JP2816899B2 - Underground structure evaluation method by three-dimensional particle motion analysis of released elastic waves during well drilling - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a so-called Lissajous pattern which is a waveform representing three-dimensional particle motion of an elastic wave (acoustic emission) generated during drilling of a well. Based on the discovery of the fact that the surface is oriented in the direction, the elastic wave is detected using a triaxial elastic wave detector in the downhole, and appropriate information processing is performed. (Fracture zones, cracks, stratum boundaries, etc.) in advance, and (2) grasp the underground structure around the well (VS
The present invention relates to a method for evaluating underground structure by three-dimensional particle motion analysis of emitted elastic waves during drilling of a well, which enables P).

[Conventional technology]

Conventionally, means for detecting an underground structure using a three-axis detector by sound waves or the like include those described in JP-A-63-32085, JP-A-62-90495 and JP-A-62-59792. There is one described in Japanese Patent Publication No. In addition, so-called seismic data such as vibration waves or elastic waves generated from drill bits during drilling are acquired through a geophone installed on the ground surface, and at the same time, vibration waves generated by drilling are transmitted through a pilot sensor installed at the reference point at the top of the rigging organization. And the vibration propagation time of the data obtained between these two points is corrected.
The so-called TOMEX that analyzes the underground structure based on this
A technique called a law is known.

This conventional technology called the TOMEX method is the eleventh
Drilling bit 1, well 2, drilling rig 3 as shown
In addition to the basic configuration described above, a geophone 4 is installed on the ground surface remote from the drilling rig 3, and the drilling bit 1 uses a direct wave and a reflected wave of a vibration wave generated at the time of drilling as a reference point. Location and the geophone 4
By supplementing at the installation position and taking a mutual correlation between them, for example, a stratum boundary existing near the well 2 is detected. That is, the vibration propagation time between the excavation bit 1 and the reference point is corrected for the observation data at the reference point, and a vibration wave meter from the excavation bit 1 position, that is, input wave data is obtained. Then, a cross-correlation process is performed on the input wave data and the data obtained by the geophone 4 on the ground surface to obtain a record similar to the case of an impulse epicenter.

[Problems to be solved by the invention]

The TOMEX method determines the propagation distance between the direct wave from the tip of the drill bit and the reflected wave from the reflective surface to the detector, and correlates the reference signal from the drill rig with the detected signal. It is assumed that the reflection surface is estimated by analysis. However, regarding the analysis time, it is usually necessary to analyze a signal over a long time of about one hour, which is not only suitable as a real-time monitoring method desired in the field, When the drilling rate of the well is large, there is a disadvantage that the analysis itself cannot be performed.

In addition, in such a conventional TOMEX method, in the usual method, only the reflected wave is used for the inclined reflecting surface, so it is difficult to estimate the reflection surface. However, there is a drawback in that it requires separate introduction of a sensor array and migration analysis. As a result, not only is the cost increased, but also the amount of analysis is increased, and the accuracy and the performance are reduced.

Further, in this method, it is necessary to estimate the sound emitted from the tip of the drill bit using the signal detected in the rig, and therefore, it is necessary to estimate the transfer function of the drill system with high accuracy.

Furthermore, this method has a decisive drawback in that it is strongly affected by noise propagating on the ground surface and a high attenuation layer near the ground surface, resulting in a reduction in resolution due to loss of noise and high-frequency components.

[Means for solving the above problems]

In order to solve the above-mentioned drawbacks, the present inventors performed three-dimensional acoustic emission measurement (AE measurement) in a well at the time of excavating a well, and as a result, the three-dimensional Lissajous shape obtained by this measurement was determined to be a specific shape. At frequency, it was found to have a particular polarization direction. That is, the direction of polarization obtained from the principal axis analysis of the Lissajous pattern in a limited frequency band was extremely consistent with the direction of the drill bit in the drilling well. This is because, in the AE signal, the longitudinal wave component that reaches directly than the reflected wave or the transverse wave whose mode has been converted is dominant, and the Lissajous pattern shows an extremely good directionality.

Therefore, by analyzing this Lissajous pattern, it is possible to detect the direction of the excavation bit very easily and with a simple configuration, and furthermore, by utilizing this information, the weak reflected wave from the stratum boundary can be detected. And the delay time is evaluated, it is possible to detect a stratum boundary existing at a higher depth in the traveling direction during excavation.

[Action]

The present invention uses a drill bit for well drilling as an AE signal source, and performs a three-dimensional particle motion analysis on the AE signal obtained by a three-axis AE sensor. Since the polarization direction obtained from the principal axis analysis matches the direction of the actual drill bit, the direction of the drill bit is determined by this analysis, and the result is used to determine the relative direction of the direct wave and the reflected wave. The stratum boundary existing in the excavation traveling direction is evaluated from the traveling direction and the arrival time difference.

