scholarly journals Rock Physics Model and Seismic Dispersion and Attenuation in Gas Hydrate-Bearing Sediments

2021 ◽  
Vol 9 ◽  
Author(s):  
Zhiqi Guo ◽  
Xueying Wang ◽  
Jian Jiao ◽  
Haifeng Chen
Keyword(s):  
Gas Hydrate ◽  
Rock Physics ◽  
P Wave ◽  
Pore Filling ◽  
P Wave Velocity ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Rock Physics Model ◽  
Hydrate Bearing Sediments ◽  
Dispersion And Attenuation

A rock physics model was established to calculate the P-wave velocity dispersion and attenuation caused by the squirt flow of fluids in gas hydrate-bearing sediments. The critical hydrate saturation parameter was introduced to describe different ways of hydrate concentration, including the mode of pore filling and the co-existence mode of pore filling and particle cementation. Rock physical modeling results indicate that the P-wave velocity is insensitive to the increase in gas hydrate saturation for the mode of pore filling, while it increases rapidly with increasing gas hydrate saturation for the co-existence mode of pore filling and particle cementation. Meanwhile, seismic modeling results show that both the PP and mode-converted PS reflections are insensitive to the gas hydrate saturation that is lower than the critical value, while they tend to change obviously for the hydrate saturation that is higher than the critical value. These can be interpreted that only when gas hydrate begins to be part of solid matrix at high gas hydrate saturation, it represents observable impact on elastic properties of the gas hydrate-bearing sediments. Synthetic seismograms are calculated for a 2D heterogeneous model where the gas hydrate saturation varies vertically and layer thickness of the gas hydrate-bearing sediment varies laterally. Modeling results show that larger thickness of the gas hydrate-bearing layer generally corresponds to stronger reflection amplitudes from the bottom simulating reflector.

Estimating the amount of gas hydrate and free gas from marine seismic data

Geophysics ◽  
2000 ◽  
Vol 65 (2) ◽  
pp. 565-573 ◽  
Author(s):  
Christine Ecker ◽  
Jack Dvorkin ◽  
Amos M. Nur
Keyword(s):  
Gas Hydrate ◽  
Seismic Data ◽  
Rock Physics ◽  
Free Gas ◽  
Interval Velocity ◽  
Blake Ridge ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Rock Physics Model ◽  
Marine Seismic

Marine seismic data and well‐log measurements at the Blake Ridge offshore South Carolina show that prominent seismic bottom‐simulating reflectors (BSRs) are caused by sediment layers with gas hydrate overlying sediments with free gas. We apply a theoretical rock‐physics model to 2-D Blake Ridge marine seismic data to determine gas‐hydrate and free‐gas saturation. High‐porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity; effective pressure; mineralogy; elastic properties of the pore‐filling material; and water, gas, and gas‐hydrate saturation of the pore space. To apply this model to seismic data, we first obtain interval velocity using stacking velocity analysis. Next, all input parameters to the rock‐physics model, except porosity and water, gas, and gas hydrate saturation, are estimated from geologic information. To estimate porosity and saturation from interval velocity, we first assume that the entire sediment does not contain gas hydrate or free gas. Then we use the rock‐physics model to calculate porosity directly from the interval velocity. Such porosity profiles appear to have anomalies where gas hydrate and free gas are present (as compared to typical profiles expected and obtained in sediment without gas hydrate or gas). Porosity is underestimated in the hydrate region and is overestimated in the free‐gas region. We calculate the porosity residuals by subtracting a typical porosity profile (without gas hydrate and gas) from that with anomalies. Next we use the rock‐physics model to eliminate these anomalies by introducing gas‐hydrate or gas saturation. As a result, we obtain the desired 2-D saturation map. The maximum gas‐hydrate saturation thus obtained is between 13% and 18% of the pore space (depending on the version of the model used). These saturation values are consistent with those measured in the Blake Ridge wells (away from the seismic line), which are about 12%. Free‐gas saturation varies between 1% and 2%. The saturation estimates are extremely sensitive to the input velocity values. Therefore, accurate velocity determination is crucial for correct reservoir characterization.

