Identification Of Gas Hydrate Research Articles

Three-Dimensional Amplitude versus Offset Analysis for Gas Hydrate Identification at Woolsey Mound: Gulf of Mexico

The Gulf of Mexico Hydrates Research Consortium selected the Mississippi Canyon Lease Block 118 (MC118) as a multi-sensor, multi-discipline seafloor observatory for gas hydrate research with geochemical, geophysical, and biological methods. Woolsey Mound is a one-kilometer diameter hydrate complex where gas hydrates outcrop at the sea floor. The hydrate mound is connected to an underlying salt diapir through a network of shallow crestal faults. This research aims to identify the base of the hydrate stability zone without regionally extensive bottom simulating reflectors (BSRs). This study analyzes two collocated 3D seismic datasets collected four years apart. To identify the base of the hydrate stability zone in the absence of BSRs, shallow discontinuous bright spots were targeted. These bright spots may mark the base of the hydrate stability field in the study area. These bright spots are hypothesized to produce an amplitude versus offset (AVO) response due to the trapping of free gas beneath the gas hydrate. AVO analyses were conducted on pre-stacked 3D volume and decreasing amplitude values with an increasing offset, i.e., Class 4 AVO anomalies were observed. A comparison of a time-lapse analysis and the AVO analysis was conducted to investigate the changes in the strength of the AVO curve over time. The changes in the strength are correlated with the decrease in hydrate concentrations over time.

Identification and Characterization of Gas Hydrate using Marine Seismic and Well Data: South Makassar Strait case study

Gas hydrate has the potential as an alternative unconventional resource to current oil and gas. Identification of gas hydrate is made possible by applying the seismic method with the main indicator of the bottom simulating reflector (BSR). BSR is characterized on the seismic section by the presence of high amplitudes, reverse polarity reflections, and cutting through the stratigraphy. In this study, the presence and thickness of the gas hydrate zone have been identified using five 2D seismic tracks, namely CM02-57, SM93-06 SM93-08, SM93-09 and SM93-11, and the SULTAN-1 well. The gas hydrate zone was identified at a depth of about 2300 m. The existence of the gas hydrate zone is followed by the presence of a gas chimney indicator as a free gas migration path. The gas hydrate zones are estimated to be 15 m, 28 m, 33 m, 23 m, and 30 m thick from the respective seismic tracks CM02-57, SM93-06 SM93-08, SM93-09 and SM93-11.

Morphology identification of gas hydrate based on a machine learning method and its applications on saturation estimation

SUMMARY Proper identification of hydrate morphology is an essential pre-condition for the quantification and exploitation of gas hydrate resources. However, the morphology results from core-based analysis and resistivity-based imaging could be discontinuous in hydrate-bearing intervals. Rock physical model-based methods could predict morphology within complete hydrate-bearing intervals, but the accuracy is not much satisfactory in some cases. In this study, we propose a machine learning (ML) method using the wavelet twin support vector machine (WTWSVM) to accurately differentiate the pore-filling and grain-displacing hydrate. By employing different combinations of well logs as the inputs of the WTWSVM, we find the optimal one for the data set in Hydrate Ridge, offshore Oregon is the combination of gamma-ray, resistivity, compressional and shear wave velocity logs, with an accuracy of 88.6 per cent and F1-score of 82.89 per cent. Compared with the two traditional rock-physics-based methods and three typical ML algorithms, the WTWSVM with those optimal inputs performs better in terms of accuracy and F1-score. We then use the WTWSVM to predict the hydrate morphology in the hydrate-bearing intervals at an unlabelled (i.e. unidentified hydrate morphology) site 1250F and a partially labelled (i.e. only a portion of the hydrate and its morphology is identified by IR images) site 1247B at Hydrate Ridge. Finally, the hydrate-morphology-related rock physics models are employed to construct 3-D crossplots of density, compressional and shear wave velocity, on which hydrate concentration, as well as other reservoir parameters, are estimated through projecting. The proposed WTWSVM method and workflow are proved to be valid based on the good agreement between the reservoir parameters from core measurement and elastic properties.

