Small Amount Of Free Gas Research Articles

Seafloor Methane Seepage Related to Salt Diapirism in the Northwestern Part of the German North Sea

This study focuses on seafloor methane seep sites and their distribution in the northwestern part of the German North Sea. Methane seepage is a common phenomenon along marine shelves and known to occur in the North Sea, but proof of their existence was lacking in the study area. Using a ship-based multibeam echosounder we detected a minimum of 166 flares that are indicative for free gas releases from the seafloor in the German “Entenschnabel” area, which are not related to morphologic expressions at the seafloor. However, a group of small depressions was detected lacking water column anomalies but with indications of dissolved fluid release. Spatial analysis revealed that flares were not randomly distributed but show a relation to locations of subsurface salt diapirs. More than 60% of all flares were found in the vicinity of the salt diapir “Berta”. Dissolved methane concentrations of ∼100 nM in bottom waters were ten times the background value in the “Entenschnabel” area (CH4< 10 nM), supporting the finding of enhanced seepage activity in this part of our study area. Furthermore, locations of flares were often related to acoustic blanking and high amplitude reflections in sediment profiler echograms, most prominently observed at location Berta. These hydroacoustic signatures are interpreted to result from increased free gas concentrations in the sediments. Electromagnetic seabed mapping depicts local sediment conductivity anomalies below a flare cluster at Berta, which can be explained by small amounts of free gas in the sediment. In our area of interest, ten abandoned well sites were included in our mapping campaign, but flare observations were spatially not related to these wells. Naturally seeping methane is presumably transported to the seafloor along sub-vertical faults, which have formed concurrently to the updoming salt. Due to the shallow water depths of 30 to 50 m in the study area, flares were observed to reach close to the sea surface and a slight oversaturation of surface waters with methane in the flare-rich northeastern part of the working area indicates that part of the released methane through seepage may contribute to the atmospheric inventory.

Methods for simultaneously evaluating reserve and permeability of undersaturated coalbed methane reservoirs using production data during the dewatering stage

In this work, a flowing material balance equation (FMBE) is established for undersaturated coalbed methane (CBM) reservoirs, which considers immobile free gas expansion effect at the dewatering stage. Based on the established FMBE, five straight-line methods are proposed to determine the control area, initial water reserve, initial free gas reserve, initial adsorbed gas reserve, original gas in place, as well as permeability at the same time. Subsequently, the proposed FMBE methods for undersaturated CBM reservoirs are validated against a reservoir simulation software with and without considering free gas expansion. Finally, the proposed methods are applied in a field case when considering free gas expansion effect. Validation cases show that the straight-line relationships for the proposed five FMBE methods are excellent, and good agreements are obtained among the actual reserves and permeabilities and those evaluated by the proposed five FMBE methods, indicating the proposed five FMBE methods are effective and rational for CBM reservoirs. Results show that a small amount of free gas will result in a great deviation in reserve evaluation; hence, the immobile free gas expansion effect should be considered when establishing the material balance equation of undersaturated CBM reservoirs at the dewatering stage.

The value of ultrasound in diagnosis of neonatal upper and lower gastrointestinal perforation

