Physical Properties Of Hydrate-bearing Sediments Research Articles

Pore-scale analysis of hydrate saturation on the physical parameters of hydrate-bearing sediment with different particle shapes

Methane hydrate is the most potential novel energy source with huge reserves to be developed, and is the priority of countries' future energy development strategies. Evaluating the basic physical characteristics of hydrate reservoirs, especially permeability, thermal, and electrical conductivity, is essential for clarifying the main controlling factors of physical characteristics and formulating a reasonable development strategy. Accordingly, 16 cementing hydrate-bearing models with different particle shapes were constructed account of a new three dimensions morphologic modeling algorithm for hydrate in this study. Based on the above models, the evolution of pore space distribution and basic physical properties of hydrate-bearing sediments (HBS) have been analyzed via pore network modeling (PNM) and finite volume method (FVM). The results show that (1) the quantity of pores and throats of HBS with round particles is higher, and the average radius is smaller than those of HBS with irregular particles, (2) increasing hydrate saturation leads to a decrease in absolute permeability, with a more rapid decrease in permeability for HBS with irregular particles, (3) the thermal conductivity of HBS with round particles exceeds those of HBS with irregular particles, and the their curves remain parallel with increasing hydrate saturation, (4) the resistivity index of HBS with round particles increases faster as hydrate forms, and the conductivity is always smaller than those of HBS with irregular particles.

Modeling the physical properties of hydrate‐bearing sediments: Considering the effects of occurrence patterns

Investigating the dependence of the physical properties of hydrate-bearing sediments (HBS) on hydrate occurrence patterns and saturation levels is crucial for exploring gas hydrate resources. However, precise analysis of these effects remains challenging due to limitations in current experimental techniques and numerical modeling methods that cannot accurately control the hydrate saturation and distribution patterns. In this work, to address such issues, we propose a novel hybrid modeling approach integrating X-ray CT imaging technology, morphological operation algorithm, and quartet structure generation set method. Then, we generate 75 samples containing pore-floating, cementing, as well as bridging hydrates, each with predefined saturation levels, and comprehensively investigate the effects of the hydrate distribution patterns and saturation on the pore radius, coordination number, correlation functions, permeability, electrical conductivity, and elastic moduli of HBS. The findings indicate that the heterogeneity of pore and throat radius distributions varies across different hydrate types. The increase in pore-floating hydrate leads to the most rapid decline in the pore and throat radii, tortuosity, and pore-space correlation, but it makes the average coordination number larger while the others decrease the number. Moreover, the cementing patterns cause the weakest damage to the permeability and electrical conductivity with low hydrate saturation. When the hydrate saturation is larger than about 45%, the bridging pattern has the greatest effect on the mass transport properties. Furthermore, the pore-floating and bridging patterns cause the largest and smallest increase in the elastic moduli, respectively. The hydrate occurrence pattern causes a larger effect on the bulk modulus than the shear modulus.

Visual experimental study on hydrate occurrence patterns and growth habits in porous media

The distribution and morphology of natural gas hydrates in pores have significant impacts on the macroscale physical properties of hydrate-bearing sediments. This paper investigated hydrate morphology characteristics using a micromodel, and the memory effect was used to accelerate the hydrate formation process. It is found that the initial gas and water saturation play a significant role in hydrate occurrence patterns and growth habits. When water is the initial main continuous phase, the hydrate prefers to grow towards the center of pores during hydrate formation. When gas is the initial main continuous phase, hydrates grow towards the center of pores and along the wall, and the mass transfer happens between the water and gas phases. It can be realized that the hydrate occurrence pattern is determined by the growth habit, which is related to mass transfer. These findings provide a theoretical foundation for future natural gas hydrate development.

Pore-scale observations of natural hydrate-bearing sediments via pressure core sub-coring and micro-CT scanning

Both intra-pore hydrate morphology and inter-pore hydrate distribution influence the physical properties of hydrate-bearing sediments, yet there has been no pore-scale observations of hydrate habit under pressure in preserved pressure core samples so far. We present for the first time a pore-scale micro-CT study of natural hydrate-bearing cores that were acquired from Green Canyon Block 955 in UT-GOM2-1 Expedition and preserved within hydrate pressure–temperature stability conditions throughout sub-sampling and imaging processes. Measured hydrate saturation in the sub-samples, taken from units expected to have in-situ saturation of 80% or more, ranges from 3 ± 1% to 56 ± 11% as interpreted from micro-CT images. Pore-scale observations of gas hydrate in the sub-samples suggest that hydrate in silty sediments at the Gulf of Mexico is pore-invasive rather than particle displacive, and hydrate particles in these natural water-saturated samples are pore-filling with no evidence of grain-coating. Hydrate can form a connected 3D network and provide mechanical support for the sediments even without cementation. The technical breakthrough to directly visualize particle-level hydrate pore habits in natural sediments reported here sheds light on future investigations of pressure- and temperature-sensitive processes including hydrate-bearing sediments, dissolved gases, and other biochemical processes in the deep-sea environment.

