CN114563827A - Small-sized hydraulic fracturing monitoring method and monitoring device based on same well - Google Patents
Small-sized hydraulic fracturing monitoring method and monitoring device based on same well Download PDFInfo
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Abstract
The invention belongs to the technical field of hydraulic fracturing monitoring, and particularly relates to a small-sized hydraulic fracturing monitoring method based on the same well, which comprises the following steps: distributing N detectors in a fracturing well, and establishing a three-dimensional same-well monitoring model; calculating to obtain cross-correlation waveforms corresponding to the H pairs of non-repetitive detectors, and taking absolute values of the H-channel cross-correlation waveforms to obtain cross-correlation waveforms with the H-channel absolute values; converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z relative to the well axis, and establishing a speed model and discretization to obtain a discretized speed model; calculating to obtain the theoretical direct wave travel time difference from each grid node to the H pairs of non-repetitive detectors; and (4) superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the travel time difference offset of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z and determine the event position.
Description
Technical Field
The invention relates to the technical field of hydraulic fracturing monitoring, in particular to a small-sized hydraulic fracturing monitoring method and device based on the same well.
Background
Hydraulic fracturing is one of the key technologies to improve the exploitation of geothermal heat, oil and gas and energy. However, this process carries environmental risks, one of which is induced seismic activity that accompanies hydraulic fracture growth. Therefore, monitoring the distribution and development of fractures is essential for the safe exploitation of resources.
Small acoustic emission events with low energy in the frequency range of 1kHz to 200kHz correspond to micro-cracks on the millimeter to decimeter scale. Although not causing potential damage because a single acoustic emission event is too small, by monitoring such small acoustic emission events, the exact location, extension and direction of potential damage can be determined as early as possible, and instabilities in the rock are observed before any damage becomes visible, providing valuable information for small-scale dynamic processes. However, monitoring low acoustic emission events at high frequencies places high demands on the recording unit, while attenuation of low acoustic emission events in the high frequency range is very severe, and the geophones need to be very close to the hydraulic fracturing test.
The microseism monitoring technology is one of effective means for monitoring hydraulic fracturing, and the spatial arrangement of a monitoring system is a key factor influencing the positioning effect and the monitoring effect. According to the arrangement condition of the detectors, the microseism monitoring can be divided into ground monitoring, shallow well monitoring and in-well monitoring. In the ground monitoring and shallow well monitoring, the detector is placed on the ground or in a shallow well of 100-200 m and is far away from a fracturing section. The ground monitoring is also easily influenced by the complex near-surface, so that the micro-seismic signals are seriously attenuated, the signal-to-noise ratio is low, and the micro-seismic event picking difficulty is high. In-well monitoring, the geophones are placed in one or more monitoring wells, closer to the fracture zone. However, the monitoring in the well has the problems of insufficient observation direction and observation angle caused by the limitation of the length of the geophone array, great construction difficulty, higher layout cost and the like. In addition, microseismic monitoring techniques are often used to measure large-scale deformations, and microcracks cannot be detected due to limitations in frequency range and sensitivity. Accordingly, microseismic monitoring techniques generally do not consider small acoustic emission events in the stability assessment and interpretation of rock geomechanical conditions.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a small hydraulic fracture monitoring method based on the same well, which comprises the following steps:
arranging N detectors in a fracturing well, taking the fracturing well as a monitoring well, picking up and recording original waveforms of N sound emission events, and establishing a three-dimensional same-well monitoring model;
according to the original waveform of the N-channel acoustic emission events, calculating to obtain cross-correlation waveforms corresponding to H pairs of non-repetitive wave detectors, wherein each pair of non-repetitive wave detectors corresponds to 1 channel of cross-correlation waveforms to further obtain H-channel cross-correlation waveforms, and taking absolute values of the H-channel cross-correlation waveforms to obtain H-channel cross-correlation waveforms after taking the absolute values; wherein H ═ N (N-1)/2;
converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z relative to the well axis, establishing a speed model and discretizing the two-dimensional plane to-be-detected area, and obtaining a discretized speed model;
calculating the theoretical direct wave travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane to-be-detected area to obtain a travel time table containing the theoretical direct wave travel time from each grid node to all the detectors, and further calculating to obtain the theoretical direct wave travel time difference from each grid node to H pairs of non-repeating detectors;
and superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the theoretical direct wave travel time difference offset to obtain a final two-dimensional interference imaging section about R-Z, and determining the event position according to the final two-dimensional interference imaging section about R-Z.
