Presence Of Free Gas Research Articles

Identification of shallow gas by seismic data and AVO processing: Example from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea

This study investigates the evidence of shallow gas from a newly collected dataset comprising 2D multi-channel seismic (MCS), single-channel seismic (SCS), Chirp sub-bottom profiler (SBP), and multi-beam echo sounder (MBES) data from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea. Various indicators of shallow gas were identified in this part the shelf, including seismic chimneys, acoustic flares in the water column, pockmarks, enhanced reflections, and acoustic blanking. Seismic chimneys, which are related to fluid leakage within the subsurface, are characterized weak to high amplitude, upturned or concave-upward internal reflection with polarity reversal in MCS section, whereas they are faint or transparent on SCS section with 700 Hz and Chirp SBP sections with 3.5 kHz frequency. Acoustic flares, which are diagnostics of active gas venting, were detected only on the Chirp SBP sections. There is also evidence of small-scale depressions, immediately below the acoustic flares; these are interpreted as formed by the sudden explosion of gas. Enhanced reflections (~3 km-long) concordant with stratification are observed at different levels. Acoustic blanking anomalies, caused by absorption of acoustic energy due to the presence of gas, are seen in the range 50–200 ms TWT below seabed. Amplitude versus offset (AVO) analysis indicates the presence of free gas ca. 350 ms TWT below sea surface using the MCS dataset recorded by a 600 m-long streamer cable. The AVO cross-section shows that the lower repetitive signals in the MCS data are due to the gas plume, not lithological contrast. Our study highlights that AVO analysis is an extremely useful tool for identifying free gas, helping to discriminate water-gas contacts and bright events among the chaotic signals on the MCS data. • We identify shallow gas based on seismic data from SW shelf of Ulleung Basin. • Seismic chimneys, gas flares, enhanced reflections are most common. • We compare acoustic anomalies of shallow gas with different resolutions. • We verify the presence of gas by AVO analysis.

Identification of gas inflow zones in the COSC-1 borehole (Jämtland, central Sweden) by drilling mud gas monitoring, downhole geophysical logging and drill core analysis

On-line monitoring of drilling mud gas was for the first time applied during continuous wireline coring of the COSC-1 borehole (Jämtland, central Sweden) to analyse formation gases and to identify inflow gas zones. Nearly complete gas records were obtained with 3 m depth resolution from 662 m (installation of the separator for gas extraction) to 1709 m and 6 m resolution from 1709 m to 2490 m depth (COSC-1 final depth: 2496 m) for H2, CH4, CO2, and He. Between 662 m and 1400 m, both He and CH4 form broad peaks superimposed by several spike-like features. Zones with gas spikes coincide with high resistivity intervals from dual laterolog (DLL) geophysical borehole logging and show fractures in borehole televiewer (BHTV) images, drill core scans, and visual core inspection. Therefore, we assume gas inflow through open fractures where DLLd/DLLs ratios >1.5 imply the presence of free gas. The correlation between helium and DLLd/DLLs ratios no longer appears at depths greater than ~1550 m, probably because the formation gases are dissolved in formation fluids at higher pressure. Below 1550 m depth, the He concentration drops significantly, whereas the CH4 concentration remains relatively high and H2 and CO2 reach maximum values. The high amount of H2 and CH4 at depths below 1616 m, from where friction between the casing and the drill string was reported, imply that these gases are most certainly artificially generated at depths below 1616 m and at least partly of artificial origin at shallower depths.

Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea

Abstract Integrated three-dimensional seismic, logging, sediment cores, and geochemical testing data were collected from Guangzhou Marine Geological Survey 3 and 4 hydrate drilling expeditions and used in this study for a comprehensive investigation of the geological and geophysical features and accumulation mechanism of hydrates in the first offshore gas-hydrate production test region (GHPTR) in the Shenhu area of the South China Sea. Seismic signatures indicative of disseminated hydrates and free gas include the bottom simulating reflector (BSR), gas chimney, and mud diapir associated with enhanced seismic reflections, acoustic blanking, masking, and chaotic appearance have been observed. The acoustic travel-time responses, density, and compensated neutron three porosity log analysis, high-precision grid tomography inversion analysis, and constrained sparse spike inversion confirm the presence of free gas below the gas-hydrate-bearing zone (GHBZ). Free-gas-bearing zones have significantly different p-wave impedances and low-velocity anomalies than the overlying GHBZ and surrounding strata. These anomalous zones are controlled by the structural attitude of the reservoir strata, which are characterized as inter-bedded stratigraphic units. Variations in the type and geological characteristics of the hydrocarbon migration pathways were observed between sites W18 and W19 on the western ridge and sites W11 and W17 on the eastern ridge in the GMGS study area. The efficiency of gas migration in the western ridge may be higher than that in the eastern ridge, resulting in variations in hydrate gas types, thickness of the GHBZ, and gas migration flux and accumulation. Except for site W11, hydrates were recovered below the structure I inferred BSR at sites W17, W18, and W19. The gas-hydrate stability zone calculations reveal that the structure I hydrate stability zone differs from the BSR depth and is generally shallower than the base of the logging anomaly, indicating the coexistence of structure I and II hydrates. The BSR is not indicative of the BGHSZ; it is rather regarded as a transitional indicator of structure I and II gas hydrates in the GHPTR. The appearance of free gas and hydrates below the structure I inferred BSR indicates that the Shenhu area is characterized by a complex hydrate formation and accumulation system resulting from the supply of biogenic and thermogenic gases. Despite fine-grained host sediments predominating the GHPTR, the coupling of favorable conditions including efficient hydrocarbon generation, sufficient gas supply, multiple pathways for gas migration, and relatively high reservoir porosity have led to the development of highly saturated gas-hydrate accumulations within relatively thick sedimentary sections, which demonstrates a significant resource potential.

On the use of Fredlund gas–fluid compressibility relationship to model medium-dense gassy sand behavior

This paper presents the influence of gas bubbles trapped in a soil mass on the stress–strain–strength response of medium-dense sands. A hypoplastic constitutive sand model enhanced with the intergranular strain concept was coupled with the Fredlund gas–fluid compressibility relationship to capture gassy soil behavior. Boundary value element representations in a finite element platform of oedometer and saturated drained and undrained triaxial compression tests are performed for the calibration of soil parameters. For the numerical simulation of gassy soil behavior, pore fluid compressibility is modified to account for the presence of free gas in the pore fluid. The gassy soil mechanical response is studied by using only one set of parameters determined from the saturated soil response. The testing bed for this evaluation is a laboratory experimental program conducted on sands retrieved from the Oakridge Landfill, a sanitary landfill located in South Carolina, USA. The hypoplasticity sand model specialized with the Fredlund relationship reproduces reasonably well the stress–strain–strength response of these sands for a wide range of loading conditions and at a reasonable level of testing for the calibration of constitutive parameters. It is found that a slight reduction in the degree of saturation significantly decreases the undrained shear strength of the soil and causes changes in volume (i.e., drained-like behavior).

Methane occurrence, emissions and hazards in the Upper Silesian Coal Basin, Poland