A rock-physics model to determine the pore microstructure of cracked porous rocks

2020 ◽  
Vol 223 (1) ◽  
pp. 622-631
Author(s):  
Lin Zhang ◽  
Jing Ba ◽  
José M Carcione
Keyword(s):  
Crack Density ◽  
Rock Physics ◽  
Differential Pressure ◽  
P Wave ◽  
Porous Rocks ◽  
S Wave ◽  
Wave Velocities ◽  
Pore Microstructure ◽  
Physics Model ◽  
Rock Physics Model

SUMMARY Determining rock microstructure remains challenging, since a proper rock-physics model is needed to establish the relation between pore microstructure and elastic and transport properties. We present a model to estimate pore microstructure based on porosity, ultrasonic velocities and permeability, assuming that the microstructure consists on randomly oriented stiff equant pores and penny-shaped cracks. The stiff pore and crack porosity varying with differential pressure is estimated from the measured total porosity on the basis of a dual porosity model. The aspect ratio of pores and cracks and the crack density as a function of differential pressure are obtained from dry-rock P- and S-wave velocities, by using a differential effective medium model. These results are used to invert the pore radius from the matrix permeability by using a circular pore model. Above a crack density of 0.13, the crack radius can be estimated from permeability, and below that threshold, the radius is estimated from P-wave velocities, taking into account the wave dispersion induced by local fluid flow between pores and cracks. The approach is applied to experimental data for dry and saturated Fontainebleau sandstone and Chelmsford Granite.

scholarly journals Seismic refraction tracks porosity generation and possible CO2production at depth under a headwater catchment

2020 ◽  
Vol 117 (32) ◽  
pp. 18991-18997 ◽  
Author(s):  
Xin Gu ◽  
Gary Mavko ◽  
Lisa Ma ◽  
David Oakley ◽  
Natalie Accardo ◽  
...  
Keyword(s):  
Water Table ◽  
Pyrite Oxidation ◽  
Seismic Refraction ◽  
Rock Physics ◽  
P Wave ◽  
Indirect Measurements ◽  
Deep Rock ◽  
Physics Model ◽  
Rock Physics Model ◽  
Bedrock Aquifers

In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water table under valley and ridge. We attribute this gas largely to CO2produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems.

A predictive anisotropic rock-physics model for estimating elastic rock properties of unconventional shale reservoirs

2019 ◽  
Vol 38 (5) ◽  
pp. 358-365 ◽  
Author(s):  
Colin M. Sayers ◽  
Sagnik Dasgupta
Keyword(s):  
Accurate Determination ◽  
Rock Physics ◽  
Seismic Inversion ◽  
P Wave ◽  
Rock Properties ◽  
Anisotropic Rock ◽  
Rock Classification ◽  
Shale Reservoirs ◽  
Physics Model ◽  
Rock Physics Model

This paper presents a predictive rock-physics model for unconventional shale reservoirs based on an extended Maxwell scheme. This model accounts for intrinsic anisotropy of rock matrix and heterogeneities and shape-induced anisotropy arising because the dimensions of kerogen inclusions and pores are larger parallel to the bedding plane than perpendicular to this plane. The model relates the results of seismic amplitude variation with offset inversion, such as P- and S-impedance, to the composition of the rock and enables identification of rock classes such as calcareous, argillaceous, siliceous, and mixed shales. This allows the choice of locations with the best potential for economic production of hydrocarbons. While this can be done using well data, prestack inversion of seismic P-wave data allows identification of the best locations before the wells are drilled. The results clearly show the ambiguity in rock classification obtained using poststack inversion of P-wave seismic data and demonstrate the need for prestack seismic inversion. The model provides estimates of formation anisotropy, as required for accurate determination of P- and S-impedance, and shows that anisotropy is a function not only of clay content but also other components of the rock as well as the aspect ratio of kerogen and pores. Estimates of minimum horizontal stress based on the model demonstrate the need to identify rock class and estimate anisotropy to determine the location of any stress barriers that may inhibit hydraulic fracture growth.