Objective To explore the value of ultrasound in the differential diagnosis of neonatal upper and lower gastrointestinal tract(GIT)perforation. Methods We retrospectively reviewed the ultrasound findings of 42 neonates of surgery-confirmed neonatal GIT perforation in our hospital from January 1, 2015 to December 31, 2018.The accuracy of ultrasound for detecting GIT perforation and the ultrasound features of upper and lower GIT perforation were evaluated. Results (1)Of the 42 neonates with GIT perforation, 1 case didn′t undergo ultrasound, 2 cases were missed, and 1 case was misdiagnosed.Thirty-eight neonates were diagnosed of GIT perforation by ultrasound preoperatively, with a detection rate of 92.7%(38/41). The locations of GIT perforation were identified by ultrasound in 30 cases(78.9%, 30/38), including 11 cases of upper GIT perforation and 19 cases of lower GIT perforation.(2)A common sonographic finding of GIT perforation in 38 cases was pneumoperitoneum, which appeared as an echogenic line with posterior reverberation artifact under diaphragm or anterior to hepatic/splenic surface and a stratosphere sign in M-mode sonography.Free gas changed position when the patient′s position was changed, and didn′t change due to respiratory change.Besides, free gas dispersed with compression on abdomen, and gathered without compression.(3)Upper GIT perforation was showed that poor filling of the stomach cavity, and the abdominal free gas sharply increased.Lower GIT perforation was characterized by collapsed bowel, blurred and interrupted intestinal wall structure, and more accompanied with intestinal obstruction.(4)There was no significant difference of detection rate between ultrasound and X-ray in diagnosing GIT perforation[92.7%(38/41)vs.83.3%(35/42)](P>0.05), whereas ultrasound more sensitive for a very small amount of free gas in the early stage of perforation.(5)Helicobacter pylori infection was found in two cases of GIT perforation. Conclusion Ultrasound can be used for differential diagnosis of upper and lower GIT perforation, and could be recommended as the first choice for detecting GIT perforation in neonatal patients. Key words: Ultrasound; Upper gastrointestinal tract perforation; Lower gastrointestinal tract perforation; Intestinal obstruction

Possible link between weak bottom simulating reflections and gas hydrate systems in fractures and macropores of fine-grained sediments: Results from the Hikurangi Margin, New Zealand

Small amounts of free gas in interstitial sediment pores are known to significantly lower compressional (P-) wave velocity (Vp). This effect, combined with moderately elevated Vp from the presence of gas hydrates, is usually thought to be the cause for the often observed strong negative reflection coefficients of bottom simulating reflections (BSRs) at the base of gas hydrate stability (BGHS). At several locations however, weak BSRs have been observed, which are difficult to reconcile with a presence of gas in sediment pores. We here present a rock physics model for weak BSRs on the Hikurangi Margin east of New Zealand. Thin sections of a fine-grained mudstone sample from a submarine outcrop in the vicinity of a weak BSR show macroscopic porosity in the form of fractures and intrafossil macropores. We apply the Kuster-Toksöz theory to predict seismic velocities for a rock with water-saturated interstitial micropores and gas or hydrates in macroscopic pore space simulating fractures or compliant macropores. We match field observations of a weak BSR with a reflection coefficient of −0.016 with two end-member models; (1) rocks with gas hydrate-filled voids with a concentration of <4% of bulk sediment overlying water-filled voids, or (2) fully gas-saturated voids at a concentration of <2% beneath water-filled voids. A natural system is likely to consist of a combination of these end-members and of macroporosity filled with a mixture of water and gas or hydrate. Our results suggest weak BSRs may be caused by gas hydrate systems in fractures and macropores of fine-grained sediments with fully water-saturated interstitial pore space. Gas may be supplied into the macroscopic pore space by diffusion-driven short-range migration of methane generated within the gas hydrate stability field or, our favoured model based on additional geologic considerations, long-range advective migration from deeper sources along fractures.

Foldback reflectors near methane hydrate bottom‐simulating reflectors: Indicators of gas distribution from 3D seismic images in the eastern Nankai Trough

AbstractUnderstanding of fluid behavior and gas distribution in the shallow subsurface are important considerations in gas hydrate formation and the global carbon cycle. Estimation of gas distribution based on reflection seismic surveys, however, is difficult because the boundary of a gas‐bearing zone is indistinct and not systematically defined. This study reports distinctive features related to gas‐hydrate distribution and possible fluid migration in high‐resolution 3D seismic‐reflection data from sediments of the eastern Nankai Trough. These features, here termed foldback reflectors (FBRs), descend in accordion shaped reflectors near the edges of bottom‐simulating reflectors (BSRs). FBRs generally correspond to lateral boundaries between two seismic facies, a ‘dimmed’ facies with relatively low amplitude and subdued high‐frequency components beneath the BSR and the contrasting facies around the BSR. The dimmed facies corresponds to areas of anomalously low velocity consistent with a small amount of free gas. FBR is mostly developed in well‐stratified formations in uplifted regions. Dip directions of the FBR appear to be restricted by orientation of the host formations. Edges of the FBR often correspond to high‐amplitude layers. Such occurrences of FBR suggest that regional uplift and layer‐parallel fluid migration are related to the formation of FBR as well as BSR.