〔Example〕

The present invention will be described based on a conceptual diagram of one embodiment. First
The figure is a conceptual diagram in the case of AE triaxial measurement according to the present invention.

In the figure, 11 is a drill bit, 12 is a drilling well, 13 is a drilling rig, 14 is a measurement well for monitoring, 15 is a triaxial AE sensor, 16
Is a measuring vehicle. In order to understand the concept of the system, an outline of measurement by the measuring vehicle 16 is shown in FIG. That is, in the figure, reference numeral 17 denotes a three-axis main amplifier (3 Ch. Main Am
p.) and 18 are the fixing motor controller (Fixing Motor C
ontroller), 19 is an electronic compass (Electric Comp)
ass Driver), 20 is an automatic data collection system (Automati
c is a data acquisition system, 21 is a personal computer, 22 is a recording disk, 23 is a data recorder, 24 is a digital storage oscilloscope, 25 is an RMS DC converter, and 26 is an Xt recorder. With the purpose of drilling a 380m well based on the system with this configuration, 313m-355m
The drill bit 11 is turned between the rocks to generate rock crushing sound due to drilling, and the triaxial AE provided in the monitoring well 15 drilled at a distance of about 80 m from the drilling well 12.
The AE signal was collected by the measurement sensor 14. This wellbore AE
The sensor 14 was installed in the monitoring well 15 at a depth of 206 m.

In such a measurement system, the three-axis AE sensor
Fig. 2 shows the result of 3D particle motion analysis on the AE signal obtained by Fig. 14, and Fig. 2 shows the Lissajous pattern of the raw AE signal, and Fig. 3 shows the result of filtering with a cutoff frequency of 80Hz. Shown in 2 (a) and 2 (b), FIG. 2 (a) shows the horizontal Lissajous pattern, and FIG. 2 (b) shows the vertical Lissajous pattern, and it is assumed that the drill bit 11 is in the direction of the arrow. . Comparing and examining the Lissajous patterns shown in FIGS. 2 and 3, the Lissajous pattern of the raw data (FIG. 2) does not show much polarization, but the Lissajous pattern resulting from the filtering shows an elliptical shape (FIG. FIG. 3) shows that the direction of the main axis coincides with the direction of the actual drill bit 11.

In addition, spectral matrix analysis of AE data [JCSamsom,
“Matrix and Stokes Vector Representation of Detec
tors for Polarized Waveforms: Theory and Some Appli
cations to Teleseimic Waves, "Geophycs.JRAstr.So
c., 51, pp. 583-603 (1977)], a vertical projection of the eigenvectors is as shown in FIG. 4 as an example. This figure shows the polarization direction as a function of frequency.As is clear from this figure, the AE Lissajous pattern in the frequency range of 100 Hz or less, which is sufficiently lower than the resonance frequency of the AE sensor 10, is oriented in the direction of the drill bit. It shows that it is polarized. This seems to be due to the following reasons.

(A) As for the elastic wave generated by the drill bit, the longitudinal wave is superior to the shear wave.

(B) The reflection, refraction, and mode conversion waves of the elastic wave at the stratum boundary are relatively small.

Therefore, in a frequency range sufficiently lower than the resonance frequency of the sensor, the direction of the drill bit can be obtained by three-dimensional Lissajous analysis of the AE signal.

Based on the above results, the inventors of the present invention performed AE through a low-pass filter of f _c = 80 Hz to detect the direction of a drill bit.
Principal component analysis (PCA) of the Lissajous pattern was performed. The results are shown in FIG. FIG. 5A shows the result of analyzing the horizontal component, and FIG. 5B shows the result of analyzing the vertical component. As apparent from FIGS. 5 (a) and 5 (b), the drilling bit direction can be detected with extremely high precision by the PCA in both the horizontal and vertical directions. In this example, as a result of comparison with the actual drill bit direction, the detection error was within 15 ° in both the horizontal and vertical planes. This error is considered to be the effect of underground velocity structure and added noise.

In addition, in this example, it was found that, although not always, waves of strong energy generated by friction between the drill pipe and the wellbore sometimes affected the detection of the position of the drill bit. That is, the intensity of the friction is included in the RMS (effective value) signal of the AE and is generated as a constant periodic signal corresponding to the rotation of the drill pipe, and is evaluated by the energy density distribution ratio (EDDR '). it can. Fig. 6 shows EDDR '
And the angle error of the drill bit position. In FIG. 6, it can be seen that the orientation error tends to increase when EDDR 'is 10 or more.

