Applied Geophysics

Trong các bể khí tự nhiên không đồng nhất, khí thường hiện diện dưới dạng các túi nhỏ giống như mảng được nhúng trong ma trận chủ lưu có bão hòa nước. Loại không đồng nhất này, còn được gọi là "bão hòa patchy", gây ra sự phân tán và suy giảm tốc độ địa chấn đáng kể. Để thiết lập mối quan hệ giữa phản ứng địa chấn và loại chất lỏng, chúng tôi đã thiết kế một mô hình vật lý đá cho các bể carbonat. Đầu tiên, chúng tôi thực hiện quét CT và phân tích phân bố chất lỏng trong các đá có độ bão hòa một phần. Sau đó, chúng tôi dự đoán mối quan hệ định lượng giữa phản ứng sóng ở các dải tần số khác nhau và các thuộc tính lithological cơ bản cũng như các chất lỏng trong lỗ rỗng. Một mẫu vật lý đá đã được xây dựng dựa trên phân tích mỏng của cấu trúc lỗ rỗng và nghịch đảo địa chấn. Phương pháp này đã được áp dụng cho các bể khí limestone ở khối bờ phải sông Amu Darya. Dựa trên nghịch đảo tham số đàn hồi trước và sau khi chồng chất, dữ liệu địa chấn đã được sử dụng để ước tính độ rỗng của đá và độ bão hòa khí. Kết quả mô hình tương đồng tốt với chế độ sản xuất của các giếng khoan.

When a seismic wave propagates through subsurface viscoelastic media, the formation absorbs the high-frequency energy of the seismic wave more strongly than the low-frequency energy. As the depth and the offset increase, the difference between the logarithmic spectral areas with high and low frequencies gradually increases. Based on this seismic wave characteristic, we have developed a novel Q-estimation method based on logarithmic spectral area difference of high and low frequencies (referred to as the LSAD_LH method). In this paper, we derive the theoretical relationship between the Q value and difference of logarithmic spectral areas with high and low frequencies and prove the applicability of the LSAD_LH method using a single-layer medium numerical model. To verify the sensitivity of the LSAD_LH method to bandwidth selection and noise, we compare the LSAD_LH method with two credible methods—the logarithmic spectral ratio (LSR) and logarithmic spectral area difference (LSAD) methods using a synthetic model containing the random noise. The results demonstrate that the LSAD_LH method is not highly dependent on bandwidth, and in terms of noise immunity, it is significantly better than the LSR method and has the same advantages as the LSAD method. To further highlight the advantages of the LSAD_LH method, we apply the LSAD_LH and LSAD methods to the vertical seismic profiling (VSP) numerical simulation of the multilayer media and the field zero-offset VSP data. The application of the two cases proves the applicability of the LSAD_LH method and the accuracy of the high Q-value estimation relative to the LSAD method. Moreover, via the transmission coefficient, the LSAD_LH method overcomes the weakness of the LSAD method.

There are complex heterogeneous entities in the underground medium, and the heterogeneous scale has a substantial impact on wave propagation. In this study, we used a set of 11 samples of glass beads as high-velocity heterogeneous bodies to evaluate the impact of such heterogeneous bodies on the propagation of P-wave. We vary the heterogeneous scale by changing the diameter of the glass beads from 0.18 to 11 mm while keeping the same volume proportion (10%) of the beads for the set of 11 samples. The pulse transmission method was used to record measurements at the ultrasonic frequencies of 0.34, 0.61, and 0.84 MHz in the homogeneous matrix. The relationship between P-wave field features and heterogeneity scale, P-wave velocity, and the multiple of the wave number and heterogeneous scale (ka) was observed in the laboratory, which has sparked widespread interest and research. Heterogeneous scale affects P-wave propagation, and its wave field changes are complex. The waveform, amplitude, and velocity of the recorded P-waves correlate with the heterogeneous scale. For the forward scattering while large-scale heterogeneities, noticeable direct and diffracted waves are observed in the laboratory, which indicates that the influence of direct and diffracted waves cannot be ignored for large-scale heterogeneities. The relationship between velocity and ka shows frequency dependence; the reason is that the magnitude of change in velocity caused by wave number is different from that caused by heterogeneous scale. According to the change in the recorded waveform, amplitude variation, or the relationship between the velocity measured at different frequencies and the heterogeneous scale, the identified turning points of the ray approximation are all around ka = 10. When ka is less than 1, the velocity changes slowly and gradually approaches the effective medium velocity. The ray velocity measured for heterogeneous media with large velocity perturbations in the laboratory is significantly smaller than the velocity predicted by the perturbation theory.