The Experimental Study on a New Type Low Temperature Water- based Composite Alcohol Drilling Fluid

Based on the characteristics of gas hydrate bearing permafrost in western China, and the application of alcohols drilling fluid, the performances of composite alcohol drilling fluid composed by polyethylene glycol and propylene glycol at low temperatures were comprehensively studied in this paper. The results show that the composite alcohol drilling fluid has a reasonable density, good low-temperature rheology, lubricity, shale hydration inhibition and hydrate formation inhibition, which can effectively inhibit shale expansion in a long time, and prevent hydrate formation from a small amount of free gas released from gas hydrate decomposition around the wellbore in the circulation of drilling fluid. Therefore, the composite alcohol drilling fluid has great significance for safe and high efficiency drilling in the permafrost gas hydrate bearing formation.

Pressure Waves Propagation in an Elastic Shell with Polydisperse Liquid-Gas Mixture

The target of the paper is modeling of the influence of small amount of free gas on pressure wave propagation in a cylindrical elastic shell, filled with viscous liquid. Heterogeneity of the bubble population and the basic sources of wave attenuation (wall friction, heat exchange between gas in microbubbles and surrounding liquid, viscous and acoustic losses) are taken into account. The dispersion equation for the waveguide is studied numerically and effect of the bubble size distribution on the longitudinal wave propagation is discussed.

Updates in neonatal gastrointestinal US

Sonography has come to play a significant role in imaging of the gastrointestinal (GI) tract in neonates. Although plain radiographs of the abdomen remain essential in some situations, the information obtained from radiographs regarding the bowel itself is often limited. US has several advantages over radiographs in that it can provide detailed information about the bowel wall regarding echogenicity (including intramural gas) and thickness; real-time images can provide information regarding peristalsis, and Doppler techniques can provide information regarding perfusion. US can also clarify whether a gasless area on a radiograph is the result of collapsed bowel, fluid-filled bowel, free fluid or a focal fluid collection. Separation of bowel gas on abdominal radiographs is a nonspecific appearance, and US might elucidate whether this is a result of true bowel wall thickening or fluid within the bowel lumen or in the peritoneal cavity. Furthermore, US can easily depict pneumoperitoneum caused by very small amounts of free gas at least as accurately as plain radiographs and probably even more accurately. To make the best use of US in the evaluation of the neonatal GI tract one has to optimize technique. The wider field of view obtained with vector or curved-array transducers is essential to evaluate the abdomen as a whole and to delineate the overall relationship of the bowel to other structures and free fluid or focal fluid collections. However, the features of the GI tract itself are often better depicted with the high-megahertz linear-array transducers. It is essential to magnify these images so as to depict only an individual or just a few bowel loops at any one time and to ensure that the focal zone is directed at the level of maximum interest in the GI tract. A relative limitation of US is the presence of large amounts of bowel gas but the technique of gentle graded compression can be useful in displacing the gas for better visualization. In neonates with severe abdominal tenderness, abdominal US might cause significant discomfort but the use of a large amount of gel on the abdominal wall can facilitate the examination by enabling images to be obtained without the transducer actually touching the abdomen. US should not be attempted in any neonate who is labile or unstable. There is a wide range of clinical situations in which abdominal US is invaluable for assessment of the neonate with known or suspected GI tract pathology. For example, in neonates with non-bilious vomiting, US can easily differentiate those with hypertrophic pyloric stenosis from those with gastric foveolar hyperplasia or other rarer causes of gastric outlet obstruction. In neonates with bilious vomiting US is the modality of choice to determine whether a midgut volvulus is present. In those neonates without a volvulus, US evaluation of the anatomical position of the entire duodenum (facilitated with intraluminal fluid) might elucidate the presence or absence of midgut malrotation. In the above examples US will obviate the necessity for an upper GI series. Another less common use of US is in the evaluation of the esophagus in neonates suspected of having an N-type tracheoesophageal fistula. Abdominal radiographs are still important in neonates suspected of having necrotizing enterocolitis (NEC). However, US can be invaluable in two clinical situations: first, in those neonates in whom the clinical and radiographic findings are nonspecific and the diagnosis remains indeterDisclaimer Dr. Daneman has no financial interests, investigational or off-label uses to disclose.

