Abstract: Shale reservoirs are typically very tight, and crack are only a small part of the reservoir. The directional arrangement of cracks leads to the anisotropic characteristics of shale, and the type of fluid filled in cracks affects the shale reservoir evaluation and late development. Many rock physics theories and methods typically use second- and fourth-order crack density tensors to characterize the elastic anisotropy induced by cracks as well as the normal-to-tangential crack compliance ratio to distinguish between dry and saturated cracks. This study defines an anisotropic crack fluid indicator for vertical transversely isotropy (VTI) media with vertical symmetry axis which is the integration of the normal-to-tangential crack compliance ratio in three directions. A new dimensionless fourth-order tensor, including crack fluid type, azimuth distribution, and geometric shape, is constructed by substituting the normal and tangential compliance into the fourth-order crack density tensor, which can also be used to identify the type of crack fluid in the VTI media. Using the Callovo-Oxfordian shale experimental data, the variation of the elastic properties of dry and saturated shale samples with axial stress is analyzed. The results demonstrate that the anisotropic crack fluid indicator of water-bearing shale samples is less than that of the dry shale samples and that the dimensionless fourth-order tensor of water-bearing shale samples is nearly one order of magnitude greater than that of the dry shale samples. Therefore, the anisotropic crack fluid indicator and dimensionless fourth-order tensor can reflect the crack fluid type in shale samples and can be used for shale reservoir prediction and fluid identification. PubDate: 2019-07-22

Abstract: Estimating the quality factor Q accurately significantly improves the seismic data resolution and reservoir characterization. The commonly used log-spectral ratio method uses least-squares fitting to obtain Q values and involves only the amplitude information of seismic data while neglecting phase information. This paper proposes a joint interval Q inversion method based on the spectral ratio method and employs both amplitude and phase information to improve the accuracy. Based on the assumption that Q is independent of frequency, the nonlinear relation between the Q value and the two types of information is jointly used to construct an objective function, which clarifies the quantitative relation between amplitude spectrum, phase information, and Q value. The interval Q value can be inverted by calculating the minimum value of the objective function. The model test exhibits that the proposed method has higher precision and stability than the spectral ratio method; furthermore, the application to field data demonstrates that accurate Q inversion results are consistent with reservoir characteristics. PubDate: 2019-07-20

Abstract: Existing seismic prediction methods struggle to effectively discriminate between fluids in tight gas reservoirs, such as those in the Sulige gas field in the Ordos Basin, where porosity and permeability are extremely low and the relationship between gas and water is complicated. In this paper, we have proposed a comprehensive seismic fluid identification method that combines ray-path elastic impedance (REI) inversion with fluid substitution for tight reservoirs. This approach is grounded in geophysical theory, forward modeling, and real data applications. We used geophysics experiments in tight gas reservoirs to determine that Brie’s model is better suited to calculate the elastic parameters of mixed fluids than the conventional Wood’s model. This yielded a more reasonable and accurate fluid substitution model for tight gas reservoirs. We developed a forward model and carried out inversion of REI, which reduced the non-uniqueness problem that has plagued elastic impedance inversion in the angle domain. Our well logging forward model in the ray-path domain with different fluid saturations based on a fluid substitution model proved that REI identifies fluids more accurately when the ray parameters are large. The distribution of gas saturation can be distinguished from the crossplot of REI (p= 0.10) and porosity. The inverted ray-path elastic impedance profile was further used to predict the porosity and gas saturation profile. Our new method achieved good results in the application of 2D seismic data in the western Sulige gas field. PubDate: 2019-07-19