Visualization of interactions between depressurization-induced hydrate decomposition and heat/mass transfer

Visual evidences to understand the interactions between hydrate decomposition and heat/mass transfer are currently lacking. This study proceeds from the hydrate morphology to visualize the interactions between depressurization-induced hydrate decomposition and heat/mass transfer from different scales. Reactor-scale hydrate distribution evolution shows that the dominant influencing factor of hydrate decomposition transforms from heat transfer to mass transfer. More importantly, pore-scale visual evidences suggest that the mass transfer of gas shows significant effects on hydrate morphology evolution. Specifically, the limited gas diffusion in liquid phase could lead to the hydrate morphology evolution from patchy pore-filling to “grain-bridging” during hydrate decomposition. The combination of grain-bridging hydrate together with the water layer that wraps the hydrate is termed as “hydrate bridge” in this work. It is also worth noting that the grain-bridging hydrate could accelerate fluid flow in pores according to our seepage simulation results. These findings provide visual evidences for variations in physical properties of hydrate-bearing sediments during hydrate decomposition. Since physical properties of hydrate-bearing sediments play important roles in hydrate decomposition, the hydrate morphology evolution characteristics analyzed here are valuable for hydrate exploitation in field tests.

Visualization of Interactions between Depressurization-Induced Hydrate Decomposition and Heat/Mass Transfer

Abstract Visual evidences to understand the interactions between hydrate decomposition and heat/mass transfer are currently lacking. This study proceeds from the hydrate morphology to visualize the interactions between depressurization-induced hydrate decomposition and heat/mass transfer from different scales. Reactor-scale hydrate distribution evolution shows that the dominant influencing factor of hydrate decomposition transforms from heat transfer to mass transfer. More importantly, pore-scale visual evidences suggest that the mass transfer of gas shows significant effects on hydrate morphology evolution. Specifically, the limited gas diffusion in liquid phase could lead to the hydrate morphology evolution from patchy pore-filling to “grain-bridging” during hydrate decomposition. The combination of grain-bridging hydrate together with the water layer that wraps the hydrate is termed as “hydrate bridge” in this work. It is also worth noting that the grain-bridging hydrate could accelerate fluid flow in pores according to our seepage simulation results. These findings provide visual evidences for variations in physical properties of hydrate-bearing sediments during hydrate decomposition. Since physical properties of hydrate-bearing sediments play important roles in hydrate decomposition, the hydrate morphology evolution characteristics analyzed here are valuable for hydrate exploitation in field tests.

Dependence of acoustic properties on hydrate-bearing sediments with heterogeneous distribution

The relationship between the hydrate-bearing sediments with physical properties such as wave velocity is of importance for the exploration and exploitation of natural gas hydrate. The distribution modes of naturally occurring gas hydrates in sediments are diverse and have significant influence on physical properties of rocks. However, hydrates in homogeneous distribution are in general investigated in laboratory currently. In this work, to simulate the diverse distribution modes of naturally occurring gas hydrates, a novel method to prepare the hydrate-bearing sediments with heterogeneous distribution including layered distribution was proposed by controlling the temperature, water migration, and water saturation distribution. Velocity and amplitude of P-wave are investigated to evaluate the physical properties of hydrate-bearing sediments with homogeneous or heterogeneous distribution. The results showed that the hydrate formation process apparently consists of unconsolidated and consolidated steps according to the amplitude curve. The amplitude of P-wave in the unconsolidated stage increases slowly, while it increases suddenly and rapidly in the consolidated stage. The P-wave velocity of the hydrate deposits with homogeneous distribution is faster than that with heterogeneous distribution when under a same hydrate saturation. This is also verified from the experiments performed on hydrate-bearing sediments with layered distribution, which provided direct evidence and quantification about heterogeneous degree of hydrate distribution in sediment. This difference of the P-wave velocity between heterogeneous and homogeneous hydrate-bearing sediments would provide a guidance in the future development of natural gas hydrate.

