This explains why this method is called the similar constructive method. With this method, solutions for the model of the pressure response of a CBM reservoir can be obtained. A corresponding program for the method was written, which can help engineers working on CBM reservoirs solve similar problems even without pre-training. In addition to the typical analysis curves for well testing such as the conventional log—log graph of the pressure and pressure differential and semi-log graph, another type of analysis curve, i.
On such basis, the diagnosis of the pressure response of a CBM well is more accurate. Our results might be of great significance to the theoretical study of pressure responses of CBM reservoirs. The results of this research could provide great simplicity for software developers to create well testing analysis software and well testing interpreters to interpret CBM well test data.
Digital rock physics DRP builds a bridge between pore-scale physical processes and the macroscopic physical properties of rock. Its key paradigm is to image and digitize the pore space and mineral matrix of rock, then to simulate the response of various physical fields and calculate the equivalent elastic parameters of rocks through digital rocks and mathematical methods. In this paper, a new approach is proposed to estimate the rock moduli of two-phase media with the staggered-grid high-order finite difference method FDM of the Biot theory based on DRP.
This new method not only takes into consideration the impact of fluid on elastic wave propagation in two-phase media, but it is also easy to understand and implement, improving the calculation accuracy, requiring less memory and improving on the weaknesses of conventional rock physics experiments which are time consuming and expensive. In order to estimate the rock moduli, we establish two models, and the digital rock sample is embedded in one of those. Using this method, it is possible to model the dynamic wave propagation and measure the time delay of the peak amplitude caused by the inhomogeneous structure of the digital rock sample, with the receivers set at the bottom of the two models.
The time delay allows us to estimate the effective velocity of both compressional and shear waves, and therefore calculate the rock moduli.
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Additionally, comparison between the numerical simulated results obtained through this method and experimental results indicates that they agree well. Comparison with the numerical simulated results obtained via another method tests and verifies the accuracy and feasibility of the new method. Also, the equivalence conditions between this new method and the various rock physics models are inferred.
Cracked media are a common geophysical phenomena. It is important to study the propagation characteristics in boreholes for sonic logging theory, as this can provide the basis for the sonic log interpretation.
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This paper derives velocity—stress staggered finite difference equations of elastic wave propagation in cylindrical coordinates for cracked media. The sound field in the borehole is numerically simulated using the finite-difference technique with second order in time and tenth order in space. It gives the relationship curves between the P-wave, S-wave velocity, anisotropy factor and crack density, and aspect ratio. Furthermore, it gives snapshots of the borehole acoustic wave field in cracked media with different crack densities and aspect ratios.
The calculated results show that in dry conditions the P-wave velocity in both the axial and radial directions decreases, and more rapidly in the axial direction while the crack density increases. The attenuation of the wave energy increases with the increase in crack density. In fluid-saturated cracked media, both the P-wave and S-wave velocity increases with the aspect ratio of the cracks.
The anisotropy of the P-wave decreases with the aspect ratio of the cracks. The aspect ratio of the crack does not obviously affect the energy attenuation. The coalbed gas reservoirs in the Qinshui Basin in central China are highly heterogeneous; thus, the reservoir characteristics are difficult to assess. Research on the pore structure of a reservoir can provide a basis for understanding the occurrence and seepage mechanisms of coal reservoirs, rock physics modeling and the formulation of rational development plans.
Therefore, the pore structure characteristics of the coalbed gas reservoirs in the high rank bituminous coal in the No.
The results showed that the effective porosity system of the coal reservoir was mainly composed of pores and microfractures and that the pore throat configuration of the coal reservoir was composed of pores and microthroats. A model was developed based on the porosity and microfractures of the high rank coal rock and the mercury injection and drainage curves. The mercury injection curve model and the coal permeability are well correlated and were more reliable for the analysis of coal and rock pore system connectivity than the mercury drainage curve model.
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Coal rocks with developed microfractures are highly permeable; the production levels are often high during the initial drainage stages, but they decrease rapidly. A significant portion of the natural gas remains in the strata and cannot be exploited; therefore, the ultimate recovery is rather low. Coal samples with underdeveloped microfractures have lower permeabilities.
While the initial production levels are lower, the production cycle is longer, and the ultimate recovery is higher. Therefore, the initial production levels of coal reservoirs with poorly developed microfractures in some regions of China may be low. However, over the long term, due to their higher ultimate recoveries and longer production cycles, the total gas production levels will increase. This understanding can provide an important reference for developing appropriate CBM development plans.
Spectral decomposition based on sparse constrained inversion, as proposed in recent years, is a high-resolution time-frequency domain analysis method. However, the traditional inverse spectral decomposition ISD algorithm is based on an inverse wavelet transform IWT whose mother wavelet is a Ricker wavelet function instead of an adaptive wavelet, which causes poor continuity and instability of the ISD results. However, it is frequency-decomposition based, which means that inversion results can be greatly affected by the effect of time-frequency analysis Wu et al SEG Technical Program Expanded Abstracts pp —9; Wu et al Geophys.