As one improvement of the above technical solution, the cross-correlation waveform corresponding to H pairs of non-repeating detector pairs is obtained by calculation according to the original waveform of the N-channel acoustic emission event, and each pair of non-repeating detector pairs corresponds to 1-channel cross-correlation waveform, so as to obtain an H-channel cross-correlation waveform, and an absolute value is taken from the H-channel cross-correlation waveform, so as to obtain a cross-correlation waveform after the H-channel absolute value is taken; the specific process comprises the following steps:
according to the original waveforms of the N acoustic emission events, cross-correlation waveforms corresponding to H pairs of non-repetitive detectors are obtained through calculation, wherein H is N (N-1)/2;
wherein, Cnm(τ) is a cross-correlation waveform corresponding to a certain pair of non-repeating detector pairs { n, m }, i.e. a cross-correlation waveform obtained by performing cross-correlation operation on original waveforms corresponding to any pair of detector pairs { n, m } in H pairs of non-repeating detector pairs; v. ofn(t) is the original waveform recorded by the nth detector; v. ofm(τ) is the original waveform recorded by the mth detector; tau is a time parameter; t is tmaxThe time length of the single-channel original waveform before the cross-correlation operation;
each pair of nonrepeating detectors { n, m } corresponds to 1-channel cross-correlation waveform, so as to obtain H-channel cross-correlation waveform, and an absolute value of the H-channel cross-correlation waveform is obtained, so as to obtain H-channel cross-correlation waveform after the absolute value is obtained;
wherein, the first and the second end of the pipe are connected with each other,
for a certain cross-correlation waveform Cnm(τ) taking the absolute value of the cross-correlation waveform.
As one improvement of the technical scheme, the three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model is converted into a two-dimensional coordinate system of the distance R and the depth Z to the well axis, a two-dimensional plane to-be-measured area of the distance R and the depth Z to the well axis is obtained, and a speed model and discretization are established for the two-dimensional plane to-be-measured area to obtain a discretized speed model; the specific process comprises the following steps:
converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of a distance R and a depth Z relative to a well axis;
wherein x iswAnd ywRespectively an X coordinate and a Y coordinate of the monitoring well;
the depth Z is Z in a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model;
acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z to the well axis based on the two-dimensional coordinate system of the distance R and the depth Z to the well axis obtained after conversion, and establishing a speed model and discretization for the two-dimensional plane to-be-detected area to obtain a discretized speed model;
the size of a two-dimensional plane to-be-measured area about the distance R and the depth Z from a well axis is l x h, wherein l is the length of the to-be-measured area in the R direction, h is the length of the to-be-measured area in the Z direction, and the length unit is meter (m);
assigning a value to the speed of the region to be measured to obtain a corresponding speed model, and dispersing the speed model by using a grid with the size dr x dz, wherein dr is the length of the grid in the R direction, dz is the length of the grid in the Z direction, and finally obtaining a discretized speed model with the number of grid nodes nn x mm;
wherein nn is the number of grid nodes of the discretized velocity model in the R direction, and mm is the number of grid nodes of the discretized velocity model in the Z direction.
As one improvement of the above technical solution, the calculating of the theoretical direct wave travel time from each grid node to each detector in the discretized velocity model of the two-dimensional plane region to be measured obtains a travel time table including the theoretical direct wave travel time from each grid node to all detectors, and further calculates to obtain the theoretical direct wave travel time difference from each grid node to H pairs of non-repetitive detectors; the specific process comprises the following steps:
solving a function equation according to the discretized speed model and the position of the detector, and calculating the theoretical direct wave travel time from each grid node to each detector; wherein, the theoretical direct wave travel time comprises: the theoretical direct longitudinal wave travel time and the theoretical direct transverse wave travel time;
and obtaining a travel time table containing the travel time of the theoretical direct wave from each grid node to all the detectors, and then calculating the travel time difference of the theoretical direct wave from each grid node to the H pairs of non-repeating detectors through the travel time table.
