Seismic rock physical modelling for gas hydrate-bearing sediments

Multiple physical properties of gas hydrate-bearing sediments recovered from Alaska North Slope 2018 Hydrate-01 Stratigraphic Test Well

Compressional and shear-wave velocities from gas hydrate bearing sediments: Examples from the India and Cascadia margins as well as Arctic permafrost regions

Gas hydrate exploration was conducted using logging-while-drilling (LWD) and wireline logging (WLL) at two sites (UBGH2-6 and UBGH2-10) in the Ulleung Basin, East Sea (Sea of Japan). Coring for gas hydrate sampling was also conducted. Seismic profiles revealed that the upper layers at the logged sites (∼180 m at UBGH2-6 and ∼150 m at UBGH2-10) are dominated by turbidite/hemipelagic sediments, whereas the lower-lying sediments are characterised by thick mass transport deposits. Except in the gas hydrate-bearing interval (140–160 metres below the seafloor) of UBGH2-6, the WLL velocity and resistivity data are a little higher (∼100 m/s and ∼0.4 ohm-m, respectively) than those indicated by LWD. Conversely, the density and porosity do not show a consistent pattern for the two methods. Discrepancies between LWD and WLL can be caused by the methodology, borehole condition, and gas hydrate occurrence type. The high resistivity values (∼100 ohm-m) in the gas hydrate-bearing zone of UBGH2-6 are typical of inclined fracture-filling gas hydrate. In this case, the LWD resistivity may be overestimated as a result of the data acquisition principle. The relationships between the physical properties are significantly controlled by the presence of gas hydrate. Therefore, in fracture-filling gas hydrate-bearing sediments, the WLL resistivity data are more reasonable than the LWD resistivity data. Both methods have advantages and disadvantages in terms of data quality, efficiency, and resolution. Therefore, it is necessary to compare and interpret data measured using both methods to identify the physical properties of gas hydrate-bearing sediments.

A state-dependent subloading constitutive model with unified hardening function for gas hydrate-bearing sediments

The microscale physical properties of gas hydrate-bearing sediments (HBSs) are significant for understanding their macroscale elastic responses and further facilitating seismic exploration. Several models have been developed to investigate the microscale properties of gas HBSs, whereas most of them place emphasis on the construction of the rock frame, ignoring the influence of mixing patterns of pore fluids. Based on laboratory observations, we have developed a rock-physics model that integrates the spatial distribution of gas hydrate, water, and free gas in pores; in addition, this model considers the variable stress-strain relationship of the pore fluids depending on hydrate saturation. We also attempt to incorporate the effect of temperature on the elastic properties of gas HBSs through theoretical modeling. Our approach of handling hydrate-gas spatial relationship reasonably delineates the velocity trends, ensuring that prediction results are congruent with field data. Moreover, the variable stress-strain relationship of the pore fluids allows for the achievement of better simulation results than those by conventional iso-stress and iso-strain fluid mixing schemes. Integrating the factors tied to hydrate dissociation and hydrate moduli reduction during heating processes enables the prediction of a declining trend in the velocity-temperature relation, which is congruent with laboratory-measured data. This model provides an alternative approach to predict the elastic properties of gas HBSs and can reasonably explain the effects of fluid saturation and temperature.

Pore-scale morphology and wettability characteristics of xenon hydrate in sand matrix - Laboratory visualization with micro-CT

