Impact of extremely low porosity on geothermal gradient and fluid migration in gas hydrate-bearing layers: A case study of South Shetland Islands, Antarctic Peninsula

Numerical investigation of fracturing fluid invasion into hydrate reservoirs during hydraulic-fracturing stimulation

During gas hydrate production, the upper parts of the gas hydrate-bearing layer (GHBL) behave as a seal. If parts of the upper gas hydrates are dissociated, free methane gas present in the lower parts of GHBL may leak into the upper parts of the layer. Therefore, the dissociation of methane gas in the GHBL must be monitored to ensure strong stability during gas production. To investigate the feasibility of monitoring the dissociation of gas hydrates with a seismic survey, we established a rock physics model incorporating the geological characteristics of the region around the UBGH 2–6 well in the Ulleung Basin based on LWD data and core analysis data. We performed fluid substitution modeling to build a velocity model with the dissociated parts of the gas hydrates, and generated synthetic data. Diffractions from the edges of the dissociated parts allowed detection of gas hydrate dissociations, and the dissociated locations could be identified in the migrated image. In addition, sensitivity testing showed that the dissociated part was recognizable in the migrated image, even at a width of half the wavelength. Therefore, we conclude that the stability of gas hydrates during production can be monitored using a time-lapse seismic survey.

Abstract: Geothermal gradients and present day heat flow values were evaluated for about seventy one wells in parts of the eastern Niger delta, using reservoir and corrected bottom – hole temperatures data and other data collected from the wells. The results showed that the geothermal gradients in the shallow/continental sections in the Niger delta vary between 10 - 18° C/km onshore, increasing to about 24° C/km seawards, southwards and eastwards. In the deeper (marine/paralic) section, geothermal gradients vary between 18 - 45° C/km. Heat flow values computed using Petromod 1–D modeling software and calibrated against corrected BHT and reservoir temperatures suggests that heat flow variations in this part of the Niger delta range from 29 – 55 mW/m2 (0.69 – 1.31 HFU) with an average value of 42.5 mW/m2 (1.00 HFU). Heat flow variations in the eastern Niger delta correspond closely to variations in geothermal gradients. Geothermal gradients increase eastwards, northwards and seawards from the coastal swamp. Vertically, thermal gradients in the Niger delta show a continuous and non-linear relationship with depth, increasing with diminishing sand percentages. As sand percentages decrease eastwards and seawards, thermal gradient increases. Lower heat flow values (< 40 mW/m2) occur in the western and north central parts of the study area. Higher heat flow values (40 - 55 mW/m2) occur in the eastern and northwestern parts of the study area. A significant regional trend of eastward increase in heat flow is observed in the area. Other regional heat flow trends includes; an eastwards and westwards increase in heat flow from the central parts of the central swamp and an increase in heat flow from the western parts of the coastal swamp to the shallow offshore. Vertical and lateral variations in thermal gradients and heat flow values in parts of the eastern Niger delta are influenced by certain mechanisms and geological factors which include lithological variations, variations in basement heat flow, temporal changes in thermal gradients and heat flow, related to thicker sedmentary sequence, prior to erosion and evidenced by unconformities, fluid redistribution by migration of fluids and different scales of fluid migration in the sub-surface and overpressures.

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.

