of the required detection area.

2. The seismic exploration method based on structural constraints of drilling information according to claim 1, wherein The number of acquisition time periods in Step 1 is at least 3.

3. The seismic exploration method based on structural constraints of drilling information according to claim 1, characterized in that, The time intervals between the acquisition moments within each acquisition time period in Step 1 are the same.

4. The seismic exploration method based on structural constraints of drilling information according to claim 1, characterized in that, The specific objective function used in the first inversion in Step 3 is: where m T1 is the inversion imaging result of the first numbered acquisition period, and W d is the covariance matrix of the data fitting term, where d is the forward operator and dbos,1 is the seismic data of the first numbered acquisition period.

5. The seismic exploration method based on structural constraints of drilling information according to claim 1, characterized in that The specific objective function used in the second inversion in Step 4 is: where m T2 is the inversion imaging result of the second numbered acquisition period, W d is the covariance matrix of the data fitting term, d is the forward operator, dbos,2 is the seismic data of the second numbered acquisition period, β is the regularization parameter of the gradient structure constraint, W m is the covariance matrix of the borehole information structure constraint, m T1+T2 represents the model constraint information of the borehole information for the first and second numbered acquisition periods.

6. The seismic exploration method based on structural constraints of drilling information according to claim 1, characterized in that, The specific objective function used in the third inversion in Step 5 is: where m T3 is the inversion imaging result of the third numbered acquisition period, W d is the covariance matrix of the data fitting term, d is the forward operator, dbos,3 is the seismic data of the third numbered acquisition period, β is the regularization parameter of the gradient structure constraint, W m is the covariance matrix of the borehole information structure constraint, m T1+T2+T3 represents the model constraint information of the borehole information for the three acquisition periods numbered from the first to the third.

7. The seismic exploration method based on structural constraints of drilling information according to claim 1, characterized in that, The borehole information is the lithological properties and elastic parameters obtained during the drilling process.

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