The paper presents the variability of methane emissions into mining excavations and the atmosphere in the Upper Silesian Coal Basin (USCB) (Poland) against the background of natural and mining factors. Emissions of methane from exploited coal seams have become a serious problem in the USCB due to the growing methane hazard in coal mines and emissions of methane into the atmosphere. In the mid-nineties 753 million m3 of methane was emitted annually from the USCB mines. Despite a significant drop in coal production over the next 20 years, methane emissions have not decreased sharply; on the contrary, in recent years they have begun to grow, and, in 2016, reached the level of 933 million m3 per year. This represents an increase of 180 million m3 per year since the mid-nineties. One of the important reasons for this phenomenon is the constantly increasing depth of coal exploitation, which in many mines now exceeds 1000 m; this is the depth corresponding to the deep methane zone, where the volume of accumulated methane in the coal seams is particularly high. Factors influencing the volume of methane emissions in relation to mine workings can be divided roughly into two groups: natural (geological) and anthropogenic (mining-related). Natural factors include methane content in coal seams, gas pressure, the presence of free gas in fault zones, related fissures and porous sandstone, the migration of methane through faults and fissures, and the presence of a continuous and impermeable Miocene overburden. Mining factors include the depth of exploitation and the concentration of coal production as expressed in terms of the length, height, and advance of walls. The interdependence of these factors means that, despite the decline in coal production, methane emissions, both total and specific, are increasing. This problem cannot be neglected, especially since it may grow worse in future. One measure to prevent the growth of methane emissions and the associated hazard may be the intensification of mine methane drainage and the economic use of captured mining gas, which is already done in the USCB.

Multi-channel 2D seismic constraints on pore pressure- and vertical stress-related gas hydrate in the deep offshore of the Mahanadi Basin, India

Estimation of pore pressure and in-situ vertical stress magnitude is essential for understanding the geomechanical behavior of the gas hydrate sediments in deep offshore of the Mahanadi basin. The basin located at northern side of eastern continental margin of India (ECMI) contains gas hydrate in clay/silt sediments. The pore pressure and vertical stress are mapped on two 2D-multi channel seismic data of lines (MH-38A and MH-38B) with aid of information of three wells (namely NGHP-01-19, NGHP-01-09 and NGHP-01-08). Initially, the coefficients of best fit curve have been computed from velocity-effective stress plot for the individual wells and applied on the seismic velocity to transform into the effective stress. The vertical stress has been computed from both the seismic and log data. Therefore, the pore pressure is predicted by subtracting the effective stress from the vertical stress. The pore pressure have been mapped in gas hydrate stability zone (GHSZ) and sediments below bottom simulating reflector (BSR). The pore pressure and vertical stress gradient are 10.11 MPa/km and 10.67 MPa/km, respectively. The pore pressure and vertical stress from seismic data are closely matched at well location with excellent goodness of fit (R2) varying from 0.82 to 0.95. Normal pressure is observed in the gas hydrate bearing sediments but slightly high pressures are noticed below the BSR indicating presence of free gas. The pore pressure from seismic data will guide drillers for choosing the mud weight during well drilling and casing optimizing in other part of deep offshore in the Mahanadi basin.

Computed tomography improves the differentiation of infectious mediastinitis from normal postoperative changes after sternotomy in cardiac surgery.

To identify CT parameters independently associated with infectious mediastinitis after cardiac surgery and to improve the discrimination of patients with acute infection from those with normal postoperative changes. In this single-center, retrospective, observational cohort study, we evaluated thoracic CT scans of poststernotomy cardiac surgery patients. Inclusion criteria were clinically suspected mediastinitis, unclear CT signs (e.g., retrosternal mass), and subsequent deep revision surgery. Revision surgery and microbiological samples determined the mediastinitis status. Overall, 22 qualitative and quantitative CT imaging parameters were assessed and associated with infectious mediastinitis in univariate and multivariate regression models. Discriminative capacity and incremental value of the CT features to available clinical parameters were determined by AUC and likelihood-ratio tests, respectively. Overall 105 patients (82% men; 67.0 ± 10.3years) underwent CT and deep revision surgery. Mediastinitis was confirmed in 83/105 (79%) patients. Among available clinical parameters, only C-reactive protein (CRP) was independently associated with infectious mediastinitis (multivariate odds ratio (OR) (per standard deviation) = 2.3; p < 0.001). In the CT, the presence of free gas, pleural effusions, and brachiocephalic lymph node size were independently associated with mediastinitis (multivariate ORs = 1.3-6.3; p < 0.001-0.039). Addition of these CT parameters to CRP increased the model fit significantly (X2 = 17.9; p < 0.001; AUC, 0.83 vs. 0.73). The presence of free gas, pleural effusions, and brachiocephalic lymph node size in CT is independently associated with infectious mediastinitis in poststernotomy patients with retrosternal mass. These imaging features may help to differentiate mediastinitis from normal postoperative changes beyond traditional clinical parameters such as CRP. • Presence of free gas, pleural effusions, and brachiocephalic lymph node size on CT are associated independently with infectious mediastinitis. • Combination of these CT parameters increases the discriminatory capacity of clinical parameters such as CRP.

