Journal of Geophysics and Engineering - Latest Papers

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Latest articles for Journal of Geophysics and Engineering
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A method to construct high-precision complex pore digital rock

Tue, 10/30/2018 - 8:00pm
X-ray CT scanning is one of the main methods to study the pore structure of the rocks. However, low resolution is currently one of the major challenges of this method, which does not allow for the identification of microscopic pores. Even though adopting images of higher resolutions could reflect the microscopic pores of the rocks, due to the small size of the scanned rock sample in the x-ray CT scanning method, the rock parameters cannot be effectively obtained, especially for the rocks with strong heterogeneity. This would cause the loss of important pore information, such as fractures’ information. Focusing on this problem, the current research tries to propose a method for constructing high-precision digital rocks of complexly porous rocks. This method is based on the simultaneous application of x-ray CT scanning images, nuclear magnetic resonance measurements, as well as mercury injection experimental data and fractal discrete fracture network. These methods effectively com...

Cross-rhombus stencil-based finite-difference methods for 2D acoustic modeling and reverse-time migration on rectangular grids

Wed, 10/24/2018 - 7:00pm
The recently developed cross-rhombus stencil-based time–space domain finite-difference (FD) method for modeling two-dimensional acoustic equations, controls the temporal and spatial dispersions synchronously and outperforms cross-stencil-based FD (CS-FD). However, it is only applicable to modeling on equally spaced grids and extensive application is hindered. In this work, we extend it further and develop novel cross-rhombus stencil-based FD (CRS-FD) for modeling on arbitrarily rectangular grids. Two kinds of rhombus stencils involving grid points both on and off the axis are developed first to achieve fourth order and sixth order FD accuracies, respectively, for solving the second order temporal derivative on rectangular grids. The plane wave theory is then used to derive the time–space-domain dispersion relations, when the temporal and spatial derivatives are solved by the new rhombus-stencil-based temporal high order FD and the cross-stencil-based spatial high order FD, respe...

Application of the power mean to modeling the elastic properties of reservoir rocks

Wed, 10/24/2018 - 7:00pm
The relationship between seismic velocity and porosity is one of the most important research topics in exploration geophysics. A strong correlation, if it exists, can be very useful for quantitative seismic interpretation of direct hydrocarbon indicators. The Voigt and Reuss bounds are often plotted alongside the data for quality control and interpretation. The relative position of the data within the bounds may supply important information regarding the pore shape, stress condition, lithology, and diagenesis of porous rocks. It is found that the Voigt bound and Reuss bound are actually special cases of the weighted power mean, with the power parameters being 1 and −1, respectively. The geometric mean is a special case of the power mean with the power parameter being zero. The power mean is a monotonic function of the power parameter. For the construction of a rock physics template, the region between the Voigt and Reuss bounds can be evenly divided by varying the power paramete...

Permeability enhancement performance and its control factors by auger mining of extremely thin coal seams

Mon, 10/15/2018 - 7:00pm
In order to improve both the pressure relief in deep coal mining and the permeability of coal seams for gas drainage, the auger mining (AM) method was proposed for mining extremely thin coal seams underneath a major coal seam to enhance its permeability and improve its pressure relief for subsequent major mining. The method was applied to Chiyu Coal Mine in China as a case study. Based on the geological conditions of the mine, the influences of AM parameters such as AM width, intermediate coal pillar width and distance between adjacent temporary wide pillars on the pressure distributions and permeability enhancement were studied in detail and the optimal set of parameters for the mining operation were obtained. Based on our simulations, it was found that using the proposed method with the optimal parameters derived, the permeability of the upper protected coal layer will increase by more than 671 times the initial permeability. This demonstrated that the proposed new AM method i...

High-resolution shallow reflection seismic integrated with other geophysical methods for hydrogeological prospecting in the Nylsvley Nature Reserve, South Africa

Mon, 10/15/2018 - 7:00pm
Two perpendicular high-resolution shallow seismic and resistivity profiles were acquired to assist in imaging the near-subsurface sedimentary architecture for hydrogeological prospecting in the Nylsvley Nature Reserve along the Nyl River floodplain in South Africa. We deployed 48 channels of 14 Hz geophones with 1–2 m dense source-receiver spacing providing fold coverage of 24 (line 2) and 48 (line 1). The resistivity profiles were acquired with a Schlumberger electrode configuration with 2–3 m electrode spacing and a total of 40 steel electrodes. The raw seismic shot records were characterized by low-frequency, high-amplitude source-generated noise (surface and guided waves). Reflections from bedrock were not obvious. To enhance the reflected seismic signal, we employed an extensive seismic processing workflow which enhanced the seismic reflectivity on the stacked sections. The seismic reflection interpretation was constrained and integrated with seismic refraction and resistiv...

