RU2674524C1 - Method of seismic exploration - Google Patents

Abstract

FIELD: geophysics.SUBSTANCE: invention relates to the geophysics and can be used in seismic surveys. Claimed is the seismic survey method, which is based on the oscillations repeated excitation, the seismic records recording, the records conversion into the media images and the differential source records or generated from such records images formation. According to the claimed method, it is proposed to carry out each subsequent excitation and registration of oscillations after the previous one with an interval of time that is in an order of magnitude shorter than the anomalous object formation time. Then, from the registered wave fields or generated from them images, form a difference after equalizing the maximum intensity of wave fields or images adjacent in time and by the difference wave field or difference image judge on the change in the anomalous object properties, its size and location in space. At that, in one of the method modifications, forming the total wave field or image by summing the difference wave fields or images, obtained by adjacent in time wave fields or images subtraction. In the another method modification, simultaneously recording the oscillations inside the medium and in the near-surface zone and, before the wave fields subtraction, their equalization is carried out by recorded inside the medium seismic records. In the another method modification, from each of the registered wave fields subtracting the wave field formed by mathematical modeling using a model of the medium that does not contain the anomalous object. Obtained in this way the differential wave fields are summed up among themselves and on the total wave field, or on the obtained from it medium image, selecting the anomalous object and judging on its properties change with time. To implement the method into production, it is proposed to use the existing seismic records standard interpretative processing packages.EFFECT: increase in the anomalous objects selection reliability, whose properties, spatial position and size are quickly changing in time.4 cl

Description

The invention relates to geophysical methods for studying the geological environment and is intended mainly to detect abnormal objects located inside the environment, to study the rapid changes in time of their condition, location and size. Such objects may be:

трещины, сформировавшиеся в результате гидроразрыва или тектонических движений;

cracks formed as a result of hydraulic fracturing or tectonic movements;

пустоты или несанкционированно роющиеся в приповерхностной зоне подземные туннели;

voids or underground tunnels rummaging in the near-surface zone;

потенциальные очаги землетрясений, краткосрочный прогноз которых остается проблемой до настоящего времени.

potential sources of earthquakes, the short-term forecast of which remains a problem to date.

Anomalous objects, the size of which is comparable with the prevailing seismic wavelength, may not be detected on seismic records against the background of strong physically possible waves of different types and classes. The kinematic and dynamic characteristics of diffracted waves, most often associated with anomalous objects, can be close to the characteristics of other, stronger, waves recorded at close times to the target waves associated with the object under study. Under these conditions, it is very difficult to separate the target waves and other waves, which in this case are interference waves.

Known in seismic surveying methods for subtracting noise-waves dominating in intensity have significant limitations under the conditions of many waves that are interference: sequential subtraction of each wave leads to distortions of weaker waves remaining in the wave field, among which there are weak target waves associated with the detected object.

There is a method of searching for hydrocarbon deposits confined to fissure-cavernous reservoirs, in which two cubes of seismic data are obtained using pre-mortem migration along the Fresnel: a migrated cube of reflected waves (reflectors) and a cube of diffracted waves (diffractors). In this case, a cube of diffractors is obtained by subtracting the reflected waves from the total wave field and subsequent focusing of the scattered waves. The amplitude and spectral attributes of the seismic field are calculated from the cube of diffractors. By integrated amplitude characteristics, a three-dimensional model of scattering objects of the geological environment (cube index of acoustic heterogeneity) is obtained. Through a joint interpretation of the migrated cube of reflectors and the cube of the index of acoustic heterogeneity, we obtain the distribution of the instantaneous amplitudes of the scattered waves over the cross section of the cube in the intervals corresponding to the studied productive levels of the geological section. Objects with maximum values of the acoustic heterogeneity index are identified that correspond to the zones of development of fractured-cavernous reservoirs (RF Patent No. 2451951, 2010; Kharakhinov and Shlenkin, 2015).

The main disadvantage of this method is a single observation of the wave seismic field, which makes it impossible to study the changes in the properties of the detected anomalous object in time. In addition, it has an obvious difficulty, consisting in the need to form a field of reflected waves in order to subsequently subtract them from the general field for the diagnosis of diffractors in a difference field.