A comparative study of machine learning methods for gas hydrate identification

Gas hydrates are a kind of efficient and clean energy that is recognized as the ideal alternative energy for fossil fuels in the future. Accurate identification of gas hydrate reservoirs is a prerequisite for the application of gas hydrate resources. Artificial intelligence algorithms have been widely applied to solve geological research problems and have obtained good results. Therefore, we use machine learning algorithms to analyze whether sediments contain gas hydrates in the Oregon Hydrate Ridge. In this paper, several commonly used machine learning algorithms are selected, including the random forest, bagging, decision tree, AdaBoost, support vector machine (SVM), k-nearest neighbor (KNN) and gradient boosting decision tree (GBDT). The P- and S-wave velocities (Vp and Vs, respectively) that have high sensitivities to hydrate changes are analyzed, the parameters of different algorithm models are optimized through training, and the classification effects of different algorithm models are compared in detail. Finally, the results show that these algorithms can better distinguish whether there are hydrates in the sediments. Compared with other algorithms, the random forest has the highest accuracy, precision and f1 score in the results of testing hydrate identification models; the AdaBoost has the highest recall; and the KNN has the closest area under curve (AUC) value to 1. The combination of artificial intelligence and resource exploration greatly improves the efficiency and accuracy of hydrate identification, which provides strong support for the subsequent development and utilization of gas hydrate resources.

Identification of gas hydrate on bottom-simulating reflector characteristics with 2D seismic pre-stack time migration of North Bali Waters

Gas hydrate is a physical compound composed of gas molecules that are formed in a seabed layer characterised by high pressure and low temperature. It is known as one of the alternative non-conventional hydrocarbons besides petroleum and natural gas. One of the identified areas of gas hydrate stability zone is in the North Bali Waters. The North Bali Waters is part of the North East Java Basin, which has oil and gas exploration and production, both conventional and non-conventional. One method of identifying the content of gas hydrates is by looking at the appearance of the Bottom Simulating Reflector (BSR) as shown on the Pre-Stack Time Migrated seismic sections. The detection of gas hydrate zone is determined by the presence of high amplitude, reversed polarity reflection and cross-cut reflection of sedimentary layer. This study aims to determine the existence of a BSR in the waters of North Bali. The procedures for analysing the existence of Bottom Simulating Reflector in this study are pre-processing, processing, and interpretation of 2D marine seismic data. The result shows gas hydrates found with indicated Bottom Simulating Reflector on CDP 35-812 at TWT depth of 1526-1582 ms, characterised by high amplitude-reverse polarity.

Morphology identification of gas hydrate from pointwise Lipschitz regularity for P- and S-wave velocity

The accurate morphology identification of gas hydrate-bearing sediments (GHBS) has great significance in practical exploitation and subsequent resource evaluation. Previous studies have disclosed two main morphologies for gas hydrate in sediments: pore- and fracture-filling. However, the existing identification methods of gas hydrate’s morphology rarely consider their intrinsic differences in distribution characteristics. In this paper, a new method is proposed to identify the morphology of hydrate according to the scattered distribution of fracture dips for fracture-filling GHBS. Firstly, numerical simulations are performed to study the relationships between the morphology of hydrate and the sonic velocities. Considering the dip variation is within a certain range for fracture-filling hydrate, the theoretical curves show that the resulting mutation degrees between P- and S-wave velocities are inconsistent in fracture-filling GHBS, which is different from pore-filling GHBS. Then the modified estimation method for pointwise Lipschitz exponent α is introduced to capture their differences. The cross plots of Lipschitz exponent for P-wave velocity, α(Vp), and Lipschitz exponent for S-wave velocity, α(Vs), indicate that most of the dots representing pore-filling GHBS are evenly distributed near the line α(Vp)=α(Vs), while the dots representing fracture-filling GHBS are scattered outside the line α(Vp)=α(Vs). Based on these characteristics, a ratio method is put forward to differentiate the two types of hydrate. These hypotheses and methods are verified using the measured P- and S-wave velocities log data at different sites in Leg 204, Ocean Drilling Program (ODP), in the United States. Finally, the results of this new method agree closely with core data.

Gas hydrate identification and distribution in the Mexican Ridges, Mexico

We have mapped gas hydrates, free gas, and bottom-simulating-reflector (BSR) distributions in an area of the Mexican Ridges, central Gulf of Mexico, Mexico, revealing the relationship between these three elements and the tectonostratigraphy. The three elements are more visible when the host rock is a high-porosity sandstone because there is a large seismic impedance contrast between the solid gas hydrates above and the free gas below, which manifests itself on the seismic as a BSR. Gas hydrates are identified in the well as higher resistivity sandstone layers with a strong positive amplitude. When the host rock has a higher shale content with lower porosity, the impedance contrast is lower and the BSR is weak or not visible. The study area in Mexican Ridges is an anticline where gas hydrates and free gas are trapped on the crest after migrating through the dipping layers and faults from synclines that are generated in calcareous shale. The main seal is mass transport complex deposits from the Pliocene; when they are not deposited at the crest of the anticline, there is gas escape to the seafloor in the form of a gas chimney. In this way, we established a complete petroleum system for gas hydrates and free gas on the Mexican Ridges.