scholarly journals The Acoustic Properties of Sandy and Clayey Hydrate-Bearing Sediments

Energies ◽  
2019 ◽  
Vol 12 (10) ◽  
pp. 1825
Author(s):  
Xiao-Hui Wang ◽  
Qiang Xu ◽  
Ya-Nan He ◽  
Yun-Fei Wang ◽  
Yi-Fei Sun ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Gas Hydrate ◽  
Acoustic Properties ◽  
P Wave ◽  
S Wave ◽  
Hydrate Saturation ◽  
New Apparatus ◽  
Sandy Sediments ◽  
Hydrate Bearing Sediments ◽  
And Control

Natural gas hydrates samples are rare and difficult to store and transport at in situ pressure and temperature conditions, resulting in difficulty to characterize natural hydrate-bearing sediments and to identify hydrate accumulation position and saturation at the field scale. A new apparatus was designed to study the acoustic properties of seafloor recovered cores with and without hydrate. To protect the natural frames of recovered cores and control hydrate distribution, the addition of water into cores was performed by injecting water vapor. The results show that hydrate saturation and types of host sediments are the two most important factors that govern the elastic properties of hydrate-bearing sediments. When gas hydrate saturation adds approximately to 5–25%, the corresponding P-wave velocity (Vp) increases from 1.94 to 3.93 km/s and S-wave velocity (Vs) increases from 1.14 to 2.23 km/s for sandy specimens; Vp and Vs for clayey samples are 1.72–2.13 km/s and 1.10–1.32 km/s, respectively. The acoustic properties of sandy sediments can be significantly changed by the formation/dissociation of gas hydrate, while these only minorly change for clayey specimens.

Seismic reflections of gas hydrate from perturbational forward modeling

Geophysics ◽  
2006 ◽  
Vol 71 (6) ◽  
pp. F165-F171 ◽  
Author(s):  
Ingrid Cordon ◽  
Jack Dvorkin ◽  
Gary Mavko
Keyword(s):  
Reservoir Characterization ◽  
Rock Physics ◽  
Clay Content ◽  
The Arctic ◽  
Seismic Amplitude ◽  
Hydrate Reservoir ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Rock Physics Model ◽  
Seismic Reflections

We perturb the elastic properties and attenuation in the Arctic Mallik methane-hydrate reservoir to produce a set of plausible seismic signatures away from the existing well. These perturbations are driven by the changes we impose on porosity, clay content, hydrate saturation, and geometry. The key is a data-guided, theoretical, rock-physics model that we adopt to link velocity and attenuation to porosity, mineralogy, and amount of hydrate. We find that the seismic amplitude is very sensitive to the hydrate saturation in the host sand and its porosity as well as the porosity of the overburden shale. However, changes to the amount of clay in the sand only weakly alter the amplitude. Attenuation, which may be substantial, must be taken into account during hydrate reservoir characterization because it lowers the amplitude to an extent that may affect the hydrate-volume prediction. The spatial structure of the reservoir affects the seismic reflection: A thinly-layered reservoir produces a noticeably different amplitude than a massive reservoir with the same hydrate volume.