As one improvement of the above technical solution, the cross-correlation waveform obtained by offsetting and superposing the absolute value of the H-channel according to the travel time difference of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z, and determining an event position according to the final two-dimensional interference imaging section about R-Z; the specific process comprises the following steps:
superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the travel time difference offset of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z;
wherein S isαβ(x) Is an R-Z two-dimensional interference imaging section about a direct wave alpha and a direct wave beta; t is tn,α(x) And tm,β(x) Respectively the theoretical travel time of the direct wave alpha from each grid node to the nth detector and the theoretical travel time of the direct wave beta from each grid node to the mth detector, wherein the direct wave alpha and the direct wave beta can both reach the longitudinal wave or the transverse wave;
from the resulting two-dimensional interferometric imaging profile S about R-Zαβ(x) Determining the position of the event according to the position of the middle maximum imaging value; wherein the event location is the location of the mini hydraulic fracturing event.
The invention also provides a small hydraulic fracturing monitoring device based on the same well, which comprises:
the three-dimensional same-well detection model building module is used for laying N detectors in a fracturing well, taking the fracturing well as a monitoring well, picking and recording original waveforms of N sound emission events, and building a three-dimensional same-well monitoring model;
the cross-correlation waveform absolute value obtaining module is used for calculating to obtain a cross-correlation waveform corresponding to H pairs of non-repetitive wave detectors according to the original waveform of the N channels of acoustic emission events, wherein each pair of non-repetitive wave detectors corresponds to 1 channel of cross-correlation waveform to further obtain an H channel cross-correlation waveform, and obtaining an absolute value of the H channel cross-correlation waveform to obtain a cross-correlation waveform after the H channel absolute value is obtained; wherein H ═ N (N-1)/2;
the model conversion and dispersion module is used for converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z relative to the well axis, establishing a speed model and discretization for the two-dimensional plane to-be-detected area, and obtaining a discretized speed model;
the direct wave travel time difference acquisition module is used for calculating theoretical direct wave travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane to-be-detected region, obtaining a travel time table containing the theoretical direct wave travel time from each grid node to all the detectors, and further calculating the theoretical direct wave travel time difference from each grid node to H pairs of non-repetitive detectors; and
and the event determining module is used for superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the travel time difference offset of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z, and determining the event position according to the final two-dimensional interference imaging section about R-Z.
Compared with the prior art, the invention has the beneficial effects that:
the method of the invention is a method for monitoring the small-sized hydraulic fracturing event by using the same well, and introduces an interference imaging method to position and image the acoustic emission event, thereby solving the near-well monitoring problem of the small acoustic emission event with high frequency and low energy, realizing the identification of micron-scale to decimeter-scale microcracks, being beneficial to predetermining areas which are easy to be damaged, providing valuable information about small-scale dynamic processes, instability in the rock can be observed before any damage becomes visible for the purposes of rock geomechanical stability assessment and interpretation, revealing in-depth information useful for risk assessment and stope planning, making conventional monitoring of hydraulic fracturing with a same-well monitoring system feasible for underground production, facilitating safe exploitation of resources, meanwhile, the number of the monitoring wells is reduced, and the construction difficulty and the production cost of water conservancy fracturing monitoring are reduced to a greater extent.
Drawings
FIG. 1 is a flow chart of a method of the invention for monitoring a small hydraulic fracture based on the same well;
FIG. 2 is a schematic diagram of the relative spatial locations of receiver-events of a three-dimensional co-well monitoring model in accordance with an embodiment of the present invention;
FIG. 3(a) is a waveform diagram of the vertical component of the vibration velocity of 10 seismic traces in the embodiment of the present invention;
FIG. 3(b) is a vertical component waveform of the rear 8 seismic wave vibration velocities in an embodiment of the present invention;
FIG. 4(a) is a cross-correlation waveform diagram of the 1 st original waveform and other original waveforms in the vertical component of the vibration velocity of the seismic wave in the embodiment of the present invention;
FIG. 4(b) is a cross-correlation waveform diagram of the absolute value of the 1 st original waveform and the other original waveforms in the vertical component of the vibration velocity of the seismic wave in the embodiment of the present invention;
FIG. 5 is a schematic diagram of a discretized velocity model of a two-dimensional planar region-to-be-measured according to an embodiment of the present invention;
FIG. 6 is a cross-sectional view of a two-dimensional interferometric imaging for R-Z according to an embodiment of the invention;
FIG. 7 is a schematic diagram of a two-dimensional hyperbolic overlay positioning in accordance with an embodiment of the present invention.
Detailed Description
The invention will now be further described with reference to the accompanying drawings.
