RU2507396C9 - Method for determining parameters of hydraulic fracturing crack system - Google Patents

Abstract

FIELD: mining.

SUBSTANCE: method involves excitation of elastic vibrations by a vibration source in a well crossing hydraulic fracturing cracks, recording at receiving points at least in one neighbouring well of resonant vibrations emitted with a hydraulic fracturing crack system at excitation in drilling fluid of elastic vibrations, and determination of parameters of the crack system as per resonant vibrations occurring in the cracks. Excitation of vibrations in the well and their recording is performed before and after hydraulic fracturing. Besides, for each fixed source-receiver pair there formed is a difference seismic record of the records received before and after hydraulic fracturing; signals emitted by the crack system are separated on the difference seismic record, and parameters of cracks are determined as per the above signals.

EFFECT: improving reliable determination of spatial orientation of a hydraulic fracturing crack system and its dimensions.

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Description

The invention relates to methods for downhole seismic exploration, capable of solving problems associated with monitoring the state of hydraulic fractures (Fracturing) by examining seismic vibrations emitted by cracks.

Hydraulic fracturing is an effective way to intensify hydrocarbon production from a borehole by increasing the permeability of the reservoir after the formation of hydraulic fractures.

To determine the geometry of the fractures formed as a result of hydraulic fracturing and their development over time (monitoring), various technologies and techniques are currently used. The most widely known methods for visualizing fracturing, based on the registration of passive seismic-acoustic emission (SE). These methods provide an estimate of the spatial orientation of the crack and its length during fracturing operations.

Seismic monitoring using passive SE is currently being intensively developed worldwide (Maxwell S.C., Urbancic T.I., 2001; Kuznetsov O.L. et al., 2006). In this case, a method based on a passive location, which provides a 3D overview of the lower space with an areal reception system mounted on a day surface, is considered the most promising.

The known technology of passive seismic exploration "Seismic location of emission centers - SLEE" to listen to the SE (Kuznetsov OL and others, 2006). This technology differs from the standard MOGT-4D technology in that, based on the use of SE waves, the possibility of continuous and unlimited in time listening to technogenic processes in the geological environment is realized. However, the drawback of the PLE, as well as any other passive seismic technology, is the low level of the signal emitted by both the directly investigated object, for example, a crack formed as a result of hydraulic fracturing, and smaller cracks formed in the zone of high stresses in the vicinity of the edge of the hydraulic fracture. In this case, the hydraulic fracture itself, which is the cause of SE, functions as a pump, since when it opens, the fluid is drawn into the cavity of the fracture, and when it collapses, it is squeezed out of it. From this it follows that the opening and collapse of the cavity of the crack, which is an elastic system, can be carried out more intensively with an external seismic-acoustic impact on the crack. Such an effect can be carried out by immersing in a well crossing a hydraulic fracture, a source of pulsed or continuous vibrations. Energy transfer from a source to a fracture capable of operating in a radiation mode will occur directly through the fluid contained in the well.

A known method for determining the parameters of a hydraulic fracturing system, including the wave (seismoacoustic) effect of elastic vibrations on the reservoir through the fluid contained in the well intersecting the hydraulic fractures (V. Dyblenko et al., 2009). Such an impact on the formation, according to the authors of this method, leads to the initiation of additional crack formation and vibration hydraulic fracturing, which results in seismic emission (SE), which is proposed to be recorded. By registering solar cells with three-component geophones located on the earth's surface, the spatial location of the vertices of the resulting cracks is determined.

The disadvantage of this known method is the need to create large pressure amplitudes necessary for the destruction of the rock at which seismic emission occurs. It is significant that seismic emission is associated precisely with the fracture process and the emergence of new cracks, and not with vibrations of existing cracks. In addition, registration of solar cells is carried out on the earth's surface. With a high level of interference characteristic of observations on the earth's surface, it is very difficult to isolate seismic signals associated with solar cells. In addition, the use of three-component reception in the presence of a low-velocity zone (ZMS) may be ineffective due to the difficulties of wave separation using the directivity characteristics of the first kind in connection with the subvertical output of rays for waves of various types and classes.

