Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

Gas hydrate model and free gas model are established, and two-phase theory (TPT) for numerical simulation of elastic wave velocity is adopted to investigate the unconsolidated deep-water sedimentary strata in Shenhu area, South China Sea. The relationships between compression wave (P wave) velocity and gas hydrate saturation, free gas saturation, and sediment porosity at site SH2 are studied, respectively, and gas hydrate saturation of research area is estimated by gas hydrate model. In depth of 50 to 245 m below seafloor (mbsf), as sediment porosity decreases, P wave velocity increases gradually; as gas hydrate saturation increases, P wave velocity increases gradually; as free gas saturation increases, P wave velocity decreases. This rule is almost consistent with the previous research result. In depth of 195 to 220 mbsf, the actual measurement of P wave velocity increases significantly relative to the P wave velocity of saturated water modeling, and this layer is determined to be rich in gas hydrate. The average value of gas hydrate saturation estimated from the TPT model is 23.2%, and the maximum saturation is 31.5%, which is basically in accordance with simplified three-phase equation (STPE), effective medium theory (EMT), resistivity log (Rt), and chloride anomaly method.

Download Full-text

Multiscale fracture density analysis at Stromboli Volcano, Italy: implications to flank stability

10.5194/egusphere-egu21-8872 ◽

2021 ◽

Author(s):

Thomas Alcock ◽

Sergio Vinciguerra ◽

Phillip Benson ◽

Federico Vagnon

Keyword(s):

Electrical Resistivity ◽

Wave Velocity ◽

Rift Zone ◽

Pulse Generator ◽

Average Length ◽

Crack Density ◽

P Wave ◽

Fracture Density ◽

Stromboli Volcano ◽

P Wave Velocity

<p>Stromboli volcano has experienced four sector collapses over the past 13 thousand years, resulting in the formation of the Sciara del Fuoco (SDF) horseshoe-shaped depression and an inferred NE / SW striking rift zone across the SDF and the western sector of the island. These events have resulted in the formation of steep depressions on the slopes on the volcano where episodes of instability are continuously being observed and recorded. This study aims to quantify the fracture density inside and outside the rift zone to identify potential damaged zones that could reduce the edifice strength and promote fracturing. In order to do so we have carried out a multiscale analysis, by integrating satellite observations, field work and seismic and electrical resistivity analyses on cm scales blocks belonging to 11 lava units from the main volcanic cycles that have built the volcano edifice, ie. Paleostromboli, Nestromboli and Vancori. 0.5 m resolution Pleiades satellite data has been first used to highlight 23635 distinct linear features across the island. Fracture density has been calculated using Fracpaq based on the Mauldon et al (2001) method to determine the average fracture density of a given area on the basis of the average length of drawn segments within a predetermined circular area. 41.8 % of total fracture density is found around intrusions and fissures, with the summit area and the slopes of SDF having the highest average fracture density of 5.279&#160; . Density, porosity, P- wave velocity in dry and wet conditions and electrical resistivity (in wet conditions) were measured&#160; via an ultrasonic pulse generator and acquisition system (Pundit) and an on purpose built measuring quadrupole on cm scale blocks of lavas collected from both within and outside the proposed rift zone to assess the physical state and the crack damage of the different lava units.&#160; Preliminary results show that P-wave velocity between ~ 2.25 km/s < Vp < 5km/s decreases with porosity while there is high variability electrical resistivity with 21.7 < &#961; < 590 Ohm * m. This is presumably due to the lavas texture and the variable content of bubble/vesicles porosity and crack damage, that is reflected by an effective overall porosity between 0 and 9 %. Higher porosity is generally mirrored by lower p-wave velocity values. Neostromboli blocks show the most variability in both P-wave velocity and electrical resistivity. Further work will assess crack density throughout optical analyses and systematically investigate the UCS and elastic moduli. This integrated approach is expected to provide a multiscale fracture density and allow to develop further laboratory testing on how slip surfaces can evolve to a flank collapse at Stromboli.</p>

Download Full-text

Assessment of engineering behaviour of an intensely weathered swelling mudstone under full range of seasonal variation and the relationships among measured parameters

Canadian Geotechnical Journal ◽

10.1139/cgj-2017-0582 ◽

2018 ◽

Vol 55 (12) ◽

pp. 1837-1849 ◽

Cited By ~ 5

Author(s):

Zhixiong Zeng ◽

Lingwei Kong ◽

Min Wang ◽

Hossain Md. Sayem

Keyword(s):

Electrical Resistivity ◽

Shear Strength ◽

Wave Velocity ◽

Full Range ◽

Aggregate Size ◽

Physical And Mechanical Properties ◽

Inverse Correlation ◽

Strong Relationship ◽

P Wave ◽

P Wave Velocity

An experimental study was conducted to investigate the physical and mechanical properties of an intensely weathered mudstone from Northeast China after wetting–drying (W–D), freezing–thawing (F–T), and wetting–drying–freezing–thawing (W–D–F–T) cycles. These cyclic climatic processes have significant effects on the volume, microstructure, stress–strain behaviour, shear strength, electrical resistivity, and P-wave velocity of the samples. The variation in electrical resistivity exhibits an inverse correlation with the volume change, and a strong relationship can be observed between the electrical resistivity and porosity. The cohesion decreases with increasing number of cycles, while the internal friction angle slightly increases; these relationships can be caused by the presence of cracks and large voids and by the increase in the aggregate size and density during the drying and freezing processes, respectively. Moreover, the W–D–F–T cycles have a greater influence on the shear strength than do either the W–D or F–T cycles. This phenomenon is similar to that observed in the P-wave velocity, and the relationships between the shear strength parameters and P-wave velocity are also explored. This study provides nondestructive methods of predicting the deformation and shear strength of mudstones in seasonally frozen regions.

Download Full-text

Identifying the morphologies of gas hydrate distribution using P-wave velocity and density: a test from the GMGS2 expedition in the South China Sea

Journal of Geophysics and Engineering ◽

10.1088/1742-2140/aaaba1 ◽

2018 ◽

Vol 15 (3) ◽

pp. 1008-1022 ◽

Cited By ~ 4

Author(s):

Tao Liu ◽

Xuewei Liu

Keyword(s):

South China Sea ◽

Wave Velocity ◽

South China ◽

Gas Hydrate ◽

The South China Sea ◽

P Wave ◽

The South ◽

China Sea ◽

P Wave Velocity

Download Full-text

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.

Download Full-text