Pore- and fracture-filling gas hydrates were identified from the core samples at several sites during the second Guangzhou Marine Geological Survey (GMGS2) expedition. Well logs indicated that gas hydrate occurred in three distinct layers at site GMGS2-08. The gas hydrate saturations calculated from well-log data and the seismic responses for the three gas hydrate-bearing layers, especially within the middle carbonate layer, were poorly known. We estimated gas hydrate saturations using isotropic and anisotropic models based on the mineral composition of the sediments and the effective medium theory. In the upper and lower gas hydrate-bearing layers, saturations estimated from anisotropic models are close to those estimated from pressures cores and chlorinity data. The average saturation using an anisotropic model in the upper (fracture-dominated) hydrate layer is approximately 10% with a maximum value of 25%. In the lower (fracture-dominated) layer, the horizontal and vertical gas hydrate-filled fractures and visible gas hydrate were formed with a maximum saturation of approximately 85%. For the middle layer, well logs show high P-wave velocity, density, high resistivity as well as low gamma ray, porosity, and drilling rate, together indicating a carbonate layer containing gas hydrate. The hydrate saturations calculated from isotropic models assuming hydrate formed at grain contacts are less than 20%, which fit well with two values calculated from chlorinity data for this layer. The upper gas hydrate layer shows no clear seismic response and probably consisted of small fractures filled with gas hydrate. The middle carbonate and lower fracture-filled gas hydrate-bearing layers show pull-up reflections, with the carbonate layer exhibiting relatively higher amplitudes. Pore-filling gas hydrate was also identified just above the depth of the bottom-simulating-reflector (BSR) from the GMGS2-05 drill site. Below the BSR, the push-down reflections, polarity reversal, and enhanced reflections indicate the occurrence of free gas in the study area.

A triaxial system is designed with a temperature range from −20 °C to 25 °C and a pressure range from 0 MPa to 30 MPa in order to improve the understanding of the mechanical properties of gas hydrate-bearing sediments. The mechanical properties of synthetic gas hydrate-bearing sediments (gas hydrate-kaolin clay mixture) were measured by using current experimental apparatus. The results indicate that: (1) the failure strength of gas hydrate-bearing sediments strongly depends on the temperature. The sediment’s strength increases with the decreases of temperature. (2) The maximum deviator stress increases linearly with the confining pressure at a low-pressure stage. However, it fluctuates at a high-pressure stage. (3) Maximum deviator stress increases with increasing strain rate, whereas the strain-stress curve has no tremendous change until the axial strain reaches approximately 0.5%. (4) The internal friction angles of gas hydrate-bearing sediments are not sensitive to kaolin volume ratio. The cohesion shows a high kaolin volume ratio dependency.

Quantification of hydrates in permafrost sediments using conventional seismic techniques has always been a major challenge in the study of the climate-driven evolution of gas hydrate-bearing permafrost sediments due to almost identical acoustic properties of hydrates and ice. In this article, a coupled geophysical-geothermal scheme is developed, for the first time, to predict hydrate saturation in gas hydrate-bearing permafrost sediments by utilising their geophysical and geothermal responses. The scheme includes a geophysical part which interprets the measured elastic wave velocities using a rock-physics model, coupled with a geothermal part, interpreting the measured effective thermal conductivity (ETC) using a new pore-scale model. By conducting a series of sensitivity analyses, it is shown that the ETC model is able to incorporate the effect of the hydrate pore-scale habit and hydrate/ice-forced heave as well as the effect of unfrozen water saturation under frozen conditions. Given that the geophysical and geothermal responses depend on the overburden pressure, the elastic wave velocities and ETC of methane hydrate-bearing permafrost sediment samples were measured at different effective overburden pressures and the results were provided. These experimental data together with the results of our recent study on the geophysical and geothermal responses of gas hydrate-bearing permafrost sediment samples at different hydrate saturations are used to validate the performance of the coupled scheme. By comparing the predicted saturations with those obtained experimentally, it is shown that the coupled scheme is able to quantify the saturation of the co-existing phases with an acceptable accuracy in a wide range of hydrate saturations and at different overburden pressures.