A known method and system for monitoring fluid-filled areas in a medium based on boundary waves propagating over their surface (RU 2327154). In this method, resonant vibrations are recorded in a fluid-filled well intersected by a fracture when excited by any vibration source, and the lowest resonant frequencies of such vibrations are analyzed. The lower resonant frequencies determine the size of the crack. It is implicitly assumed that the lower resonant frequencies are associated with resonant oscillations due to boundary waves propagating along the crack filled with liquid. However, the authors do not take into account that low-frequency resonances in a well can arise not only due to boundary waves in a fracture, but also due to resonances of a hydraulic wave (Stoneley wave) propagating along the wellbore.

Due to the great depth of the wells, the resonant frequencies of fluid oscillations in the well associated with the hydraulic wave may be lower or overlap the lower frequencies of resonances associated with boundary waves in the fracture, and thus be misinterpreted.

The closest technical solution (prototype) can be considered a technique and system for performing cross-hole studies, which allow recording seismic vibrations that occur during hydraulic fracturing caused by intensification of inflow into the well, injection of fluid into the well, and other causes that lead to the destruction of rocks with the formation of cracks in them (RU 2439621). Registration of microseismic oscillations by one or several multicomponent sensors allows determining the location of hydraulic fracturing, as well as judging the pressure fluctuation in the well, fracture growth geometry and the main direction of stress in the formation.

The main disadvantage of the prototype is the registration directly of the total signal generated as a result of interference of the exciting signal emitted by a source immersed in the well and resonant vibrations emitted by the resulting hydraulic fracturing system. It is very difficult to isolate resonance vibrations from such a total signal in a pure form, as well as to determine the parameters of a crack system from such total oscillations.

The purpose of the invention is to increase the reliability of determining the spatial orientation of the fracture system and its dimensions.

This goal is achieved by the fact that in the method for determining the parameters of a hydraulic fracture system, including the excitation of elastic vibrations by a vibration source in a well crossing hydraulic fractures, registration at resonance points in at least one neighboring well of resonant vibrations emitted by the hydraulic fracturing system when excited in drilling fluid elastic vibrations, and determination of the parameters of the system of fractures from the resonant vibrations that occur in the cracks, the excitation of vibrations in the well and their istratsiyu carried out before and after hydraulic fracturing. In this case, for each fixed source-receiver pair, a differential seismic record is formed from the records obtained before and after hydraulic fracturing, the signals emitted by the crack system are extracted from the differential seismic record, and these parameters are used to judge the crack parameters. In contrast to the method (RU 2327254), it does not analyze the resonant vibrations of the fluid in the well that the fracture crosses, but analyzes the amplitudes of the seismic-acoustic waves emitted by the fracture into the external elastic medium under the action of a pressure source in the well that it intersects and recorded in another well. In this case, the resonant frequency of the seismic vibrations of the emitter in the form of a system of hydraulic fractures is found by exciting continuous fluid vibrations in the well in a sufficiently wide range of seismic frequencies. The lower limit of the operating frequency range of continuous oscillations excited in the liquid is taken to be less than the expected resonant frequencies of the fracture system, and its upper limit is taken to be larger than these resonant frequencies. The size of the cracks is judged by the resonant frequencies emitted by the fracture system. In one embodiment of the invention, seismic vibrations emitted by a hydraulic fracture system are recorded in wells located in different directions from a well intersecting hydraulic fractures, and the parameters of the fracture system are judged by the kinematic and dynamic parameters of the recorded signals. In another embodiment of the invention, the vibrations emitted by the hydraulic fracturing system are recorded not only in one of the neighboring wells, but also at the receiving points located in the near-surface zone.

The essence of the invention is as follows.