A known method for determining the parameters of a hydraulic fracture system, which includes the excitation of elastic vibrations by a source of oscillation in the well crossing the hydraulic fractures, registration at the points of reception in at least one neighboring well of resonant vibrations emitted by the hydraulic fracture system when elastic vibrations are excited in the drilling fluid, and determination of the parameters of the crack system from the resonant oscillations occurring in the cracks. Excitation of vibrations in the well and their registration is 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 parameters of the cracks (RF Patent No. 2507396, 2012).

The main disadvantage of this method is the excitation and registration of seismic vibrations only in neighboring wells, which does not allow, due to the limited aperture of observations from seismic records, to form a spatial image of the medium containing the desired anomalous object, in this case, a fracture system. In addition, a single registration of seismic vibrations after hydraulic fracturing does not allow us to trace in time the process of further development of cracks. The fundamental limitation of this method is its attachment to expensive deep wells, which can be drilled far from everywhere.

There is a known method of seismic monitoring, called “4D seismic exploration” (four-dimensional seismic exploration), in which a fourth coordinate (time) is added to the three spatial coordinates, since repeated observations are supposed to track changes in time of the state of the studied object, for example, the state of the developed oil reservoir ( Jack, 1998). In this method, the images of the geological environment obtained at different times are compared and their changes are used to judge the changes in time of the physical properties of the studied objects.

The disadvantage of this method is that it allows you to see on the images of the environment obtained at different times, only those changes that are caused by extended in space and quite contrasting objects. These changes are superimposed by background images formed from physically possible strong body waves under the conditions of a model that does not even include an anomalous target. Small in size, low-contrast objects, whose properties change over time, can be omitted altogether.

There is a known method of 4D seismic monitoring, in which sequential performance of conventional 3D surveys is carried out at long intervals with subsequent subtraction of wave fields (Ampilov, Baturin, 2013). Subtraction of the wave fields obtained with an interval of 13 years made it possible, for example, to see zones with the most pronounced changes in the difference wave field. These changes were associated with the injection of water through injection wells to maintain reservoir pressure during the displacement of oil by water. As a result, it was possible to map the zones of water distribution, to take into account the data obtained in the filtration model of the field and to determine the places for drilling additional injection wells.

The disadvantage of this method is the fixation in it of only the final result in the investigated anomalous object by comparing the wave fields with a very large time interval (13 years). The process of changes over time in the state of the anomalous object with such a single repeat of observations is impossible to trace.

Closest to the proposed seismic exploration method (prototype) is a seismic monitoring method in which seismic wave fields are repeatedly excited and recorded, wave fields are subtracted, and time-domain wave field attributes associated with the anomalous object under study are tracked by difference wave fields (Van Gestel et al., 2008). In this method, the excitation and registration of wave seismic fields was carried out once every six months, while the starting (background) wave field obtained before the start of the process of pumping water into the formation from injection wells was subtracted from subsequent wave fields. On the difference wave fields, it was possible to trace in time the process of water penetration into the formation. In addition, it was possible to mark the places where water penetrates poorly, and then adjust the drilling of injection and production wells. This technology made it possible to optimize the well drilling program, increase the production rate, and also reduce drilling costs.

The disadvantage of this method is the low frequency of repetition of excitation and registration of wave fields, which does not allow to trace the fast-moving changes in the properties of the anomalous object, its size and position in space. In addition, subtracting the background field from subsequent wave fields at large intervals in obtaining each of these fields can lead to distortions of each of the wave fields caused by changes in the excitation and reception conditions when the weather and time of the year change. Subtraction of the background part of the wave field for these reasons may not be completely effective.

The purpose of the invention is the expansion of the functionality of the method when detecting abnormal objects that quickly change their physical properties, sizes and location in time.