Results from application in Southwest China demonstrate that the high frequency-resolution of ISD is helpful in the extraction of gas-induced frequency anomalies from post-stack data, and its high time-resolution helps to obtain high time-resolution gas-induced dispersion results from FAVO inversion. The results of FAVO-ISD not only agree better with well information, but also supply more credible boundary information for the reservoirs. Non-penetrating surface flaws play a key role in the fracture process of rock-like material, and could cause localized collapse and even failure of the materials.
Until now, the mechanism and the effect of surface crack propagation have remained unclear. In this paper, compression tests on gypsum a soft rock material are conducted to investigate crack propagation and coalescence due to non-penetrating surface flaws and their effect on the material strength. Specimens are tested under dual pre-existing surface flaws with various combinations of depth and spacing. When the pre-existing flaw penetrates completely through the specimen, the spacing has a small effect on the specimen strength.
A larger flaw depth ratio could advance the occurrence of the peak load PL and result in a smaller specimen residual strength. The failure process of the specimen is divided into several stages featured by a stepped decline of the load value after PL, which is closely related to the initiation and propagation of secondary cracks. In addition, the spalling failure of a portion of the surface caused by coalescence of cracks can be regarded as indicating the failure of the specimen, and two possible types of spalling formation are briefly discussed.
Fractal dimension can effectively describe the microstructure of self-similar fractal porous media, and it is of great significance to predict their macroscopic mechanical properties by different fractal dimensions. Based on fractal theory and the Mori—Tanaka method, homogenization equations of fractal porous media were deduced by tensors in this paper, and the parameters affecting the prediction results and the macro mechanical properties were discussed.
When the pore fractal dimension is close to 1. The analytical solution in this paper can be utilized to predict the macroscopic mechanical properties of fractal porous media and to provide theoretical support for parameters selection of numerical simulation and scale upgrading. The present article outlines an innovative derivation of the well-known CRS traveltime formula. This is made possible by approximating the isochrones tangent to a small reflecting element embedded in a 3D homogeneous auxiliary medium.
The arising paraxial traveltime formula is then parametrised by six attributes, each one characterising geometrically the wavefront of the reference normal-incident ray at the emergence point of the datum plane. For a layered medium, the assignment of the six attributes for each azimuthal direction around describes locally the emerging wavefront and establishes the mapping between reflecting elements of the two media, the auxiliary and the real, which respond, however, with different traveltimes. This 3D homeomorphism provides the time correction that gives rise to the CRS traveltime formula with eight attributes.
For time imaging applications, the traveltime profile must be numerically shaped by improving iteratively the value of the eight attributes, so as to intercept, without the need of a velocity model, the largest number of coherent data in the volume of seismic traces gathered in the midpoint-offset domain. In order to study the response characteristics of coal permeability to pore pressure, seepage experiments under different simulated in situ stresses on loading and unloading paths are carried out using the self-developed Gas Flow and Displacement Testing Apparatus GFDTA system.
Based on the analysis of the experimental data, the relationship between average pore pressure and permeability is found to basically obey the function distribution of a two degree polynomial. In this paper, two aspects of the relationship between permeability and pore pressure are explained: the Klinbenberg effect and expansion, and the penetration of the initial fracture.
Under low pore pressure, the decrease in the Klinbenberg effect is the main reason for the decrease in permeability with increased pore pressure. Under relatively high pore pressure, the increase in pore pressure leads to the initial fracture expansion and penetration of the coal sample, which causes an increase in permeability. In order to evaluate the sensitivity of the permeability response to pore pressure changes, the permeability dispersion and pore pressure sensitivity coefficients are defined. After the sensitivity analysis, it was concluded that the loading history changed the fracture structure of the original coal sample and reduced its permeability sensitivity to pore pressure.
Under low pore pressure, the Klinbenberg effect is the reason for the decrease in pore pressure sensitivity. Lastly, the permeability—pore pressure relationship is divided into three stages to describe the different response characteristics individually. In this work we test four classification methods for litho-fluid facies identification in a clastic reservoir located in the offshore Nile Delta. The ultimate goal of this study is to find an optimal classification method for the area under examination.
The geologic context of the investigated area allows us to consider three different facies in the classification: shales, brine sands and gas sands. The depth at which the reservoir zone is located — m produces a significant overlap of the P- and S-wave impedances of brine sands and gas sands that makes discrimination between these two litho-fluid classes particularly problematic. The classification is performed on the feature space defined by the elastic properties that are derived from recorded reflection seismic data by means of amplitude versus angle Bayesian inversion. As classification methods we test both deterministic and probabilistic approaches: the quadratic discriminant analysis and the neural network methods belong to the first group, whereas the standard Bayesian approach and the Bayesian approach that includes a 1D Markov chain a priori model to constrain the vertical continuity of litho-fluid facies belong to the second group.
The ability of each method to discriminate the different facies is evaluated both on synthetic seismic data computed on the basis of available borehole information and on field seismic data.