The excitation of elastic vibrations in the fluid contained in the well crossing the fracture leads to the formation of intense hydraulic waves acting on the inhomogeneities crossed by the wellbore. The most contrasting heterogeneities are hydraulic fractures formed as a result of hydraulic fracturing. Single cracks forming a system of closely spaced cracks are capable of independently emitting vibrations under the influence of hydraulic waves excited by a vibration source immersed in the fluid contained in the well. If the vibrations are excited by a source located in the borehole directly in the interval contained within the fracture system, then the vibrations of the fractures themselves occur synchronously with the excitation of the vibrations by the source, initiating seismic acoustic emission of the cracks. Thus, the mode of excitation of vibrations by a system of hydraulic fractures from a passive one goes into the active mode, in which the intensity of seismic-acoustic radiation from hydraulic fractures increases. The three-component registration of this signal in neighboring wells, and not on the earth’s surface, allows a more reliable determination of the spatial orientation of the fracture system and its dimensions. A system of cracks with finite dimensions and elastic properties is characterized by its inherent resonant frequencies formed by the partial frequency of each of the cracks included in the system of cracks. Taking into account a priori information on the elastic properties of the rocks that make up the productive horizon, as well as the fluids contained in the cracks, allows us to judge the size of the system of cracks (or one of the predominant cracks) by the recorded resonant frequencies. A vibration source that excites the vibration of a fracture (or a system of fractures) as a whole, by itself emits seismic vibrations into the surrounding space regardless of the existence of hydraulic fractures. Therefore, registering in other wells the vibrations of such a source before and after the formation of hydraulic fractures, it is possible to remove vibrations associated directly with the source from seismic records, leaving only the vibrations associated with the hydraulic fracturing system in differential records. Simultaneous registration of vibrations emitted by hydraulic fractures in wells and in the near-surface zone allows achieving a total effect, in contrast to the registration of vibrations either in the well or in the near-surface zone. The fact is that in a well a signal is recorded in the least distorted form and can be used in subsequent processing of downhole and ground-based observations. However, ground-based registration of fluctuations in the number of receiving points sufficient to solve inverse problems is usually easier to provide than by recording oscillations in the well with small-channel VSP probes.

The method is as follows.

A source of seismic vibrations recorded in at least one of the neighboring wells is immersed in a fluid located in the well in which the hydraulic fracturing is carried out using a three-component VSP probe. In this case, the excitation and registration of oscillations is carried out before and after the hydraulic fracturing. After the formation of a system of cracks by hydraulic fracturing, each of the cracks becomes a source of vibrations, which are excited by the action of a hydro wave on it, an excited source immersed in a fluid contained in the same well.

Unlike wave fields observed before hydraulic fracturing, waves emitted from hydraulic fractures will also be recorded in neighboring wells. By changing the frequency of the vibrations excited in the fluid by the source in one of the neighboring wells, by recording the vibrations emitted by the hydraulic fracturing system, the resonance frequencies characteristic of the fractures as autonomous oscillation sources are determined. The resonant frequency is determined by the maximum amplitude spectra of wave seismic fields recorded in neighboring wells, referred to the amplitude spectrum of pressure recorded in a well with a hydraulic fracture in a certain frequency range. The frequency range of oscillations that is then formed, emitted by the source of continuous oscillations that have a wave (seismoacoustic) effect on hydraulic fractures, is taken so that this range obviously contains the resonant frequencies inherent in the system of cracks. Thus, the radiation from the system of cracks will turn out to be substantially more intense than with passive SC. For each fixed source-receiver pair, oscillations are recorded before and after hydraulic fracturing. Subtracting the records one from the other will allow us to get rid of the registration of waves that are not related to the hydraulic fracturing system, which will significantly simplify the selection of waves excited directly by the cracks when they are exposed to the waves emitted by a source located in the fluid.

As sources of oscillations in the proposed method, you can use hydrodynamic generators type GDV2V-20, GDV2V-30, as well as jet pumps. As a submersible vibration source, a known type of hydraulic vibrator can be used (Turpening et al., 2000). The quality of contact of each of the instruments of the VSP probe with the walls of the well should be so high that spurious oscillations of the instrument housings at the contact are excluded. This is achieved by using shoes in downhole tools that are rigidly attached to the body of each device (Shekhtman G.A., Kasimov A.N.O., Redekop V.A., 2012).

The signals emitted by hydraulic fractures are extracted by applying standard processing procedures contained in packages designed for processing and interpretation of VSP records. The determination of the interval values of the propagation velocities of longitudinal and transverse waves, as well as the use of density data obtained as a result of gamma-gamma-ray logging, will allow the most unambiguous connection of vibration parameters emitted by cracks with the size of cracks and their orientation in space.