Assessment of Free Gas in the Tibial Condyle Bone of the Human Knee by Diode Laser Spectroscopy With Possible Application to Arthrosis Diagnostics

The gas in scattering media absorption spectroscopy (GASMAS) technique was used to investigate the possible presence of free gas pores in human condyle bones, extracted from the tibia in knee revision surgical procedures in response to arthrosis development. Following earlier studies showing free gas presence in femoral head bones, decaying because of inadequate blood flow, we investigated condyle bone, where the arthrosis degeneration process, affecting the cartilage cup, is of different origin. Only a minor gas presence was observed in affected condyle samples, in accordance with our finding for non-arthrosis samples. Further, studies of fresh or initially frozen samples showed very similar signals, suggesting that the handling procedure has only a minor importance in these studies. In contrast to the situation for femoral heads, the monitoring of a gas presence would add only little in assessing the arthrosis status in the view of elective surgical procedures.

Effect of CO2 on P- and S-wave velocities at seismic and ultrasonic frequencies

Time-lapse seismic is a widely used technology for monitoring the geological sequestration of carbon dioxide (CO2), consisting of mapping its movement in the subsurface and of demonstrating that the CO2 is safely stored in the reservoir (Xue and Ohsumi, 2004). In this work, the effect of CO2 on P- and S-wave velocities was investigated. Laboratory measurements were performed with Castlegate sandstone both at seismic frequencies (1–155 Hz), and at ultrasonic frequency (around 500 kHz). Different CO2 saturations between 2% and 10% were obtained by controlled depressurization of CO2-saturated water with which the sandstone sample had been saturated with. At seismic frequencies, the results of the experiments revealed that P-wave velocity is strongly reduced in the presence of free gas CO2 in the pore space, whereas at ultrasonic frequency, the P-wave velocity changed only slightly. Therefore, the presence of free CO2 gas increased significantly the P-wave dispersion between seismic and ultrasonic frequencies. The S-wave velocity, on the other hand, was hardly affected by the pore fluid at seismic frequencies. At seismic frequencies, P- and S-wave velocities were consistent with the Biot–Gassmann model. The P-wave velocity dispersion and corresponding attenuation were simulated by applying the Cole–Cole model. The transition frequency was found around 200 kHz.

Downhole log evidence for the coexistence of structure II gas hydrate and free gas below the bottom simulating reflector in the South China Sea

Abstract Stratigraphic layered pore-filling gas hydrates are identified above the bottom simulating reflector (BSR) using the well log and core data acquired at Sites W11 and W17 during the third gas hydrate drilling expedition conducted by China's Geological Survey/Guangzhou Marine Geological Survey (GMGS3) in the South China Sea. A seismic profile near Site W17, reveal the presence of two BSRs (i.e., double BSR), which we show to relate to zones of structure I gas hydrate (I-BSR) and structure II gas hydrate (II-BSR). Well log data from Site W17 between the “I-BSR” (projected depth of 250 mbsf) and “II-BSR” (projected depth of 330 mbsf) showed anomalous responses for gas hydrate-bearing sediments with high resistivity, high S-wave velocity, and alternating high and low P-wave velocities. Pressure core data support the interpretation that structure II gas hydrate occurs at a depth of 263 mbsf at Site W17. The cross-plot between log-derived neutron and density porosities reveals a free gas-bearing layer at a depth of 258–270 mbsf, suggesting gas hydrate coexists with free gas between the “I-BSR” and the “II-BSR.” Synthetic seismograms generated from the P-wave velocity and density logs further support the presence of free gas in this section. Based on the coexistence of hydrate, free gas and water, the simplified three-phase equation (STPE) was modified to simultaneously estimate free gas and hydrate saturations beneath the “I-BSR” from P-wave and S-wave velocity logs, assuming uniform or patchy distributions of free gas. The estimated free gas and hydrate saturations, together with gas compositions from pressure core samples, collectively indicate that structure II gas hydrate and free gas are interbedded and coexist below the “I-BSR” at Site W17. Our study of the coexistence of gas hydrate and free gas between the double BSR at Site W17 provides new insights into gas hydrate systems in nature that contain more complex gas chemistries.