Gas hydrates with high saturation in clayey silt sediments have been identified from several gas hydrate drilling expeditions in the South China Sea (SCS). However, the spatial distribution is not well delineated due to the limitation of two dimensional or pseudo-three dimensional seismic data. The three dimensional (3D) seismic data acquired by oil & gas exploration was reprocessed using high-density velocity analysis, which can be used to show the detailed mapping and characterization of high concentration gas hydrate in the LW3 area, SCS. The well log data and core samples have indicated that pore-filling gas hydrates have various saturations and thickness in the study area, but the spatial variations are still poorly defined. High-resolution broadband inversion without well constraint was conducted based on the high-resolution velocity analysis, which dramatically increased the accuracy in detecting thin gas hydrate-bearing layers. Rock physical models were involved to quantitatively predict gas hydrate saturation, which has been used to describe the gas hydrate-bearing layers with a thickness ranging from 5 to 6 m. Multi-attribute analysis extracted along the base of gas hydrate stability zone (BGHSZ) was used to predict the gas hydrate distribution. Gas hydrate distribution identified from the reprocessed high-resolution 3D seismic data indicate that gas hydrate-bearing layers are mainly located at four ridges of the migrating canyons in the Liwan 3 area. The thickness and saturation of gas hydrate distribution at the third and fourth ridges are better than the other two ridges. The gas hydrate saturation is over 40 % with a thickness exceeding to 70 m. Moreover, the thin gas hydrate layer is interbedded with thick layer at the forth ridge. However, the gas hydrate saturation is over 70 % with a thickness of 30 m at the third ridge. The gas hydrate distributions exhibit obvious heterogeneous at each canyon zone. The accuracy prediction and delineation of gas hydrate distribution are very important for assessing gas hydrate test production responses and scenarios.

Increasing the accuracy of estimated porosity and saturation for gas hydrate reservoir by integrating geostatistical inversion and lithofacies constraints

Three MT soundings were carried out on the James Ross island, next to the Antarctic Peninsula (Trinity Peninsula) during the Antarctic Summer campaign 1991–1992. The region is at about 150 km in the SE direction from a rift (Bransfield strait). The sounding sites are almost free of glacier cover. The James Ross island had volcanic origin during the Pliocene period. The equipment used for these soundings was battery powered, using Cu-CuSO4 electrodes, induction coils and a Flux-gate magnetometer. The results of the MT soundings show that the region of Seymour and James Ross islands is affected by the above mentioned rift. From Seymour island, the permafrost becomes thinner and the sub-permafrost has more conductance, while the James Ross island and the Antarctic Peninsula is approached, suggesting a possible increase of geothermic gradient and heat flow from Seymour to James Ross island. MT soundings also suggest the presence of an intercalated conductive layer at 55–60 km depth, probably in the Upper Mantle, cosindering the possible lithospheric attenuation produced by the rift. This conductive layer could be the asthenosphere. However, the scarce information obtained over 1000 sec of period do not permit to assure its presence.

Quantitative seismic characterization for gas hydrate- and free gas-bearing sediments in the Shenhu area, South China sea

In the Krishna-Godavari (K-G) offshore basin, India, a 130 m thick fracture-filling and near-seafloor paleo-cold seep-related gas hydrate-bearing layer (GHBL) was encountered by drilling at Site NGHP-01-10 (Site 10) and nearby piston sampling of authigenic carbonates and shells. Our analyses of drilling cores and pore-water show that authigenic carbonates and shells are widely distributed within 200 mbsf at Site 10, with two separate intervals of high chloride concentrations up to 663 mM. This indicates that the GHBL is a young system of multistage formation related to periodically active cold seeps. This study combines core, well logging and seismic data to gain insight into the fine characteristics and detailed formation process of such a thick system. Seismic imaging of new chimney-like structures, growth faults and multiple stacked mass transport deposits (MTDs) illustrates that the system is located in the chaotic reflection strata. Synthetic seismogram shows that multiple MTDs repeatedly control the paleo-cold seeps and further influence the hydrae system. Based on a buried vent with a high amplitude reflection consistent with seafloor polarity, and its high density and high velocity similar to authigenic carbonates, a new and larger paleo-cold seep-related hydrate system is defined to the southeast of Site 10. These two thick systems probably formed in stages due to the clear stratifications on the seismic data, 2D anisotropic saturations and internal chimney-like structures. They are originated from diapirism and growth faulting, and their lateral extent depends on the fracture zone width of the anticline ridge. After formation, they are then buried by multiple MTDs and have already been upshifted by sedimentation. Although the cold seep near Site 10 is not active and the hydrate system is currently only in the chloride diffusion stage, the underlying gas accumulation means that new hydrate systems and cold seeps may form in the future. Our results suggest that the processes of formation, sedimentation, upward shift and diffusion of hydrate systems have been circulating near Site 10, which could better interpret the formation and dynamic evolution of the multilayered or thick GHBL found at drill sites around the world.