This goal is achieved by the fact that in the method of seismic exploration, based on the repeated excitation of oscillations, registration of seismic records and the formation of differential records, the excitation and registration of oscillations is carried out with a time interval an order of magnitude shorter than the formation time of the anomalous object from the recorded wave fields or generated from them of images form the difference after equalizing the maximum intensity of the time-adjacent wave fields or images, and the difference wave Liu or difference image judging the change in time of the anomalous properties of the object, its size and location in the object space. Moreover, in one of the modifications of the method, a total wave field or image is formed by summing the difference wave fields or images obtained by subtracting the time-adjacent wave fields or images. In another modification of the method, oscillations inside the medium and in the near-surface zone are simultaneously recorded, and before the wave fields are subtracted, their equalization is carried out using seismic records recorded inside the medium. In yet another modification of the method, the wave field generated by mathematical modeling is subtracted from each of the registered wave fields. The difference wave fields obtained in this way are summarized among themselves and an anomalous object is isolated on the total wave field or on the image of the medium obtained from it and judged about the change in its properties over time.

Compared with the prototype of the proposed method for seismic exploration in accordance with the invention is characterized by the following significant differences:

периодичностью повторных наблюдений, включающих возбуждение и регистрацию колебаний, которая позволяет оперативно изучать быстропротекающие процессы внутри среды путем достаточно высокой частоты воздействий;

the frequency of repeated observations, including the excitation and registration of vibrations, which allows you to quickly study fast-moving processes inside the medium through a fairly high frequency of effects;

более высоким отношением сигнал/помеха, которое достигается путем впервые предложенного суммирования разностных волновых полей, на которых полезный сигнал уже предварительно выделен путем акта вычитания фоновых волновых полей;

a higher signal-to-noise ratio, which is achieved by the first proposed summation of difference wave fields, on which the useful signal is already preliminarily selected by the act of subtracting the background wave fields;

возможностью выделять слабоконтрастные аномальные объекты благодаря суммированию сигналов, излучаемых именно этими объектами. Сущность предлагаемого способа состоит в следующем.

the ability to highlight low-contrast anomalous objects due to the summation of the signals emitted by these objects. The essence of the proposed method is as follows.

Small in size anomalous objects located inside the geological environment can be sources of diffracted waves, as well as their own resonant vibrations. A change in time of the physical properties of these objects, their size and orientation in space will inevitably affect the dynamic and kinematic characteristics of the waves associated with such objects. However, against the background of physically possible waves of a different type and class (longitudinal, transverse, exchange, surface, etc.), it is not only difficult to trace changes in time of seismic pulses associated with anomalous objects, but it can be difficult to isolate these pulses in general because of their low intensity . Upon repeated excitation and registration of seismic oscillations in wave fields, only those changes that are invisible to the eye will take place, which are caused by changes in the properties of the anomalous object over time. If we subtract these wave fields, having previously equalized them in intensity, then the difference wave field will be a reflection of changes in wave fields associated only with an anomalous object. It is significant at the same time that almost all waves unrelated to the anomalous object are subtracted, while changes in the local wave fields caused directly by the anomalous object occur due to the act of subtraction, and such changes can be foreseen by knowing the frequency response of the subtraction of a pair of seismic pulses. With small time shifts that occur in the proposed method, this frequency response is, as you know, a high-pass broadband filter. If the dimensions, shape and orientation of the anomalous object vary greatly in time, then the difference signal associated with such an object will have a rather complex shape. In this case, strong waves that are not associated with an anomalous object will be in phase, and subtraction will lead to their almost complete suppression. After subtraction of the wave fields, the difference seismogram can be transformed by migration into an image of the medium, which, due to the absence of regular interference waves, will contain only an image of an anomalous object

In accordance with the invention, the background wave fields obtained by full-wave simulation can be subtracted from the observed wave fields. However, it is quite difficult to achieve complete identity of such a wave field and the registered wave field, since this will require detailed information on the structure of the medium containing the anomalous object.