Pneumomediastinum and pneumopericardium following blunt thoracic trauma: much ado about nothing?

Pneumomediastinum is the hallmark of intrathoracic aerodigestive trauma, but rare following blunt injury. review of blunt thoracic trauma (BTC) for the incidence and outcome of patients with pneumomediastinum or pneumopericardium (PM/PC) on Computerised Tomographic scanning. Admissions to the level I trauma ICU at IALCH, Durban, ZA following BTC from April 2007 to March 2014. Patients with Chest-CT-scan were analysed. Variables included age, sex, mechanism of injury, and Injury Severity Score (ISS). Specific injury patterns: isolated thoracic trauma, flail chest, bilateral injury and presence of haemothorax or pneumothorax were analysed. Three hundred and eighty-nine patients were included. Males (70.9%) accounted for the majority of patients. The median Injury Severity Score was 32 (IQR 24-41). Motor vehicle collisions accounted for 94% of injury mechanisms. Twenty-three (5.9%) were identified with pneumomediastinum, 6 (1.5%) with both pneumomediastinum and pneumopericardium, and 1 (0.2%) with isolated pneumopericardium. No patient required surgery for thoracic trauma. Increasing age (p<0.001) and a flail chest (p=0.005) were significant associations. The mortality rate was almost identical in those with or without air within the mediastinum. No patient died from a missed mediastinal aero-digestive injury. The presence of PM/PC following BTC is incidental and benign. Increased injury severity with a flail chest is associated with a significant increase in the presence of free gas within the mediastinum. In the absence of complications, no obvious injury to the intrathoracic aero-digestive tract on CT scanning, and no difference in mortality, a conservative management policy is warranted.

Reconstruction of repeated Quaternary slope failures in the northern South China Sea

A series of Quaternary mass transport deposits (MTDs) with a total areal extent of ~11,000 km2 and a conservative volume estimate of ~1035 km3 are observed in the Pearl River Mouth Basin (PRMB), northern South China Sea (SCS), based on integrated analysis of extensive 2-D/3-D seismic data, multibeam bathymetric data and ODP/IODP wells. Unlike other large MTDs worldwide, the MTDs in this study are not directly sourced from slopes built on major deltaic depositional systems. Moreover, they are remarkable in that the whole PRMB is a giant debris flow corridor that has transected a broad subsided continental margin. The MTDs in this study have a large evacuation zone on the slope and a depositional zone in the ocean basin connected by a narrow pathway. Six events of MTDs (MTDa - MTDd on the slope and MTDo1 - MTDo2 in the ocean basin) have been identified as being separated by basal erosional surfaces defined by strong negative seismic reflections. Similar ages and volumes (depleted volume of MTDb on the slope and the depositional volume of MTDo2) suggest that MTDo1 and MTDo2 correlate with MTDa and MTDb, respectively. The major factor controlling the occurrence of slope failures is argued to be the presence of a key reflection package. This package is interpreted to consist of gas-charged, coarse-grained sediments deposited during a relative lowstand of sea level, overlain by a unit of fine-grained sediments. Other contributing factors that influenced the morphology and position of the MTDs include the presence of free gas, normal faulting in the slope sediments, and seafloor relief related to igneous activity. From MTDd to MTDb, slope failures are shown to have retrogressed upslope with a decreasing time interval between events. The events, dimensions, susceptible factors and formation processes of slope failures in the study area are demonstrated for the first time in the petroliferous basin of northern SCS. Their recognition is important for geo-hazard assessment (e.g. tsunami) in this area. Moreover, this study highlights that the lithostratigraphy of the slope succession, especially the coarse-grained sediments in the deepwater area, can be an important component for the occurrence of slope failures, and may be a key factor in slope stability on other continental margins too.

