RU2282876C1 - Method of seismic prospecting of cracking areas of rocks during process of hydraulic fault - Google Patents

Abstract

FIELD: geophysics.

SUBSTANCE: method can be used for detection of spatial location of cracking areas of rocks. Method is based upon registration of elastic oscillations caused by perforation during hydraulic fault. Elastic waves from the sources are modeled preliminary, are subject to noise suppression and processed. Elastic waves caused by perforation are registered according to selected approach. Data received are used for revision of speed model and for determination of static and dynamic corrections at points of reception. The results are used in the future when focusing and storing registered elastic waves caused by hydraulic fault onto depth surface. Results of storage are registered in frames in preset interval of registration time and the frames are used for creating image on screen in form of film, which presents time and spatial change in intensity of cracking at preset depth surface. Results are given in form of sectional views and maps, which allow mapping of areas of cracking of rocks.

EFFECT: improved truth of results.

5 dwg

Description

The invention relates to seismic exploration and can be used to detect the spatial position of rock cracking zones during hydraulic fracturing, which is carried out to increase the influx of oil or gas, as well as to create artificial storage facilities, as well as penetrating cavities, channels, and for other engineering purposes.

There is a method of seismic exploration of objects scattering elastic waves, including the excitation of a seismic signal, registration of a seismic field and data processing, while directing both reflected and scattered components of the wave field, pre-modeling scattered waves from possible objects of reflection, diffraction and scattering, and choose the option of selection of reflected, diffracted and scattered waves, which provides the best selection of objects against the background of interference, and then refine the parameters on p cial data to obtain the total amplitude spectrum depending on the time and frequency for different cone angles, and use parameters for processing the received data and receiving real anomaly amplitude spectra in the form of sections and maps for the subsequent selection of objects, the elastic wave scattering. (See RF Patent No. 2248014 C1, dated March 22, 2004, “A Method for Seismic Exploration of Objects Scattering Elastic Waves,” patent holder: Forum Forum Closed Joint-Stock Company, authors V. A. Kochnev, V. S. Polyakov, I.S. Bekhterev, I.I. Bekhterev).

The disadvantage of this method is that it cannot detect the dynamics of hydraulic fracturing. In addition, it requires additional costs for drilling small wells and for blasting and special observations before and after hydraulic fracturing.

A known method for determining the position of seismic energy sources around the well. Several seismic receivers, each of which is equipped with mutually orthogonal seismic sensors, are located along the axis of the device, capable of transmitting seismic signals in real time to the surface via a conventional cable. A method is also proposed for calculating the position of microearthquakes with respect to receivers. The method uses data from all sensors and an objective function to minimize position error. The source of seismic energy is a microseismic event associated with hydraulic fracturing around the well in which the instrument is placed, or a remote well. (See US Patent No. 5747750 of May 5, 1998, patent holder of Exxon Production Research Company (Houston, TX), authors Bailey; Jeffrey R. (Houston, TX); Ringo; Marion M. (League City, TX) - prototype )

This method has the following disadvantages.

The method can be implemented only in the presence of deep wells using specialized instruments. When registering the hydraulic fracturing process in one well, high accuracy of determining the coordinates of microearthquakes in the plan cannot be achieved. Thus, to improve accuracy, the method requires several wells, which limits its capabilities, since in a real situation there are not always deep wells located at the desired distance and direction from the studied object.

Currently, there is no way to obtain reliable results when registering elastic vibrations not in the well, but on the surface. This is because seismic waves, especially high-frequency ones, come to the surface very attenuated when passing through the upper layers. In addition, a significant level of seismic noise is present on the surface and in the upper layer. And this leads to the fact that when registering seismic waves on the surface, it was not possible to register waves from hydraulic fracturing and to establish the actual development of the hydraulic fracturing zone, and, consequently, the rock cracking zone. This leads to incorrect decisions and unjustified economic costs.

