75th EAGE Conference & Exhibition - Workshops

Since huge amounts of methane are bounded in natural gas hydrates occurring at all active and passive continental margins and in permafrost regions, the production of natural gas from hydrate bearing sediments becomes more and more of interest. Three different methods for the release of methane gas from destabilized hydrate are discussed in principle: thermal stimulation, depressurization and chemical stimulation. This study focusses on the thermal stimulation using a counter-current heat-exchange reactor for the in situ combustion of methane. The heat of the flameless, catalytic oxidation of methane was used for the decomposition of hydrates in sand in a pilot plant scale within a large reservoir simulator (LARS). The promising results of the latest reactor test for which LARS was filled with sand, and ca. 80 % of the pore space was saturated with methane hydrate are presented in this study. The data analysis showed that 15 % of the methane gas released from hydrates would have to be used for the successful dissociation of all hydrates in the sediment using catalytic combustion of methane.

Gas hydrates are a geohazard and a potential energy resource, yet the estimated global volume varies by over four orders of magnitude. This is because mapping hydrates with seismic methods alone is difficult, showing only the edges of the hydrate stability zone. Another technique, marine controlled source electromagnetic (CSEM) techniques, are more apt in estimating hydrate saturations than seismic methods. In 2008, an extensive CSEM data set was collected in the Gulf of Mexico (GoM) aimed at developing the CSEM technique to map marine gas hydrates. Preliminary 1D apparent resistivity pseudo-section results of the data show lateral variations in resistivity that may be attributed to gas hydrate, deeper salt bodies, carbonates, and water saturated channel sands. However, the exact resistivity values and geometries of hydrate emplacement within the surrounding sedimentary architecture requires 2D or even 3D modeling and inversion before final interpretations of the CSEM data and volume estimates of gas hydrate saturation can be made. Here we apply a newly developed parallel goal-oriented adaptive finite-element modeling algorithm (referred to as MARE2DEM) for efficient 2.5D imaging of the CSEM data and present preliminary inversion results for the GoM survey.

Whether gas hydrates are considered as hazard or resource, we need to find it first. We developed an integrated, seismic-based, five-step workflow (Dutta et al 2010; Dutta and Dai, 2007) to delineate and quantify gas hydrates using an approach very similar to finding hydrocarbon. The approach is primarily based on seismic characterization, geologic analysis and seismic inversion that is constrained with rock physics principles. These are performed within the gas hydrate stability zone. The introduction of gas hydrates in the shallow unconsolidated sediments tends to enhance both the stiffness and rigidity of the hosting rocks. Gas hydrate drilling worldwide has indicated that the increases in the stiffness and rigidity are somewhat proportional to the concentration of gas hydrates in the porous space of the sediments. This provides basis for the gas hydrate characterization and quantification using seismic information. In this presentation, we will review the technology, and demonstrate its application, using multiple examples from the Gulf of Mexico (GOM). Based on our models for gas hydrate exploration, a set of wells were drilled by the Consortium of USA – DOE and several oil companies in the deep water GOM (e.g. Green Canyon, Walker Ridge, Atwater Valley) and the model

In the last decades, permafrost gas hydrates have beneficiated from an increasing attention among researchers and industries around the world. However, little work has been done on characterizing this resource at the reservoir scale. In this study, we used cutting edge stochastic inversion software and we developed a cascade stochastic Bayesian algorithm to simulate the gas hydrate grade (product of porosity and gas hydrate saturation) on a 3D seismic cube at the Mallik field, in the Mackenzie Delta, Canada. Firstly, the 3D seismic data are stochastically inverted for acoustic impedance leading to multiple high-resolution AI realizations conditioned to the seismic and the well-log data from wells 2L- and 5L-38. Secondly, a petrophysical inversion is performed in a stochastic Bayesian framework using gas hydrate grade logs as hard data, and randomly selected AI scenarios as secondary data. For the later inversion, an in-situ petrophysical relationship linking gas hydrate grades to acoustic impedance is built using upscaled well data. The results are thus multiple 3D gas hydrate grade realizations conditioned to all available data, and reflecting a great part of the model uncertainty. These models allow calculating the total gas volume with its associated uncertainty for the studied region.  

