Azimuthal anisotropy analysis of wide-azimuth P-wave seismic data for fracture orientation and density characterization in a tight gas reservoir

The role of anisotropy in fracture detection has dramatically increased with the advent of wide-azimuth (WAZ) and high-density seismic acquisition. Fracture prediction using horizontal transverse isotropy (HTI) anisotropic theory is a useful tool for identifying reservoir characteristics. We have developed an approach for fracture density and orientation estimation based on the combination of a velocity variation with azimuth (VVAZ) and an amplitude variation with azimuth (AVAZ) analysis workflow. First, we sort the prestack WAZ data into offset vector tile (OVT) sectors and perform VVAZ inversion by elliptical velocity fitting of measured azimuth-differential traveltimes. In this step, we can predict the fast P-wave velocity, slow P-wave velocity, and fracture orientation. Second, we apply AVAZ inversion to extract more accurate predictions of the anisotropic gradient and fracture orientation. We implement the method with 3D prestack WAZ seismic data acquired in the Sichuan Basin, from the southwest part of China. The field data example indicates that the inversion results agree with geologic information and well-log imaging data, which confirm the effectiveness of this technology.

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Using Seismic Data to Predict Shale Pore Pressure and Overpressure Sweet Spots: A Case Study from the Lower Silurian Longmaxi Formation in Sichuan Basin, China

10.2118/208098-ms ◽

2021 ◽

Author(s):

Sheng Chen ◽

Qingcai Zeng ◽

Xiujiao Wang ◽

Qing Yang ◽

Chunmeng Dai ◽

...

Keyword(s):

Prediction Model ◽

Wave Velocity ◽

Shale Gas ◽

Seismic Data ◽

P Wave ◽

Formation Pressure ◽

S Wave ◽

P Wave Velocity ◽

New Formation ◽

Sweet Spots

Abstract Practices of marine shale gas exploration and development in south China have proved that formation overpressure is the main controlling factor of shale gas enrichment and an indicator of good preservation condition. Accurate prediction of formation pressure before drilling is necessary for drilling safety and important for sweet spots predicting and horizontal wells deploying. However, the existing prediction methods of formation pore pressures all have defects, the prediction accuracy unsatisfactory for shale gas development. By means of rock mechanics analysis and related formulas, we derived a formula for calculating formation pore pressures. Through regional rock physical analysis, we determined and optimized the relevant parameters in the formula, and established a new formation pressure prediction model considering P-wave velocity, S-wave velocity and density. Based on regional exploration wells and 3D seismic data, we carried out pre-stack seismic inversion to obtain high-precision P-wave velocity, S-wave velocity and density data volumes. We utilized the new formation pressure prediction model to predict the pressure and the spatial distribution of overpressure sweet spots. Then, we applied the measured pressure data of three new wells to verify the predicted formation pressure by seismic data. The result shows that the new method has a higher accuracy. This method is qualified for safe drilling and prediction of overpressure sweet spots for shale gas development, so it is worthy of promotion.

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Evaluating Rock Brittleness from Radial P-wave Velocity Variation in the LWD Condition

77th EAGE Conference and Exhibition 2015 ◽

10.3997/2214-4609.201412854 ◽

2015 ◽

Author(s):

Y.D. Su ◽

X.M. Tang ◽

S. Xu ◽

C.X. Zhuang

Keyword(s):

Wave Velocity ◽

P Wave ◽

Velocity Variation ◽

P Wave Velocity ◽

Rock Brittleness

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Azimuthal anisotropy of the P-wave velocity in the hypocentral volume of the Krn Mt. (Slovenia) earthquake sequence

Journal of Applied Geophysics ◽

10.1016/j.jappgeo.2003.08.007 ◽

2003 ◽

Vol 54 (3-4) ◽

pp. 257-264 ◽

Cited By ~ 1

Author(s):

Marijan Herak ◽

Mladen Živčić ◽

Davorka Herak

Keyword(s):

Wave Velocity ◽

Azimuthal Anisotropy ◽

P Wave ◽

Earthquake Sequence ◽

P Wave Velocity

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Elastic reflectivity vectors and the geometry of intercept-gradient crossplots

The Leading Edge ◽

10.1190/tle38100762.1 ◽

2019 ◽

Vol 38 (10) ◽

pp. 762-769

Author(s):

Patrick Connolly

Keyword(s):