The question of a possible short-term earthquake forecast by the proposed method merits a separate consideration. The basis of modern ideas about the occurrence of the earthquake source is the provisions of the mechanics of destruction. It follows from them that a crack formed in an elastic medium at some moment loses stability and begins to spread like an avalanche. In a heterogeneous material, before the formation of a large crack, phenomena that precede this process — precursors — certainly appear in seismology as foreshocks. The intensity of foreshocks decreases over time, and then, after their almost complete disappearance, the stage of instability begins, characterized by an avalanche-like propagation of cracks. Thus, the earthquake is accompanied by a similar avalanche-like crack propagation. It is significant that the avalanche-like cracking occurs exclusively in the fault zone, where the earthquakes took place earlier. Thus, the forecast area is confined to such earthquake-prone zones. Short-term changes are those that occur as a result of the already begun but still hidden development of a large crack. At this stage (1-3 days before the earthquake), the current seismic situation is analyzed by changes in seismic records. These changes can be most confidently seen by isolating in pure form the waves associated with the potential hypocenter of the earthquake, in the vicinity of which anomalous changes in the elastic properties occur. Subtraction of wave fields, performed in accordance with the proposed method, is able to provide more confident observations in the wave field, due to a change in time of the elastic properties of the source, caused by an avalanche-like change in its continuity. The method is as follows.

To obtain records using modern seismic recording equipment capable of detecting weak signals against a background of strong interference. This condition assumes a sufficiently wide instantaneous dynamic range of the seismic channels used. Such an apparatus allows, after subtracting the total wave fields, to obtain undistorted weak signals on the difference records associated with the anomalous object under study. The location of sources and receivers of seismic vibrations implies that the waves illuminate a region of the medium containing the desired anomalous object. You can reasonably locate the sources and receivers of oscillations by mathematical modeling, which allows you to make sure that the object under study is reliably illuminated by the target waves. The use of diffracted waves for this purpose facilitates the design of an appropriate observation system. The presence of deep wells at the observation site allows borehole-surface observations capable of providing subsequent interpretative processing with a sufficiently accurate velocity model required to transfer images from the time scale to the deep scale. In addition, equalization of seismic records by their intensity is most reliably carried out by seismic signals recorded inside the medium.

The sequence of procedures in accordance with the invention is as follows:

при помощи источников колебаний и современной кабельной или бескабельной сейсмической аппаратуры периодически проводят возбуждение и регистрацию сейсмических колебаний с периодом, на порядок меньшим ожидаемой продолжительности изменений упругих свойств изучаемого объекта, его размеров или положения в пространстве;

using vibration sources and modern cable or cableless seismic equipment, the seismic vibrations are periodically excited and recorded with a period that is an order of magnitude shorter than the expected duration of changes in the elastic properties of the studied object, its size or position in space;

уравнивают по интенсивности соседние по времени сейсмические записи и вычитают одну из другой, после чего наблюдают остаточное разностное поле в ожидаемой области регистрации, которую можно получить по модели среды (если она имеется) путем трассирования сейсмических лучей;

equalize the seismic records adjacent in time in intensity and subtract one from the other, after which the residual difference field in the expected recording region is observed, which can be obtained from the medium model (if any) by tracing seismic rays;

в том случае, если на разностной сейсмограмме не получено сигнала, соответствующего аномальному объекту, аналогичным образом обрабатывают последующую пару записей;

if the signal corresponding to the anomalous object is not received on the differential seismogram, the next pair of records is processed in the same way;

начиная с момента, когда на разностной сейсмической записи получен значимый по интенсивности сигнал, соответствующий аномальному объекту, суммируют последующие соседние по времени разностные записи;

starting from the moment when a signal of significant intensity corresponding to the anomalous object is received on the difference seismic record, the subsequent time difference records, neighboring in time, are summarized;

если доступна достаточно детальная модель среды, вмещающей аномальный объект, то рассчитывают по известным программам полное двумерное или трехмерное полноволновое поле, которое вычитают из периодически получаемых записей, и затем суммируют полученные разностные записи;

if a sufficiently detailed model of the medium containing the anomalous object is available, then a full two-dimensional or three-dimensional full-wave field is calculated using known programs, which is subtracted from periodically obtained records, and then the resulting difference records are summed;