A semi-analytical model for the relationship between pressure and saturation in the CBM reservoirs

At present, analyzing and predicting CBM production performance remains challenging, which can be attributed to the complex gas-water two phase flow and dynamic nature of petro-physical properties. For the sake of simplicity, the saturation distribution in coal seams is generally overlooked and characterized by average water saturation in previous literature, which one can easily handle analytically, semi-analytically, and numerically. However, average water saturation is inadequate to obtain the precise value of pseudo pressure for water/gas phase, which may generate large deviation compared with actual production behavior. To our best knowledge, the relationship between pressure and saturation in coal seams is still lacking in the petroleum industry. Thus, the main objective of this research is to gain a clear understanding of this issue.In this work, on the basis of rigorous derivation, a semi-analytical model for the relationship between pressure and saturation is developed, which comprehensively accounts for the presence of free gas at the early production stage, matrix shrinkage and gas desorption. Notably, the stress dependence effect is incorporated during the entire production process of CBM wells. Subsequently, employing the gas-water two phase relative permeability data, we present an iterative numerical algorithm to solve the model. Moreover, the proposed semi-analytical model is successfully verified against a numerical reservoir simulator. After that, we shed light on the influences of physical parameters upon the saturation distribution versus pressure and achieve a variety of new insights.This research, for the first time, presents a semi-analytical model to quantify the relationship between pressure and saturation, which fills the gap for the theory of gas-water two phase flow in CBM reservoirs. The proposed model is less data-intensive than performing a numerical simulation and turns out to be simple and powerful in the application. In addition, the new model can significantly contribute to the obtainment of precise pseudo pressure for water/gas phase, therefore lays the theoretical basis for the accurate production prediction for CBM wells.

Issues and Challenges With Controlling Large Drawdown in the First Offshore Methane-Hydrate Production Test

Summary The first offshore methane-hydrate production test was conducted in the Eastern Nankai Trough area of Japan in 2013, subjecting a gas-hydrates reservoir to large drawdowns by reducing bottomhole pressure (BHP) for in-situ dissociation of gas hydrates. This pioneering test has proved the feasibility of the depressurization method through demonstration of gas production from a deepwater gas-hydrates reservoir. Approximately 119 500 std m3 of gas was produced during a continuous flow period of 6 days. However, reservoir response to a range of drawdown conditions was not attainable, which is important for reservoir evaluation, because drawdown became uncontrollable after unintended water production through the gas line occurred. Gas and water released from the dissociation of gas hydrates were separated by use of a downhole gas-separation system. The separated gas and water were produced to surface by means of two dedicated gas and water lines. Drawdown was executed by pumping out water into the water line by use of an electrical submersible pump (ESP). Drawdown control was designed to regulate the liquid level (or hydrostatic pressure) in the gas line by controlling the ESP frequency and the water-line surface backpressure. Analysis of production data supported by flow simulations indicated that continuous water production through the gas line was the main reason for the loss of drawdown control. The trigger of the water production was that the water column in the gas line reached surface because of the rising water level resulting from the produced gas, which also lightened the water column and lowered the BHP. Consequently, the continuous water production made it difficult to regulate the drawdown as intended. The analysis concluded that the risk of water production through the gas line could be significantly lowered if a choke valve was installed at the surface gas line and/or the ESP had high tolerance to the presence of free gas. This first field trial has provided valuable information in understanding the methane-hydrate production system to further improve/develop strategies in controlling large drawdown in the system.