Elastic Property ◽

Elastic Properties ◽

Wave Velocity ◽

Seismic Data ◽

Measurement Errors ◽

Rock Physics ◽

P Wave ◽

S Wave ◽

P Wave Velocity ◽

Well Log Analysis

Reflectivities of elastic properties can be expressed as a sum of the reflectivities of P-wave velocity, S-wave velocity, and density, as can the amplitude-variation-with-offset (AVO) parameters, intercept, gradient, and curvature. This common format allows elastic property reflectivities to be expressed as a sum of AVO parameters. Most AVO studies are conducted using a two-term approximation, so it is helpful to reduce the three-term expressions for elastic reflectivities to two by assuming a relationship between P-wave velocity and density. Reduced to two AVO components, elastic property reflectivities can be represented as vectors on intercept-gradient crossplots. Normalizing the lengths of the vectors allows them to serve as basis vectors such that the position of any point in intercept-gradient space can be inferred directly from changes in elastic properties. This provides a direct link between properties commonly used in rock physics and attributes that can be measured from seismic data. The theory is best exploited by constructing new seismic data sets from combinations of intercept and gradient data at various projection angles. Elastic property reflectivity theory can be transferred to the impedance domain to aid in the analysis of well data to help inform the choice of projection angles. Because of the effects of gradient measurement errors, seismic projection angles are unlikely to be the same as theoretical angles or angles derived from well-log analysis, so seismic data will need to be scanned through a range of angles to find the optimum.

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Semiglobal viscoacoustic full-waveform inversion

Geophysics ◽

10.1190/geo2017-0773.1 ◽

2019 ◽

Vol 84 (2) ◽

pp. R271-R293 ◽

Cited By ~ 9

Author(s):

Nuno V. da Silva ◽

Gang Yao ◽

Michael Warner

Keyword(s):

Physical Properties ◽

Wave Velocity ◽

Seismic Data ◽

Waveform Inversion ◽

P Wave ◽

Full Waveform Inversion ◽

Data Set ◽

Intrinsic Attenuation ◽

Full Waveform ◽

P Wave Velocity

Full-waveform inversion deals with estimating physical properties of the earth’s subsurface by matching simulated to recorded seismic data. Intrinsic attenuation in the medium leads to the dispersion of propagating waves and the absorption of energy — media with this type of rheology are not perfectly elastic. Accounting for that effect is necessary to simulate wave propagation in realistic geologic media, leading to the need to estimate intrinsic attenuation from the seismic data. That increases the complexity of the constitutive laws leading to additional issues related to the ill-posed nature of the inverse problem. In particular, the joint estimation of several physical properties increases the null space of the parameter space, leading to a larger domain of ambiguity and increasing the number of different models that can equally well explain the data. We have evaluated a method for the joint inversion of velocity and intrinsic attenuation using semiglobal inversion; this combines quantum particle-swarm optimization for the estimation of the intrinsic attenuation with nested gradient-descent iterations for the estimation of the P-wave velocity. This approach takes advantage of the fact that some physical properties, and in particular the intrinsic attenuation, can be represented using a reduced basis, substantially decreasing the dimension of the search space. We determine the feasibility of the method and its robustness to ambiguity with 2D synthetic examples. The 3D inversion of a field data set for a geologic medium with transversely isotropic anisotropy in velocity indicates the feasibility of the method for inverting large-scale real seismic data and improving the data fitting. The principal benefits of the semiglobal multiparameter inversion are the recovery of the intrinsic attenuation from the data and the recovery of the true undispersed infinite-frequency P-wave velocity, while mitigating ambiguity between the estimated parameters.

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Iterative P-wave velocity inversion using Markov chain Monte Carlo method

10.5194/egusphere-egu2020-6298 ◽

2020 ◽

Author(s):

Hyunggu Jun ◽

Hyeong-Tae Jou ◽

Han-Joon Kim ◽

Sang Hoon Lee

Keyword(s):

Wave Velocity ◽

Seismic Data ◽

Velocity Structure ◽

Low Frequency ◽

P Wave ◽

Velocity Analysis ◽

Seismic Section ◽

Traveltime Tomography ◽

P Wave Velocity ◽

Frequency Components

<p>Imaging the subsurface structure through seismic data needs various information and one of the most important information is the subsurface P-wave velocity. The P-wave velocity structure mainly influences on the location of the reflectors during the subsurface imaging, thus many algorithms has been developed to invert the accurate P-wave velocity such as conventional velocity analysis, traveltime tomography, migration velocity analysis (MVA) and full waveform inversion (FWI). Among those methods, conventional velocity analysis and MVA can be widely applied to the seismic data but generate the velocity with low resolution. On the other hands, the traveltime tomography and FWI can invert relatively accurate velocity structure, but they essentially need long offset seismic data containing sufficiently low frequency components. Recently, the stochastic method such as Markov chain Monte Carlo (McMC) inversion was applied to invert the accurate P-wave velocity with the seismic data without long offset or low frequency components. This method uses global optimization instead of local optimization and poststack seismic data instead of prestack seismic data. Therefore, it can avoid the problem of the local minima and limitation of the offset. However, the accuracy of the poststack seismic section directly affects the McMC inversion result. In this study, we tried to overcome the dependency of the McMC inversion on the poststack seismic section and iterative workflow was applied to the McMC inversion to invert the accurate P-wave velocity from the simple background velocity and inaccurate poststack seismic section. The numerical test showed that the suggested method could successfully invert the subsurface P-wave velocity.</p>