Abstract: Seismic AVAZ inversion method based on an orthorhombic model can be used to invert anisotropy parameters of the Longmaxi shale gas reservoir in the Sichuan Basin.. As traditional seismic inversion workflow does not suffi ciently consider the influence of fracture orientation, we predict fracture orientation using the method based on the Fourier series to correct pre-stacked azimuth gathers to guarantee the accuracy of input data, and then conduct seismic AVAZ inversion based on the VTI constraints and Bayesian framework to predict anisotropy parameters of the shale gas reservoir in the study area.We further analyze the rock physical relation between anisotropy parameters and fracture compliance and mineral content for quantitative interpretation of seismic inversion results. Research results reveal that the inverted anisotropy parameters are related to P- and S-wave respectively, and thus can be used to distinguish the effect of fracture and fluids by the joint interpretation. Meanwhile high values of anisotropy parameters correspond to high values of fracture compliance, so the anisotropy parameters can reflect the development of fractures in reservoir. There is two sets of data from different sources, including the content of brittle mineral quartz obtained from well data and the anisotropy parameters inverted from seismic data, also show the positive correlation. This further indicates high content of brittle mineral makes fractures developing in shale reservoir and enhances seismic anisotropy of the shale reservoir. The inversion results demonstrate the characterization of fractures and brittleness for the Longmaxi shale gas reservoir in the Sichuan Basin. PubDate: 2019-07-18

Abstract: Wave field extrapolation is critical in reverse time migration (RTM). At present, wavefield extrapolation in RTM imaging for tunnels is mostly carried out via the finite difference method. However, complex tunnel models, such as those for karst and fault fracture zones, are constructed using regular grids with straight curves which can easily cause numerical dispersion and reduce imaging accuracy. In this study, the wavefield extrapolation for tunnel RTM was conducted using the finite element method, where an unstructured mesh is employed as the body-fitted partition in a complex model. The Poynting vector calculation equation suitable for the unstructured mesh finite element method was established to suppress low-frequency noise interference. The tunnel space was considered during the wavefield extrapolation to suppress mirror artefacts by using the flexibility of mesh generation. Finally, the influence of the survey layouts (one-sidewall and two-sidewall) on the tunnel imaging results was explored. The RTM results for a simple tunnel model with an inclined interface show that the method based on unstructured meshes can effectively suppress low-frequency noise and mirror artefacts, thus obtaining clearer imaging results. Also, the two-sidewall tunnel survey layout more accurately obtains the real position of the inclined interface ahead of the tunnel face. Complex tunnel numerical modelling and actual data migration results further illustrate the effectiveness of the finite element unstructured mesh method. PubDate: 2019-07-16

Abstract: The development of the Tongchuan city, Shaanxi, located in northwestern region of China is restricted by water resources. The application of direct current resistivity method and induced polarization (IP) sounding method are usually used to find urban groundwater. These methods, however, are not effective due to its complicated topography and geological conditions. The application practice shows that the audio magnetotelluric method (AMT) is of large depth of exploration, high work efficiency and high lateral resolution. To investigate the distribution of groundwater resources, we deployed three audio-frequency magnetotelluric profiles in the city area. The impedance tensor information of AMT data is obtained using SSMT2000. AMT data dimension analysis reveals that the two-dimensional structure features of the observation area are obvious. The main structure of the observation area is about 45° northeast by structural trend analysis. A shallow two-dimensional electrical profile of 1 km in Tongchuan City is obtained by a two-dimensional nonlinear conjugate gradient inversion.. Finally, combined with regional geological information, the geological structure characteristics reflected by the electrical profile were obtained with detailed the characteristics of water-rich structures in the area. The influence of the structure on the distribution of groundwater was analyzed and the water-rich areas were trapped. That contributes prospective development of Tongchuan city. PubDate: 2019-07-15

