RU2370791C2 - Detection method of generation or existing of one crack, filled with liquid, in medium - Google Patents

Abstract

FIELD: mining.

SUBSTANCE: medium oscillations are registered. Frequencies of registered oscillations are compared with existing frequencies of medium oscillations in the absence of cracks, filled with liquid. Besides frequencies of inpounded interface waves, spreading over surfaces, at least one crack, filled with liquid, are determined as corresponding to substract resonance frequencies, at least, one crack, filled with liquid. Group velocity of the interface wave is calculated, that depends on crack width and properties of construction element and fluid. Typical size of the crack along spreading of the inpounded interface wave is calculated. Generation or existing of at least one crack, filled with liquid, is detected.

EFFECT: creation of effective detection method of generation or existing of cracks filled with liquid in the medium.

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Description

FIELD OF THE INVENTION

The present invention generally relates to monitoring liquid-filled regions in a medium, in particular to detecting the nucleation or existence of liquid-filled regions in a medium based on boundary waves propagating over their surfaces.

State of the art

As used herein, the term “fluid-filled region” is a generic term for any fluid-filled discontinuity in a continuous medium. In particular, the fluid-filled areas may be cracks, layers, faults, shears. Used in the description, the term "monitoring" is a generic term for actions to detect, observe, predict, analyze or determine the main characteristics.

The present invention is applicable to a wide variety of environments (in particular, underground formations, structural elements, bones) and liquids (in particular, water, oil).

Monitoring of fluid-filled cracks is critical in many areas of human activity, such as mining, medicine, and construction. In this case, fluid-filled cracks in the medium can be both desirable and undesirable. Desired fractures include artificially created fractures, such as hydraulic fractures, designed to increase oil production efficiency or pre-treat ore during mining. Undesirable fluid-filled cracks include, in particular, large-scale natural underground cracks in the vicinity of cities and industrial facilities, cracks in building structures and bones.

In the oil industry, the formation of hydraulic fractures is widely used to intensify oil production through the formation or expansion of channels from the well to the reservoir. Fracturing cracks form by injecting a fluid (also called a fracturing fluid) into a well under pressure. As a result of this, one or more cracks of normal separation are formed and filled with liquid in the underground formation, which usually leads to an increase in the intensity of oil production.

The fracturing fluid contains a proppant, a proppant whose small particles are added to the fluid to keep the crack open after the fluid has been stopped and the pressure is relieved to create a high-throughput drainage layer in the formation. Such particles are particles of sand or ceramic material. For efficient use, the crack must propagate inside the reservoir and not go into the adjacent layers, and also have sufficient dimensions. Thus, the determination of the characteristic dimensions of a fluid-filled crack is an important step in ensuring the optimization of the production process.

Sometimes the proppant forms an impenetrable structure near the tip of the crack, causing the crack to stop growing. Finding out when a crack stops growing is important for an operator who needs to determine when proppant pumping has stopped.

Monitoring of fluid-filled areas is also of great importance in the context of detecting the origin or existence of large-scale natural cracks in underground formations that can cause erosion of the earth's surface, cracks in various elements of building structures, such as slabs, building foundations, various types of supports that can lead to to the destruction of structural elements, as well as the exploration of underground layers filled with liquid, and determine their characteristics.

Currently, the nucleation or existence of fluid-filled cracks is determined using various technologies and techniques. The method of indirect determination is most widely used, based on the analysis of the characteristics of pressure changes during development and production, which is described, for example, in Reservoir Stimulation, Third Edition, MJEconomides and KGNolte (Ed.), Chichester, UK, Wiley, 2000. A more reliable method is the acoustic crack investigation technique used in the field and based on location of events using passive acoustic radiation. Such technology is described, for example, in the publication D. Barree, MKFisher, RAWoodroof, "A practical guide to hydraulic fracture diagnostic technologies", material SPE 77442, presented at the Annual Technical Conference and Exhibition in San Antonio, Texas, USA, 09/28/2002 - 02/10/2002.

All of the listed technologies and techniques involve complex preliminary processing of the collected data, which is required for the subsequent determination of the geometric characteristics of the fracture based on the models. As a result, the complexity of data processing, characteristic of the mentioned technologies and methods, does not allow to quickly interpret the measurement data and greatly limits the ability to determine the geometric characteristics of cracks in real time.

From US 5206836, a method is known for detecting the nucleation or existence of liquid-filled cracks by registering vibrations in a fluid filling a crack with a resonant frequency and comparing the frequencies of the recorded vibrations with the available vibrational frequencies of the medium in the absence of liquid-filled cracks.