Oil reservoirs with low permeability and porosity that are in the middle and late exploitation periods in China’s onshore oil fields are mostly in the high-water-cut production stage. This stage is associated with severely non-uniform local-velocity flow profiles and dispersed-phase concentration (of oil droplets) in oil-water two-phase flow, which makes it difficult to measure water holdup in oil wells. In this study, we use an ultrasonic method based on a transmission-type sensor in oil-water two-phase flow to measure water holdup in low-velocity and high water-cut conditions. First, we optimize the excitation frequency of the ultrasonic sensor by calculating the sensitivity of the ultrasonic field using the finite element method for multiphysics coupling. Then we calculate the change trend of sound pressure level attenuation ratio with the increase in oil holdup to verify the feasibility of the employed diameter for the ultrasonic sensor. Based on the results, we then investigate the effects of oil-droplet diameter and distribution on the ultrasonic field. To further understand the measurement characteristics of the ultrasonic sensor, we perform a flow loop test on vertical upward oil-water two-phase flow and measure the responses of the optimized ultrasonic sensor. The results show that the ultrasonic sensor yields poor resolution for a dispersed oil slug in water flow (D OS/W flow), but the resolution is favorable for dispersed oil in water flow (D O/W flow) and very fine dispersed oil in water flow (VFD O/W flow). This research demonstrates the potential application of a pulsed-transmission ultrasonic method for measuring the fraction of individual components in oil-water two-phase flow with a low mixture velocity and high water cut.

Although high resolution can be provided by electrical logging, the measured electrical log range is narrow and is limited to near the well. Borehole-surface electric potential measurements are able to detect a wide enough range but its resolution is limited, particularly for reservoirs with complex oil and water distribution or complicated structure. In this study, we attempt to accurately locate the 3-D reservoir water and oil distribution by combining borehole-surface and crosswell electric potentials. First, the distributions of oil and water in both vertical and horizontal directions are detected by the borehole-surface and crosswell electric potential methods, respectively, and then the measured crosswell potential result is used to calibrate the measured borehole-surface electric potential data to improve vertical resolution so that the residual oil distribution is determined in a lower half-space with three dimensions. The evaluation of residual oil distribution is obtained by investigation of differences between the simulation results of the reservoir with and without water flooding. The finite difference numerical simulation results prove that the spatial residual oil distribution can be effectively determined by combining the crosswell and borehole-surface electric potentials.

The absence of low-frequency information in seismic data is one of the most difficult problems in elastic full waveform inversion. Without low-frequency data, it is difficult to recover the long-wavelength components of subsurface models and the inversion converges to local minima. To solve this problem, the elastic envelope inversion method is introduced. Based on the elastic envelope operator that is capable of retrieving lowfrequency signals hidden in multicomponent data, the proposed method uses the envelope of multicomponent seismic signals to construct a misfit function and then recover the longwavelength components of the subsurface model. Numerical tests verify that the elastic envelope method reduces the inversion nonlinearity and provides better starting models for the subsequent conventional elastic full waveform inversion and elastic depth migration, even when low frequencies are missing in multicomponent data and the starting model is far from the true model. Numerical tests also suggest that the proposed method is more effective in reconstructing the long-wavelength components of the S-wave velocity model. The inversion of synthetic data based on the Marmousi-2 model shows that the resolution of conventional elastic full waveform inversion improves after using the starting model obtained using the elastic envelope method. Finally, the limitations of the elastic envelope inversion method are discussed.

The full-waveform inversion method is a high-precision inversion method based on the minimization of the misfit between the synthetic seismograms and the observed data. However, this method suffers from cycle skipping in the time domain or phase wrapping in the frequency because of the inaccurate initial velocity or the lack of low-frequency information. furthermore, the object scale of inversion is affected by the observation system and wavelet bandwidth, the inversion for large-scale structures is a strongly nonlinear problem that is considerably difficult to solve. In this study, we modify the unwrapping algorithm to obtain accurate unwrapped instantaneous phase, then using this phase conducts the inversion for reducing the strong nonlinearity. The normal instantaneous phases are measured as modulo 2π, leading the loss of true phase information. The path integral algorithm can be used to unwrap the instantaneous phase of the seismograms having time series and one-dimensional (1D) signal characteristics. However, the unwrapped phase is easily affected by the numerical simulation and phase calculations, resulting in the low resolution of inversion parameters. To increase the noise resistance and ensure the inversion accuracy, we present an improved unwrapping method by adding an envelope into the path integral unwrapping algorithm for restricting the phase mutation points, getting accurate instantaneous phase. The objective function constructed by unwrapping instantaneous phase is less affected by the local minimum, thereby making it suitable for full-waveform inversion. Further, the corresponding instantaneous phase inversion formulas are provided. Using the improved algorithm, we can invert the low-wavenumber components of the underneath structure and ensure the accuracy of the inverted velocity. Finally, the numerical tests of the 2D Marmousi model and 3D SEG/EAGE salt model prove the accuracy of the proposed algorithm and the ability to restore large-scale low-wavenumber structures, respectively.