As shown in fig. 1, the invention provides a small-sized hydraulic fracture monitoring method based on the same well, by the method, the near-well monitoring problem of a high-frequency and low-energy small acoustic emission event is solved, and the construction difficulty and the production cost of hydraulic fracture monitoring are reduced to a certain extent; the method comprises the following steps:
step S110) distributing N detectors in the fracturing well, taking the fracturing well as a monitoring well at the moment, picking up and recording original waveforms of N acoustic emission events, and establishing a three-dimensional same-well monitoring model;
step S120) according to the original waveform of the N channels of acoustic emission events, calculating to obtain cross-correlation waveforms corresponding to H pairs of non-repetitive wave detectors, wherein each pair of non-repetitive wave detectors corresponds to 1 channel of cross-correlation waveforms, further obtaining H channels of cross-correlation waveforms, and taking absolute values of the H channels of cross-correlation waveforms to obtain H channels of cross-correlation waveforms after the absolute values are taken; wherein H ═ N (N-1)/2;
specifically, according to the original waveform of the N acoustic emission events, cross-correlation waveforms corresponding to H pairs of non-repeating detector pairs are calculated, where H is N (N-1)/2;
wherein, Cnm(τ) is a cross-correlation waveform corresponding to a certain pair of non-repeating detector pairs { n, m }, i.e. a cross-correlation waveform obtained by performing cross-correlation operation on original waveforms corresponding to any pair of detector pairs { n, m } in H pairs of non-repeating detector pairs; v. ofn(t) is the original waveform recorded by the nth detector; v. ofm(τ) is the original waveform recorded by the mth detector; tau is a time parameter; t is tmaxThe time length of the single-channel original waveform before the cross-correlation operation;
each pair of nonrepeating detectors { n, m } corresponds to 1-channel cross-correlation waveform, so as to obtain H-channel cross-correlation waveform, and an absolute value of the H-channel cross-correlation waveform is obtained, so as to obtain H-channel cross-correlation waveform after the absolute value is obtained;
wherein the content of the first and second substances,
for a certain cross-correlation waveform Cnm(τ) taking the cross-correlation waveform after the absolute value;
step S130) converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z relative to the well axis, establishing a speed model and discretizing the two-dimensional plane to-be-detected area, and obtaining a discretized speed model;
specifically, a three-dimensional X-Y-Z coordinate system is converted to a two-dimensional coordinate system of distance R and depth Z about the well axis;
wherein x iswAnd ywRespectively an X coordinate and a Y coordinate of the monitoring well;
the depth Z is Z in a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model;
acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z to the well axis based on the two-dimensional coordinate system of the distance R and the depth Z to the well axis obtained after conversion, and establishing a speed model and discretization for the two-dimensional plane to-be-detected area;
wherein the size of a two-dimensional plane region to be measured with respect to a distance R to a well axis and a depth Z is l x h, wherein l is the length of the region to be measured in the R direction, h is the length of the region to be measured in the Z direction, and the length unit is meter (m),
assigning a value to the speed of the region to be measured to obtain a corresponding speed model, and dispersing the speed model by using a grid with the size dr x dz, wherein dr is the length of the grid in the R direction, dz is the length of the grid in the Z direction, and finally obtaining a discretized speed model with the number of grid nodes nn x mm;
wherein nn is the number of grid nodes of the discretized velocity model in the R direction, and mm is the number of grid nodes of the discretized velocity model in the Z direction;
step S140) calculating the theoretical direct wave travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane region to be measured to obtain a travel time table containing the theoretical direct wave travel time from each grid node to all the detectors, and further calculating to obtain the theoretical direct wave travel time difference from each grid node to H pairs of non-repeating detectors;
specifically, according to the discretized speed model and the position of the detector, solving a function equation, and calculating the theoretical direct wave travel time from each grid node to each detector, wherein the theoretical direct wave travel time comprises: the theoretical direct longitudinal wave travel time and the theoretical direct transverse wave travel time;
and obtaining a travel time table containing the travel time of the theoretical direct wave from each grid node to all the detectors, and then calculating the travel time difference of the theoretical direct wave from each grid node to the H pairs of non-repetitive detectors through the travel time table.