Spontaneous Subcutaneous Emphysema with Pneumo-Mediastinum: A Case Report

Aim: To report a rare case of Spontaneous subcutaneous emphysema and pneumo-mediastinum Introduction: Spontaneous subcutaneous emphysema and pneumo-mediastinum may be defined as the presence of free gas or air in the subcutaneous tissue or mediastinal structures without an apparent precipitating cause. It most commonly occurs in adolescent in good physical health group devoid of severe existing lung pathology. Case Report: In our case, a middle aged married housewife was referred to Department of Medicine, Peoples Medical University Hospital Nawabshah, Sindh, Pakistan, by her local GP for the worsening of her condition due to development of acute neck pain, difficulty in breathing, eating and swallowing (mainly for solid foods) with swelling of neck. She was not complaining of any respiratory symptoms. The chest and neck radiographs showed subcutaneous emphysema and pneumo-mediastinum, there was no any evidence of air leakage from esophagus. She was subsequently put on free fluids, light diet, antibiotics, analgesia and other supportive measures along with close observation. After three days of admission, her clinical symptoms were alleviated to a great extent. She was discharged well from hospital after four days. Conclusion: Spontaneous subcutaneous emphysema with pneumo-mediastinum is medical and surgical emergency. Diagnosis may be made by routine chest X-rays and CT scan of the chest. Prompt diagnosis and immediate management may affect the morbidity and mortality outcomes in this condition.

Pneumatosis Cystoides Intestinalis, Is the Emergency Surgery Inevitable: Case Report

Pneumatosis cystoides intestinalis is a rare clinical phenomenon in which there is presence of gas filled cysts in the gut wall. Its exact cause is not known but this condition is commonly associated with smoking and other factors leading to raised intra-abdominal pressure. Sometimes these gasses filled cysts may rupture on their own or secondary to some surgical intervention leading to presence of free gas in the abdomen or retroperitoneum. We present here a case report of the patient who presented to the emergency room with pain abdomen and a large amount of free gas in the abdomen seen on erect X-ray of the abdomen. Here comes the challenge to the surgeon, whether to operate or not. Contrast Enhanced Computerised Tomography (CECT) scan of the abdomen is a better modality for diagnosing Pneumatosis cystoides intestinalis (PCI) and it can prevent unnecessary surgeries. However, the management of this condition is mainly nonsurgical, and oxygen forms the mainstay of therapy.

Evidence for gas hydrate occurrences in the Canadian Arctic Beaufort Sea within permafrost-associated shelf and deep-water marine environments

The presence of a wedge of offshore permafrost on the shelf of the Canadian Beaufort Sea has been previously recognized and the consequence of a prolonged occurrence of such permafrost is the possibility of an underlying gas hydrate regime. We present the first evidence for wide-spread occurrences of gas hydrates across the shelf in water depths of 60–100 m using 3D and 2D multichannel seismic (MCS) data. A reflection with a polarity opposite to the seafloor was identified ∼1000 m below the seafloor that mimics some of the bottom-simulating reflections (BSRs) in marine gas hydrate regimes. However, the reflection is not truly bottom-simulating, as its depth is controlled by offshore permafrost. The depth of the reflection decreases with increasing water depth, as predicted from thermal modeling of the late Wisconsin transgression. The reflection crosscuts strata and defines a zone of enhanced reflectivity beneath it, which originates from free gas accumulated at the phase boundary over time as permafrost and associated gas hydrate stability zones thin in response to the transgression. The wide-spread gas hydrate occurrence beneath permafrost has implications on the region including drilling hazards associated with the presence of free gas, possible overpressure, lateral migration of fluids and expulsion at the seafloor. In contrast to the permafrost-associated gas hydrates, a deep-water marine BSR was also identified on MCS profiles. The MCS data show a polarity-reversed seismic reflection associated with a low-velocity zone beneath it. The seismic data coverage in the southern Beaufort Sea shows that the deep-water marine BSR is not uniformly present across the entire region. The regional discrepancy of the BSR occurrence between the US Alaska portion and the Mackenzie Delta region may be a result of high sedimentation rates expected for the central Mackenzie delta and high abundance of mass-transport deposits that prohibit gas to accumulate within and beneath the gas hydrate stability zone.