Based on the above results, the inventors of the present invention have applied not only the direct wave from the drill bit 11 but also the counter wave to evaluate the underground structure. That is,
As described above, longitudinal waves are more prominent than transverse waves in the AE signal resulting from rock destruction due to excavation, so by detecting the direct and reflected waves of this AE signal, it is possible to analyze the underground structure. Become.

Therefore, the inventors of the present application used a method shown in FIG.
VSP measurement was performed. FIG. 7 shows only the measurement concept, and its basic measurement configuration is not different from that shown in FIG. Accordingly, the same reference numerals are used for the same components as those used in FIG. In addition,
20 is a conceptually indicated stratum boundary. As shown in FIG. 7, this stratum boundary can be evaluated from both the relative traveling direction of the direct wave and the reflected wave and the time delay. That is, in order to estimate the cross-spectral analysis of the particle motion as an evaluation of the arrival time difference, “(a) the drill bit generates a longitudinal wave as a point signal source, and (b) the mode conversion into a shear wave at the stratum boundary. The energy is negligibly small compared to the reflection of the longitudinal wave. " Under such a premise, the vector of the particle motion by the three-axis AE sensor can be expressed as follows.

here, Are the particle motion vectors of the longitudinal wave that has arrived directly and the longitudinal wave that has arrived by reflection, respectively. Also, Is a vector of noise components. Assuming that the reflected longitudinal wave has a time delay τ and a difference in the traveling direction in the vertical plane θ, the difference between the time delay and the traveling direction is the particle motion vector of the wave from the virtual reflecting surface. Can be obtained by using the concept of Where the particle motion vector Can be calculated from the three-dimensional particle motion if the direction of the actual AE signal source is known. That is, the cross spectrum of the direct wave and the virtual reflected wave is represented by the angular frequency ω and the relative direction φ of the reflecting surface.
And the following equation is derived.

The relative direction of the reflective surface is Can be obtained from the relative direction φ of the virtual reflecting surface at the peak of From the phase of

When the result obtained by this method is applied to the filtered AE signal, the result is as shown in FIG.

As can be seen from FIG. 8, the displayed cross spectrum It can be seen that there are several peaks in the norm.
That is, the reflection surface is estimated in the direction of φ that gives the maximum value to the norm of the cross spectrum.

9 and 10 show the resulting distance and direction changes as a function of excavation depth, respectively, by selecting the maximum value of the cross spectrum in the frequency range 20-40 Hz. This change is due to the reflection layer in this field being 385m
Is assumed, and this is indicated by a broken line. In FIG. 9, the distance seems to be slightly shifted, but it can be seen from the result compared with FIG. 10 that the direction coincides with the actual silt plane.

In this embodiment, monitoring wells are separately provided in addition to the drilling wells, but the number of monitoring wells is not limited to this. By installing a plurality of these wells, the accuracy of three-dimensional underground analysis is significantly improved.

〔The invention's effect〕

The present invention uses a drilling bit during drilling as a signal source, supplements it with a three-axis AE sensor, and performs three-dimensional analysis, whereby the Lissajous pattern of the filtered AE signal is polarized in the direction of the drilling bit, making it easy to drill. The bit direction can be determined.

In addition, the existence of the reflecting surface can be easily known from the time difference between the direct wave and the reflected wave from the drill bit, thereby contributing to the analysis of the underground structure in the direction of the excavation.

That is, according to the present invention, the reflection surface can be easily estimated by the cross spectrum analysis of the particle motion detected by the downhole three-axis AE sensor, so that the underground structure can be estimated independently of the excavation work, Moreover, the analysis of the signal obtained in a time of only a few minutes can provide an extremely excellent effect that the underground structure can be analyzed.
This means that real-time monitoring can be easily performed at the site, and as a result, when excavating for underground structure evaluation, it is possible to make advance predictions about the existence of crush zones, reservoirs, etc. effective.

In addition, since the underground structure can be evaluated using the propagation distance difference and the propagation direction difference between the direct wave and the reflected wave, it is possible to estimate the slope underground. There is an extremely excellent effect such as enabling highly accurate estimation.

Also, unlike the conventional TOMEX method, the present invention does not require the estimation of the signal emitted from the tip of the drill bit, so that the calculation time can be shortened. Uncertain elements such as functions can be reduced.