Step S150) according to the travel time difference offset of the theoretical direct wave, superposing the cross-correlation waveform of the H-channel absolute value to obtain a final two-dimensional interference imaging section about R-Z;
specifically, a cross-correlation waveform obtained after H-channel absolute value is superposed according to theoretical direct wave travel time difference offset to obtain a final two-dimensional interference imaging section about R-Z;
wherein，Sαβ(x) Is an R-Z two-dimensional interference imaging section about a direct wave alpha and a direct wave beta; t is tn,α(x) And tm,β(x) Respectively the theoretical travel time of the direct wave alpha from each grid node to the nth detector and the theoretical travel time of the direct wave beta from each grid node to the mth detector, wherein the direct wave alpha and the direct wave beta can both reach the longitudinal wave or the transverse wave; wherein, tn,α(x) The alpha in the intermediate can be up to longitudinal wave or up to transverse wave; t is tm,β(x) Beta in the intermediate can also be up to longitudinal wave or up to transverse wave; namely alpha can be direct longitudinal wave or direct transverse wave, beta can be direct longitudinal wave or direct transverse wave;
step S160) determines the event location from the resulting two-dimensional interferometric imaging profile about R-Z.
In particular, according to a final two-dimensional interferometric imaging profile S with respect to R-Zαβ(x) Determining the position of the event according to the position of the middle maximum imaging value; wherein the event location is the location of the mini hydraulic fracturing event.
The invention also provides a small hydraulic fracturing monitoring device based on the same well, which comprises:
the three-dimensional same-well detection model establishing module is used for distributing N detectors in a fractured well, taking the fractured well as a monitoring well at the moment, picking and recording original waveforms of N sound emission events, and establishing a three-dimensional same-well monitoring model;
the cross-correlation waveform absolute value taking module is used for calculating to obtain cross-correlation waveforms corresponding to H pairs of non-repeating wave detectors according to the original waveforms of the N channels of acoustic emission events, wherein each pair of non-repeating wave detectors corresponds to 1 channel of cross-correlation waveforms, so as to obtain H channels of cross-correlation waveforms, and taking absolute values of the H channels of cross-correlation waveforms to obtain H channels of cross-correlation waveforms after the absolute values are taken; wherein H ═ N (N-1)/2;
the model conversion and dispersion module is used for converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z relative to the well axis, establishing a speed model and discretization for the two-dimensional plane to-be-detected area, and obtaining a discretized speed model;
the direct wave travel time difference acquisition module is used for calculating theoretical direct wave travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane to-be-detected region, obtaining a travel time table containing the theoretical direct wave travel time from each grid node to all the detectors, and further calculating the theoretical direct wave travel time difference from each grid node to H pairs of non-repetitive detectors; and
and the event determining module is used for superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the travel time difference offset of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z, and determining the event position according to the final two-dimensional interference imaging section about R-Z.
Example 1.
In this specific embodiment, a schematic diagram of the relative spatial position of the receiver-event of the three-dimensional same-well monitoring model is shown in fig. 2, the region to be measured is a cube with an origin (0, 0, 0) as a vertex, three coordinate axes are adjacent edges with the size of 10m, the coordinates of the event position are (4.8m, 3.8m, 3.5m), the X coordinate of the monitoring well is 4.45m, the Y coordinate is 3.45m, 1 detector is respectively arranged at the positions with the depth Z of 2.5m and 3m in the monitoring well, 1 detector is arranged at intervals of 0.5m in the depth of 6m to 9.5m, and 10 detectors are provided in total, which means that the detector array completely covers the vertical direction of the event.
The seismic source mechanism of the event is a double-force couple source, a vertical component waveform diagram of 10-channel seismic wave vibration speed shown in fig. 3(a) is recorded, and it can be seen that the intensity of the waveform of the first 2 channels is obviously greater than that of the waveform of the second 8 channels, the low-amplitude wave which arrives at first is a direct longitudinal wave, and the high-amplitude wave which arrives at later is a direct transverse wave. Combining the vertical component waveform diagram of the vibration velocity of the rear 8 seismic waves as shown in fig. 3(b), it can be found that the waveform polarities of the direct longitudinal waves and the direct transverse waves in the front 2 seismic waves are respectively different from those of the direct longitudinal waves and the direct transverse waves in the rear 8 seismic waves, and the amplitude of each seismic wave gradually decreases as the position of the geophone from the event is farther.
For a high frequency acoustic emission event,
there are a total of 45 non-repeating detector pairs of 10 detectors.
Using the formula:
and calculating 45-channel cross-correlation waveforms of the vertical component of the vibration velocity of the seismic waves.