Erratum to: An application of rock physics modeling to quantify the seismic response of gas hydrate-bearing sediments in Makran accretionary prism, offshore, Pakistan

Naturally occurring gas hydrates are potential future energy source. A significant amount of gas hydrates is interpreted through seismic reflection data in the form of bottom simulating reflector (BSR) present in the sediments of the convergent continental margin of Pakistan. However, the seismic character of these hydratebearing unconsolidated sediments is not properly investigated. Since no direct measurements are available for quantitative estimation of gas hydrate and free gas in these sediments, therefore detailed knowledge of seismic velocities is essential. Seismic velocities of the gas hydrate-bearing sediments in the study area are estimated by using the effective medium theory and the fluid substitution modeling. The results show that the presence of gas hydrates increases the stiffness of the unconsolidated sediments; whereas the presence of free gas decreases the stiffness of these sediments. It is noted that seismic velocities and density of hydrate-bearing sediments are highly affected by saturation and distribution pattern of gas hydrates. The hydrate-bearing sediments seem to be characterized not only by high P-wave velocity (about 2800 m/s) but also by anomalously low S-wave velocity (about 850 m/s). As pure gas-hydrates have much higher seismic velocities than those of host sediments, presence of gas-hydrate increases the seismic velocities, whereas free-gas below the hydrate-bearing sediments decreases the velocities. Seismic reflection from the BSR exhibits a wide range of amplitude variation with offset characteristics, which depend upon the saturation and distribution of hydrates above and free gas below the BSR. We have also demonstrated that some attributes like acoustic and shear impedances, and AVO can be used as important proxies to detect gas hydrate saturation.

Gas hydrate and free gas estimation from seismic analysis offshore Chiloé island (Chile)

In this study one seismic section offshore Chiloe Island was analyzed to better define the seismic character of the hydrate-bearing sediments. The velocity analysis was used to estimate the gas-phase concentration and relate it to the geological features. The velocity model allowed us to recognize two important layers that characterize hydrate- and free gas-bearing sediments above and below the BSR respectively: one located above the BSR, characterized by high velocity (1,800-2,200 m/s) and a second one, below the BSR, characterized by low velocity (1,600-1,700 m/s). A weak reflector at about 100 m below the BSR marks the base of the second layer. AVO analysis and offset stack sections confirming that the reflector interpreted as BGR is related to free gas presence in the pore space. The velocity field is affected by lateral variation, showing maximum (above the BSR) and minimum (below the BSR) values in the sector. Here, the highest gas hydrate and free gas concentrations were calculated, obtaining 9.5% and 0.5% of total volume respectively. A variable BSR depth (from 300 to 600 mbsf) can be justified supposing a variable geothermal gradient (from 25 to 45 °C/km).

Resistivity image beneath an area of active methane seeps in the west Svalbard continental slope

The Arctic continental margin contains large amounts of methane in the form of methane hydrates. The west Svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone. The presence of bottom simulating reflectors (BSRs) on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area. Resistivity obtained from marine controlled source electromagnetic (CSEM) data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments. We acquired two CSEM lines in the west Svalbard continental slope, extending from the edge of the continental shelf (250 m water depth) to water depths of around 800 m. High resistivities (5–12 Ωm) observed above the BSR support the presence of gas hydrate in water depths greater than 600 m. High resistivities (3–4 Ωm) at 390–600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone (GHSZ) of the continental slope. In addition, high resistivities (4–8 Ωm) landward of the GHSZ are coincident with high-amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas. Pore space saturation estimates using a connectivity equation suggest 20–50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments. A free gas zone beneath the GHSZ (10–20 per cent gas saturation) is connected to the high free gas saturated (10–45 per cent) area at the edge of the continental shelf, where most of the seeps are observed. This evidence supports the presence of lateral free gas migration beneath the GHSZ towards the continental shelf.