Transport- reaction modeling of marine gas hydrate deposits- global results

We have developed a multi-1D numerical model of gas hydrate formation and dissolution processes in anoxic marine sediments and, by this model, we have estimated the new global gas hydrate inventory (BURWICZ E. B. et al., 2011). The reaction-transport model contains various chemical compounds (solid organic carbon, dissolved methane, inorganic carbon, and sulfates, gas hydrates, and free methane gas). The rates of POC degradation, anaerobic methane oxidation, sulfate reduction, and methanogenesis are kinetically controlled. Gas hydrate stability zone (GHSZ) is defined as a combination of pressure, temperature, and (to a smaller degree) salinity conditions. The lower boundary of the GHSZ is defined as the intersection of gas hydrate and methane gas solubilities. The diffusion equations are solved using a fully-implicit finite-differences method, while all transport processes are resolved by a Semi-Lagrangian scheme. Global input data sets (1°x1° resolution) were compiled from various oceanographic, geological and geophysical sources. The entire model was implemented in Matlab

Thermal State of the Blake Ridge Gas Hydrate Stability Zone (GHSZ) - Insights on Gas Hydrate Dynamics from a New Multi-Phase Numerical Model

Marine sediments of the Blake Ridge province exhibit clearly defined geophysical indications for the presence of gas hydrates and a free gas phase. Despite being one of the world’s best-studied gas hydrate provinces and having been drilled during Ocean Drilling Program (ODP) Leg 164, discrepancies between previous model predictions and reported chemical profiles as well as hydrate concentrations result in uncertainty regarding methane sources and a possible co-existence between hydrates and free gas near the base of the gas hydrate stability zone (GHSZ). Here, by using a new multi-phase finite element (FE) numerical model, we investigate different scenarios of gas hydrate formation from both single and mixed methane sources (in-situ biogenic formation and a deep methane flux). Moreover, we explore the evolution of the GHSZ base for the past 10 Myr using reconstructed sedimentation rates and non-steady-state P-T solutions. We conclude that (1) the present-day base of the GHSZ predicted by our model is located at the depth of ~450 mbsf, thereby resolving a previously reported inconsistency between the location of the BSR at ODP Site 997 and the theoretical base of the GHSZ in the Blake Ridge region, (2) a single in-situ methane source results in a good fit between the simulated and measured geochemical profiles including the anaerobic oxidation of methane (AOM) zone, and (3) previously suggested 4 vol.%–7 vol.% gas hydrate concentrations would require a deep methane flux of ~170 mM (corresponds to the mass of methane flux of 1.6 × 10−11 kg s−1 m−2) in addition to methane generated in-situ by organic carbon (POC) degradation at the cost of deteriorating the fit between observed and modelled geochemical profiles

3-D basin-scale reconstruction of natural gas hydrate system of the Green Canyon, Gulf of Mexico

Our study presents a basin-scale 3D modeling solution, quantifying and exploring gas hydrate accumulations in the marine environment around the Green Canyon (GC955) area, Gulf of Mexico. It is the first modeling study that considers the full complexity of gas hydrate formation in a natural geological system. Overall, it comprises a comprehensive basin re-construction, accounting for depositional and transient thermal history of the basin, source rock maturation, petroleum components generation, expulsion and migration, salt tectonics and associated multi-stage fault development. The resulting 3D gas hydrate distribution in the Green Canyon area is consistent with independent borehole observations. An important mechanism identified in this study and leading to high gas hydrate saturation (> 80 vol. %) at the base of the gas hydrate stability zone (GHSZ), is the recycling of gas hydrate and free gas enhanced by high Neogene sedimentation rates in the region. Our model predicts the rapid development of secondary intra-salt mini-basins situated on top of the allochthonous salt deposits which leads to significant sediment subsidence and an ensuing dislocation of the lower GHSZ boundary. Consequently, large amounts of gas hydrates located in the deepest parts of the basin dissociate and the released free methane gas migrates upwards to recharge the GHSZ. In total, we have predicted the gas hydrate budget for the Green Canyon area that amounts to ∼3,256 Mt of gas hydrate which is equivalent to ∼340 Mt of carbon (∼7 x 1011 m3 of CH4 at STP conditions), and consists mostly of biogenic hydrates

Estimating the gas hydrate recovery prospects in the western Black Sea basin based on the 3D multiphase flow of fluid and gas components within highly permeable paleo-channel-levee systems