Abstract: Few seismic exploration work was carried out in Tibetan Plateau due to the characteristics of alpine hypoxia and harsh environmental protection needs. Complex near surface geological conditions, especially the signal shielding and static correction of permafrost make the quality of seismic data is not ideal, the signal to noise ratio (SNR) is low, and deep target horizon imaging is difficult. These data cannot provide high quality information for oil and gas geological survey and structural sedimentary research in the area. To solve the issue of seismic exploration in Tibetan Plateau, this test used low frequency vibroseis wide-line and high-density acquisition scheme. In view of the actual situation of the study area, the terrain, the source and the different observation system were simulated, and the processing technique was adopted to improve the quality of seismic data. Low-frequency components with a minimum of 1.5Hz of vibroseis ensure the deep geological target imaging quality in the area, the seismic profile wave group is clear, and the SNR is relatively high, which can meet the needs of oil and gas exploration. Seismic data can provide the support for the development of oil and gas survey in the Tibet plateau. PubDate: 2019-07-13

Abstract: The time-frequency domain electromagnetic (TFEM) sounding technique can directly detect the oil and gas characteristics through the anomaly of resistivity and polarizability. In recent years, it has made some breakthroughs in the hydrocarbon detection. In order to predict the petroliferous property of the Ili basin, The TFEM was carried out. According to the geological structure characteristics of the study area, a Two-Dimensional layered medium model is constructed, and the forward modeling is carried out. We use the forward modeling results to guide the field construction and ensure the quality of the field data collection. We use the model inversion results to identify and distinguish the resolution of the geoelectric information, and provide a reliable basis for data processing. Based on that, the key technologies such as 2D 2-D resistivity tomography imaging inversion, polarimetric constrained inversion have been developed, and we obtained the abundant geological and geophysical information. Though the analysis of the electrical logging data in the study area, the characteristics of TFEM anomalies of the hydrocarbon reservoirs in the Ili basin are summarized. And more, the oil-gas properties of the Permian and Triassic layers have been predicted, and the next favorable exploration targets have been optimized. PubDate: 2019-07-11

Abstract: Carbon-bearing stratum normally features low resistance and high polarization. If the lithostratigraphy of the exploration area contains large amounts of carbon, the induced polarization anomaly caused by metal sulfide ore bodies will be inundated by the high polarization of carbon-containing wall rock. In this work, we adopted time-domain induced polarization (TDIP) and controlled-source audio-frequency magnetotellurics (CSAMT) on deep prospecting of the carbon-bearing stratum of the Ar Horqin Banner, Inner Mongolia. The underground medium is divided into target geologic bodies according to the geological information within the known exploration line borehole, and the physical properties of various target geologic bodies are calculated using weighted averages to build a geologic-geophysical model that can fit the observation data. Consequently, we can determine the range and morphological characteristics of the electrical properties of the ore-bearing geologic bodies in the inversion results in the study area. Then we can use the characteristics summarized from the known exploration line to interpret unknown exploration line. Results indicated that, when the difference in physical properties between the ore body and interference wall rock is not clear, the geologic body can be classifi ed via the paragenetic (associated) assemblage relations of the underground medium. Geological interpretation is guided by the comprehensive physical properties of ore-bearing geologic bodies to avoid interferences. PubDate: 2019-07-09

Abstract: The time-frequency electromagnetic prospecting system has both the characteristics of frequency sounding and transient electromagnetic sounding system, can obtain apparent resistivity both in frequency domain and time domain, therefore, thus, more accurate and credible geoelectric information can be obtained. After analyzing, there are mainly three typical types of noise in time-frequency electromagnetic observed data, they are 50Hz power frequency and its harmonics interference noise, high-frequency random impulse noise and low frequency interference noise. In view of characteristics of these three types of noise and the time-frequency electromagnetic prospecting signal, we adopted frequency domain band stop filtering method remove the 50 Hz power frequency and its harmonics interference noise, proposed a segmentation and extension median filtering method and a fitting fixed extreme EMD method to remove the high frequency random impulse noise and the low frequency interference noise separately, and proposed a median filtering window size test selection method based on variance and skewness coefficient of sample signal. Moreover, we designed a denoising processing fl ow as handle 50Hz power frequency and its harmonics interference noise at fist step, handle high frequency random impulse noise at second step, handle low frequency interference noise at last step, it can give full advantages of the denoising handling methods. By theoretical analysis and verification by practical observation data experiments, the methods what we adopted and proposed and the processing flow we designed can effectively remove the three types of interference noises effectively and avoid method error, can preserve phase and amplitude information of effective signal accurately, finally, high quality and satisfactory time domain signal waveform can obtained as a result, and lays a good foundation for extracting accurate transient electromagnetic attenuation curve in time domain for the Time-Frequency electromagnetic prospecting data. PubDate: 2019-06-20