Fig. 4(a) is a cross-correlation waveform diagram of the 1 st original waveform and other original waveforms in the vertical component of the seismic wave vibration velocity, and it can be found that the polarity of the 1 st cross-correlation waveform is different from the polarity of the 8 th cross-correlation waveform, and when the cross-correlation waveforms with different polarities are directly used for offset stacking positioning, the peak values in the cross-correlation waveforms cannot be coherently stacked, so that the positioning result is affected.
Using the formula:
and taking an absolute value of the 45-channel cross-correlation waveform of the vertical component of the vibration velocity of the seismic wave.
Fig. 4(b) is a cross-correlation waveform diagram after taking absolute values of the 1 st original waveform and other original waveforms, and it can be found that the polarities of all the absolute-value cross-correlation waveforms are consistent, which means that the polarities of the cross-correlation waveforms are corrected.
Using the formula:
Z＝Z
and converting the three-dimensional X-Y-Z coordinate system into a two-dimensional coordinate system of a distance R and a depth Z from the well axis, and calculating to obtain that the distance from the event to the well axis is about 0.5m, namely the actual position of the event in the two-dimensional coordinate system is (0.5m, 3.5m), and the size of the area to be measured of the two-dimensional plane is 10 m. A velocity model is established for the two-dimensional plane region to be measured, the longitudinal wave velocity and the transverse wave velocity are 4500m/s and 2500m/s respectively, and the velocity model is discretized by a grid with the size of 0.05m × 0.05m, so that a discretized velocity model schematic diagram of the two-dimensional plane region to be measured about R-Z shown in fig. 5 is obtained. The distance of 10 points in fig. 5 corresponds to the actual distance of 1 point.
Using the formula:
the number of grid nodes of the discretized velocity model obtained by calculation is 201 × 201, that is, the number of grid nodes of the discretized velocity model is 201 in both the R direction and the Z direction.
Because the waveform of the direct shear wave in the original waveform of the vertical component of the vibration velocity of the seismic wave is strong, interference imaging is performed only by using the direct shear wave during imaging in the embodiment. Then, a distance function equation is solved according to the discretized speed model and the detector position, the theoretical direct transverse wave travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane region to be measured is calculated, a travel time table containing the theoretical direct transverse wave travel time from each grid node to all the detectors is further obtained, and the theoretical direct transverse wave travel time difference from each grid node to H pairs of non-repetitive detectors is calculated through the travel time table.
Using the formula:
and calculating to obtain an interference imaging section of the direct transverse wave, wherein alpha and beta are taken as the direct transverse wave.
Wherein, FIG. 6 is a two-dimensional interference imaging cross-sectional view about R-Z, the lower right hand small image in the figure is an enlarged view of a rectangular area of the coordinate range (0 m-1 m, 3 m-4 m), and the black circle represents the true position of the event. The final localization result of the event obtained from the position of the maximum imaging value in the map is (0.5m, 3.5m), which is consistent with the true position of the event, and it can be found that the acoustic emission event of high frequency can be accurately localized by this method.
It should be noted that the three-dimensional interference imaging cross-sectional view is formed by stacking a plurality of hyperboloids, the corresponding two-dimensional interference imaging cross-sectional view is formed by stacking a plurality of hyperboloids, fig. 7 is a two-dimensional hyperboloid stacking positioning schematic diagram, hyperboloids in different directions and shapes are stacked and constrained to obtain a final event position, and an energy smearing trace is left at a corresponding position in the interference imaging cross-sectional view to form an artifact, but at this time, the two-dimensional imaging cross-sectional view is about a distance R and a depth Z to a well axis, the Z axis is equivalent to the well axis, so that only the Z axis, namely the energy smearing trace on the right side of the well axis, correspondingly appears in fig. 6, and the artifact in fig. 6 mainly extends obliquely along the Z axis because the energies of hyperboloids (R) and (c) in fig. 7 are weaker than the energies of hyperboloids (R).