Gas hydrate deposits are abundant in the Black Sea region and confirmed by direct observations as well as geophysical evidence, such as continuous bottom simulating reflectors (BSRs). Although those gas hydrate accumulations have been well-studied for almost two decades, the migration pathways of methane that charge the gas hydrate stability zone (GHSZ) in the region are unknown. The aim of this study is to explore the most probable gas migration scenarios within a three-dimensional finite element grid based on seismic surveys and available basin cross-sections. We have used the commercial software PetroMod TM(Schlumberger) to perform a set of sensitivity studies that narrow the gap between the wide range of sediment properties affecting the multi-phase flow in porous media. The high-resolution model domain focuses on the Danube deep-sea fan and associated buried sandy channel-levee systems whereas the total extension of the model domain covers a larger area of the western Black Sea basin. Such a large model domain allows for investigating biogenic as well as thermogenic methane generation and a permeability driven migration of the free phase of methane on a basin scale to confirm the hypothesis of efficient methane migration into the gas hydrate reservoir layers by horizontal flow along the carrier beds

Gas hydrate distribution and hydrocarbon maturation north of the Knipovich Ridge, western Svalbard margin

A bottom-simulating reflector (BSR) occurs west of Svalbard in water depths exceeding 600 m, indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern North Atlantic margin. Regional BSR mapping shows the presence of hydrate and free gas in several areas, with the largest area located north of the Knipovich Ridge, a slow-spreading ridge segment of the Mid Atlantic Ridge system. Here, heat flow is high (up to 330 mW m-2), increasing towards the ridge axis. The coinciding maxima in across-margin BSR width and heat flow suggest that the Knipovich Ridge influenced methane generation in this area. This is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination. To evaluate the source rock potential on the western Svalbard margin, we applied 1D petroleum system modeling at three sites. The modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons. The bulk petroleum mass produced since the Eocene is at least 5 kt and could be as high as ~0.2 Mt. Most likely, source rocks are Miocene organic-rich sediments and a potential Eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters. Thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the Knipovich Ridge. The presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the Svalbard margin varies spatially

Basin-scale estimates on petroleum components generation in the Western Black Sea basin based on 3-D numerical modelling

Highlights • Total amount of generated biogenic methane is estimated at ~3100 Gt. • Total amount of generated thermogenic methane is estimated at ~1,560 Gt. • The Maykop formation is partially productive in the central basin and not yet fully productive towards the basin peripherals. A new numerical model reconstructing the depositional history (98–0 Ma) of the Western Black Sea sub-basin is presented. The model accounts for changing boundary conditions (i.e. water depth, bottom water temperature, heat flow evolution over time) and estimates the rates and total amounts of the in-situ biogenic methane generation and thermally-driven organic matter maturation in the source rocks. The overall thermogenic and biogenic gas generation predicted by the model is estimated at ~1560 Gt and ~3100 Gt, respectively

A new numerical reaction-transport model of marine gas hydrate deposits

Introduction We have developed a new multi 1-D numerical model to investigate and understand the processes of gas hydrate formation and dissolution in anoxic marine sediments under a wide range of conditions. By this reaction-transport model we are able to investigate a various aspects of gas hydrate dynamics: sediment compaction which results in expulsion of pore fluids containing various chemical species, reduction in porosity and permeability of the sediment matrix due to hydrate formation, time-resolved evolution of pressure and temperature regimes, multiphase flow of compressible pore fluids, gas hydrate, and a free gas, thermal-blanketing effect due to vigorous sedimentation of cold impermeable layers, gas hydrate dissolution as a response to a slowing down sedimentation, and the effects of salinity variations on the thickness of the Gas Hydrate Stability Zone (GHSZ). Numerical model The reaction-transport model contains various chemical compounds (solid organic carbon, dissolved in pore water methane, dissolved inorganic carbon, dissolved sulfates, gas hydrates, and free methane gas). We consider a reference frame which extends from the seafloor to the bottom of the GHSZ (defined as a combination of pressure, temperature, and salinity conditions) plus 50m of Free Gas Zone lying directly beneath. However, the upper part of sediment column (10 cm) is not considered in the model due to strong bioturbation processes which might potentially have an impact on the gradients of dissolved chemical species. Initially, the system is filled by compressible pore fluids of a given salinity (consistent with a value at the sediment-water interface). As the upper boundary conditions, we have applied constant concentrations of dissolved methane, dissolved inorganic carbon, and sulfate according to the mean values in the ocean. At the beginning of each time-step, a new sediment layer is deposited at the top of sediment column according to a given sedimentation rate, lithological type, and initial porosity at the surface. Transport processes have been split into the advection and diffusion part and solved separately for every chemical compound. Multiphase flow of dissolved chemical species and free gas phase has been solved by finite-volumes method according to the Darcy’s law. Molecular diffusion of dissolved species is controlled by changes in concentration gradients and has been solved by finite-elements method. Reaction module contains kinetically controlled rates of methanogenesis, sulfate reduction, methane oxidation, and POC degradation. POC decay via microbial sulfate reduction takes place until the dissolved sulfate pool in ambient pore waters is depleted. Below the sulfate penetration depth, POC is microbially decomposed into methane and CO2. Upward diffusing dissolved methane is consumed by anaerobic oxidation within the sulfate-methane transition zone. This reaction module has been evaluated previously by Wallmann et al., 2006, Marquardt et al., 2010, and Burwicz et al., 2011. Applications Dynamic un-steady state compaction allows us to investigate gas hydrate formation and dissolution in terms of changing parameters (e.g. sedimentation rate or permeability of deposited sediments). By depositing sediment layers of a different grain size (‘sandwich-like’ scenario), we have observed that lithology of potential hydrate-bearing layers (e.g. coarse-grained sands vs. shales) results in preferential hydrate accumulation in the first ones which stays in agreement with field observations. We have also investigated the effect of slowing down sedimentation rates on gas hydrate dissolution. We have concluded that slow deposition of sediment layers at the top of sediment column and, as a result, a decrease in POC input in time, result in undersaturated in CH4 pore waters causing hydrate destabilization. This scenario clearly shows the importance of constraining a time-resolved sedimentation history in gas hydrate simulations which are coupled with climate models. By depositing thick layers of cold low-permeable sediments on top of the column, we have investigated the temperature variations within sediments, known as ‘thermal blanketing’ effect, which has an impact on previously formed hydrates