Transition from acceleration waves to strong discontinuities in fluid-saturated solids: drained versus undrained behaviour

The evolution of compression waves propagating in a fluid-saturated granular solid is considered. The pore fluid is assumed to consist of a liquid with a small amount of free gas. The stiffness of such a solid increases with increasing pressure. This property leads to the transformation of continuous compression waves into shock fronts after a finite time of propagation. The aim of the study is to calculate the critical distance covered by a continuous wave before it loses continuity. Critical distances are calculated for weak discontinuities (acceleration waves) propagating into a quiescent region. In numerical examples, the pressure dependence of the stiffness is taken in a form typical of granular solids. Emphasis is placed on the influence of free gas in the pore fluid and the permeability of the skeleton. Comparison of locally undrained and drained behaviour reveals that the drained model with low permeability turns out to be misleading for the calculation of the critical distance of a compression wave.

0366: Sonography in Necrotizing Enterocolitis

Plain radiographs of the abdomen remain the current standard method for evaluation of neonates suspected or known to have necrotizing enterocolitis. Despite this sonography does play an important role in this condition. It is well documented that sonography is more accurate in delineating free fluid and focal fluid collections in the abdomen than radiographs. Furthermore sonography may even be more accurate in certain circumstances than radiographs in depicted very small amounts of free gas in the peritoneal cavity. Sonography is also more accurate than radiographs in depicting bowel wall thickness (thickening or thinning), intramural gas and portal venous gas. This is particularly important when the plain radiographs are nonspecific and clinicians are uncertain whether to initiate therapy for necrotizing enterocolitis. Further work is required to better define the role of sonography in this condition and and to determine which neonates are likely to benefit from evaluation with sonography.

Critical distances for the formation of strong discontinuities in fluid-saturated solids

An increase in the stiffness of a solid in compression is known to lead to the steepening of the profiles of compression waves and, as a consequence, to the formation of strong discontinuities from continuous waves propagating in the solid. In this paper, the critical distance required for a continuous wave to turn into a shock wave is calculated from the evolution equation for a weak discontinuity (acceleration wave) propagating into a quiescent region. Infinite growth of the amplitude of an acceleration wave in a finite time signifies the transition to a strong discontinuity. Relations between the critical distances for plane, cylindrical and spherical waves are established. Numerical examples are presented for a particular case of the pressure-dependent stiffness typical of granular solids such as sand or soil, with emphasis placed on the influence of a small amount of free gas in the pore fluid.

On well-posedness of the dynamic problem for an anisotropic fluid-saturated solid

It is known that a high degree of anisotropy in the constitutive behaviour of a solid may result in the loss of hyperbolicity of the dynamic equations in the form of either complex-conjugate or purely imaginary characteristic wave speeds (flutter ill-posedness and shear band formation, respectively). In the present paper we investigate the characteristic wave speeds in the dynamic problem for a transversely isotropic fluid-saturated porous solid. Three cases are considered: a dry solid and a saturated solid under locally undrained and drained conditions. It is shown that, for given constitutive parameters of the solid skeleton, the dynamic problem for a drained solid may become ill-posed due to the flutter-type loss of hyperbolicity, while the dynamic equations for a dry and an undrained solids remain hyperbolic. For a given solid skeleton, the characteristic wave speeds are strongly influenced by the pore fluid compressibility which, in turn, is extremely sensitive to the presence of a small amount of free gas.