разностные записи или суммарные разностные записи преобразуют путем миграции в изображение среды, используя известные алгоритмы дифракционной томографии. Внедрение предлагаемого изобретения в практику сейсморазведки не требует создания новых технических средств или математического обеспечения и может быть начато уже в настоящее время. Его использование позволит обнаруживать слабоконтрастные аномальные объекты и объекты, размеры которых соизмеримы с преобладающей длиной сейсмической волны. Суммирование не исходных записей или их изображений, а разностных записей или разностных изображений, полученных при повторных воздействиях и регистрации сигналов, позволит обоснованно обнаруживать слабоконтрастные аномальные объекты с меняющимися во времени физическими свойствами. На стадии подземной разработки месторождений любых ископаемых это, к примеру, позволит проводить мониторинг, нацеленный на контроль за изменением содержимого вмещающих пород (контроль за фронтом горения, обводнением и др.). Технический результат - повышение надежности и разрешающей способности сейсморазведки.

difference records or total difference records are converted by migration into an image of the medium using known diffraction tomography algorithms. The introduction of the proposed invention into the practice of seismic exploration does not require the creation of new technical means or mathematical support and can be started now. Its use will make it possible to detect low-contrast anomalous objects and objects whose sizes are commensurate with the predominant seismic wavelength. The summation of not the original records or their images, but the difference records or differential images obtained by repeated exposure and registration of signals, will reasonably detect low-contrast anomalous objects with physical properties changing over time. At the stage of underground mining of deposits of any minerals, this, for example, will allow for monitoring aimed at monitoring changes in the contents of the host rocks (control of the combustion front, watering, etc.). The technical result is an increase in the reliability and resolution of seismic exploration.

The proposed method, considered in detail as applied to ground-based seismic exploration, without a significant change in its essence can be quite applied in marine and downhole seismic surveys, as well as in downhole-seismic exploration.

In the method of vertical seismic profiling (VSP), the proposed method can be used in a wide variety of modern modifications of the method (Shekhtman, 2017).

INFORMATION SOURCES

1. Ampilov Yu.P., Baturin D.G. The latest 4D seismic monitoring technologies in the development of offshore oil and gas fields // Seismic exploration technologies, No. 2, 2013, p. 31-36.

2. Baranov V.D., Erokhin G.N., Kremlin A. N., Rodin S.V. A method of searching for hydrocarbon deposits confined to fractured cavernous reservoirs // RF Patent No. 2451951, 2010.

3. Vershinina A.V., Muzychenko E.L., Kurin E.A., Denisov M.S. A method for predicting and subtracting multiple waves from seismic data obtained using bottom loggers // Seismic exploration technologies. 2012, No.3, p. 56-64.

4. Kasimov A.N. O., Shekhtman G.A., Maksimov G.A., Kasimov S.A., Chertenkov M.V., Stenin V.P. The method of determining the parameters of the fracturing system // Patent of the Russian Federation No. 2507396, 2012.

5. Levyant VB, Mottl VV. The method of seismic exploration of massive geological rocks // RF Patent No. 2168187, 1999.

6. Kharakhinov V.V., Shlenkin S.I. Fractured reservoirs of oil and gas // M .: Scientific World, 2015. - 284 p.

7. Shekhtman G.A. Vertical seismic profiling. - M .: EAGE Geomodel LLC, 2017. - 284 p.

8. Jack Ian Time-Lapse Seismic in Reservoir Management //! 998 Distinguished Instructor Short Course, SEG, Tulsa.

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Claims (4)

1. The method of seismic exploration, based on the re-excitation and registration of seismic vibrations with the subsequent formation of differential wave seismic fields, characterized in that in order to expand the functionality of the method when detecting anomalous objects that quickly change their physical properties, sizes and location in time, the excitation and the oscillations are recorded with an interval of time, an order of magnitude shorter than the formation time of the anomalous object, from the registered wave fields or the formed image formed thereon after equalizing the difference of the maximum intensity of neighboring time wavefields or images, and a difference of the wave field or the difference image judging the time variation of the anomalous properties of the object, its size and location in space. 2. The method according to p. 1, characterized in that they form the total wave field or image of the medium by summing the difference wave fields or images obtained by subtracting the time-adjacent wave fields or images. 3. The method according to p. 1, in which oscillations inside the medium and in the near-surface zone are simultaneously recorded and before the wave fields are subtracted, their equalization is carried out according to seismic records recorded inside the medium. 4. The method according to p. 1, characterized in that the wave field generated by mathematical modeling according to a model that does not contain an anomalous object is subtracted from the registered seismic wave field.