Basin-scale gas hydrate-free gas re-cycling process derived from 3D numerical modeling at the Green Canyon province, Gulf of Mexico

Gas migration pathways in the Gulf of Mexico are strongly influenced by the extensive formation and time evolution of salt canopies, welds and sheets. This multi-level salt system (known as the Louann Salt formation) deposited mostly within Callovian age (upper Middle Jurassic) and mobilized during late Miocene up to Pliocene-Pleistocene times controls the extension and direction of petroleum components migration over the entire history of the basin which, in return, has a major impact on potential gas transportation into the gas hydrate stability zone (GHSZ). In the context of gas hydrate formation, presence of extensive salt deposits tends to bend gas migration pathways from vertical (typical for the Gulf of Mexico region) towards rather horizontal and dispersed. However, amalgamation of two or more salt structures often results in re-focusing of the flow towards the local topographic subsalt heights. Together with the formation of local sediment discontinuity structures such as faults developing at the rims and tops of rootless salt deposits related to further stages of allochthonous salt mobilization, new high-permeability migration pathways develop and act as direct connection for the thermogenic gas to the GHSZ. Our study presents the 3D modeling solution quantifying and exploring the gas hydrate accumulation potential in the marine environment experiencing salt tectonics such as the Green Canyon, Gulf of Mexico. This modeling study evaluates the potential of bio- and thermogenic gas hydrate formation within Pliocene-Pleistocene reservoir layers based on full basin re-construction which accounts for depositional and transient thermal history of the basin, source rock maturation, petroleum generation, expulsion and migration, salt tectonics and associated faults development. Based on a numerical study calibrated with the existing field data, we present a new distribution pattern of gas hydrates attributed to both microbial and thermogenic origin. We present here an explanation for a formation mechanism of large gas hydrate amounts (> 70 vol. %) wide-spread at the base of the stability zone as a result of the gas hydrate-free gas recycling process enhanced by very high Neogene sedimentation rates in the region. We suggest that the rapid development of secondary intra-salt mini-basins situated on top of the allochthonous salt deposits and following sediment subsidence caused a consequent dislocation of the GHSZ lower boundary and led to efficient gas hydrate dissociation process followed by a free gas re-charge into the GHSZ

Frontiers of 3D basin-scale modeling of natural gas hydrate systems

Numerical modeling of natural gas hydrate systems requires an innovative and complex approach. The variability of parameters present in natural geological settings and the lack of wide spread high-quality 3D seismic data are the main factors limiting large-scale numerical simulations. Here, we present the outcome of a joint academic-industry project on testing the feasibility of a newly developed simulation-module included in the commercial software PetroMod TM for modeling the formation of natural gas hydrate deposits at two locations in the Gulf of Mexico. The project aimed at the scientific assessment of required input data quality and validity, choice of the computational methods, and calibration with the field data