Abstract: The density inversion of gravity gradiometry data has attracted considerable attention; however, in large datasets, the multiplicity and low depth resolution as well as efficiency are constrained by time and computer memory requirements. To solve these problems, we improve the reweighting focusing inversion and probability tomography inversion with joint multiple tensors and prior information constraints, and assess the inversion results, computing efficiency, and dataset size. A Message Passing Interface (MPI)-Open Multi-Processing (OpenMP)-Computed Unified Device Architecture (CUDA) multilevel hybrid parallel inversion, named Hybrinv for short, is proposed. Using model and real data from the Vinton Dome, we confirm that Hybrinv can be used to compute the density distribution. For data size of 100×100×20, the hybrid parallel algorithm is fast and based on the run time and scalability we infer that it can be used to process the large-scale data. PubDate: 2019-06-14

Abstract: The conventional fast converted-wave imaging method directly uses backward P-and converted S-wavefield to produce joint images. However, this image is accompanied by strong background noises, because the wavefields in all propagation directions contribute to it. Given this issue, we improve the conventional imaging method in the two aspects. First, the amplitude-preserved P- and S-wavefield are obtained by using an improved space-domain wavefield separation scheme to decouple the original elastic wavefield. Second, a converted-wave imaging condition is constructed based on the directional-wavefield separation and only the wavefields propagating in the same directions used for cross-correlation imaging, resulting in effectively eliminating the imaging artifacts of the wavefields with different directions; Complex-wavefield extrapolation is adopted to decompose the decoupled P- and S-wavefield into directional-wavefields during backward propagation, this improves the efficiency of the directional-wavefield separation. Experiments on synthetic data show that the improved method generates more accurate converted-wave images than the conventional one. Moreover, the improved method has application potential in micro-seismic and passive-source exploration due to its source-independent characteristic. PubDate: 2019-06-08

Abstract: We normalize data from 43 Chinese observatories and select data from ten Chinese observatories with most continuous records to assess the secular variations (SVs) and geomagnetic jerks by calculating the deviations between annual observed and CHAOS-6 model monthly means. The variations in the north, east, and vertical eigendirections are studied by using the covariance matrix of the residuals, and we find that the vertical direction is strongly affected by magnetospheric ring currents. To obtain noise-free data, we rely on the covariance matrix of the residuals to remove the noise contributions from the largest eigenvalue or vectors owing to ring currents. Finally, we compare the data from the ten Chinese observatories to seven European observatories. Clearly, the covariance matrix method can simulate the SVs of Dst, the jerk of the northward component in 2014 and that of the eastward component in 2003.5 in China are highly agree with that of Vertically downward component in Europe, compare to CHAOS-6, covariance matrix method can show more details of SVs. PubDate: 2019-06-08

Abstract: The anisotropy of a geologic formation can reflect the direction of fractures and ground stress, which is an important metric that guides the exploration and development of oil and gas reservoirs. Cross-dipole acoustic logging is the main method used to detect anisotropy with borehole geophysics. In this paper, a stepwise inversion method for three anisotropy parameters in a horizontal transversely isotropic (HTI) formation is proposed, which turns one 3D operation of simultaneous inversion into three 1D operations. The scheme’s stability and reliability were tested by numerically simulated data using a finite-difference method, and by field logging data. The inversion results of the simulated data show that the stepwise inversion method can stably obtain the fast shear azimuth and the anisotropy parameters in both fast and slow formations with strong and weak anisotropy, and it performed well even with noisy data. In particular, the results of the fast shear azimuth inversion were very stable and reliable. The inversion results of field logging data were consistent with those given by existing commercial software, which used simultaneous inversion, for both fast and slow formations. Where large difference was observed between our stepwise method and the commercial software, our analysis suggests that the fast shear azimuth of our inversion was more reasonable, which I reinforces its superior performance and practicality. PubDate: 2019-06-08