Pore-scale evidence of ion exclusion during methane hydrate growth and evolution of hydrate pore-habit in sandy sediments

Hydrate-bearing sediments constitute complex hydrological systems, within which the pore habit of hydrate significantly affects all physical properties. However, there is scarce pore-scale evidence of the evolution of hydrate growth in porous media and the influence of water salinity in the micro-morphology of hydrate. In this paper, we used X-ray computed micro-tomography (CT) to monitor three experiments of methane hydrate growth in sand partially saturated with either NaBr or KI brine under excess-gas conditions. Gas, brine and hydrate are observed to co-exist at the pore-scale during the evolution of hydrate pore-habit towards three-phase equilibrium. The experiments and data analysis reveal that (1) hydrate forms as a porous medium mixed with inclusions of brine and gas, where hydrate and brine evolve gradually into separate phases as hydrate cages exclude salt ions, (2) hydrate growth mobilizes water over fairly long distances resulting in heterogeneous hydrate distribution, and (3) hydrate can exhibit interconnecting pore-habit at local hydrate saturations higher than ~50%, even if grown under excess-gas conditions. The results imply that hydrate micro-morphology and pore habit evolve during hydrate formation and, we suggest, so do the physical properties of hydrate-bearing sediments. Our findings provide new insights into the micro-morphology of hydrate and evolution of the properties of hydrate-bearing sediments synthesized in the laboratory and in natural systems.

Pore habit of methane hydrate and its evolution in sediment matrix – Laboratory visualization with phase-contrast micro-CT

Abstract Describing the pore habit of methane hydrate in sediment matrices is essential for understanding natural distribution of methane hydrate, methane trace (transport and solidification) in the hydrate stability zone, physical properties of hydrate-bearing sediments, and the associated influence on potential gas production. Pore habit visualization in natural media at pore scale even with laboratory synthesized cores has been challenging due to the similar densities of methane hydrate from pore liquid. In this work, we used phase-contrast assisted micro-CT with potassium iodine-doped brine to visualize four phases: sand particles, pore fluid, methane hydrate and methane gas. This study visualizes the pore habit of methane hydrate at various stages including during hydrate formation in excess-gas systems, its evolution after brine injection to replace pore fluid, and hydrate formation in excess-water systems. Hydrate tends to adopt round and smooth surfaces when in contact with water while exhibits relatively angular interfaces when in contact with methane gas. Hydrate formation in excess-gas systems results in a partial cementing and partial mineral-coating pore habit, while hydrate in excess-water systems develops mainly as pore-filling, and locally cementing or mineral-coating where big gas pockets exist at the initial state. Pore liquid replacement from methane gas to brine triggers a shift of hydrate pore habit towards pore-filling. Methane hydrate evolution over time produces bigger hydrate particles but with less contact area with sand particles. The effects of hydrate pore habit become less important as hydrate particle size exceeds the pore size. Additionally, hydrate formation could trap residual methane gas and brine as inclusions.

Dynamical mechanism of hydrate formation resulting from free gas migration into seeping seafloor sediments

Generating hydrate-bearing samples of nature-approximation in the lab is critically important for evaluating the integrated physical properties of hydrate-bearing sediments, accurately assessing the potential of marine hydrate resources and establishing a safe and efficient gas production technique; however, current database on generating hydrate-bearing samples of nature-approximation in the lab are still limited due to experimental challenges. Here we report the dynamical formation of gas hydrate resulting from free gas migration into seeping seafloor sediments through visual observation using a magnetic resonance imaging (MRI) device. The effects of core temperature, pore pressure, gas flow rate, and solution salinity are preliminarily explored. The saturations of water, gas, and hydrate are calculated based on the mean intensity (MI) of obtained MRI images. The residual water saturation variation along with gas flow direction during hydrate formation are analyzed. Also, the residual water saturation distribution in the pore space are visually presented.

Pore-scale identification of multi-phase spatial distribution of hydrate bearing sediment

The pore-scale location of hydrates exerts strong control on the macroscale physical properties of hydrate-bearing sediments. Many microfocus x-ray computed tomography (CT) devices have been used to detect the pore-scale spatial distribution of water, free natural gas, hydrate and sediment within the pore spaces of hydrate-bearing sediments. However, it is not easy to identify these four phases for intersection of grayscale intervals in CT images based on the usually obtained bimodal or trimodal histogram of grayscale in the threshold segmentation method, where four thresholds are required. To help deal with the overlapping areas of the grayscale in CT images, the Markov random field (MRF) is employed to promote the identification result of each phase, in the concept of maximum a posteriori (MAP) probability based on Bayes' theorem. After validation in the example of Berea sandstone (voids and solid), this method behaves well in hydrate-bearing sediment, which is consistent with the hydrate saturation of the experiment result, by finding the appropriate grayscale intervals and potential energy function.