HOW GAS HYDRATE SATURATION AND MORPHOLOGY CONTROL SEISMIC ATTENUATION: A CASE STUDY FROM THE SOUTH HYDRATE RIDGE

2021 ◽  
pp. 1-63
Author(s):  
Aoshuang Ji ◽  
Tieyuan Zhu ◽  
Hector Marin-Moreno ◽  
Xiong Lei
Keyword(s):  
Gas Hydrate ◽  
Wave Attenuation ◽  
Rock Physics ◽  
Seismic Profile ◽  
Seismic Attenuation ◽  
P Wave ◽  
Frequency Range ◽  
The South ◽  
Hydrate Ridge ◽  
Hydrate Saturation

Prior studies have shown an ambiguous relationship between gas hydrate saturation and seismic attenuation in different regions, but the effect of gas hydrate morphology on seismic attenuation of hydrate-bearing sediments was often overlooked. Here we combine seismic data with rock physics modeling to elucidate how gas hydrate saturation and morphology may control seismic attenuation. To extract P-wave attenuation, we process both the vertical seismic profile (VSP) data within a frequency range of 30 – 150 Hz and sonic logging data within 10 – 15 kHz from three wells in the south Hydrate Ridge, offshore of Oregon (USA), collected during Ocean Drilling Program (ODP) Leg 204 in 2000. We calculate P-wave attenuation using spectral matching and centroid frequency shift methods, and use Archie's relationship to derive gas hydrate saturation from the resistivity data above the bottom simulating reflection (BSR) at the same wells. To interpret observed seismic attenuation in terms of the effects of both gas hydrate saturation and morphology, we employ the Hydrate-Bearing Effective Sediment (HBES) rock physics model. By comparing the observed and model-predicted attenuation values, we infer that: (1) seismic attenuation appears to not be dominated by any single factor, instead, its variation is likely governed by both gas hydrate saturation and morphology; (2) the relationship between seismic attenuation and gas hydrate saturation varies with different hydrate morphologies; (3) the squirt flow, occurring at different compliances of adjacent pores driven by pressure gradients, may be responsible for the significantly large or small attenuation over a broad frequency range.

Hydrate quantification: Integrating full-waveform inversion, seismic attributes, and rock physics

2016 ◽  
Vol 4 (1) ◽  
pp. SA55-SA71 ◽  
Author(s):  
P. Jaiswal
Keyword(s):  
Velocity Field ◽  
Waveform Inversion ◽  
Rock Physics ◽  
Well Logs ◽  
Full Waveform Inversion ◽  
Secondary Porosity ◽  
Full Waveform ◽  
Hydrate Saturation ◽  
Physics Model ◽  
Rock Physics Model

Hydrate quantification from seismic data is a two-pronged challenge. The first is creating a velocity field with high enough resolution and accuracy such that it is a meaningful representation of hydrate variability in the host sediments. The second is constructing a rock-physics model that accounts for the appropriate growth of the hydrate and allows for the interpretation of the velocity field in terms of hydrate saturation. In this paper, both challenges are addressed in a quantification workflow that uses 2D seismic and colocated well logs. The study area is situated in the Krishna-Godavari Basin, offshore eastern Indian coast, where hydrate was discovered in the National Gas Hydrate Program Expedition 01 (NGHP-01). The workflow hinges on a rock-physics model that expresses total hydrate saturation in terms of primary (matrix) and secondary (fractures, faults, voids, etc.) porosities and their respective primary and secondary saturations and incorporates hydrate-filled secondary porosity into the rock as an additional grain type using the Hashin-Shtrikman bounds. The model is first applied to a set of well logs at a colocated site, NGHP-01-10, following which the application is extended into the seismic domain by (1) the incoherency attribute as a proxy for secondary porosity and (2) a full-waveform inversion-based P-wave velocity ([Formula: see text]) model as a proxy for primary saturation. The remaining — the primary porosity and secondary saturation — are assumed to remain the same across the seismic profile as at the site NGHP-01-10. The resulting, seismically estimated, hydrate saturation compares well with saturations from core depressurization at colocated sites NGHP-01-10 and NGHP-01-13. The quantification workflow presented here is potentially adaptable to other geographical areas with the caveat that empirical relations between porosity, saturation, and seismic attributes may have to be locally established.

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