Marine methane hydrate in sands has huge potential as an unconventional gas resource; however, no field test of their production potential had been conducted. Here, we report the world’s first ...offshore methane hydrate production test conducted at the eastern Nankai Trough and show key findings toward future commercial production. Geological analysis indicates that hydrate saturation reaches 80% and permeability in the presence of hydrate ranges from 0.01 to 10 mdarcies. Permeable (1–10 mdarcies) highly hydrate-saturated layers enable depressurization-induced gas production of approximately 20,000 Sm3/D with water of 200 m3/D. Numerical analysis reveals that the dissociation zone expands laterally 25 m at the front after 6 days. Gas rate is expected to increase with time, owing to the expansion of the dissociation zone. It is found that permeable highly hydrate-saturated layers increase the gas–water ratio of the production fluid. The identification of such layers is critically important to increase the energy efficiency and the technical feasibility of depressurization-induced gas production from hydrate reservoirs.
Gas production tests have been conducted on artificial sandy sediments saturated by methane hydrate and water using a unique apparatus referred to as High-pressure Giant Unit for Methane-hydrate ...Analyses (HiGUMA), which is the world's largest reservoir simulating vessel intended for gas hydrate analysis. The gas recovery factor was investigated at various depressurization schemes, including one-step depressurization, multistep depressurization, and depressurization below the quadruple point of methane hydrate. The gas production rate increased during the depressurization process with sediment temperature reduction; however, the rate decrease and stabilized at a very low level after the temperature reached a newly established equilibrium condition. This result indicates that an appropriate heat of the hydrate-bearing sediments is a crucial factor for driving hydrate dissociation. The potential economic recovery factor was 14% for 4.6 MPa of production pressure in the one-step depressurization. In the multistep depressurization, the recovery factor was increased with a reduction in production pressure and showed values of 13%, 31%, and 40% for 4.0 MPa, 3.1 MPa, and 2.5 MPa, respectively. However, depressurization above the quadruple point could not dissociate all the existing hydrate due to the lack of heat. In contrast, it was determined that 65% of the in-place methane could be produced when the production pressure was decreased to 2.1 MPa, which is below the quadruple point, because the latent heat of ice formation was efficiently used for hydrate dissociation. The results show that intentional ice formation by adjusting production pressure can potentially enhance methane hydrate recovery at a comparable level of conventional natural gas production.
To obtain basic information for methane hydrate (MH) reservoir characterization at the first offshore production test site (AT1) located on the northwestern slope of the Daini-Atsumi Knoll in the ...eastern Nankai Trough, extensive geophysical logging and pressure coring using a hybrid pressure coring system were conducted in 2012 at a monitoring well (AT1-MC) and a coring well (AT1-C). The MH-concentrated zone (MHCZ), which was confirmed by geophysical logging at AT1-MC, has a 60-m-thick turbidite assemblage with sublayers ranging from a few tens to hundreds of centimeters thickness. The turbidite assemblage is composed of lobe/sheet-type sequences in the upper part and relatively thick channel-sand sequences in the lower part. Well-to-well correlations of sandy layers between two monitoring wells within 40 m of one another exhibited fairly good lateral continuity of sand layers in the upper part of the reservoir. This suggests an ideal reservoir for the production test.
The validity of MH pore saturation (Sh) evaluated from geophysical logging data were confirmed by comparing with those evaluated by pressure core analysis. In the upper part of the MHCZ, Sh values estimated from resistivity logs showed distinct differences between the sand and mud layers, compared with Sh values from nuclear magnetic resonance (NMR) logs. Resistivity logs have higher vertical resolution than NMR logs; therefore, they are favorable for these types of thin-bed evaluations. In the upper part, Sh values of 50%–80% were observed in sandy layers, which is in fairly good agreement with core-derived Sh values. In the lower part of the MHCZ, Sh values estimated from both resistivity and NMR logs showed higher background values and relatively smoother curves than those for the upper part. In the lower part, Sh values of 50%–80% were also observed in sandy layers, and they showed good agreement with the core-derived Sh values.
•Geologic settings in the β-MH-concentrated zone.•Well distribution in 2012 pre-drilling and formation evaluation method.•Characteristics of confirmed MH-bearing formation.•Lateral continuity of sand layers confirmed by well-to-well correlations.•Reservoir properties suggested from well log and core data comparisons.
•The cyclic depressurization method, a new enhanced gas hydrate recovery, is proposed.•The method is a sustainable heat supply method driven by the geothermal heat flow.•The recovery factor of a ...hypothetical field increased from 42.4% to 71.5%.•The number of operating wells was reduced to less than one-third.
