On the origin of multiple BSRs in the Danube deep-sea fan, Black Sea

Zander, Timo; Haeckel, Matthias; Berndt, Christian; Chi, Wu‐Cheng; Klaucke, Ingo; Bialas, J.; Klaeschen, Dirk; Koch, Stephanie; Atgin, O.

doi:10.1016/j.epsl.2017.01.006

Cited by 69 publications

(112 citation statements)

References 41 publications

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“…BSRs may present various forms such as a reverse polarity, they may be semicontinuous, or show aligned terminations of high amplitudes (Davies et al, 2017). The BSR has been mapped out in the western Black Sea by Popescu et al (2007) who also indicate the presence of multiple BSRs in the Danube deep-sea fan (Popescu et al, 2006) that could be related to past stability conditions (Zander et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

Ker

Thomas

Riboulot

et al. 2019

Geochem Geophys Geosyst

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A comprehensive characterization of gas hydrate system offshore the western Black Sea was performed through an integrated analysis of geophysical data. We detected the bottom‐simulating reflector (BSR), which marks, in this area, the base of gas hydrate stability. The observed BSR depth does not fit the theoretical steady state base of gas hydrate stability zone (BGHSZ). We show that the disparity between the BSR and predicted BGHSZ is the result of a transient state of the hydrate system due to the ongoing reequilibrium since the Last Glacial Maximum. When gas hydrates are brought outside the stability zone due to changes in temperature and sea level, their dissociation generates an increase in interstitial pore pressure. This process is favorable to the recrystallization of gas hydrates and delays the upward migration of the hydrate stability zone explaining the anomalously deep BSR. The BSR depth, which is commonly used to derive geothermal gradient values by assuming steady state conditions, is used here to derive the maximum excess pore pressure at the BGHSZ. Derived excess pore pressure values of 1–2 MPa are probably the result of the low permeability of hydrate‐bearing sediments. Higher pore pressure values derived at the location of a fault system could cause hydrofracturing enabling the free gas to cross the gas hydrate stability zone and emerge at the seafloor, forming the flares observed in close vicinity to where the shallow gas hydrates were sampled.

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Section: Introductionmentioning

confidence: 99%

Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

Ker

Thomas

Riboulot

et al. 2019

Geochem Geophys Geosyst

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“…At t = t 0 , we assume a hydrostatic pressure at point A of P w | z=0,t=0 = P s f = 15 MPa, corresponding to a water depth of roughly 1500m, and a bottom water temperature of T | z=0,t=0 = T s f = 4 0 C, corresponding to glacial conditions in the Black Sea [48]. We assume that the initial pressure distribution within the computational domain follows a hydrostatic gradient, and the initial temperature distribution follows a steady state geothermal gradient of 35 0 C/km.…”

Section: Problem Settingmentioning

confidence: 99%

An All-At-Once Newton Strategy for Marine Methane Hydrate Reservoir Models

2020

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Marine gas hydrate systems are characterized by highly dynamic transport-reaction processes in an essentially water-saturated porous medium that are coupled to thermodynamic phase transitions between solid gas hydrates, free gas and dissolved methane in the aqueous phase. These phase transitions are highly nonlinear and strongly coupled, and cause the mathematical model to rapidly switch the phase states and pose serious convergence issues for the classical Newton's method. One of the common methods of dealing with such phase transitions is the primary variable switching (PVS) method where the choice of the primary variables is adapted locally to the phase state 'outside' the Newton loop. In order to ensure that the phase states are determined accurately, the PVS strategy requires an additional iterative loop, which can get quite expensive for highly nonlinear problems. For methane hydrate reservoir models, the PVS method shows poor convergence behaviour and often leads to extremely small time step sizes. In order to overcome this issue, we have developed a nonlinear complementary constraints method (NCP) for handling phase transitions, and implemented it within a non-smooth Newtons linearization scheme using an active-set strategy. Here, we present our numerical scheme and show its robustness through field scale applications based on the highly dynamic geological setting of the Black Sea.

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“…Economically viable hydrate reservoirs occur in sands and coarse silts with permeability that is high enough to sustain gas flow towards the well during production. The paleo Danube deep-sea fan in the Black Sea offers the best conditions for hydrate production in Europe because it contains sandy sediments and a widespread bottom simulating reflector (BSR) in seismic data that indicates the presence of gas hydrates (Popescu et al, 2006, Zander et al, 2017.…”

Section: Accepted Manuscriptmentioning

confidence: 99%

“…Several older channels can be identified from the bathymetry, such as a channel westwards of the Danube channel which was named SUGAR channel (Fig. 2;Zander et al, 2017).…”

Section: A C C E P T E D Accepted Manuscriptmentioning

confidence: 99%

“…For the observed bottom water temperature of 9 °C ) and a salinity of 22.3 (Özsoy and Ünlüata, 1997) the upper limit of the gas hydrate stability zone is located at a water depth of 721 m. The pore water salinity decreases rapidly in shallow depth below the seafloor to a level of 3-5 (Soulet et al, 2010), which shifts the phase boundary for methane hydrates upwards. Indirect indicators for gas hydrates exist in the form of a BSR which is observed in reflection seismic data (Popescu et al, 2006;Zander et al, 2017), and gas hydrate was sampled during a research cruise in 2015 (Ker and Riboulot, 2015).…”

Section: A C C E P T E D Accepted Manuscriptmentioning

confidence: 99%

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Potential impacts of gas hydrate exploitation on slope stability in the Danube deep-sea fan, Black Sea

Zander

Choi

Vanneste

et al. 2018

Marine and Petroleum Geology

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Potential impacts of gas hydrate exploitation on slope stability in the Danube deep-sea fan, AbstractMethane production from gas hydrate reservoirs is only economically viable for hydrate reservoirs in permeable sediments. The most suitable known prospect in European waters is the paleo Danube deep-sea fan in the Bulgarian exclusive economic zone in the Black Sea where a gas hydrate reservoir is found 60 m below the seafloor in water depths of about 1500 m. To investigate the hazards associated with gas production-induced slope failures we carried out a slope stability analysis for this area. Screening of the area based on multibeam bathymetry data shows that the area is overall stable with some critical slopes at the inner levees of the paleo channels. Hydrate production using the depressurization method will increase the effective stresses in the reservoir beyond pre-consolidation stress, which results in sediment compaction and seafloor subsidence. The modeling results show that subsidence would locally be in the order of up to 0.4 m, but it remains confined to the immediate vicinity above the production site. Our simulations show that the Factor of Safety against slope failure (1.27) is not affected by the production process, and it is more likely that a landslide is triggered by an earthquake than by production itself. If a landslide were to happen, the mobilized sediments on the most likely failure plane could generate a landslide that would hit the production site with velocities of up to 10 m s -1 . This case study shows that even in the case of production from very shallow gas hydrate reservoirs the threat of naturally occurring slope failures may be greater than that of hydrate production itself and has to be considered carefully in hazard assessments.

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On the origin of multiple BSRs in the Danube deep-sea fan, Black Sea

Cited by 69 publications

References 41 publications

Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

An All-At-Once Newton Strategy for Marine Methane Hydrate Reservoir Models

Potential impacts of gas hydrate exploitation on slope stability in the Danube deep-sea fan, Black Sea

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