The objective of the invention is to provide a method for seismic exploration, which provides for the selection of the optimal monitoring system and such processing of elastic waves recorded on the surface during fracking, which allows the processing process to accumulate weak waves on a given deep surface against a background of strong interference and thereby identify active sources radiation of elastic waves.

The problem is solved in that in a seismic exploration method including recording elastic vibrations from perforation and hydraulic fracturing, elastic waves from these sources are preliminarily modeled, they are noisy, and they are processed by selecting processing parameters and parameters of the recording system in such a way that the best signal-to-signal ratio is ensured noise at a given limit on the number of recording channels. According to the selected observation scheme, elastic waves from perforation are recorded. The data obtained are used to refine the speed model and determine the static and dynamic corrections at the points of reception. In the future, these results are used when accumulating registered elastic waves from hydraulic fracturing on a given deep surface. The accumulation results are recorded in frames after a predetermined recording time interval, and the frames are used for image on the screen in the form of a film that creates an idea of the temporal and spatial changes in the intensity of elastic waves on a given deep surface. In addition, the results are presented in the form of sections and maps.

The analysis of the prior art by the applicant, including a search by patent and scientific and technical sources of information and identification of sources containing information about analogues of the claimed method, made it possible to establish that the applicant did not find a source characterized by the features of the proposed seismic exploration method.

The definition from the list of identified analogues of the prototype allowed us to establish a set of essential features in relation to perceived by the applicant the technical result of the distinguishing features in the claimed method of seismic exploration, set forth in the claims.

Comparison of the proposed method with the prototype and other analogs of a similar purpose shows that the proposed method has novelty and significant differences.

The proposed method allows you to correctly select the parameters of the recording arrangement and the parameters of the processing and accumulation of elastic vibrations on predetermined deep surfaces, based on the results of modeling, perforation and subsequent refinement of the parameters.

The method of seismic exploration of zones of cracking of rocks during hydraulic fracturing is illustrated by the drawings.

Figure 1 presents: a - observation diagram of elastic vibrations, b - model seismograms from sources located at a depth of 2475 meters and excited sequentially at random intervals, without superimposed noises, c - model seismograms from sources located at a depth of 2475 meters and excited sequentially at random intervals, with superimposed noise Ac / As = 1, where Ac is the mean square value of the signal amplitudes, Ash is the mean square noise amplitude.

Figure 2 shows the results of the extraction of deep objects on the model data on the surface: a - without noise, b - with noise Ac / As = 1, c - with noise Ac / As = 0.5, d - with noise Ac / As = 0.25.

Figure 3 shows: a - seismic tracks that recorded the perforation of the column after entering kinematic corrections. The white line indicates the arrival times of the characteristic phase of arrivals used to determine the static corrections,

b is the total field accumulated during the focusing of the wave field recorded on the day surface from perforation to the level of 2475 meters. Radius step 25 meters.

Figure 4 shows a fragment of a seismic wave field from hydraulic fracturing in the time interval from 3200 to 3400 s.

Figure 5 shows:

a - the result of the accumulation of waves from hydraulic fracturing at a depth of 2475 m in the time interval from 0 to 2000 s;

b - the same, in the range from 0 to 3000 s;

s - the same, in the range from 0 to 4000 s;

d - the same, in the range from 0 to 5000 s. Dots indicate reception points that run through 50 meters.

The method is as follows.

Elastic waves from the proposed hydraulic fracturing are simulated in a medium with approximately known parameters, a certain type and noise level are modeled, and they are additively added to the model wave fields (Figure 1). Spend processing, selecting the necessary parameters, achieving the best selection of known model objects emitting elastic waves (Figure 2). If necessary, the recording system is changed and the model experiment is repeated again, achieving the most accurate results of the restoration of model objects generating elastic waves.

The observation system chosen in this way is used to record elastic vibrations from the perforation of the column. Clarify the speed model of the environment and determine the static and dynamic corrections at the points of reception (Figure 3, a). Check the correctness of the parameters for restoring the position of the source of perforation (Figure 3, b). Elastic waves are continuously recorded during hydraulic fracturing (an example of a registered field fragment is shown in FIG. 4).

