Summary
The current oil and gas exploration targets have the characteristics of "hidden, fragmented, thin, and small", which put forward higher requirements for migration imaging technology. Due to the special acquisition method, borehole seismic has the advantages of high data resolution, rich wave field information, and less interference. In theory, it can realize high-precision imaging of "hidden, fragmented, thin, and small" complex reservoirs around the well. The well type has great restrictions on the distribution of seismic sources. In practice, in addition to vertical wells, there are also various well types such as inclined wells, curved wells, and horizontal wells. The locations of the same depth seismic sources are different in different well types. The spatial distribution of the number of seismic sources in different well types is also different, which leads to significant differences in the propagation paths of seismic waves and affects the imaging quality. However, there is no qualitative or quantitative understanding of the impact of the current well type on migration imaging. Based on the viscoacoustic reverse time migration imaging method, this paper compares seismic migration imaging in theoretical models under different well type conditions. As a result, the influence of well type on migration quality was analyzed. The numerical calculation results clarified the qualitative relationship between the well type and the seismic migration imaging quality and effective imaging range in the well, and the results also provided corresponding theoretical support for the design of the seismic acquisition system in the well.
The current hydrocarbon exploration targets are concealed, scattered, thin, and small. These characteristics put forward higher requirements for the migration imaging technique. Owing to the special acquisition method, the data derived from borehole seismic have the advantages of high field rich resolution information, and less interference. In theory, borehole seismic can be used to realize high-precision imaging of complex reservoirs, such as concealed, scattered, thin and small ones around the well. Well types greatly limit the layout of the seismic sources.In practice, besides vertical wells, there are also many types of wells, such as inclined wells, curved inclined wells, and horizontal wells. For different well types, the seismic sources at the same depth have different positions and the same number of seismic sources have different spatial distributions,leading to significantly different seismic wave propagation paths and further affecting the imaging quality. However, there is no qualitative or quantitative understanding of the effects of well types on migration imaging currently. effects of well types on migration quality by comparing the seismic migration imaging results of theoretical models under various well types.The numerical results provide the qualitative relationships between well types and borehole seismic migration imaging quality and effective imaging range.The results also provide corresponding theoretical support for the design of a borehole seismic acquisition system.There is no qualitative or quantitative understanding of the effects of well types on migration imaging currently. Using the visco-acoustic inverse time migration imaging method, this study analyzed the effects of well types on migration quality by comparing the seismic migration models of or imaging the results under various well types. The numerical results provide the qualitative relationships between well types and borehole seismic migration imaging quality and effective imaging range. The results also provide corresponding theoretical support for the design of a borehole seismic acquisition system.There is no qualitative or quantitative understanding of the effects of well types on migration imaging currently. Using the visco-acoustic inverse time migration imaging method, this study analyzed the effects of well types on migration quality by comparing the seismic migration models of or imaging the results under various well types. The numerical results provide the qualitative relationships between well types and borehole seismic migration imaging quality and effective imaging range. The results also provide corresponding theoretical support for the design of a borehole seismic acquisition system.This study analyzed the effects of well types on migration quality by comparing the seismic migration imaging results of theoretical models under various well types. The numerical results provide the qualitative relationships between well types and borehole seismic migration al imaging responsive range quality and provide corresponding theoretical support for the design of a borehole seismic acquisition system.This study analyzed the effects of well types on migration quality by comparing the seismic migration imaging results of theoretical models under various well types. The numerical results provide the qualitative relationships between well types and borehole seismic migration al imaging responsive range quality and provide corresponding theoretical support for the design of a borehole seismic acquisition system.
Preface
At present, with the continuous improvement of the exploration degree of oil and gas-bearing basins in China, the difficulty of exploration has gradually increased, and complex structures, complex lithologies, and deep/ultra-deep oil and gas reservoirs have gradually become the main exploration objects[1], and the geological targets of exploration have " hidden The characteristics of ", fragmented, thin, and small", such as complex small fault blocks, compact clastic rocks, etc. [ 2-3 ] , put forward higher requirements for seismic migration imaging technology. Borehole seismic has the advantages of high signal-to-noise ratio and strong reservoir identification ability. In addition, it can adapt to any arrangement of geophones, can reduce the loss of surface element data volume, and can theoretically realize high-precision imaging of "hidden, broken, thin, small" reservoirs around the well.
Borehole seismic technology can be divided into borehole seismic while drilling and detonation borehole seismic according to the type of source. Earthquake while drilling, that is, relying on the idea of earthquake while drilling, using the vibration of the drill bit as the source of well observation [ 4 ] , based on this idea, many scholars at home and abroad have done related research on the rotary drill bit and earthquake while drilling [ 5 ⇓ ⇓ ⇓ - 9 ] . But seismic while drilling has the disadvantage of limited detection depth, which is not conducive to seismic exploration in deep complex structural regions, thus developing the detonation well seismic that excites the source in the well. For earthquakes in detonation wells, because it needs to excite strong energy while protecting the wellbore, it has high requirements on the seismic source, so there are few related studies and mainly focus on shallow tomography, seismic research in coal seam wells and engineering geophysical prospecting. field [ 9 ⇓ ⇓ ⇓ ⇓ ⇓ ⇓ - 16 ] . In the field of petroleum seismic exploration, there are relatively few studies on borehole seismic technology.