The result provided by this method is achieved by interpreting the recorded fluctuations in the fluid pressure in the well based on the waves propagating in the fluid inside the fracture.

Thus, at present, in the considered field of technology there is a need for fast reliable methods for monitoring areas filled with liquid, providing the ability to perform monitoring in real time.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an effective method for detecting the nucleation or existence of fluid-filled cracks in a medium.

To solve this problem, in accordance with the invention, a method is proposed for detecting the nucleation or existence of at least one fluid-filled crack in a medium, comprising the steps of recording medium vibrations, comparing the frequencies of recorded vibrations with the available vibration frequencies of the medium in the absence of liquid-filled cracks and detecting nucleation or the existence of at least one fluid-filled crack based on the presence of frequencies of detected oscillations of the medium, significantly less the lowest available vibrational frequencies of the medium.

In a preferred embodiment, the medium is an underground formation, the mouth of the fracture adjacent to the injection well. At the stage of registration of vibrations, fluctuations in the fluid pressure in the injection well are recorded and the lowest resonant frequencies that correspond to vibrations of at least one fluid-filled fracture are isolated. At the stage of determining wave characteristics, the frequencies ν ^{(n) of} standing boundary waves (n is an integer) propagating along the surfaces of at least one fluid-filled crack are determined as corresponding to the selected resonant vibrational frequencies of at least one fluid-filled crack and the group velocity V ( ν ⁽ⁿ⁾ , w) of the boundary wave, which depends on the properties of the formation and fluid.

In another preferred embodiment, the medium is a part of the structural element immersed in the liquid, and at least one fluid-filled crack intersects the surface of the part of the structural element immersed in the liquid. At the stage of registration of vibrations, the sensor is fixed at the intersection of at least one fluid-filled crack with the surface of the part of the structural element immersed in the fluid, the vibrations are recorded by this sensor, and the lowest resonant frequencies that correspond to the vibrations of at least one fluid-filled crack are detected. At the stage of determining wave characteristics, the frequencies ν ^{(n) of} standing boundary waves (n is an integer) propagating along the surfaces of at least one fluid-filled crack are determined as corresponding to the selected resonant vibrational frequencies of at least one fluid-filled crack and the group velocity V ( ν ⁽ⁿ⁾ , w) of the boundary wave, which depends on the properties of the structural element and the fluid.

One of the main distinguishing features of the proposed method is that the interpretation of the recorded vibrations is performed on the basis of boundary waves propagating on the surfaces of the liquid-filled areas, which, in particular, allows you to carry out all the steps of the method in real time.

Brief Description of the Drawings

The above and other features and advantages of the present invention are disclosed in the following description of preferred embodiments thereof, given with reference to the drawings in which

Figure 1 is a flowchart of a method for detecting the nucleation or existence of a fluid-filled crack according to the invention;

Figure 2 - dispersion curves for waves propagating along the crack;

Figure 3 - dependence of the recorded fluid pressure on time;

Figure 4 - Fourier transform of the dependence of figure 3.

Description of the invention

In accordance with the foregoing, the detection of the origin or existence of fluid-filled cracks is of great importance in various fields of human activity. Next, with reference to figure 1 sets out corresponding to the present invention a method that allows you to effectively determine the origin or existence of fluid-filled cracks.

In step 1 of FIG. 1, vibrations of a fluid-filled crack are recorded. In particular, these oscillations can be excited in advance for the purpose of their subsequent registration. Registration and excitation of crack oscillations can be performed by any suitable means known in the art. A number of methods and means of exciting and registering vibrations of a fluid-filled crack are described in more detail below. It should be noted that crack vibrations can be recorded not only directly, but also by recording vibrations or other phenomena induced by crack vibrations.

In step 2 of FIG. 1, the frequencies of the detected oscillations are compared with the available frequencies.

In step 3 of FIG. 1, the nucleation or existence of at least one fluid-filled crack is detected based on the presence of the frequencies of the detected oscillations of the medium, significantly lower than the available frequencies of the vibrations of the medium.

The described method, due to the simplicity of the corresponding data processing, in particular computing, can be implemented in real time.

When registering vibrations of a fluid-filled fracture, fluid pressure fluctuations in the injection well induced by the vibrations of the fracture are recorded. Preferably, a fracture is excited prior to registration by an abrupt change in fluid pressure in the injection well. The most obvious way to provide an abrupt change in fluid pressure is to stop fluid injection for a few seconds. To record fluid pressure fluctuations in the injection well, any known suitable high-speed data acquisition system can be used, for example, allowing one recording in 5 ms. In particular, registration of fluid pressure fluctuations can be carried out using one or more sensors installed near a fluid-filled fracture, for example, in an injection well.