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P‐wave velocity and permeability distribution of sandstones from a fractured tight gas reservoir

Geophysics ◽

10.1190/1.1451673 ◽

2002 ◽

Vol 67 (1) ◽

pp. 241-253 ◽

Cited By ~ 16

Author(s):

Helmut Dürrast ◽

P. N. J. Rasolofosaon ◽

Siegfried Siegesmund

Keyword(s):

Velocity Distribution ◽

Wave Velocity ◽

Confining Pressure ◽

P Wave ◽

Tight Gas ◽

Rock Fabric ◽

The Core ◽

Wave Velocities ◽

P Wave Velocity ◽

Tight Gas Reservoir

Fractures are an important fabric element in many tight gas reservoirs because they provide the necessary channels for fluid flow in rocks which usually have low matrix permeabilities. Several sandstone samples of such a reservoir type were chosen for a combined study of rock fabric elements and petrophysical properties. Geological investigations of the distribution and orientation of the fractures and sedimentary layering were performed. In addition, laboratory measurements were carried out to determine the directional dependence of the permeability and P‐wave velocities. Higher permeability values are generally in the plane of the nearly horizontal sedimentary layering with regard to the core axis. With the occurrence of subvertical fractures, however, the highest permeabilities were determined to be parallel to the core axis. Compressional wave velocities were measured on spherical samples in more than 100 directions to get the VP symmetry without prior assumptions. Below 50 MPa confining pressure, all samples show a monoclinic symmetry of the P wave velocity distribution, caused by sedimentary layering, fractures, and crossbedding. At higher confining pressure, sedimentary layering is approximately the only effective fabric element, resulting in a more transverse isotropic VP symmetry. Using the geological‐petrophysical model introduced here, the complex symmetry of the VP distributions can only be explained by the rock fabric elements. Furthermore, water saturation increases the velocities and decreases the anisotropy but does not change VP symmetry. This indicates that at this state, all fabric elements, including the fractures, have an influence on P‐wave velocity distribution.

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Identification of gas hydrate dissociation from wireline-log data in the Shenhu area, South China Sea

Geophysics ◽

10.1190/geo2011-0324.1 ◽

2012 ◽

Vol 77 (3) ◽

pp. B125-B134 ◽

Cited By ~ 16

Author(s):

Xiujuan Wang ◽

Myung Lee ◽

Shiguo Wu ◽

Shengxiong Yang

Keyword(s):

Wave Velocity ◽

Gas Hydrate ◽

Seismic Data ◽

Pore Space ◽

P Wave ◽

Well Log ◽

Hydrate Dissociation ◽

Free Gas ◽

P Wave Velocity

Wireline logs were acquired in eight wells during China’s first gas hydrate drilling expedition (GMGS-1) in April–June of 2007. Well logs obtained from site SH3 indicated gas hydrate was present in the depth range of 195–206 m below seafloor with a maximum pore-space gas hydrate saturation, calculated from pore water freshening, of about 26%. Assuming gas hydrate is uniformly distributed in the sediments, resistivity calculations using Archie’s equation yielded hydrate-saturation trends similar to those from chloride concentrations. However, the measured compressional (P-wave) velocities decreased sharply at the depth between 194 and 199 mbsf, dropping as low as [Formula: see text], indicating the presence of free gas in the pore space, possibly caused by the dissociation of gas hydrate during drilling. Because surface seismic data acquired prior to drilling were not influenced by the in situ gas hydrate dissociation, surface seismic data could be used to identify the cause of the low P-wave velocity observed in the well log. To determine whether the low well-log P-wave velocity was caused by in situ free gas or by gas hydrate dissociation, synthetic seismograms were generated using the measured well-log P-wave velocity along with velocities calculated assuming both gas hydrate and free gas in the pore space. Comparing the surface seismic data with various synthetic seismograms suggested that low P-wave velocities were likely caused by the dissociation of in situ gas hydrate during drilling.

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