Identification of gas hydrates and bottom-simulating reflectors in far-offset seismic images

The presence of marine gas hydrates is routinely inferred based on the identification of bottom simulating reflectors (BSRs) in common-depth-point (CDP) seismic images. Additional seismic studies such as amplitude variation with offset (AVO) analysis can be applied for corroboration. Although confirmation is needed by drilling and sampling, seismic analysis has proven to be a cost-effective approach to identify the presence of marine gas hydrates. Single-channel far-offset seismic images are investigated for what appears to be a more reliable and cost-effective indicator for the presence of BSRs than traditional CDP processing or AVO analysis. A nontraditional approach for processing seismic data is taken to be more relevant to imaging the gas/gas hydrate contact. Instead of applying the traditional CDP seismic processing workflows from the oil industry, we more carefully review the significant amount of information existing in the data to explore how the character of the data changes as offset angle increases. Three cases from different environments are selected for detailed analysis. These include (1) stratigraphy running parallel with the ocean bottom, (2) a potential BSR, running parallel to the ocean bottom, and cross-cutting dipping reflections, and (3) a suspected thermal intrusion without a recognizable BSR. This investigation considers recently collected multichannel seismic data from the deep waters of the central Aleutian Basin beneath the Bering Sea, the preprocessing of the data sets, and the methodology for processing and display to generate single-channel seismic images. Descriptions are provided for the single-channel near- and far-offset seismic images for the example cases. Results indicate that BSRs related to marine gas hydrates, and originating due to the presence of free gas, are more easily and uniquely identifiable from single-channel displays of far-offset seismic images than from traditional CDP displays.

Identification of gas hydrate based on velocity cross plot analysis

Gas hydrate is regarded as a kind of important energy resource in the recent decades. The identification of gas hydrate is a hot issue for researchers all over the world in order to exploration and exploitation. Previous research shows some methods to identify gas hydrate, but these methods also have shortcomings in different actual conditions. It is still a difficult problem to distinguish gas hydrate accurately from marine sediments. Here we propose a cross plot analysis of velocity to identify gas hydrate. Firstly, we simulate different minerals bearing sediments by rock physics models to obtain different physical properties. After a series of experiments, the results show that there are obvious differences between gas hydrate bearing sediments and other minerals bearing sediments in P-wave and S-wave velocity. Therefore, the cross plot of velocity can be used to distinguish gas hydrate qualitatively. Finally, we verify the cross plot method using the data in the Ocean Drilling Program (ODP) Leg 204 expedition of the Hydrate Ridge along the Oregon continental margin, where gas hydrate is confirmed via logging. The hydrate identification results of actual velocity data by cross plot analysis are generally in keeping with the results of core samples. Moreover, using the cross plot method also can estimate gas hydrate saturation range, which is in accordance with the measurement of chloride (Cl−) concentration in most cases.

A new approach for the identification of gas hydrate in marine sediments

It has always been the focus of researchers to accurately identify gas hydrate location. Geophysical prospecting is a widely used method for gas hydrate exploration, which has high credibility, especially seismic exploration technology is most generally used. In our study, we analyze the different physical properties of gas hydrate and other minerals bearing in unconsolidated marine sediments based on the effective medium theory. Then, a new attribute is put forward to discriminate gas hydrate. The logging data in the Dongsha area of the South China Sea and the Hydrate Ridge along the Oregon continental margin are applied to validate this method. Our test results are basically in line with actual situation, which provides a new approach in hydrate identification.

Seismic identification of gas hydrates: A case study from Sakarya Canyon, western Black Sea

Seismic identification of gas hydrates: A case study from Sakarya Canyon, western Black Sea

Infrared Spectra of Gas Hydrates from First-Principles.

The infrared spectra of sII gas hydrates have been computed using density functional theory for the first time, at equilibrium, and under pressure. It is also the first account of a full vibrational analysis (both guest and host vibrations) for gas hydrates with hydrocarbon guest molecules. Five hydrate structures were investigated: empty, propane, isobutane, ethane-methane, and propane-methane sII hydrates. The computed IR spectra are in good agreement with available experimental and theoretical results. The OH stretching frequencies were found to increase, while the H-bond stretching and H2O libration frequencies decreased with an increase in guest size and cage occupancy and with a decrease in pressure. The H2O bending vibrations are relatively independent of guest size, cage occupancy, pressure, temperature, and crystal structure. The guest vibrational modes, especially the bending modes, also have minimal pressure dependence. We have also provided more quantitative evidence that gas hydrate material properties are defined by their hydrogen bond properties, by linking H-bond strength to Young's modulus. The results and ensuing vibrational analysis presented in this paper are a valuable contribution to the ongoing efforts into developing more accurate gas hydrate identification and characterization methods in the laboratory, in industry/nature, and even in outer space.