A model to predict the elastic properties of gas hydrate-bearing sediments

ABSTRACT: Wellbore integrity in a gas hydrate bearing formation during drilling remains great challenge as a complex solid-fluid and decomposed hydrate process interaction is involved. A wellbore temperature increase and/or depressurization processes my induce additional stresses and extra fluid flow, which in turn may trigger wellbore instability and unexpected fracturing. In order to understand and quantify the thermal-hydraulic-mechanical behavior with hydrate decomposition (THMD) process and the wellbore responses to the pressure and temperature variation under a given in-situ stress regime, a fully coupled THMD model is proposed and developed in this paper. A simplified semi-analytical solution for the induced stresses, pore pressure, and temperature change at the wellbore and inside the formation is presented. A linear Mohr-Coulomb criterion is utilized to define the onset of the wellbore instability in which a hydrate saturation depending cohesion is also incorporated. Laplace transformation is applied and a numerical method is used to solve the transformed homogeneous PDE. We conclude that thermally and hydration induced stresses may affect wellbore integrity during hydrate gas production due to wellbore pressure reduction and temperature increase. Unlike those stress perturbation in the conventional gas reservoir, critical temperature and pressure must be surpassed and additional incremental stresses may be induced due to the hydrate hydrate decomposition. Also additional fluid mass and energy transfer may take place by which both temperature and pore pressure may be induced due to the hydrate decomposition/recomposition. In additional the hydrate saturation change due to the typical drilling strategy applied may reduce the hydrate formation significantly,and this may also affect the production pressure control and management. Thus managing pressure for both maximum production and wellbore integrity must be optimized accordingly. 1. INTRODUCTION Wellbore stability has been a long-observed and widely studied problem during drilling and production in petroleum industries. Stresses around a created borehole due to primarily the in-situ stress from the far-field may increased during drilling and production when the wellbore pressure is reduced. Once such an induced stress state exceeds a strength, measured or estimated, a borehole stability issue, i.e. the borehole lost its integrity, will be brought up. Other than affected by the intact formation strength and initial in-situ stresses, wellbore orientation, formation hydraulic and thermal conditions, and drilling mud chemistry are all critical factors in stability analysis [Bradley, 1979; Santarilli et al., 1986, Detournay and Cheng, 1988; Ewy and Cook, 1990; Wang et al., 1996; Gassemi and Liu, 2005; Neyan, 2010]. Producing hydrate gas either by depressurization or/and increasing wellbore temperature has been proposed for gas exploitation in gas hydrate-bearing sediments (GHBS)[Masuda et al., 1997; Moridis, 2003; Moridis et al., 2007]. Wellbore stability has been reported due to a much more complex mechanical responses maybe provoked near a borehole to the exploiting procedure mentioned [Hyoto et al., 2002; 2008; 2013; 2014; Li et al., 2019,Wang, 2019]. The most economical approach in producing hydrate gas is to change the ice-like hydrate into fluid by reducing the pressure or increase the temperature so that a critical equilibrium condition is exceeded [Kim, 1987; Holder and Hand; 1982]. Such a procedure on the other hand will dissociate the hydrate so that newly created fluid (water and /or gas) mass may alter the conventional equilibrium condition. The new dynamic equilibrium system due the hydrate phase change may affect the mechanical response of the entire formation and particularly in the vicinity of the borehole, where phase change shall take place first and persist until the solid hydrate is decomposed completely. Furthermore the formation strength may be reduced significantly after hydrate dissociation [Masui et al., 2005; 2008; Hyoto et al., 2002; 2008; 2013, 2014; Li et al., 2019; Priest et al.; 2019]. A coupled thermal-hydraulic-mechanical model with hydrate dissociation (THMD) is proposed and numerical methods are typically utilized, i.e. Uchida et al., [2013]; Freij-Ayoub et al., 2017]. Although analytical solutions are convenient and a tool easy to use, but difficult to be tractable, a semi-analytical solution is developed in this paper for borehole mechanical responses under depressurization. A analytical solution and fully coupled THMD model are required for drilling and well integrity design during drilling and production; A semi-analytical solution is developed by incorporating THMD model including phase change and hydrate saturation-dependent cohesion. Specifically the solution is applicable specifically for the situations when the critical temperature/pressure are surpassed, i.e. the solid hydrate dissociation initiates and is decomposed into water and gas phase. Unlike those in the conventional reservoirs, the cohesion in GHBS is weakened significantly with hydrate saturation reduction [Koyno et al.,2002; 2008; 2013; 2014; Li et al., 2019]. gas and water saturation are balanced between fluid extraction at a producing well and supply from either the adjacent formation and those decomposed in each pore volume. The porosities based on the pore volume in the solid phase from the porous skeleton and combined solid hydrate+skeleton should be defined, as