Abstract: To better retain useful weak low-frequency magnetotelluric (MT) signals with strong interference during MT data processing, we propose a SVM-CEEMDWT based MT data signal-noise separation method, which extracts the weak MT signal affected by strong interference. First, the approximate entropy, fuzzy entropy, sample entropy, and Lempel-Ziv (LZ) complexity are extracted from the magnetotelluric data. Then, four robust parameters are used as the inputs to the support vector machine (SVM) to train the sample library and build a model based on the different complexity of signals. Based on this model, we can only consider time series with strong interference when using the complementary ensemble empirical mode decomposition (CEEMD) and wavelet threshold (WT) for noise suppression. Simulation results suggest that the SVM based on the robust parameters can distinguish the time periods with strong interference well before noise suppression. Compared with the CEEMDWT, the proposed SVM-CEEMDWT method retains more low-frequency low-variability information, and the apparent resistivity curve is smoother and more continuous. Moreover, the results better reflect the deep electrical structure in the field. PubDate: 2019-06-08

Abstract: We simulated hydraulic fracturing in different lithologic rocks in the horizontal drilling by using the true physical model experiment and large rock specimens, carried out the real-time dynamic monitoring with adding tracer and then did post-fracturing cutting and so on. Based on this monitoring results, we compared and assessed the factors affecting expansion in shale, shell limestone, and tight sandstone and the fracture expansion in these rocks. In shale, the reformed reservoir volume is the highest, fracture network is formed in the process of fracturing. In tight sandstone, the fracture surface boundaries are curved, and the fracture surface area accounts for 25–50% of the entire specimen. In shell limestone, the complexity of the fracture morphology is between shale and tight sandstone, but no fracture network is developed. Brittleness controls the fracture surface area. In highly brittle rocks, the fracture surface area is high. Fracture toughness mainly affects the initiation and propagation of cracks. A fracture network is formed only if bedding planes are present and are more weaker than their corresponding matrix. The horizontal in situ deviatoric stress affects the crack propagation direction, and different lithologies have different horizontal in situ deviatoric stress thresholds. Low fluid injection rate facilitates the formation of complex cracks, whereas high fluid injection rate favors the development of fractures. Fluid injection weakly controls the complexity of hydraulic fracturing in low-brittleness rocks, whereas low-viscosity fracturing fluids favor the formation of complex cracks owing to easy enter microcracks and micro-pore. Displacement has a greater impact on high brittle rocks than low brittle rocks. PubDate: 2019-06-01

Abstract: Reverse time migration (RTM) is an indispensable but computationally intensive seismic exploration technique. Graphics processing units (GPUs) by NVIDIA® offer the option for parallel computations and speed improvements in such high-density processes. With increasing seismic imaging space, the problems associated with multi-GPU techniques need to be addressed. We propose an efficient scheme for multi-GPU programming based on the features of the compute-unified device Architecture (CUDA) using GPU hardware, including concurrent kernel execution, CUDA streams, and peer-to-peer (P2P) communication between the different GPUs. In addition, by adjusting the computing time for imaging during RTM, the data communication times between GPUs become negligible. This means that the overall computation efficiency improves linearly, as the number of GPUs increases. We introduce the multi-GPU scheme by using the acoustic wave propagation and then describe the implementation of RTM in tilted transversely isotropic (TTI) media. Next, we compare the multi-GPU and the unified memory schemes. The results suggest that the proposed multi-GPU scheme is superior and, with increasing number of GPUs, the computational efficiency improves linearly. PubDate: 2019-03-01