Using the obtained parameters, the wave fields are focused and accumulated on a given deep surface. After a given interval of registration time, frames of the accumulated field of amplitudes are fixed and the film is mounted. They watch a movie, get information about the process change in time and space. The most characteristic frames are reproduced in the form of maps or sections. 5 shows frames obtained after 2000; 3000; 4000; 5000 s They also give an idea of the development of hydraulic fracturing.

Liquid injection into the formation under high overpressure, relative to the formation, causes cracking of the rocks in the part where resistance arises and where the rocks are most weakened. That is, the flow of the injected fluid will go in the direction, as a rule, in the cracks that arose as a result of previous technological and geological processes of pressure and motion caused by deeper layers. In the conditions of Western Siberia, where the first experiments were carried out, the cracks are directed submeridionally. In our example, the movement was first in the north. No intense cracking was observed during the first 2000 s (Figure 5, a). This can be explained by the good permeability of the rocks that arose in the process of oil production. The well has been in operation for more than five years. The cracking process began to appear at a distance of more than 100 m, where the resistance to fluid movement increased. The fact that the fluid did not go southward can be explained by the claying of the formation. This created an insurmountable silent barrier.

It was experimentally established that seismic waves occur during fracturing. Over the years, they have been observed in deep wells at high frequencies above 100 Hz. At such frequencies, seismic waves attenuate strongly in the upper layers and waves at frequencies below 70 Hz are actually recorded.

Nevertheless, if rock cracking occurs (click-type pulses), then along with high ones it also contains mid-frequency seismic components. The amplitude of some of them at these frequencies can be significant. The superposition of a large number of high-frequency components also adds up to lower frequency signals. Liquid injection is carried out under high pressure and from 30 to 90 m ^{3 of} liquid and filler is pumped in 30 minutes. The spent energy goes to the destruction of the rock and creates elastic vibrations that reach the surface.

The success of detecting waves on the surface is determined by a powerful system of directional selection, consisting of 360 groups of receivers in the experiment, which made it possible to increase the signal-to-noise ratio by 18 times. In addition to spatial accumulation, temporary accumulation is taking place. In our example, in the interval of 5000 s at each point, 50,000 accumulations were performed in increments of 0.1 s. This makes it possible to increase the signal-to-noise ratio hundreds of times and thereby increase the reliability of estimating the level of signals in the formation at each point and restore the picture of the level of accumulated deep signals at each time point, and the totality of the pictures allows you to see the spatial development of hydraulic fracturing.

The advantage of this method

1. The method allows to record micro-earthquakes from hydraulic fracturing on the surface using standard instruments and equipment used in seismic exploration.

2. The method allows you to select the optimal monitoring system. As a rule, it is symmetric with respect to the initial fracture point. This makes it possible to single out a set of microearthquakes at large distances in any directions from the initial fracture point.

3. The processing method makes it possible to determine and take into account the velocity model and static and dynamic corrections and thereby allows to obtain a reliable picture of the rock cracking zone.

Claims (1)

A method for seismic exploration of rock cracking zones during hydraulic fracturing by pumping fluid and other components under high pressure into them, including recording seismic waves during and after hydraulic fracturing, characterized in that the observation system and processing parameters based on seismic wave modeling are selected from the planned hydraulic fracturing, record seismic waves on the day surface from perforation of the casing and from hydraulic fracturing according to the selected observation system, determined According to seismic data from the perforation of the column, a velocity model of the deep part of the medium and the delay of the seismic waves and their attenuation in the upper part of the section under the receiving points are focused, the waves recorded on the day surface from hydraulic fracturing are focused on the given deep surface, taking into account the speed model and corrections obtained during data processing perforations, depict the result in the form of sections, maps and video images, allowing you to see the wave process accompanying cracking in space and time.

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