Discrete element simulation of the control technology of large section roadway along a fault to drivage under strong mining

Mon, 10/15/2018 - 7:00pm
The fault fracture zone has is characterized by low strength, easy deformation and easy activation under the influence of high-strength mining, resulting in instability of the roadway coal rock mass in the affected area and thus severe deformation of the surrounding rock. Aiming to prevent serious deformation of a large section roadway along a fault in response to drivage, combined with specific geological conditions and supporting engineering practice of the panel of a coal mine, UDEC polygonal methods are used to compare and analyze the fracture development, stress distribution and deformation of roadway surrounding rocks and the bearing force of the supporting body of the original support scheme and the new support scheme. In the numerical simulation scheme, the study area is divided into polygonal blocks and the mechanical parameters of the blocks and contacts in the Voronoi program are determined by fitting the rock mass properties obtained through laboratory tests. A time-...

An improved hybrid absorbing boundary condition for wave equation modeling

Sun, 09/16/2018 - 7:00pm
A recently developed hybrid absorbing boundary condition (ABC) can significantly suppress artificial boundary reflections by inserting a transition area between the boundary and the inner area. The wavefield in the transition area is computed by linearly weighting the wavefield from one-way wave equation (OWWE) and that from two-way wave equation. This leads to a smooth variation from the computational area to the boundary via the transition area and thus greatly reduces boundary reflections. In this paper, we propose two techniques to further enhance the absorbing effect. First, we widen the boundary from one point to several points. Second, we adopt nonlinear weighting coefficients instead of linear ones. Numerical simulations with 2D acoustic finite-difference modeling, combined with Clayton–Engquist OWWE, demonstrate that the proposed techniques can significantly improve the absorption effect without increasing computational cost. Tests using Higdon OWWE-based ABC suggest th...

Data assimilation of a legacy 4D seismic in a brown field

Sun, 09/16/2018 - 7:00pm
The quantitative use of four-dimensional (4D) seismic data in an assisted history matching process is still a major challenge in the industry. The problem becomes even more challenging when the quality of the seismic is deficient. Legacy 4D seismic data, i.e. 4D data from seismic acquisitions that were not originally parameterized for time-lapse processing, are typically regarded as not suitable for quantitative approaches. On the other hand, this kind of data are very common in the petroleum industry. Therefore, there is a practical interest in determining whether these data can bring useful quantitative information to improve reservoir characterization through assisted history matching. In this work, we investigate the use of low-quality 4D seismic data (legacy surveys with low repeatability) in an ensemble data assimilation process. We use data from a carbonate reservoir in Campos Basis with a long production history (38 years). The seismic data are the acoustic impedance fro...

A least squares method based on quaternions to derive absolute orientation of geophones with AHRS

Sun, 09/16/2018 - 7:00pm
In this paper, we developed a least squares method based on quaternions to get the relative rotation between geophone pairs, and combined with an attitude and heading reference system (AHRS) we can derive the orientation of all geophones in geographical coordinates. This method can be used in vertical seismic profile, or the free-fall ocean-bottom seismometer, or deep-tow acoustics and geophysics system. Compared with the traditional geophone reorientation method, this method can be applied in cases where seismic source location is unknown or a priori information is missing. An AHRS is installed in a geophone as the reference, and used a least squares method we can obtain the relative rotation between the reference and other geophones. Therefore, the orientation of all geophones in geographical coordinates can be derived with minimal additional hardware. In addition to apply the method in data processing, we developed a geophone reorientation scheme and estimated the unce...

Microseismic data SNR enhancement using local projection denoising method

Wed, 09/12/2018 - 7:00pm
It is an important research topic in the field of microseismic data to estimate clean seismic event signals from noisy single-channel records. This is also essential in other applications including the identification and positioning of events. Due to the low signal-to-noise ratio (SNR) of microseismic data, denoising processing is an indispensable procedure. In this paper, we present a novel denoising technique for SNR enhancement of microseismic data based on a local projection (LP) algorithm. Using the LP algorithm, a one-dimensional seismic signal can be processed to form a high-dimensional phase space in which signal and noise have different attractor orbits. Utilizing these different distribution characteristics, we can achieve denoising by reconstructing the phase space of the signal. The LP algorithm requires the determination of three key parameters: embedding dimension, delay time and neighborhood radius. We implement an estimation method for these three parameters for ...