The registration of the signals generated by hydraulic fractures in neighboring wells makes it possible to evaluate the parameters of these signals in the least distorted form. Reliable information on the parameters of these signals can be used when processing recordings by ground-based geophones, recording seismic acoustic emission simultaneously with the registration of these signals by VSP probes (for example, when deconvolving recordings obtained inside the environment and on the earth's surface). Thus, one of the variants of the invention is its modification, in which solar cells are recorded in wells and in the near-surface zone. A significant difference between this modification and the prototype is that the elastic vibrations of the cracks formed during hydraulic fracturing are recorded not only on the earth's surface, but also inside the medium. In this case, the number of receiving points is taken greater than the number of determined parameters of hydraulic fractures. The concept of “near-surface” zone in this case means the upper part of the section (VChR). By immersing the seismic receivers in shallow wells located in the high-frequency field, it is possible to get rid of surface interference waves and microseisms that impede the selection of useful signals at the processing stage due to the limited instantaneous dynamic range of seismic recording in serial seismic recording equipment.

Let us give an example of model studies aimed at evaluating the parameters of oscillations emitted by a hydraulic fracture. This example shows the feasibility of the invention in terms of the ability to assess the size of the hydraulic fracture by the parameters of the oscillations emitted by it.

According to the technology of creating hydraulic fractures, they are filled with a liquid, which is a waveguide through which acoustic energy can propagate. The main part of the acoustic field energy in the fracture is transferred by the main symmetric mode - the Krauklis wave (Krauklis P.V., 1962). In the long-wavelength limit with respect to crack opening δ (δω / c _f << 1), the wave number of the Krauklis wave k _Kr (ω) is given by the following expression (Krauklis P.V., 1962; Derov A.V., Maksimov G.A. , 2008)

$$k_{Kr}(\omega )={\left(\frac{\omega }{c_s}\right)}^{2/3}{\left(\frac{\Delta }{\delta }\right)}^{\mathrm{one}/3}\Delta =\frac{{\rho }_f/{\rho }_s}{\mathrm{one}-{(c_s/c_l)}^2}(\mathrm{one})$$

Here, ρ _f and c _f denote the density and speed of sound in a fluid filling a hydraulic fracture, and ρ _s , c _s , c _l denote the density and velocities of transverse and longitudinal waves in an elastic medium.

For the simplest estimation of the resonance frequency of a Krauklis wave in a hydraulic fracture, a disk-shaped crack with a radius R can be considered. For cracks of a different shape, an assessment can also be performed, but it is much more complicated. The radial resonance condition for a disk crack has the form:

$$k_{Kr}(\omega )R=\pi n/2n=\mathrm{1,2}...(2)$$

The main resonance n = 1 corresponds to the resonant frequency f = ω / 2π

$$f=\frac{\mathrm{one}}{\mathrm{four}}\sqrt{\frac{\pi }{2}}\frac{c_s}{R}{\left(\frac{\delta }{R\Delta }\right)}^{\mathrm{one}/2}(3)$$

Assuming further that the characteristic aspect ratio for hydraulic fractures is of the order of δ / R ~ 10 ^-3 ÷ 10 ^-4 , for typical values of the elastic parameters of the medium we obtain the following estimate:

$$f=({10}^{-2}-3\cdot {10}^{-3})\frac{c_s}{R}(\mathrm{four})$$

Table 1 shows the resonance frequency estimates for cracks with characteristic radii and aspect ratios.

Table 1. Estimation of the resonant frequency of the Krauklis wave for cracks of different sizes R = 1 m R = 3 m R = 10 m R = 50 m δ / R = 10 ^-2 50 Hz 20 Hz 5 Hz 1 Hz δ / R = 10 ^-3 20 Hz 6 Hz 2 Hz 0.4 Hz δ / R = 10 ^-4 5 Hz 1.7 Hz 0.5 Hz 0.1 Hz

Since the length of a body longitudinal wave in an elastic medium at a resonant frequency substantially (by two orders of magnitude) exceeds the geometrical size of the crack, in the far wave zone r >> c _l / f (at distances of more than 2 km for frequencies above 5 Hz) the crack radiation at these frequencies will be mainly monopolistic in nature, the amplitude of which can be estimated by varying the total volume of the fluid V (t) filling the crack. The potential of an effective point source of longitudinal waves can be written as

$$\varphi =-\frac{\dot{V}(t-r/c_l)}{\mathrm{four}\pi r}(5)$$

Formula (5) makes it possible to estimate the amplitudes of the longitudinal waves emitted by a hydraulic fracture, since the change in the volume of fluid is easily controlled when it flows into the well.