The cyclic depressurization method, which uses alternating depressurization and shut-in periods over decades, has been proposed to achieve sustainable gas production from methane hydrate reservoirs. Numerical simulations were conducted to investigate the dissociation and reformation behaviors of methane hydrate during depressurization and shut-in periods. A high gas production rate was obtained for a few years after primary depressurization; however, the production rate drastically decreased because the sensible heat of the reservoir was exhausted owing to hydrate dissociation. During the shut-in period after 10years of production, methane hydrate continued to dissociate owing to the geothermal heat flow for a few decades and then started to reform in accordance with pressure recovery. Case studies with shut-in periods of 10–30years showed that 20years of shut-in was the most effective period before the next depressurization. A conceptual operation plan for a hypothetical field showed that the production time increased to 120years from 70years when the cyclic depressurization method was considered. The recovery factor increased from 42.4% to 71.5%. The number of operating wells was reduced to less than one-third compared with the operation with normal depressurization method only. The results suggest that the cyclic depressurization method is a sustainable heat supply method driven by the geothermal heat flow and is both economically and environmentally sound.
Oceanic methane hydrate (MH) deposits have been found at high saturations within reservoir-quality sands in the Eastern Nankai Trough and the Gulf of Mexico. This study investigates the key factors ...for the success of depressurization-induced gas production from such oceanic MH deposits. A numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from MH deposits. We calculated the hydrate dissociation behavior and gas/water production performance during depressurization for a hypothetical MH well. Simulation runs were conducted under various initial reservoir conditions of MH saturation, temperature, and absolute permeability. A productivity function (PF) was introduced as an indicator of gas productivity, which is a function of gas production rate, water production rate, and discount rate. The simulations showed that recovery factors over 36% and maximum gas production rates over 450 000 Sm3/d were expected for the most suitable conditions of a class 3 deposit (i.e., an isolated MH deposit that is not in contact with any hydrate-free zone of mobile fluids). However, gas productivity was affected by formation temperature and initial effective permeability. The values of PF increased with increasing formation temperature when the initial permeability of the deposit was higher than a threshold value (the threshold permeability); however, it decreased for the deposit below the threshold permeability. The threshold permeability was estimated to be between 1 and 10 mD in the class 3 deposit. These results suggest that key factors for the success of depressurization-induced gas production from oceanic MH are as follows: (1) The initial effective permeability of the MH deposit is higher than the threshold value, and (2) the temperature of the MH deposit is as high as possible.
The study of mechanical properties of marine sediments is essential for the prediction of the occurrence of geohazards (e.g., subsea landslides and seafloor subsidence) and the design of submarine ...structures for offshore industry. In this study, triaxial compression tests of gas-hydrate-bearing sandy sediments and clayey-silty sediments were conducted. The sediments were recovered by pressure coring in the Eastern Nankai Trough, the area of the first Japanese offshore production test. Soil index properties were measured and revealed porosity of 40%–50%, with porosity decreasing gradually with greater depth below the seafloor. The mean particle size was less than 10 μm for clayey-silty sediments and approximately 100 μm for sandy sediments. Permeability, estimated by a consolidation process of triaxial testing and with X-ray diffraction analysis, depended on the content of fines, which consisted chiefly of mica, kaolinite, and smectite. The results of undrained compression tests for clayey-silty sediments showed positive excess pore pressure under all test conditions. This mechanical behavior indicates that the core samples are normally consolidated sediments. Drained compression tests showed that the strength and stiffness of sandy sediments increase with hydrate saturation. Furthermore, the volumetric strain of hydrate-bearing sediments changed from compression to dilative. This result was obtained for hydrate saturation values (Sh) of more than 70%. The shear strength of hydrate-bearing turbidite sediments of the Eastern Nankai Trough is shown to be a function of the confining pressure.
•Triaxial tests on sediments recovered at the Daini-Atsumi Knoll site are reported.•Permeability of sandy sediments with and without hydrates are tens of millidarcies.•70% and 79% hydrate saturation sediments were drained compression.•Effective friction angle for the sediments without hydrate is 30°–37°.•Strengthening of sediments is mainly caused by the cohesion induced by hydrates.
The India National Gas Hydrate Program Expedition 02 (NGHP-02) discovered gas hydrate at high saturation in sand reservoirs at several sites in the deepwater Bay of Bengal. To assess the potential ...response of those deposits to scientific depressurization experiments, comprehensive geologic models were constructed to enable numerical simulation for two sites. Both sites (NGHP-02-09 and NGHP-02-16) feature thick sequences of thinly-interbedded reservoir and non-reservoir facies at sub-seafloor depths less than 300 m and sub-sea depths of 2400 m or more. These settings pose significant challenges to current modeling capabilities. First, the thinly-interbedded reservoir architecture complicates the determination of basic reservoir parameters from both log and core data due to measurement resolution issues. Secondly, the fine scale variation in sediment properties imparts great contrasts in key parameters over very short distances, creating high gradients at multiple scales and varying orientations that necessitate careful design of high-definition simulation grids. Thirdly, the deposits include internal sources of water, as well as a range of complex boundary conditions, including variable permeability within the overlying mud-rich “seals,” that complicate reservoir depressurization. Lastly, because of the unique combination of great water depth and relatively shallow sub-seafloor depth: models designed to maximize the dissociation rate impose large pressure drawdowns on relatively low-strength sediments. This condition renders the proper evaluation and integration of the geomechanical response to hydrate dissociation critical. In this report, we review the history of gas hydrate reservoir simulation, discuss methods for creating geologic input models, and summarize the key findings and implications of the collaborative NGHP-02 numerical simulation effort. Together, the studies confirm the viability of the modeled accumulations for scientific testing and identify key challenges related to the selection of specific test sites and the design of test wells.