Abstract: To identify reflector fractures near borehole by using dipole-source reflected-shear-wave logging, we need to understand the relation between the amplitude of the reflected shear wave and the source radiation, borehole conditions, and attenuation owing to the surrounding formations. To assess the effect of these factors on the amplitude of the reflected waves, we first studied the radiation performance and radiation direction of the dipole source in fast, medium, and slow formations by using the asymptotic solution in the far field of the borehole. Then, the relation between the fracture parameters, and the reflected-shear-wave amplitude as well as the ratio of the reflected-shear-wave amplitude to the direct-wave amplitude (relative amplitude, RA) was evaluated by the three-dimensional finite-difference (3D FDTD) method. Finally, the fracture detection capability of the dipole reflected-shear-wave logging tool in different formations was analyzed by using the RA. The results suggest that the radiation amplitude of the SH-wave in the slow formation is weaker than those in the fast and medium formations, and the amplitude of the reflected shear wave is lower. However, the RA in the slow formation is close to or even greater than in the fast and medium formations, which means that dipole-source shear-wave logging has the same or even better fracture detection capability in the slow formation as in the fast and medium formations. In addition, when RA is small, there is a good correlation between the RA and the various fracture parameters in the different types of formation, which can be used in determining the lower limit of the fracture parameters identified by reflection logging. PubDate: 2019-03-01

Abstract: In pyrite-bearing shaly sandstones, the distortion in the resistivity logging response of formations in high-frequency resistivity logging because of dispersion hinders the calculation of reservoir saturation. To eliminate the effect of resistivity dispersion of pyrite and shale, and to avoid factors, such as mineral composition, content, and distribution in natural cores, we synthesized twelve samples with dispersed pyrite and shale grains at high temperature and pressure (60 MPa and 120 °C). We performed experiments at different water salinities and oil saturations, and different frequency to assess the effect of frequency on the conductivity of pyrite-bearing shaly sandstones. Both the dispersed pyrite and shale grains show dispersion, and the real part of the complex resistivity decreases with increasing frequency. Based on symmetrical effective medium conductivity theory and the experimental data, the effective medium dispersion model for the real part of the complex resistivity for pyrite-bearing shaly sandstones is established considering the effect of pyrite and shale content on resistivity dispersion. Simulations suggest that the predicted resistivity dispersion by the model in pyrite-bearing shaly sandstones for variable frequency, and pyrite and shale content agrees with the experimental results. The proposed model can successfully predict the dispersion of pyrite-bearing shaly sandstones. Finally, the resistivity dispersion correction plot for the conductivity of pyrite and shale grains of 0.062 S/m and 0.031 S/m, respectively, is established based on the frequency applied to various electric logs, and the correction method for the high-frequency resistivity log response is given to obtain the real formation resistivity. PubDate: 2019-03-01

Abstract: Airgun arrays are widely used in marine seismic exploration because signatures excited by airgun arrays have high energy and high-peak bubble ratio, whereas the considerable length and width of the array and ghost reflections make the airgun array signature directional. As a result, the relation of the reflection amplitude with the incident and azimuth angles is variable. This means that the directivity of the airgun array results in a nonstationary wavelet and distorts the relation of the amplitude variation with the incident and azimuth angles. To remove the directivity effect, we propose a nonstationary inversion-based directional deconvolution. At first, the signature as a function of take-off angle and azimuth angle is calculated using the spatial configuration of the airgun array and the near-field signatures. Then, based on the velocity model, the time-variant take-off angles are estimated and directional filters are designed using the take-off angles. Finally, the directivity-dependent signatures are shaped to the signature right below the airgun array using nonstationary inversion in the directional deconvolution. PubDate: 2019-03-01