New technique for accurate porosity estimation from logging-while-drilling nuclear magnetic resonance data, NGHP-02 expedition, offshore, India

Abstract Drilling and acquiring high-quality downhole log data in the gas hydrate–bearing sedimentary sections, found mostly in deep marine environments and at low temperatures, is often challenging because of the disseminated gas hydrates and mostly unconsolidated nature of the host sediments. Logging while drilling (LWD) technology was used in National Gas Hydrate Program Expedition 02 (NGHP-02) in 25 wellbores to acquire data needed for the comprehensive analysis of gas hydrate occurrences offshore eastern India. The LWD tools deployed included a nuclear magnetic resonance (NMR) tool along with a density measurement tool for porosity computation, a high-resolution resistivity imaging tool for fracture evaluation and depositional system identification, and a borehole acoustic tool for gas hydrate identification and pore-pressure monitoring (by measuring deviation of sonic velocity from normal compaction trend). The NMR LWD tool measures transversely relaxing signal (echoes) decay, which is calibrated and inverted into transverse relaxation times or a T2 distribution (Figure 2). The T2 distribution is then summed to compute NMR porosity. Due to the solid nature of gas hydrates at in situ conditions, they are invisible to NMR. It is fundamental for gas hydrate identification to look for zones with differences between the NMR porosity and density porosity. This paper describes a new evaluation technique that addresses the porosity “deficit problem” due to the fast T2 relaxation observed in NMR data acquired during NGHP-02. In the new algorithm, the T1-T2 distributions are jointly inverted, compared to conventional NMR processing, which inverts a T2 distribution from the echo signal using a constant T1/T2 ratio. It is observed that with the conventional method which assumes a constant T1/T2 ratio, the NMR porosity in gas hydrate bearing zones is underestimated by about 3–6 porosity units, and the derived gas hydrate saturations are overestimated by ∼8–10%.

Application of AVO attribute inversion technology to gas hydrate identification in the Shenhu Area, South China Sea

AVO (Amplitude Versus Offset) is a seismic exploration technology applied to recognize lithology and detect oil and gas through analyzing the feature of amplitude variation versus offset. Gas hydrate and free gas can cause obvious AVO anomaly. To find geophysical evidence of gas hydrate and free gas in Shenhu Area, South China Sea, AVO attribute inversion method is applied. By using the method, the multiple seismic attribute profiles and AVO intercept versus gradient (I-G) cross plot are obtained. Bottom-simulating reflector (BSR) is observed beneath the seafloor, and the AVO abnormal responses reveal various seismic indicators of gas hydrate and free gas. The final AVO analysis results indicate the existence of gas hydrate and free gas in the upper and lower layers of BSR in the study area.

Pore water geochemistry in shallow sediments from the northeastern continental slope of the South China sea

The northeastern continental slope of the South China Sea is one of the promising areas for gas hydrates, which is characterized by its variable topography and complex tectonics. In this paper, two gravity piston cores were sampled from the shallow sediments in the vicinity of the region where various morphologies of gas hydrate was recovered during the second gas hydrate expedition of Guangzhou Marine Geological Survey. Conventionally, the identification of gas hydrate was primarily dependent on the bottom simulating reflector (BSR). Apart from BSR, the anomalies in pore water geochemistry, which are highly related to various microbially-mediated processes and features of fluid from deep hydrocarbon reservoir, are also helpful for the gas hydrate exploration. In this study, upward methane flux may be responsible for distinct characteristics of pore waters observed at the two sample sites. A reactive transport model was accordingly established to compare contrastive fluid features and to estimate the upward methane flux. Since fluid migration is typically associated with complex tectonic settings of the study area, it is proposed that heterogeneous distribution of gas hydrates along the continental margin should be well considered.

Study on p-Wave Attenuation in Hydrate-Bearing Sediments Based on BISQ Model

In hydrate-bearing sediments, the elastic wave attenuation characteristics depend on the elastic properties of the sediments themselves on the one hand, and on the other hand, they also depend on the hydrate occurrence state and hydrate saturation. Since the hydrate-bearing sediments always have high porosity, so they show significant porous medium characteristics. Based on the BISQ porous medium model which is the most widely used model to study the attenuation characteristics in the porous media, we focused on p-wave attenuation in hydrate-bearing sediments in Shenhu Area, South China Sea, especially in specific seismic frequency range, which lays a foundation for the identification of gas hydrates by using seismic wave attenuation in Shenhu Area, South China Sea. Our results depict that seismic wave attenuation is an effective attribute to identify gas hydrates.