The small-scale hydraulic fracturing monitoring method based on the same well provided by the invention can accurately monitor high-frequency and low-energy small acoustic events by taking the fracturing well as a monitoring well and adopting an interference imaging method, is not only beneficial to guiding the application of the interference imaging method in the field of in-situ acoustic emission monitoring in future, but also can predetermine a region which is easy to be damaged by successfully positioning the information obtained by the small acoustic emission events, provides valuable information about a small-scale dynamic process, can observe instability in rocks before any damage becomes visible so as to achieve the purposes of evaluating and explaining the geomechanical stability of the rocks, reveals deep information useful for risk evaluation and stope planning, enables the conventional hydraulic fracturing monitoring by utilizing a monitoring system of the same well to be feasible for underground production, and simultaneously reduces the arrangement number of wells, the construction difficulty and the production cost of hydraulic fracturing monitoring are reduced to a great extent.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and are not limited. Although the present invention has been described in detail with reference to the embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (6)
1. A co-well-based small-scale hydraulic fracture monitoring method comprises the following steps:
arranging N detectors in a fracturing well, taking the fracturing well as a monitoring well, picking up and recording original waveforms of N sound emission events, and establishing a three-dimensional same-well monitoring model;
according to the original waveform of the N sound emission events, calculating to obtain cross-correlation waveforms corresponding to H pairs of non-repetitive wave detectors, wherein each pair of non-repetitive wave detectors corresponds to 1 channel of cross-correlation waveforms, further obtaining H channel of cross-correlation waveforms, and taking an absolute value of the H channel of cross-correlation waveforms to obtain H channel of cross-correlation waveforms after the absolute value is taken; wherein H ═ N (N-1)/2;
converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z relative to the well axis, establishing a speed model and discretizing the two-dimensional plane to-be-detected area, and obtaining a discretized speed model;
calculating the theoretical direct wave travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane to-be-detected area to obtain a travel time table containing the theoretical direct wave travel time from each grid node to all the detectors, and further calculating to obtain the theoretical direct wave travel time difference from each grid node to H pairs of non-repeating detectors;
and superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the travel time difference offset of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z, and determining the event position according to the final two-dimensional interference imaging section about R-Z.
2. The small-sized hydraulic fracturing monitoring method based on the same well according to claim 1, wherein the cross-correlation waveforms corresponding to H pairs of non-repeating wave detectors are obtained by calculation according to the original waveforms of N acoustic emission events, each pair of non-repeating wave detectors corresponds to 1 cross-correlation waveform, further H-channel cross-correlation waveforms are obtained, the absolute value of the H-channel cross-correlation waveforms is obtained, and the cross-correlation waveforms after the absolute value of the H-channel cross-correlation waveforms are obtained; the specific process comprises the following steps:
according to the original waveforms of the N acoustic emission events, cross-correlation waveforms corresponding to H pairs of non-repetitive detectors are obtained through calculation, wherein H is N (N-1)/2;
wherein, Cnm(τ) is a cross-correlation waveform corresponding to a certain pair of non-repeating detector pairs { n, m }, i.e. a cross-correlation waveform obtained by performing cross-correlation operation on original waveforms corresponding to any pair of detector pairs { n, m } in H pairs of non-repeating detector pairs; v. ofn(t) is the original waveform recorded by the nth detector; v. ofm(τ) is the original waveform recorded by the mth detector; tau is a time parameter; t is tmaxThe time length of the single-channel original waveform before the cross-correlation operation;
each pair of nonrepeating detectors { n, m } corresponds to 1-channel cross-correlation waveform, so as to obtain H-channel cross-correlation waveform, and an absolute value of the H-channel cross-correlation waveform is obtained, so as to obtain H-channel cross-correlation waveform after the absolute value is obtained;
wherein the content of the first and second substances,
for a certain cross-correlation waveform Cnm(τ) taking the absolute value of the cross-correlation waveform.
3. The small-scale hydraulic fracturing monitoring method based on the same well as the claim 1 is characterized in that the three-dimensional X-Y-Z coordinate system of the established three-dimensional same well monitoring model is converted into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, a two-dimensional plane to-be-measured area of the distance R and the depth Z relative to the well axis is obtained, a speed model and discretization are established for the two-dimensional plane to-be-measured area, and a discretized speed model is obtained; the specific process comprises the following steps:
converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of a distance R and a depth Z relative to a well axis;
wherein x iswAnd ywRespectively an X coordinate and a Y coordinate of the monitoring well;
the depth Z is Z in a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model;
acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z to the well axis based on the two-dimensional coordinate system of the distance R and the depth Z to the well axis obtained after conversion, and establishing a speed model and discretization for the two-dimensional plane to-be-detected area to obtain a discretized speed model;
the size of a two-dimensional plane to-be-measured area about the distance R and the depth Z from a well axis is l x h, wherein l is the length of the to-be-measured area in the R direction, h is the length of the to-be-measured area in the Z direction, and the length unit is meter (m);
assigning a value to the speed of the region to be measured to obtain a corresponding speed model, and dispersing the speed model by using a grid with the size dr x dz, wherein dr is the length of the grid in the R direction, dz is the length of the grid in the Z direction, and finally obtaining a discretized speed model with the number of grid nodes nn x mm;
wherein nn is the number of grid nodes of the discretized velocity model in the R direction, and mm is the number of grid nodes of the discretized velocity model in the Z direction.