The Research Path to Determining the Resource Potential of Marine Gas Hydrates Ray Boswell; Ray Boswell U.S. Department of Energy Search for other works by this author on: This Site Google Scholar Kelly Rose; Kelly Rose U.S. Department of Energy Search for other works by this author on: This Site Google Scholar Richard Baker Richard Baker U.S. Department of Energy Search for other works by this author on: This Site Google Scholar Paper presented at the Offshore Technology Conference, Houston, Texas, USA, May 2008. Paper Number: OTC-19414-MS https://doi.org/10.4043/19414-MS Published: May 05 2008 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Boswell, Ray, Rose, Kelly, and Richard Baker. "The Research Path to Determining the Resource Potential of Marine Gas Hydrates." Paper presented at the Offshore Technology Conference, Houston, Texas, USA, May 2008. doi: https://doi.org/10.4043/19414-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll ProceedingsOffshore Technology ConferenceOTC Offshore Technology Conference Search Advanced Search AbstractA primary goal of the U.S. National Interagency Gas Hydrates R&D program is to determine the natural gas production potential of marine gas hydrates. In pursuing this goal, four primary areas of effort are being conducted in parallel. First, are wide-ranging basic scientific investigations in both the laboratory and in the field designed to advance the understanding of the nature and behavior of gas hydrate bearing sediments (GHBS). This multi-disciplinary work has wide-ranging direct applications to resource recovery, including assisting the development of exploration and production technologies through better rock physics models for GHBS and also in providing key data for numerical simulations of productivity, reservoir geomechanical response, and other phenomena. In addition, fundamental science efforts are essential to developing a fuller understanding of the role gas hydrates play in the natural environment and the potential environmental implications of gas hydrate production, a critical precursor to commercial extraction. A second area of effort is the confirmation of resource presence and viability via a series of multi-well marine drilling expeditions. The collection of data in the field is essential to further clarifying what proportion of the likely immense in-place marine gas hydrate resource exists in accumulations of sufficient quality to represent potential commercial production prospects. A third research focus area is the integration of geologic, geophysical, and geochemical field data into an effective suite of exploration tools that can support the delineation and characterization commercial gas hydrate prospects prior to drilling. The fourth primary research focus is the development and testing of well-based extraction technologies (including drilling, completion, stimulation and production) that can safely deliver commercial gas production rates from gas hydrate reservoirs in a variety of settings. Initial efforts will take advantage of the relatively favorable economics of conducting production tests in Arctic gas-hydrate bearing sandstones with the intent of translating the knowledge gained to later testing in marine sandstone reservoirs. The full and concurrent pusuit of each of these research topics is essential to the determining the future production potential of naturally-occuring gas hydrates.IntroductionGas hydrates are a naturally occurring substance that can be found in nature wherever water exists in combination with sufficient volumes of appropriately-sized gas molecules under specific high-pressure, low-temperature conditions (Sloan and Koh, 2007). These conditions are restricted primarily to arctic regions, deep lakes, and marine continental shelves in water depth of ~500 m or more. Additional factors that influence the occurrence and nature of gas hydrates include the mineralogy and petrophysical characteristics of the host lithology, and the chemistry of both the gases and the water. Keywords: reservoir characterization, accumulation, drilling, detection, collett, saturation, marine gas hydrate, us government, prospect, hydrate Subjects: Reservoir Characterization, Non-Traditional Resources, Seismic processing and interpretation, Gas hydrates This content is only available via PDF. 2008. Offshore Technology Conference You can access this article if you purchase or spend a download.

Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea

Gas hydrates (GH) are found worldwide in marine sediments and permafrost regions. The detection and quantification of gas hydrates present in marine sediments is crucial for safe oil and gas extraction, seafloor stability assessments and for quantifying the impact of GH in climatic change. Therefore, there is a considerable interest in studying the microstructure of gas hydrate-bearing sediments. Although a large amount of research on gas hydrates has been carried out over the years, the micro-structural aspects of gas hydrate growth and the crystallite sizes of hydrate in sediments are still poorly known and understood. The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in-situ at sub-micron resolution. In this study, the nucleation and growth processes of GH were observed using synchrotron X-ray micro-computed tomography at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of clay minerals) or glass beads at varying water saturation. The process was observed on a timescale of a few minutes to many hours. Both, juvenile water as well as gas-enriched water obtained from gas hydrate decomposition was used in the experiments. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under-saturated with juvenile water the nucleation starts at the water-gas interface resulting in an initially several micrometer thick gas hydrate film; the further growth proceeds to form chains of predominantly isometric single crystals. The growth of gas hydrate from gas-enriched water clearly follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to several µm thickness between gas hydrates and the surface of the quartz grains. It appears that the initially different growth morphologies of gas hydrate tend to move towards more similar arrangements with coarsening of gas hydrate crystals. The initial microstructural findings obtained in laboratory experiments cannot be compared to the real situation in natural environments as the p-T conditions are different. However, the final microstructure of the system is comparable for both cases and experimental studies can permit to better understand the microstructures of GH aggregates and to tighten their physical properties required for physical detection and quantification. Several intrinsic properties of gas hydrates e.g. low affinity between gas hydrate and sediment grains revealed by laboratory experiments can be expected to remain unchanged in natural settings. New micro-structural models were developed based on these findings in order to explain the so-called seismic anomalies of high wave velocity and high wave attenuation characterizing GH-bearing sediments. The developed models are relevant for future efforts of quantitative rock physics modeling of effective elastic properties and electromagnetic properties of gas hydrates in sedimentary matrices. The imaging of gas hydrate microstructures was complemented by a quantitative analysis of the in-situ evolution of the crystallites size distribution of synthetic xenon hydrates and of natural gas hydrate-bearing sediments retrieved from Mallik 5L-38 research well. This study revealed that the crystal sizes of gas hydrate increase rapidly with time during the nucleation and initial growth processes. After completion of the hydrate formation process, an Ostwald ripening or normal grain growth process takes place; the number of gas hydrate crystals decreases and their intensities increase indicating the agglomeration of GH crystals into larger masses to reduce the interfacial energies of the system.

Horizon velocity analysis and pre-stack depth migration of seismic profiles collected by R/V Maurice Ewing in 1995 across the accretionary prism off SW Taiwan and along the continental slope of the northernmost South China Sea were implemented for identifying gas hydrates. Similarly, a survey of 32 ocean-bottom seismometers (OBS), with a spacing of about 500 m, was conducted for exploring gas hydrates on the accretionary prism off SW Taiwan in April 2006. Travel times of head wave, refraction, reflection and converted shear wave identified from the hydrophone, vertical and horizontal components of these OBS data were applied for imaging P-wave velocity and Poisson’s ratio of hydrate-bearing sediments. In the accretionary prism off SW Taiwan, we found hydrate-bearing sediment, with a thickness of about 100–200 m, a relatively high P-wave velocity of 1.87–2.04 km/s and a relatively low Poisson’s ratio of 0.445–0.455, below anticlinal ridges near imbricate emergent thrusts in the drainage system of the Penghu and Kaoping Canyons. Free-gas layer, with a thickness of about 30–120 m, a relatively low P-wave velocity of 1.4–1.8 km/s and a relatively high Poisson’s ratio (0.47–0.48), was also observed below most of the bottom-simulating reflectors (BSR). Subsequently, based on rock physics of the three-phase effective medium, we evaluated the hydrate saturation of about 12–30% and the free-gas saturation of about 1–4%. The highest saturation (30% and 4%) of gas hydrates is found below anticlines due to N–S trending thrust-bounded folds and NE-SW thrusting and strike-slip ramps in the lower slope of the accretionary prism. We suggest that fluid may have migrated through the relay-fault array due to decollement folding and gas hydrates have been trapped in anticlines formed by the basement rises along the thrust faults. In contrast, in the rifted continental margin of the northernmost South China Sea, P-wave velocities of 1.9–2.2 km/s and 1.3–1.6 km/s, and thicknesses of about 50–200 m and 100–200 m, respectively, for a hydrate layer and a free-gas layer were imaged below the remnant and erosional ridges in the upper continental slope. High P-wave velocity of hydrate-bearing sediment below erosional ridges may also indicate high saturation of hydrates there. Normal faults due to rifting in the South China continental crust may have provided conduits for gas migration below the erosional ridges where P-wave velocity of hydrate-bearing sediment in the passive continental margin of the northernmost South China Sea is greater than that in the active accretionary prism off SW Taiwan.