In addition to registering vibrations of a fluid-filled fracture by registering fluctuations in fluid pressure in an injection well, a variety of methods are known for recording fracture vibrations. In particular, vibrations of a fluid-filled crack can be detected by recording any vibrations induced by the vibrations of the crack. Also, registration of crack oscillations is performed by observing any natural phenomena induced by the crack oscillations, including

a) gravitational phenomena leading to a change in the acceleration of gravity in the vicinity of the formation due to large-scale displacement of the formations (here we mean applications for monitoring giant seismic-scale fractures);

b) electromagnetic phenomena leading to the excitation of an electromagnetic field in the vicinity of a crack both due to the movement of existing free charges in the medium and due to the separation of bound charges;

c) optical phenomena, for example, light generation in optical waveguides located near a crack and sensitive to mechanical stresses of the medium;

d) seismoelectric phenomena, which are the generation of an electromagnetic field in a medium due to a change in the stress state of the medium;

e) electrokinetic phenomena, which are the generation of an electromagnetic field due to the filtration of a fluid in a crack and in the pores of a medium;

f) thermodynamic phenomena, such as, for example, liquid-gas phase transitions induced by pressure fluctuations in the vicinity of a crack.

In addition, registration of crack oscillations can be done by observing any data obtained during measurements in the crack, such as data on the permeability of the formation, its electrical conductivity, etc.

According to one of the assumptions underlying the proposed method and confirmed experimentally, the characteristic size L of the crack is divided entirely into the lengths λ ^{(n) of} standing boundary waves propagating in the direction of this characteristic size. In accordance with the foregoing, the mentioned characteristic size L can be determined by the following equation

where n is an integer, ν ⁽ⁿ⁾ is the frequency of the nth standing boundary wave, V (ν ⁽ⁿ⁾ , w) is the group velocity of the boundary wave, w is the width of the crack filled with liquid. In this case, equation (1) allows one to determine more than one characteristic size of a crack filled with liquid, since each crack measurement (excluding the width) is characterized by its own set of standing boundary waves with the corresponding frequencies ν ⁽ⁿ⁾ .

Frequencies ν ⁽ⁿ⁾ included in equation (1) are isolated on the basis of recorded fluctuations in the fluid pressure in the injection well induced by vibrations of the fluid-filled fracture. These frequencies correspond to the lowest resonant frequencies of the recorded oscillations. The term “lowest resonant frequencies” referred to in the description corresponds to the presence (for example, on a spectrogram) of resonant frequencies significantly lower than expected in the absence of cracks filled with liquid. In this case, as the simulation shows, the lowest resonance modes of crack oscillations are formed by standing boundary waves propagating along the crack surfaces. An example of the allocation of the lowest resonant frequencies is given below.

The main wave characteristic of standing boundary waves propagating over the surfaces of a fluid-filled crack to be determined is the group velocity V (ν ⁽ⁿ⁾ , w) of the boundary wave, which, in addition to the width w of the crack in question, also depends on the properties of the underground formation and fluid.

The group velocity V (ν ⁽ⁿ⁾ , w) can be obtained either in the approximation of an inviscid fluid or in the approximation of a viscous Newtonian fluid. The terms “inviscid fluid” and “viscous Newtonian fluid” used in the description are known from the prior art and are described in detail in the book Theoretical Physics. T.6: Hydrodynamics, Landau L.D., Lifshits E.M., 4th ed., Fizmatlit, 1988, ISBN: 5-9221-0121-8, chapter I, pp. 13-70 for inviscid fluids, chapter II, pp. 71-136 for viscous Newtonian fluid.

Below, we derive the relations for V (ν ⁽ⁿ⁾ , w) in each of the approximations.

Derivation of the relation for the group velocity in the approximation of an inviscid fluid

Particle displacements corresponding to the boundary wave form elliptical trajectories with amplitudes harmonically changing in the longitudinal direction of the crack and decaying exponentially in its transverse direction. The speed of this wave is less than the lowest of the speed of an elastic wave in a liquid and the speed of a transverse elastic wave in a formation. The speed of the boundary wave depends on the frequency and is very low at low frequencies.

The phase velocity C _{phase of the} boundary wave propagating along the surfaces of the crack for a crack of constant width that is infinitely filled with an inviscid fluid can be determined from the following dispersion relation derived in Slow waves trapped in a fluid-filled, infinie crack: implications for volcanic tremor, V Ferrazini and K. Aki, J. Geophys. Res., 1992, No. B9, pp. 9215-9223, 1987

where ω is the circular frequency, w is the crack width, ρ and ρ _f are the densities of the formation and fluid, respectively, and

,

,

where α _f is the velocity of the elastic wave in the fluid, α is the velocity of the longitudinal elastic wave in the formation, β is the velocity of the transverse elastic wave in the formation.