The diet of non-breeding male Antarctic fur seals Arctocephalus gazella was investigated at different localities of the Antarctic Peninsula (Cierva Point and Hope Bay), South Shetland Islands (Deception Island and Potter Peninsula) and the South Orkney Islands (Laurie Island), by the analysis of 438 scats collected from January to March 2000. The composition of the diet was diverse, with both pelagic and benthic-demersal prey represented in the samples. Antarctic krill Euphausia superba was the most frequent and numerous prey at all the study sites except at Cierva Point, followed by fish, penguins and cephalopods. Antarctic krill also predominated by mass, followed by either fish or penguins. Fish were the second most important prey by mass at the Antarctic Peninsula whereas penguins were the second most important prey by mass at the South Shetland and South Orkney Islands. Among fish, Pleuragramma antarcticum was the most important species in the diet of the Antarctic fur seals at the Antarctic Peninsula whereas Gymnoscopelus nicholsi predominated at the South Shetland and South Orkney Islands. The results are compared with previous studies, and the possibility of implementing monitoring studies on the distribution/abundance of myctophids and P. antarcticum based on the analysis of the diet of the Antarctic fur seal is considered.

Fil: Blasina, Gabriela Elizabeth. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Instituto Argentino de Oceanografia. Universidad Nacional del Sur. Instituto Argentino de Oceanografia; Argentina. Universidad Nacional del Sur. Departamento de Biologia, Bioquimica y Farmacia; Argentina

In the Antarctic Peninsula, a marine gas hydrate system has been identified based on geophysical data (Lodolo et al., 1993; Tinivella et al., 2002). These data suggest gas hydrates average volume concentration of 6.0 ± 1.2% for in the accretionary wedge of the South Shetlands Islands (Tinivella, 2002). Based on legacy seismic profiles (belonging to 17 oceanographic cruises) retrieved from the Antarctic Seismic Data Library System (SDLS), a continuous Bottom Simulating Reflector (BSR) has been mapped in the accretionary wedge, between Elephant and King George islands. This BSR is located at a sub-bottom depth between ca. 250 ms TWTT in the upper slope and ca. 1s TWTT at the base of the accretionary wedge. The theoretical Base of Gas Hydrate Stability Zone (BGHSZ) calculated with a static model (León et al., 2009) for the present oceanographic conditions (pressure/bathymetry, seafloor temperature, geothermal gradient and salinity) is located 100 to 400 m shallower than this BSR level, considering available geothermal data for the area. The BSR-BGHSZ mismatch points that gas hydrates in the area seem to be in a transient state with respect to their theoretical location calculated from both pure methane and thermogenic compositions. Dynamic models developed with TOUGH+HYDRATE in the frame of ICEFLAME project (PID2020-114856RB-I00, Spanish Ministry of Science and Innovation), reveal two possible scenarios for the above mismatch between BSR and BSGHZ: isostatic rebound and/or tectonic uplift.

The Antarctic Peninsula, one of several terrains of Western Antarctica, is a Mesozoic magmatic arc at the southeastern Pacific margin. In order to investigate the structure of the crust and uppermost mantle of Antarctic Peninsula continental margin we have developed joint geophysical models by 2D gravity and magnetic modelling along two most representative and lengthy seismic refraction lines, acquired by Polish Academy of Sciences in 1980-1990th.

Lithospheric structure of an incipient rift basin: Results from receiver function analysis of Bransfield Strait, NW Antarctic Peninsula

Hydrogeochemistry of high-temperature geothermal systems in China: A review