In actual production, the design of drilling is affected by production requirements or objective factors. In addition to conventional vertical wells, there are also many types of wells, such as deviated wells, curved wells, and horizontal wells. Compared with conventional vertical wells, horizontal wells play an important role in the efficient exploration and development of unconventional oil and gas resources [ 17 ] . In marine exploration, due to the limited space of the operating platform, the well types are generally angled or space-changing inclined wells [ 18 ] . Because the excitation source of borehole seismic is located in the well, different well types make the spatial position of the source different, resulting in significant differences in the propagation paths of seismic waves, which will inevitably affect the final imaging results. Therefore, before applying borehole seismic technology, it is necessary to analyze the influence of well type on imaging results in detail, and then guide the selection of excitation well intervals in actual production.
Considering that the purpose of applying borehole seismic technology is to perform fine imaging of deep complex structures, compared with commonly used imaging algorithms such as Kirchhoff integral migration, one-way wave migration, and reverse time migration, the reverse time migration algorithm has no angle limit and is suitable for complex structures. Advantages of Media Imaging. In addition, because the seismic source is located underground, the weak energy is seriously affected by the absorption and attenuation of the formation, and the seismic wave will be accompanied by energy attenuation and waveform distortion during the propagation process, which will affect the accuracy of the migration profile. Based on the above factors, Q- compensated reverse time migration imaging should be performed for borehole seismicity to obtain high-fidelity migration imaging sections.
Early attenuation compensation is to apply inverse Q filter to seismic records to achieve the purpose of wavefield amplification [ 19-20 ] , mainly used in Kirchhoff migration and one-way wave migration [ 21-22 ] , but only for amplitude and phase Trace compensation does not consider the seismic wave propagation path, so it cannot achieve a reasonable compensation effect. The Q compensation offset based on the viscous wave equation can be compensated in the process of wave field continuation, which is more convincing. At present, the equation based on the standard linear solid (SLS) model is widely used to describe the decay process [ 23 - 24 ] , and many scholars have carried out related research on it and finally realized the stable Q- compensated reverse time migration (Q-RTM ) [ 25 ⇓ - 27 ] . Generally speaking, the equation based on the SLS model has the advantages of fast calculation speed and easy implementation, but it also has the disadvantage of mutual coupling between the attenuation term and the dispersion term. To solve this shortcoming, many scholars have developed equations based on the constant Q model [ 28-29 ] . Among them, Zhu et al [30 ] , Wu Yu et al. [ 31 ] proposed a decoupled fractional viscoacoustic wave equation based on the plane wave dispersion relation. This equation explicitly separates the attenuation term and dispersion term, and corrects the amplitude and phase at the same time.appliedin West Texas actual data [ 32 ] , and achieved good results. However, it also needs to use the pseudospectral method to solve the Laplace operator, which requires a large amount of calculation, and the decoupling of the attenuation and dispersion items is not complete. Based on this equation, many scholars at home and abroad have carried out a series of research [ 33 ⇓ ⇓ ⇓ ⇓ ⇓ - 39 ] , generally speaking, the decoupled fractional Laplace equation has high calculation accuracy, which is convenient for realizing stable Q -RTM.
In summary, borehole seismic is a geophysical prospecting method that can realize fine imaging of "hidden, broken, thin, and small" geological targets around the well. In actual production, different well types have an impact on imaging results. However, there is no qualitative or quantitative understanding of how well types affect migration imaging. In this paper, the viscoacoustic reverse time migration method is used for borehole seismic imaging, and the numerical simulation is carried out based on the fractional viscoacoustic wave equation proposed by Zhu et al. [ 30 ]. Multi-well imaging trial calculation, to explore the influence of well type on migration quality. The numerical experiment results clarified the influence relationship between the well type and the imaging section, and provided theoretical support for the design of the seismic acquisition system in the well.
1 Basic principles of viscoacoustic reverse time migration in wells
1.1 Characteristics of borehole seismicity
Borehole seismic technology is a special geophysical prospecting method that is excited in the borehole and received on the ground. Due to the particularity of the borehole seismic observation system, there is a big difference between it and the surface earthquake. The surface and borehole seismic observation systems are shown in Figure 1. It can be seen from the figure that compared with the ground seismic, since the seismic source in the well is located underground and close to the exploration target, the seismic wave excited by it reduces the loss of a low-velocity zone, avoids the generation of surface waves, and helps to acquire high-resolution and Signal-to-noise ratio seismic signal; at the same time, because there is no source interference on the ground for the earthquake in the well, a wide range of geophones can be arranged on the surface, which is conducive to high-density acquisition; and a regular observation system can be used to arrange geophones on the ground , in order to reduce the loss of surface element data volume, so as to obtain richer effective information in the acquisition area.
figure 1
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