Here C _phase is the wave propagation velocity along the crack. The wave itself, propagating along the surfaces of the crack, is an ellipsoidal movement of particles with an amplitude that decays exponentially across the crack, with the restriction that the movements of the particles are symmetrical with respect to the axis of the crack.

Typical dispersion curves for waves propagating along a crack are shown in FIG. 3.

These figures show the solutions of equation (2) for various values of w (w = 0.5 ... 5 mm) and various frequencies ν = ω / 2π. The high-frequency asymptote for all curves (the Scholte velocity) is close to the velocity of an elastic wave in a liquid. At zero frequency, the speed is zero, which is the main explanation for all the main phenomena under consideration.

In equation (2), we can use the approximation cot (x) ≈1 / x, which is true in the case of small x. Replacing cot (x) with 1 / x does not make significant changes in speed: for real crack widths (0.5-50 mm), the error in speed is a fraction of mm / s.

In the limit C _phase << α _f , which actually takes place for low frequencies, it can be shown that a good approximation of the solution of equation (2) is

,

,

The curves corresponding to exact solutions in FIG. 3 are actually close to the dependence C _phase ~ ν ^1/3 .

In the context of the present invention, it is required to calculate the group velocity V of the boundary wave

but for boundary waves propagating along the surfaces of the crack, there is a good approximation following from equations (3) and (4), namely

In the case of rectangular cracks of finite length, numerical simulation shows that the resonance modes corresponding to the lowest frequencies correspond to boundary waves propagating along the crack surfaces with a dispersion relation that is an altered form of equation (2). It was noted that the mentioned change becomes negligible in the limit of the large length / width aspect ratio, which is always the case for cracks.

According to the above, for real cracks filled with liquid, the width of which is much less than 1 cm, and low (in the above value) vibration frequencies, a good approximation of the exact relations for the group velocity V is the following equation obtained by combining equations (3) and (5)

,

,

The fluid and formation properties included in this equation are assumed to be known.

Consider a specific example of the application of the method described above for a hydraulic fracture created in a shallow well in a formation having the following elastic characteristics:

formation density ρ = 2700 kg / m ³ ;

the velocity of the longitudinal elastic wave in the formation α = 3900 m / s;

shear elastic wave velocity in the formation β = 2575 m / s.

The injected fluid is water.

Using the images obtained by means of inclination sensors, the following crack geometry was determined: the crack is elliptical, subhorizontal, half its length, i.e. the distance from the injection well to the far edge of the crack, is 55 m, half its height is 46 m and its width is 2 5 mm.

During the shutdown of fluid injection, a high-speed system for recording fluid pressure was activated both on the surface and inside the well. The dependence of the recorded fluid pressure on time is presented in figure 3.

Figure 4 presents the Fourier transform of the aforementioned dependence (for simplicity only its imaginary part is shown, the real part is essentially similar to the imaginary part). In the spectrogram presented to the naked eye, a sequence of four nonharmonic peaks is clearly visible, i.e., in the case under consideration, the resonance peaks are identified without any additional processing.

According to the method of the present invention, the “group” length of the boundary waves propagating over the surfaces of the liquid-filled crack corresponding to the frequency of each peak is calculated by dividing the group velocity of the boundary waves by said frequency. For calculations, the crack width value measured by the tilt sensors is used. Table 1 below shows the results of these calculations.

Table 1 frequency Hz "Group" wavelength, m f1 = 1.7 55.0 f2 = 5.5 26.0 f3 = 9.0 18.7 f4 = 13.0 14.7

From Table 1 it can be seen that the wavelength related to the lowest frequency (f1) is 55 m, i.e. half the length of the crack in question. The next wavelength (referring to f2) is approximately 55/2, the next wavelength (referring to f3) is approximately 55/3, and the last wavelength (referring to f4) is approximately 55/4. It follows that the length of the crack is divided entirely into the lengths of standing boundary waves. The results of calculations indicate the correctness of the approach based on standing boundary waves propagating along the surfaces of a fluid-filled crack.

In accordance with the above example and equation (1), as a characteristic size of a fluid-filled crack, we can estimate the maximum distance L _max between its edges by the formula

,

,

where V _min is the minimum resonant frequency of the crack.

The described implementation of the method according to the invention, due to the simplicity of the corresponding data collection and processing, can be implemented in real time. As applied to fracturing, the implementation of the "real-time" method means obtaining the required output during a fracturing procedure or faster.