In the near wave zone r≤≤c _l / f (at distances less than 1 km for frequencies below 5 Hz) the radiation of the fracture will be anisotropic, which allows using this anisotropy when registering seismic fields in neighboring wells to determine the orientation of the fracture.

A positive effect in the present invention is achieved mainly by switching from recording passive seismic acoustic emission of hydraulic fractures to registering active seismic acoustic radiation from hydraulic fractures, which is provided by wave action of the vibrations on the fractures by a vibration source immersed in the well crossing the hydraulic fractures, and also by that registration of vibrations is carried out before and after hydraulic fracturing. The formation of differential records from the records obtained before and after hydraulic fracturing, allows to suppress to a large extent interference waves that are not directly related to the emission of resonant vibrations directly by cracks. The registration of vibrations emitted by hydraulic fractures in neighboring wells, where the noise level is significantly lower, makes it possible to more reliably isolate the signals generated by hydraulic fractures, and by the kinematic and dynamic parameters of these signals to judge the size and spatial position of the fractures. The combination of borehole and ground-based observations can increase the signal-to-noise ratio in processing ground-based data by using reference signals, which can be used as resonance oscillations recorded in the borehole in a purer form than in the near-surface zone

The proposed method does not follow from the existing level of technology, and the combination of its essential features differs from the essential features of the known methods for determining the parameters of a fracturing system.

The use of the present invention will significantly improve the efficiency of hydrocarbon field development by more reliable assessment of the effects of increasing the permeability of productive formations provided by hydraulic fracturing.

INFORMATION SOURCES

1. Underhill WB, Lines SV., Gerez D., Feyard A. Methods and system for performing cross-hole research // RF Patent No. 243962], published January 10, 2012.

2. Derov A.V., Maksimov G.A. Excitation of hydraulic waves in a well intersected by a finite-size fracture under the action of an external seismic wave // Seismic Exploration Technologies. 2008, Vol. 4, pp. 60-63.

3. Dyblenko V. P., Kuznetsov O. L., Chirkin I. A., Rogotsky G. V., Ashchepkov Yu.S., Sharifullin R.Ya. A method of developing mineral deposits extracted through wells // RF Patent No. 2357073, published May 27, 2009.

4. Krauklis P.V. About some low-frequency oscillations of a liquid layer in an elastic medium. // PMM 1962, T.26, No. 6, p. 1111-1115.

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6. Segal A.Yu., M. Tierselin, K. Besson Method and system for monitoring fluid-filled areas in a medium based on boundary waves propagating over their surfaces // RF Patent No. 23237154, published on 20.06.2008.

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Claims (4)

1. A method for determining the parameters of a hydraulic fracturing system, including the excitation of elastic vibrations by a source of vibrations in the well crossing the hydraulic fractures, recording at the points of reception in at least one neighboring well resonant vibrations emitted by the hydraulic fracturing system when elastic vibrations are excited in the drilling fluid, and determining parameters of the system of cracks by the resonant vibrations that occur in the cracks, characterized in that, in order to increase the uniqueness of determining the parameters of fracture fracture systems, vibration excitation in the well and their registration is carried out before and after hydraulic fracturing, and for each fixed source-receiver pair, a differential seismic record is formed from the records obtained before and after hydraulic fracturing, the signals emitted by the fracture system are extracted from the differential seismic record, and these signals judge the crack parameters. 2. The method according to claim 1, characterized in that the resonant frequency of the fracture system is determined by the maximum intensity of the vibrations excited by the system of fractures by changing the frequency in the well of vibrations from the lower boundary of the range of continuous continuous vibrations to the upper boundary. 3. The method according to claim 1, characterized in that the seismic vibrations emitted by the hydraulic fracturing system are recorded in wells located in different directions from the well crossing the hydraulic fracturing, and the parameters of the fracture system are judged by the kinematic and dynamic parameters of the recorded signals. 4. The method according to claim 1, characterized in that in addition, simultaneously with the registration of vibrations in a neighboring well, vibrations are recorded at points of reception located in the near-surface zone.