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The outcomes of each classification method are compared with the known facies profile derived from well log data and the goodness of the results is quantitatively evaluated using the so-called confusion matrix. The results show that all methods return vertical facies profiles in which the main reservoir zone is correctly identified. However, the consideration of as much prior information as possible in the classification process is the winning choice for deriving a reliable and physically plausible predicted facies profile.
Abandoned roadways and roof caving zones are commonly found in residual coal, and can destroy the integrity of the coal seam and roof. Resulting from mining-induced stress, continuous collapse and fracture instability in roof caving zones RCZs jeopardize the safety and efficiency of residual coal mining. Based on the engineering geology conditions of remining face in Shenghua Mine, the roof fracture and instability features of the RCZ were analyzed through physical simulation, theoretical analysis, and field measurements.
In this case, influenced by the RCZ, the main roof across the RCZ fractured and rotated towards the goaf, greatly increasing the working resistance, and crushing the supports.
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The sudden instability of the coal pillars weakened its support of the main roof, thus resulting in long-key blocks across the RCZ and hinged roof structures, which significantly decreased the stability of the underlying immediate roof. This study establishes a mechanical model for the interactions between the surrounding rock and the supports in the RCZ, determines the reasonable working resistance, and examines the use of pre-grouting solidification restoration technology PSRT to solidify the RCZ and reinforce the coal pillars—thus increasing their bearing capacity.
Field measurements revealed no roof flaking, inhomogeneous loading or support crushing, indicating that the PSRT effectively controlled the surrounding rock of the RCZ. Compressed air energy storage CAES is a technology that uses compressed air to store surplus electricity generated from low power consumption time for use at peak times. This paper presents a thermo-mechanical modeling for the thermodynamic and mechanical responses of a lined rock cavern used for CAES.
The simulation was accomplished in COMSOL Multiphysics and comparisons of the numerical simulation and some analytical solutions validated the thermo-mechanical modeling. Significant temperature fluctuation occurred only in the concrete lining and sealing layer, and no strong fluctuation was observed in the host rock.
In the case of steel sealing, principal stresses in the sealing layer were larger than those in the concrete and host rock. The maximum compressive stresses of the three layers and the displacement on the cavern surface increased with the increase of cycle number. However, the maximum tensile stresses exhibited the opposite trend. Polymer sealing achieved a relatively larger air temperature and pressure compared with steel and air-tight concrete sealing. For concrete layer thicknesses of 0 and 0. It primarily is dipolar i.
Away from the surface the dipole becomes distorted. Ferromagnetism and rotation theories generally are discredited—ferromagnetism because the Curie point the temperature at which ferromagnetism is destroyed is reached only 20 or so kilometres about 12 miles beneath the surface, and rotation theories because apparently no fundamental relation exists between mass in motion and an associated magnetic field.
Most geomagneticians concern themselves with various dynamo theories , whereby a source of energy in the core of the Earth causes a self-sustaining magnetic field. From the core outward, these include the geomagnetic dynamo , crustal magnetization, the ionospheric dynamo, the ring current, the magnetopause current, the tail current, field-aligned currents, and auroral, or convective, electrojets. The geomagnetic dynamo is the most important source because, without the field it creates, the other sources would not exist. In the discussion that follows, each of these sources is considered and the respective causes explained.
Each of the major sources of the so-called steady field undergoes changes that produce transient variations, or disturbances. The main field has two major disturbances: quasiperiodic reversals and secular variation. The ionospheric dynamo is perturbed by seasonal and solar cycle changes as well as by solar and lunar tidal effects. The ring current responds to the solar wind the ionized atmosphere of the Sun that expands outward into space and carries with it the solar magnetic field , growing in strength when appropriate solar wind conditions exist.
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Associated with the growth of the ring current is a second phenomenon, the magnetospheric substorm, which is most clearly seen in the aurora borealis. An entirely different type of magnetic variation is caused by magnetohydrodynamic MHD waves. These waves are sinusoidal variations in the electric and magnetic fields that are coupled to changes in particle density. Each of these sources of variation is also discussed separately below.
Electric and magnetic fields are produced by a fundamental property of matter, electric charge. Electric fields are created by charges at rest relative to an observer, whereas magnetic fields are produced by moving charges. The two fields are different aspects of the electromagnetic field , which is the force that causes electric charges to interact.
The electric field , E, at any point around a distribution of charge is defined as the force per unit charge when a positive test charge is placed at that point. For point charges the electric field points radially away from a positive charge and toward a negative charge. A magnetic field is generated by moving charges—i. The magnetic induction , B, can be defined in a manner similar to E as proportional to the force per unit pole strength when a test magnetic pole is brought close to a source of magnetization. It is more common, however, to define it by the Lorentz-force equation. In this equation bold characters indicate vectors quantities that have both magnitude and direction and nonbold characters denote scalar quantities such as B , the length of the vector B.
The x indicates a cross product i. Theta is the angle between the vectors v and B. B is usually called the magnetic field in spite of the fact that this name is reserved for the quantity H, which is also used in studies of magnetic fields. For a simple line current the field is cylindrical around the current.