The effect of water saturation on P-wave dispersion in a fractured porous medium with two immiscible fluids

Wed, 09/12/2018 - 7:00pm
Most fractured reservoirs, such as carbonate rocks, are both partially saturated and contain aligned fractures. In this paper, we analyze the effects of water saturation and permeability on the dispersion of P waves from model data and actual seismic data from area S of the Tarim Basin, China. Firstly, we construct a fractured porous model by adding a set of horizontal fractures evenly distributed in a periodic layered porous model alternately saturated with natural gas and underground water. Then, we analyze the frequency-dependent velocity and attenuation of the P waves for five wells in the target layer. The numerical results show that there are two significant attenuations in different frequency bands. In the seismic frequency band from 10 Hz to 100 Hz the attenuation increase with water saturation when the P wave is at normal incidence. Finally, we investigate the frequency-dependent amplitude versus offset of P waves from model data and actual seismic data. The results sho...

Computation of residual statics in complex geological areas using simulated annealing method based on minimization of objective function

Wed, 09/12/2018 - 7:00pm
Complex geological areas having large source and receiver residual statics issues due to high contamination of noise in the seismic data, can be resolved using the simulated annealing approach. Conventional techniques are not robust enough to resolve the residual statics correction problems, thus resulting in poor image quality of seismic reflection data. This paper presents the simulated annealing method, a nonlinear technique for objective function minimization based on the problems of random sampling. The pre-stack seismic data in complex geological conditions with intense noise contamination is used, where the other linear inversion methods are ineffective and failed to produce good quality stack image. The molecular dynamics simulation method using the simulated annealing approach to minimize the objective function has successfully resolved the issue of cycle skipping/lack of reflector continuity. It is an iterative process with varying sequence of parameters within each it...

Dynamic detection and analysis of overburden deformation and failure in a mining face using distributed optical fiber sensing

Wed, 09/05/2018 - 7:00pm
The deformation and failure of overburden has been a major and challenging issue in the prevention of geological disasters in mines. In this study, Brillouin optical time-domain reflectometry is introduced to detect overburden deformation and failure. To use this technique, several boreholes are drilled in the top of a coal seam in a laneway, and optical cables are arranged in the boreholes to form a two-dimensional measurement cross section. During different periods throughout the mining process, the strain characteristics of the optical cables in the boreholes can be collected and dynamically analyzed to study the mining-induced changes in the strain in the overburden. This analysis can provide effective technical parameters for safe production in mines. A distributed optical fiber sensing (DOFS) test was conducted in the working face of a coal mine in Inner Mongolia, and a total of 25 sets of effective test data were obtained. The dynamic development characteristics of the ov...

Impact of frequency-dependent anisotropy on azimuthal P-wave reflections

Wed, 09/05/2018 - 7:00pm
Seismic anisotropy has widely been used for the characterization of fractures in a reservoir. Recent work has demonstrated the effect of seismic dispersion on producing a frequency-dependent reflection coefficient, which can be important in fracture characterization since large fractures often lead to frequency-dependent anisotropy. In this paper, we examine the impact of anisotropic dispersion on P-wave reflections based on an HTI sand model overlaid by VTI shale. Although VTI in the overburden does not lead to azimuthal anisotropy, its effect on angle dependence could significantly affect the azimuthal AVO responses at far offsets. We show a modest effect on the amplitude and large effect on the phase, the latter of which could even be mistaken for azimuthal velocity variations. We present a Bayesian inversion based on a forward modelling technique aimed at recovering water saturation, fracture density and fracture length of a HTI sand. Our results show potential of using seis...

Diffraction separation based on the projected first Fresnel zone

Thu, 08/16/2018 - 7:00pm
The importance of seismic diffractions has been recognized in subsurface imaging. Diffractions might contain useful information about small-scale geological structures and heterogeneities, e.g. faults, pinch-outs, thin lenses, fractures etc and can be very helpful for prospecting and development of fractured reservoirs. One of the most challenging task in diffraction imaging is the separation of diffractions from reflections. In this work, we present a new diffraction filter based on the projected (first) Fresnel zone (PFZ) to enhance diffraction separation and imaging. We found that the difference in the width of the PFZ appears to be a very consistent parameter for diffraction separation and thus, can be used as a robust diffraction filter. We derived the width of the PFZ from wavefront attributes, which are determined using the common-reflection-surface method. Applications to a simple gradient model as well as a complex synthetic data set validate our approach.