In addition, since measurement is performed in the wellbore, it is possible to avoid the effects of noise and the high attenuation layer near the ground surface.
Since N measurement can be performed and high-frequency components can be detected, it is possible to obtain an extremely excellent underground structure evaluation method such as an analysis with high resolution.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing one embodiment of the AE triaxial measurement according to the present invention, and FIG. 2 is a diagram showing an AE signal obtained from the above-described embodiment in the state of three-dimensional particle motion analysis as it is. FIG. 3 is a diagram showing a Lissajous pattern, FIG. 3 is a diagram showing a result obtained by filtering this at a cutoff frequency of 80 Hz, and FIG. 4 is an example of a vertical projection of an eigenvector obtained by analyzing a spectral matrix of the AE data. shows, FIG. 5 (a), (B) illustrates the results spindle component analysis (PCA) of the AE Lissajous pattern through a low-pass filter f _c = 80 Hz, of which FIG. 5 (a) Fig. 6B shows the horizontal component, Fig. 6B shows the analysis result of the vertical component, Fig. 6 shows the relationship between the energy density distribution ratio (EDDR ') and the angular error of the drill bit position. Fig. 7 shows three-axis VSP measurement using the reflected wave from the drill bit Conceptual view, FIG. 8 is,
The difference between the time delay and the traveling direction is the particle motion vector of the wave from the virtual reflecting surface 20 Figs. 9 and 10 show the result of applying the result expressed using the concept of フ ィ ル タ リ ン グ to the AE signal that has been subjected to the filtering process. Figs. 9 and 10 show the results obtained by selecting the maximum value of the cross spectrum in the frequency range of 20 to 40 Hz. FIG. 11 shows the obtained changes in distance and direction as a function of the excavation depth. FIG. 11 is a diagram showing the basic concept of conventional AE three-axis measurement called the TOMEX method. 1 ... drilling bit, 2 ... well, 3 ... drilling rig, 4 ...
... Geophone, 10 ... AE sensor, 11 ... Drill bit,
12… Drilling well, 14… AE sensor in borehole, 16… Measurement vehicle, τ: time delay, φ: relative direction, ω: angular frequency,

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiroshi Asanuma 3-40-11 Mizumori, Aoba-ku, Sendai, Miyagi Prefecture (72) Inventor Kazumi Osato 8-7-6, Kamurenjaku, Mitaka-shi, Mitaka-shi, Tokyo (58) Field (Int.Cl. ⁶ , DB name) E21B 47/024 E21B 47/12

Claims (6)

(57) [Claims] An elastic wave emitted from a drill bit during drilling is detected by a three-axis elastic wave detector in a downhole, and a filtering process is performed on the detection result in a frequency range sufficiently lower than a resonance frequency of the sensor. And detecting the direction of the drill bit by performing a principal axis component analysis of the three-dimensional particle motion of the AE signal. 2. An elastic wave emitted from a drill bit during drilling is detected by a three-axis elastic wave detector in a downhole, and in the cross spectrum analysis of three-dimensional particle motion based on the detected result, a direct wave from the bit is detected. Of underground structure by three-dimensional particle motion analysis of released elastic waves during well excavation to analyze underground structure from relative traveling directions of reflected and reflected waves and their arrival time differences. 3. An elastic wave emitted from a drill bit during drilling is detected by a 3-axis elastic wave detector in a downhole, and a direct wave from the bit is detected in a cross spectrum analysis of three-dimensional particle motion based on the detection result. 3. The well according to claim 2, wherein the formation boundary existing in the excavation traveling direction is evaluated based on both the transmission distance difference of the reflected wave from the reflection wave and the reflection surface and the difference in the propagation direction of the direct wave and the reflected wave to the sensor. Underground structure evaluation method by three-dimensional particle motion analysis of released elastic waves during excavation. 4. The cross-spectral analysis of three-dimensional particle motion as a detection result of the downhole three-axis elastic wave detector is obtained as a result of filtering at a predetermined cutoff frequency. 2. The method for detecting the direction of a drill bit according to claim 1. 5. The cross-spectral analysis of three-dimensional particle motion detected by the downhole three-axis elastic wave detector is obtained by filtering at a predetermined cutoff frequency. Item 3. An underground structure evaluation method based on three-dimensional particle motion analysis of an emitted elastic wave during well drilling according to item 2 or 3. 6. A cross-spectral analysis of three-dimensional particle motion based on a detection result of the three-axis elastic wave detector in the downhole,
Vector of particle motion by AE sensor The vector spectrum of the particle motion of the longitudinal wave that arrives directly and the longitudinal wave that arrives by reflection) and the cross spectrum of the direct wave and the virtual reflected wave are expressed by the angular frequency ω and the relative direction φ of the virtual reflecting surface.
Function, The underground structure evaluation method by three-dimensional particle motion analysis of emitted elastic waves during drilling of a well, according to claim 2 or 3, wherein the underground structure is evaluated by the following equation.