•The results of numerical simulations for two gas hydrate reservoirs located in the deepwater eastern India are summarized.•Critical issues in the development of input models for gas hydrate simulation are discussed.•Implications for the advancement of modeling capabilities for challenging reservoirs are presented and discussed.
Permeability measurements were conducted on gas hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin during India's National Gas Hydrate Program Expedition 02. Pressure ...cores collected in the deep seabed of the Indian Ocean were cut and stored under high pressure and low temperature on the D/V Chikyu using pressure core analyzing tools. A total of 25, 1.2-m storage chambers were transferred to Japan from India for pressure core studies using pressure-core analysis tools at the National Institute of Advanced Industrial Science and Technology. The sediment core's pressure and temperature were maintained within the hydrate stability conditions during the entire process of transfer and loading into a triaxial testing apparatus called TACTT to preserve the hydrate, which can simulate the in-situ effective stress state under triaxial conditions. The hydrate saturation of the samples was in the range of 50–90%. It was found that the initial effective water permeability was in range of 0.01 mD to tens of mD, depending on the hydrate saturation and the mean particle size of the host sediment. The hydrate pore space morphology is also a critical factor, and results suggest hydrates are forming in the pore centers of sand grains as pore filling, but also as a sediment grain coating. In a first for gas hydrate-bearing pressure cores, the permeability anisotropy was established via vertical and horizontal flow tests combined with a pore fluid flow simulation. The horizontal/vertical permeability ratio was found to be 4. Additionally, the effective stress dependency of permeability was investigated by performing flow tests at different effective stresses. Due to decreasing porosity and particle crushing, the permeability dropped by approximately 90% for an effective stress increase of 10 MPa.
•The initial effective permeability of hydrate-bearing sediments was measured.•The horizontal/vertical permeability ratio of the hydrate-bearing sediment is 4.•Permeability becomes approximately 1/10 when effective stress increases by 10 MPa.
Natural gas hydrates (or methane hydrates) could become a major energy source but could also exacerbate global warming, because as the climate warms, hydrate deposits deep under the oceans or in ...permafrost may release methane into the atmosphere. There are many shallow deposits of gas hydrates in fine‐grained muddy sediments on the seafloor. However, the mechanical properties of these sediments have not yet been investigated because of the engineering challenges in coring and testing at in situ temperatures and pressures. Here we present the first uniaxial and triaxial strength and stiffness measurements of pure massive natural gas hydrates and muddy sediments containing hydrate nodules obtained by pressure coring. As a result, we were able to observe the hydrate undergoing a catastrophic brittle failure. Its strength and deformation moduli were 3 and 300 MPa, respectively. Muddy sediments containing hydrate nodules had the same strength as that of hydrate‐free sediments.
Key Points
The first strength and stiffness data for shallow gas hydrate deposits were investigated by pressure core analysis
Massive natural gas hydrates show catastrophic brittle failures, with strength and deformation moduli of 3 and 300 MPa, respectively
Muddy sediments containing hydrate nodules had the same strength as that of hydrate‐free sediments
Pressure coring and analysis technology has progressed remarkably over the last decade. This technology allows for methane-hydrate-bearing sands, silts, and/or clays to be recovered from the deep-sea ...gas hydrate reservoir and then maintained in the laboratory within the hydrate stability phase boundary for evaluation and characterization. In this study, a series of pressure-core-based undrained/drained confined compression tests, unconfined (uniaxial) compression tests, isotropic loading and unloading tests, and permeability tests are conducted to investigate and characterize the intact strength, compressibility, and permeability of the gas hydrate reservoir. The consolidation tests and fluid flow tests showed that clayey silt sediments have permeability values of approximately tens of microdarcies, and sandy sediments with and without hydrates have permeability values of tens of millidarcies. The sediment from the bottom of the reservoir, which is sandy in nature with fines, also has permeability values of tens of microdarcies. From previous reports, the undrained shear strength of clayey silt sediments with low gas hydrate saturation is relatively low. Results from the drained shear tests and uniaxial compression tests show the first Mohr−Coulomb failure criteria for natural gas hydrate-bearing sediments, in which both the effective cohesion c' and the friction angle ϕ’ increase with increasing hydrate saturation. The results indicate in situ existence of pore filling and patchy gas hydrate. Finally, a new failure envelope for hydrate-bearing sediment is proposed that includes both hydrate saturation and confining pressure; it shows a good relation with the experimental results.
•Pressure-core-based geomechanical-hydrological tests is conducted for hydrate-bearing sediments.•Sandy sediments with hydrates have permeability values of tens of millidarcies.•The first Mohr−Coulomb failure criteria for natural gas hydrate-bearing sediments is established.•Both the effective cohesion and the friction angle increase with increasing hydrate saturation.
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