Diffusion coefficient of CO 2 into light hydrocarbons and interfacial tension of carbonated water–hydrocarbon system

Thu, 08/16/2018 - 7:00pm
The interfacial tension (IFT) and diffusion coefficient of CO 2 in a hydrocarbon in a carbonated water (CW) environment were estimated for pressures between 10–100 bar and at temperatures of 25 °C, 35 °C, and 45 °C using axisymmetric pendant drop shape analysis (ADSA). We developed a dynamic numerical model accounting for the changes in properties at the CW–hydrocarbon interface owing to the swelling of the hydrocarbon drop (HD). This approach eliminates an error of about 36% arising from the assumption of a static boundary/volume in conventional methods. The results indicated that the IFT between CW and the hydrocarbon is a function of the CO 2 phase density and solubility. At low pressures (<60 bar, 25 °C), IFT is directly proportional to the pressure, while at pressures larger than 60 bar, it is inversely proportional to the pressure. The physical behaviour of the CW–hydrocarbon system is driven mainly by a balance between CO 2 solubility and ...

Hydraulic fracturing of hard top coal and roof for controlling gas during the initial mining stages in longwall top coal caving: a case study

Thu, 08/16/2018 - 7:00pm
Due to the poor cavability of top coal, the connection between the high-level gas drainage lane and the gob is not sufficient during the initial mining stage in a extra-thick coal seam with hard roof and hard coal in longwall top coal caving. The gases gathering in the gob area will suddenly release and cause extrusion when the upper roof first collapses and falls, which will result in gas overrunning and affecting the safety production. Based on the propagation law of hydraulic fractures and the cause of gas overrunning, a new technology was put forward to solve the gas overrunning during the initial mining stage in longwall top coal caving operations. A method for designing the drilling hole parameters for hydraulic fracturing was formed. Hydraulic fracturing was carried out in the measure lane to weaken the top coal and roof and induce the mining pressure to break coal; through this the cavability of the top coal and roof could be improved. Simultaneously, the connection betw...

Segmental adjustment of hydraulic support setting load in hard and thick coal wall weakening: a study of numerical simulation and field measurement

Wed, 08/08/2018 - 7:00pm
The purpose of this study was to introduce a novel technology named segmental adjustment of hydraulic support setting load (SAHSSL) for pre-splitting and weakening the coal wall of a fully mechanized working face in hard and thick coal seams (HTCSs). The efficiency of SAHSSL was explored in the relatively stable and significant movement stages. A numerical simulation method using fast Lagrangian analysis of continua in three dimensions was used to evaluate the instability characteristics of the coal wall under different support setting loads (SSLs) ranging from 2000 to 8000 kN. The numerical results were verified by industrial testing of coal wall weakening in HTCSs. The results revealed that with the implementation of SAHSSL, the average coal-cutting time of the shearer reduced by about 21.7% (or 1.8 h), and the increasing resistance state was 64.8% of the 115 working cycles in the working specialty measurement of the support. Reasonable SSLs in the relatively stable stage of t...

Downhole microseismic signal denoising via empirical wavelet transform and adaptive thresholding

Wed, 08/08/2018 - 7:00pm
Downhole microseismic data have the characteristics of low signal-to-noise ratio and high frequency, which pose a major challenge to noise attenuation. In this paper, we propose a novel downhole microseismic denoising approach based on the empirical wavelet transform combined with adaptive thresholding. According to the frequency characteristics of the signal, a spectrum segmentation strategy is designed. It can adaptively decompose the signal and noise into different modes. Through analyzing the spectrum and energy of the modes, different threshold functions are applied. The mode that contains more valid signals is processed by hard thresholding to reserve the amplitude; the other modes, which contain less useful signals, are processed by modified threshold functions to maintain the continuity of the restructured signal. We have determined this approach’s potential for microseismic denoising by comparing its performance on synthetic and field data using complete ensemble empiri...

Stochastic inversion of facies and reservoir properties based on multi-point geostatistics

Thu, 08/02/2018 - 7:00pm
A stochastic inversion method that can provide reservoir models of facies and reservoir properties is proposed for seismic reservoir characterization. It is an iterative optimization process achieved by the probability perturbation method on the basis of conditional simulation with multi-point geostatistics, and it also incorporates a quantum annealing algorithm to improve the accuracy of inverted reservoir properties. The usual practice of stochastic inversion based on multi-point geostatistics is time-consuming and only generates the inverted facies. The probability perturbation method can help to reduce the number of simulations and speed up the inversion process. An inverted model of facies is generated, and a volume of pseudo reservoir properties met with a fixed error is also produced in this stage. However, the continuity of this volume is poor because random sampling is used when transforming facies models into reservoir property volumes. Moreover, the characteristics of...