Hydrate growth in granular materials: implication to hydrate bearing sediments

It is often assumed that the properties of hydratebearing sediments are affected by hydrate growth habits in pores. We test this hypothesis by stochastically simulating hydrate growth in pore space of a simple cubic packing of mono-sized spherical sediment grains, and assume three extreme nucleation preferences in the absence of heat or mass transport limitations, and without interfacial surface tension: (1) Homogeneous nucleation produces isolated nuclei and a highly porous hydrate mass; (2) Heterogeneous nucleation results in combined surface coating and framebuilding hydrate structures; and (3) Preferential heterogeneous nucleation on pre-existing hydrate surfaces leads to hydrate growth into the pore space. Spatial distribution of hydrate is analyzed to investigate the surface coating density, grain connectivity and mass density of hydrate cluster. Results show that surface coating density is predominant for heterogeneous nucleation and grain connectivity by hydrates is lower than the hydrate volume fraction in all cases. Therefore, a slow increase in stiffness is anticipated at low hydrate volume fraction. The highly porous hydrate structure formed by homogeneous nucleation and dense hydrate cluster are discussed with the relevance to the geomechanical and physical properties of hydrate-bearing sediments.

Hydrate adhesive and tensile strengths

The physical properties of hydrate-bearing sediments depend on the interaction between hydrates and minerals. In particular, hydrates prefer to nucleate on mineral surfaces, therefore, the hydrate-mineral adhesive strength and the tensile strength of the hydrate mass itself affect the mechanical response of hydrate-bearing sediments. In this study, ice and hydrates made with various guest molecules (CO2, CH4, and THF) are formed between mica and calcite substrates. Adhesive and tensile strengths are measured by applying an external pull-out force. Results show that tensile failure occurs in CO2 and CH4 hydrates when calcite is the substrate, while ice and all hydrates exhibit adhesive failure on mica. The debonding strength is higher when calcite substrates are involved rather than mica substrates. A nominal pull-out strength of 0.15 ± 0.03 MPa can be adopted for mechanical analyses of hydrate-bearing sediments.

Physical Properties of Gas Hydrates: A Review

Methane gas hydrates in sediments have been studied by several investigators as a possible future energy resource. Recent hydrate reserves have been estimated at approximately1016 m3of methane gas worldwide at standard temperature and pressure conditions. In situ dissociation of natural gas hydrate is necessary in order to commercially exploit the resource from the natural-gas-hydrate-bearing sediment. The presence of gas hydrates in sediments dramatically alters some of the normal physical properties of the sediment. These changes can be detected by field measurements and by down-hole logs. An understanding of the physical properties of hydrate-bearing sediments is necessary for interpretation of geophysical data collected in field settings, borehole, and slope stability analyses; reservoir simulation; and production models. This work reviews information available in literature related to the physical properties of sediments containing gas hydrates. A brief review of the physical properties of bulk gas hydrates is included. Detection methods, morphology, and relevant physical properties of gas-hydrate-bearing sediments are also discussed.

Scientific results from Gulf of Mexico Gas Hydrates Joint Industry Project Leg 1 drilling: Introduction and overview

The Gulf of Mexico Gas Hydrates Joint Industry Project (JIP) is a consortium of production and service companies and some government agencies formed to address the challenges that gas hydrates pose for deepwater exploration and production. In partnership with the U.S. Department of Energy and with scientific assistance from the U.S. Geological Survey and academic partners, the JIP has focused on studies to assess hazards associated with drilling the fine-grained, hydrate-bearing sediments that dominate much of the shallow subseafloor in the deepwater (>500 m) Gulf of Mexico. In preparation for an initial drilling, logging, and coring program, the JIP sponsored a multi-year research effort that included: (a) the development of borehole stability models for hydrate-bearing sediments; (b) exhaustive laboratory measurements of the physical properties of hydrate-bearing sediments; (c) refinement of new techniques for processing industry-standard 3-D seismic data to constrain gas hydrate saturations; and (d) construction of instrumentation to measure the physical properties of sediment cores that had never been removed from in situ hydrostatic pressure conditions. Following review of potential drilling sites, the JIP launched a 35-day expedition in Spring 2005 to acquire well logs and sediment cores at sites in Atwater Valley lease blocks 13/14 and Keathley Canyon lease block 151 in the northern Gulf of Mexico minibasin province. The Keathley Canyon site has a bottom simulating reflection at ∼392 m below the seafloor, while the Atwater Valley location is characterized by seafloor mounds with an underlying upwarped seismic reflection consistent with upward fluid migration and possible shoaling of the base of the gas hydrate stability (BGHS). No gas hydrate was recovered at the drill sites, but logging data, and to some extent cores, suggest the occurrence of gas hydrate in inferred coarser-grained beds and fractures, particularly between 220 and 330 m below the seafloor at the Keathley Canyon site. This paper provides an overview of the results of the initial phases of the JIP work and introduces the 15 papers that make up this special volume on the scientific results related to the 2005 logging and drilling expedition.