Pockmarks have been observed on the Chatham Rise east of New Zealand over an area of >20,000 km2. Their formation has been linked primarily to gas hydrate dissociation during glacial-stage sealevel lowstands. We here show first images from a recent high-resolution 3D seismic images targeting the sub-surface structure of a giant pockmarks. Results show in particular that the pockmark may be underlain by a large gas chimney.

This study provides an overview of geochemical assessment of deep sediment methane hydrates in several locations around the world. The U.S. Naval Research Laboratory has been involved in methane hydrate exploration of the coasts of New Zealand, Chile, Canada, Beaufort Sea on the Alaskan Shelf and in the Gulf of Mexico. These studies have shown a wide variation in the distribution and concentration of deep sediment methane hydrate deposits within and between geographic locations. General findings of these studies are not always consistent with a simple model of methane flux, suggesting the need for more thorough geochemical evaluation.

Methane is one of the most aggressive greenhouse gases driving climate change. Occurrence of marine gas hydrates depends on temperature, pressure, available gas and fresh water. Therefore changes in pressure and bottom water temperature will influence the formation or dissolution of gas hydrates. In general both parameters vary slowly and hence changes do not result in large Methane contribution to the atmosphere. An exception could be a sudden dissolution of larger quantities of gas hydrate with a related expulsion of Methane. Such focused fluid flow appears as funnel-shaped depressions at the seafloor, so called “pockmarks”. Typical dimensions are within a few hundreds of meters. However, five to twelve kilometre wide “giant pockmarks” (GP) are known as well. The mechanism of formation of GPs is not fully understood. New 2D and 3D multichannel seismic data have been acquired across two fields of giant pockmark-like features to better understand the formation of these depressions and potential for catastrophic methane release in the past.

The lack of correlation between test data on laboratory synthesised hydrate-bearing sediments has raised questions as to the interaction between the hydrate and the host sediment. The development of the Pressure Core Analysis and Transfer System (PCATS) has enabled the recovery of pressurised cores in liners and their subsequent transfer into other pressure vessels. Building on this development,, an advanced geotechnical testing apparatus has been developed to provide data on the behaviour of natural, intact, hydrate bearing sediments under in situ pressure and temperature conditions. This presentation describes the capabilities of this apparatus and presents some initial test results on pressurised cores.

The fully coupled methane hydrate model developed in Cambridge was adopted in this numerical study on gas production trial at the Eastern Nankai Trough, Japan 2013. Based on the latest experimental data of hydrate soil core samples, the clay parameters at Eastern Nankai site were successfully calibrated. With updated clay parameters and site geometry, a 50 days gas production trail was numerically simulated in FLAC2D. The geomechanical behaviour of hydrate bearing sediments under 3 different depressurization strategies were explored and discussed. The results from both axisymmetrical and plane-strain models suggest, the slope of the seabed only affects mechanical properties while no significant impact on the dissociation, temperature and pore pressure. For mechanical deformation after PT recovery, there are large settlements above the perforation zone and small uplift underneath the production zone. To validate the fully coupled model, numerical simulation with finer mesh in the hydrate production zone was carried out. The simulation results suggest good agreement between our model and JOE’s results on history matching of gas and water production during trial. Parameter sensitivity of gas production is also investigated and concluded the sea water salinity is a dominant factor for gas production.

The Hikurangi Margin, east of the North Island of New Zealand, contains a significant gas hydrate province. However, the distribution, concentration and dynamics of hydrate accumulations in the southern portion of the margin (the Pegasus Sub-basin) off the northeastern coast of the South Island are poorly constrained due to a lack of data. In late 2009 and early 2010, a seismic dataset consisting of approximately 3000 km of long-offset 2D seismic data was collected in the Pegasus Sub-basin. Bottom-Simulating Reflections (BSRs) are widespread in the data, and they are supplemented by other features that may indicate the presence of free gas and gas hydrates in zones of high concentration. We present the results of reprocessing and analysis of the data that has focussed on the identification of such specific gas-related features.