The described method is applicable both to natural cracks and to cracks created artificially. In particular, it is suitable for offshore applications where the underground formation is an underground shelf formation.

The method of the present invention involves the analysis of all crack response frequencies. Higher modes of fracture oscillations determine the response in the well similarly to the main mode, which explains, according to the proposed interpretation, the presence of higher frequency peaks in the spectrogram of fluid pressure fluctuations in the injection well. If there are several peaks, there is a larger number of equations that can be used to more accurately estimate the characteristic dimensions of the crack and to refine the properties of the system under consideration.

Another preferred embodiment of the method of the invention described above with reference to FIG. 1 is to detect the nucleation or existence of at least one fluid-filled crack in a submerged portion of a structural member, the crack intersecting the surface of this submerged portion of the structural member. Such a structural element may be a bridge support, an offshore platform support, a building foundation, etc.

Oscillations of the crack are recorded by means of a sensor fixed at the intersection of the crack with the surface of the structural element. Preferably, the vibrations of the crack are excited before registration, for example, by abrupt deformation of the edge of the crack.

Further, all operations for detecting the nucleation or existence of at least one fluid-filled crack are similar to the corresponding operations of the above-described method for detecting the nucleation or existence of at least one fluid-filled crack in an underground formation, which is a preferred embodiment of the method of the invention. According to these operations, the frequencies of standing boundary waves propagating along the surfaces of a fluid-filled crack are determined as corresponding to the lowest resonant frequencies of the detected vibrations, the group velocity of the boundary wave, which depends on the crack width w and the properties of the structural member and the fluid, is calculated, and the characteristic crack size along propagation of a standing boundary wave according to formula (1), preferably taking into account equation (6).

The registration of the low resonant frequencies generated by boundary waves propagating over the surfaces of fluid-filled cracks described above provides opportunities for a wide range of applications.

One such application is to detect the nucleation or existence of a fluid-filled crack in a medium. Below, with reference to FIG. 1, a description will be made of a method of detecting the nucleation or existence of a fluid-filled crack according to the invention.

In step 1, medium vibrations are recorded, while medium vibrations are preferably excited before registration. Registration and excitation of vibrations is performed using any suitable known means or method, the description of which is given above.

Then, in step 2, the vibration frequencies recorded in step 1 are compared with the available vibration frequencies of the medium in the absence of fluid-filled cracks. The oscillation frequencies of the medium in the absence of fluid-filled cracks are determined in advance, for example, by measurements in areas of the medium where the absence of cracks has been established for certain, or by mathematical modeling.

Then, in step 3, according to the results of the frequency comparison performed in step 2, the nucleation or existence of a crack filled with liquid in the medium is detected on the basis of the presence of the vibrational frequencies of the medium, registered in step 1, significantly lower than the available vibrational frequencies of the medium. As noted above, these low frequencies are determined by boundary waves propagating over the surfaces of a fluid-filled crack.

The described method is also applicable for detecting more than one fluid-filled crack. Due to the simplicity of data processing, this method can be performed in real time.

The method of detecting the origin or existence of liquid-filled cracks according to the invention is applicable for cracks in underground formations (for example, an underground shelf formation), cracks in submerged parts of structural elements (for example, bridge supports, supports of offshore platforms, building foundations), as well as for cracks in the bones of a person or animal.

The invention has been disclosed above with reference to specific options for its implementation. For specialists in this field of technology may be obvious and other embodiments of the invention, not changing its essence, as it is disclosed in the present description. Accordingly, the invention should be considered limited in scope only by the following claims.

Claims (3)

1. A method for detecting the nucleation or existence of at least one fluid-filled crack in a medium, comprising the steps of:
register fluctuations in the environment;
compare the frequencies of the recorded oscillations with the available frequencies of the medium in the absence of cracks filled with liquid;
in this case, the frequencies v ^{(n) of} standing boundary waves (n is an integer) propagating along the surfaces of at least one fluid-filled crack are determined as corresponding to the selected resonant vibration frequencies of at least one fluid-filled crack and the group velocity V (v ^{(n )} , w) a boundary wave, which depends on the width of the crack w and the properties of the structural element and fluid, and calculate the characteristic size of the crack along the propagation of a standing boundary wave;
detect the nucleation or existence of at least one fluid-filled crack. 2. The method according to claim 1, additionally containing a stage in which, before recording the vibrations, oscillations of the medium are excited, and the steps of the method are performed in real time. 3. The method according to claim 1, in which the environment is an underground formation, and the underground formation may be an underground shelf formation; or a part of a structural element immersed in a liquid, the structural element being a bridge support, an offshore platform support or a building foundation; or bone.

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