Numerical simulation of bubble plumes in overlying water of gas hydrate in the cold seepage active region

To study the seismic responses produced by gas hydrate bubble plumes in the cold seepage active region, we constructed a plume water body model based on random medium theory and acoustic velocity model of bubble medium. The plume water body model was forward simulated by finite difference. Seismic records of single shot show the scattered waves produced by the plume. The scattered wave energy is strong where the plume exists. Where the scattered wave energy is stronger, the minimum of travel time is always above the plume, which has no relationship with the shot’s position. Seismic records of shot gathers were processed by prestack time migration. The migration section shows that the scattered waves produced by plumes can be imaged distinctly with higher accuracy. These researches laid a foundation for further study on the seismic responses produced by plumes and provided a new approach for the identification of gas hydrate.

Quantitative interpretation for gas hydrate accumulation in the eastern Green Canyon Area, Gulf of Mexico using seismic inversion and rock physics transform

Gas hydrate can be interpreted from seismic data through observation of bottom simulating reflector (BSR). It is a challenge to interpret gas hydrate without BSR. Three-dimensional qualitative and quantitative seismic interpretations were used to characterize gas hydrate distribution and concentration in the eastern Green Canyon area of the Gulf of Mexico, where BSR is absent. The combination of qualitative and quantitative interpretation reduces ambiguities in the estimation and identification of gas hydrate. Sandy deposition and faults are qualitatively interpreted from amplitude data. The 3D acoustic impedance volume was interpreted in terms of high P-impedance hydrate zones and low P-impedance free gas zones. Gas hydrate saturation derived from P-impedance is estimated by a rock physics transform. We interpreted gas hydrate in the sand-prone sediments with a maximum saturation of approximately 50% of the pore space. Sheet-like and some bright spot gas hydrate accumulations are interpreted. The interpretation of sheet-like gas hydrate within sand-prone sediments around faults suggests that fluid moves into the sand zones laterally by conduits. Variations in depths of interpreted gas hydrate zones imply nonequilibrium conditions. Low P-impedance free gas zones within high P-impedance gas hydrate zones imply possible coexistence of hydrate and free gas within the hydrate stability zone. We propose that gas hydrate distribution and concentration are associated with structures, buried sedimentary bodies, sources of gas, and fluid flux.

Seismic Identification of Gas Hydrate and Its Distribution in Shenhu Area, South China Sea

AbstractShenhu Area of Northern South China Sea has favorable conditions for the occurrence and formation of gas hydrate, which was discovered in drilling cores and disseminated evenly within the hydrate stable zone (HSZ). Based on 3‐D seismic data from marine geophysical investigation of gas hydrate, a variety of methods for gas hydrate identification were introduced, including high‐resolution seismic imaging, impedance inversion, seismic attribute cluster analysis and neural network analysis. The applicability of these techniques were discussed and comprehensive interpretation of the results indicates that seismic attribute cluster analysis is the optimal technique for gas hydrate identification, by which the 3‐D distribution of free gas and gas hydrate is delineated and is identical with the drilling cores. Integrating the drilling cores with geological conditions, various geophysical information was used to analyze the geological cause of formation and the distributing model of free gas and gas hydrate, from which it can be concluded that faults and fractures are the important if not dominant factor in the formation and distribution of gas hydrate in Shenhu Area.

Recognition of gas hydrate using AVO-attribute crossplots based on the porous medium theory

Gas hydrate is gradually considered as a potential energy resource. The presence of gas hydrate is commonly inferred from the appearance of “bottom simulating reflector”(BSR) on seismic section. Understanding the properties of hydrate-bearing sediments and studying the AVO characteristics of BSR are of great significance. Although more and more domestic and international studies have been conducted on the subjects mentioned above, they are still in the primary stage and need a long way to go to be appled in practice, especially in the field of gas hydrate. Aiming at the identification of gas hydrate, we studied the characteristics of the AVO attributes based on the Biot’s theory when the sediments were bearing gas hydrate or free gas. The AVO attribute crossplots obtained from seismic sections with the forward simulation by means of staggered-grid finite-difference were compared with that of theoretic models. The coincidence shows that utilization of AVO attribute crossplots is an effective way to recognize gas hydrate and free gas.