In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico

Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C 1 through C 5) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg − 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH 4. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH 4 concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.

Seismic characteristics of gas hydrates at Hydrate Ridge, offshore Oregon

Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules (Sloan, 1998). Gas hydrates are widely present in permafrost and deep oceanic environments around the world. Hydrate Ridge, offshore Oregon, is one of the areas where hydrates are found in relatively high concentration. In marine environments, methane hydrates are usually stable at temperatures of 0–20° C, water depths greater than 500 m, and in sediments up to 300 m below the seafloor. Small amounts of free gas are often present below the gas-hydrate stability zone (GHSZ). The hydrate-to-free-gas contact gives a strong acoustic impedance contrast, which is evident on seismic sections as a bottom-simulating reflection (BSR). Offshore gas hydrate systems have received attention from the scientific community because of their potential to be:

茂原型水溶性ガス田の産出挙動における毛細管圧力と浮力の働きについて

The Mobara gas field is one of the dissolved-in-water type natural gas fields. This field exists in Chiba Prefecture, Japan. The wells in this field have particular production performance curves called “Mobara type”. The gas water ratio curves are initially low, and then increase to be so high as to blow up a large amount of water.The Mobara gas deposit consists of alternate layers of sandstones and mudstones. We applied mud logging to exploration of the Mobara gas deposit. As the result, a small amount of free gas was detected in the mudstones.In addition to exploration, we carried out laboratory experiments using an X-ray CT scanner and mudstone cores. In the experiments, gas and water flow rates and CT values were measured at the same time. Gas water ratio increased with incremental drop of pore pressure, and gas saturation calculated from CT values also increased.From analyzing the exploration and the experiments, following conclusions were obtained.(1) In the Mobara gas deposit, gas bubbles of extremely small size are stored in pore bodies of the mudstones.(2) Upward buoyancy forces and downward forces given by capillary pressure difference act on gas bubbles at all times. These forces are initially in equilibrium.(3) With the start of production, hydraulic gradients occur and capillary pressure actions change in mudstones. The gas bubbles begin to move through pore throats toward sandstones consequently.The above-mentioned experiments suggest that buoyancy and capillary pressure have an important effect on the Mobara type production performance.

Energy Dissipation in Transient Gaseous Cavitation

The aim of the paper is to examine nonfriction energy dissipation in transient cavitating flows and to verify whether they can be attributed to thermic exchange between gas bubbles and the surrounding liquid or to gas release and solution process. A two-dimensional (2D) numerical model for liquid flow with a small amount of free gas is proposed. The results of the numerical runs are compared with experimental data of pressure head oscillations in transient cavitating flow. Both thermic exchange and gas release can explain some of the dissipation if a relaxation process is assumed. However, the 2D model, including the thermodynamics of the gaseous phase, does not seem to fully explain the observed dissipation. The 2D model with gas release allows for a good simulation of the experimental data after a suitable calibration of the model parameters.

Multicomponent ocean bottom cable data in gas hydrate investigation offshore of Norway

A multicomponent seismic ocean bottom cable (OBC) line from the northern flank of the Storegga Slide offshore of Norway is the first OBC line ever acquired over a gas hydrate‐related bottom simulating reflector (BSR). The seismic data are of very good quality, and display strong P‐S converted wave reflections for the whole sediment column. All P‐S reflections follow the sediment layers and are therefore interpreted to be caused by increased shear modulus and density due to sediment compaction. No P‐S reflections are associated with the BSR along this line, indicating that the gas hydrate‐bearing sediments at the base of the hydrate stability zone are not stiff enough to increase the shear modulus of the sediments to produce P‐S converted wave reflections. Seismic interpretation and seismic stratigraphic modeling suggest that the BSR in this area is a contrast in compressional wave velocity only, mainly caused by small amounts of free gas confined to dipping sediment layers beneath the BSR. Gas hydrate saturation above the BSR, tentatively estimated to be about 1–1.5% of the sediment pore space, is most likely too low to distinguish between different models for microscale gas hydrate distribution.