4. The method for monitoring the small hydraulic fracture based on the same well according to claim 1, wherein the travel time of the theoretical direct travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane area to be measured is calculated to obtain a travel time table including the travel time of the theoretical direct travel time from each grid node to all the detectors, and the travel time difference of the theoretical direct travel time from each grid node to H pairs of non-repetitive detectors is calculated; the specific process comprises the following steps:
solving a function equation according to the discretized speed model and the position of the detector, and calculating the theoretical direct wave travel time from each grid node to each detector; wherein, the theoretical direct wave travel time comprises: the theoretical direct longitudinal wave travel time and the theoretical direct transverse wave travel time;
and obtaining a travel time table containing the travel time of the theoretical direct wave from each grid node to all the detectors, and then calculating the travel time difference of the theoretical direct wave from each grid node to the H pairs of non-repeating detectors through the travel time table.
5. The same-well-based small-sized hydraulic fracturing monitoring method according to claim 1, wherein the cross-correlation waveform obtained after H-channel absolute value taking is superposed according to theoretical direct wave travel time difference offset to obtain a two-dimensional interference imaging section finally related to R-Z, and an event position is determined according to the two-dimensional interference imaging section finally related to R-Z; the specific process comprises the following steps:
superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the travel time difference offset of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z;
wherein S isαβ(x) Is an R-Z two-dimensional interference imaging section about a direct wave alpha and a direct wave beta; t is tn,α(x) And tm,β(x) Respectively the theoretical travel time of the direct wave alpha from each grid node to the nth detector and the theoretical travel time of the direct wave beta from each grid node to the mth detector, wherein the direct wave alpha and the direct wave beta can both reach the longitudinal wave or the transverse wave;
from the resulting two-dimensional interferometric imaging profile S about R-Zαβ(x) Determining the position of the event according to the position of the middle maximum imaging value; wherein the event location is the location of the mini hydraulic fracturing event.
6. A small-size hydraulic fracturing monitoring devices based on with well, its characterized in that, the device includes:
the three-dimensional same-well detection model establishing module is used for distributing N detectors in a fractured well, taking the fractured well as a monitoring well at the moment, picking and recording original waveforms of N sound emission events, and establishing a three-dimensional same-well monitoring model;
the cross-correlation waveform absolute value taking module is used for calculating to obtain cross-correlation waveforms corresponding to H pairs of non-repeating wave detectors according to the original waveforms of the N channels of acoustic emission events, wherein each pair of non-repeating wave detectors corresponds to 1 channel of cross-correlation waveforms, so as to obtain H channels of cross-correlation waveforms, and taking absolute values of the H channels of cross-correlation waveforms to obtain H channels of cross-correlation waveforms after the absolute values are taken; wherein H ═ N (N-1)/2;
the model conversion and dispersion module is used for converting a three-dimensional X-Y-Z coordinate system of the established three-dimensional same-well monitoring model into a two-dimensional coordinate system of the distance R and the depth Z relative to the well axis, acquiring a two-dimensional plane to-be-detected area of the distance R and the depth Z relative to the well axis, establishing a speed model and discretization for the two-dimensional plane to-be-detected area, and obtaining a discretized speed model;
the direct wave travel time difference acquisition module is used for calculating theoretical direct wave travel time from each grid node to each detector in the discretized speed model of the two-dimensional plane to-be-detected region, obtaining a travel time table containing the theoretical direct wave travel time from each grid node to all the detectors, and further calculating the theoretical direct wave travel time difference from each grid node to H pairs of non-repetitive detectors; and
and the event determining module is used for superposing the cross-correlation waveform obtained after the H-channel absolute value is taken according to the travel time difference offset of the theoretical direct wave to obtain a final two-dimensional interference imaging section about R-Z, and determining the event position according to the final two-dimensional interference imaging section about R-Z.
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