Numerical Properties of the Discrete Gas Cavity Model for Transients

Wylie’s discrete gas cavity model offers a simple way to simulate transients in liquids with a small amount of free gas, and to model vaporous and gaseous cavitations. It uses a constant and gas-free wave speed to avoid interpolations and a weighting factor to control numerical oscillations. The model has an intriguing ability to capture features associated with pressure-dependent wave speeds. This paper describes a von Neumann analysis on this model, shows why the need for the weighting factor and how to select it, and explains why the model exhibits variable wave speed features. [S0098-2202(00)00703-3]

Vertical tectonics and the origins of BSRs along the Peru margin

The distribution of bottom simulating reflections (BSRs), which mark the base of the gas hydrate zone on marine seismic records across the Peru margin, is examined with respect to tectonic uplift, sedimentation, and carbon content of sediment. Where accretion of only deep ocean pelagic sediment containing less than 1% carbon is seismically imaged, no BSR is observed. No BSR is apparent on seismic records of carbon-rich (4–8%) sediment of rapidly subsiding basins. BSRs are abundant on records of carbon-rich sediment affected by tectonic uplift and/or rapid sedimentation. In areas of uplift, the bottom simulating reflectors contain small amounts of free gas in contact with sediment containing small amounts of gas hydrate. During uplift, free gas is released from gas hydrates by dissociation as the hydrate phase boundary shifts upward with respect to sea level. The strong effects of free gas in reducing seismic velocity and gas hydrate in increasing velocity can explain the relatively small total quantities of gas and hydrate indicated by observations of BSRs off Peru.

Seismic velocity studies of a gas hydrate bottom‐simulating reflector on the northern Cascadia continental margin: Amplitude modeling and full waveform inversion

On the northern Cascadia subduction margin, the multichannel seismic amplitude‐versus‐offset (AVO) behavior of a bottom‐simulating reflector (BSR) suggests a P wave velocity change from high‐velocity hydrate‐bearing sediment to lower velocity sediment containing a small amount of free gas. The observed nonlinear AVO behavior, constant or slightly decreasing amplitudes at near‐to‐mid offsets and a large‐amplitude increase at far offsets, can be reproduced in the models if an S wave velocity enhancement is assumed as expected from hydrate cementation. The AVO behavior of the hydrate BSR is found not to be as useful as was earlier thought for determining the amount of free gas below the BSR. This is because Poisson's ratio change below the BSR due to gas is likely very small in high‐porosity unconsolidated sediments. The uncertainty in the models is large, as there is no reliable S wave velocity information for the sediments containing hydrate or gas. AVO modeling alone is not sufficient to distinguish different velocity models across the BSR. Our interpretation of the BSR amplitude behavior is that a P wave velocity increase above the BSR is the main cause of the BSR reflection amplitude increase at large incidence angles. Caution must be taken in applying AVO analysis, as little is known about S wave velocities. However, subtle differences in BSR amplitude behavior and reflection waveform can provide constraints through very careful full waveform inversion. A well‐defined reference velocity‐depth profile is also required to represent water‐saturated sediment unaffected by either hydrate or free gas. Using full waveform velocity inversion, a high‐resolution velocity model for the hydrate BSR has been derived. The best fit model for the seismic data near the Ocean Drilling Program (ODP) sites 889/890 consists of a high‐velocity zone above the BSR and a thin low‐velocity layer below, in agreement with the ODP downhole velocity data.