Hydrate-Bearing Sediments from the Krishna−Godavari Basin: Physical Characterization, Pressure Core Testing, and Scaled Production Monitoring

Abstract

Gas hydrate-bearing sediments recovered by pressure coring from the Krishna-Godavari Basin offshore India during the 2006 National Gas Hydrate Program (NGHP) expedition were characterized using the instrumented pressure testing chamber (IPTC). The IPTC studies provided longitudinal profiles of P-and S-wave velocities, electrical conductivity, and undrained penetration resistance. The formation consisted of fine-grained clayey sediments of high specific surface and high plasticity. X-ray images showed horizontal layering, pronounced heterogeneity from milli-to centimeter scales, with the presence of high-density nodules and both horizontal and sub-vertical gas hydrate lenses. Relative to reference values for sediments without hydrates, regions of high P-and S-wave velocities, low electrical conductivity, and high undrained shear strength were identified and correlated with regions of apparent high hydrate volume fractions in all tested specimens. Physical properties were monitored during the depressurization of pressure cores. The hydrate volume fraction was estimated to be 18.2%. The evolution of P-wave amplitude, electrical conductivity, and temperature showed gas generation and freshening upon phase transformation. The effect of sampling by pressure coring and the ensuing zero-effective stress condition were discussed.

Comparison of the physical and geotechnical properties of gas-hydrate-bearing sediments from offshore India and other gas-hydrate-reservoir systems

“…11F). These values are close to that of seawater (Yun et al, 2010) reflecting the high porosity of the sediment. Measurements typically could not be completed below the SMI because of the presence of gas in the sediment.…”

“…The formation of these extremely complex rotational hydrate features that appear to be "twisted" and induced by core rotation during the drilling process, actually may reflect natural conditions because some recovered cores with these rotational hydrates were obtained as push cores, unaffected by drill-bit rotation (M. Holland, personal communication, 2008). However, sampling effects, including effective stress reduction, fracture formation, and hydrate reformation, may be responsible for some coring-related artifacts (Yun et al, 2010). Hydraulic fracturing caused by the advection of pore fluids may have caused some hydrate to form at Site NGHP-01-10 in the KG Basin (Rees et al, 2011).…”

Challenges, Uncertainties, and Issues Facing Gas Production From Gas-Hydrate Deposits

“…However, some potential is afforded by the volume change that would be expected to be associated with gas hydrate dissociation. Such internal stresses and strains generated by dissociation could enhance formation permeability locally, providing a means for both the further transmission of pressure reductions as well as the flow of gas to the wellbore (Yun et al, 2010a). Nonetheless, whereas it now appears that gas hydrate production from sand reservoirs is conceivable using largely existing processes; it is clear that much more needs to be known, and perhaps fundamentally new approaches developed, to further the prospects of production from elements lower in the gas hydrates resource pyramid.…”

“…Pressure coring has been used on a number of oceanic expeditions (Schultheiss et al, 2009) stabilizing the core pressure at the pressure the core was collected. Sampling and test equipment, such as the instrumented pressure testing chamber shown in Figure 7a (Yun et al, 2006;Yun et al, 2010b) has been built and instrumented to make measurements on the obtained pressure core without removing the pore pressure. This device can be modified to allow for other measurements to be made, however not all required measurements can be made using this apparatus.…”

Submarine Slope Failure Primed and Triggered by Bottom Water Warming in Oceanic Hydrate-Bearing Deposits

“…The excess pore pressure reduces the effective stress of sediments and the shear strength, which results in shear failure. Because a number of oceanic gas hydrate deposits are classified as low-permeability fine-grained sediments, e.g., Ulleung basin sediments as reported in Kwon et al [16], hydrate deposits in the Gulf of Mexico as reported in Francisca et al [17], and Krishna-Godavari basin in India as reported in Yun et al [18], the generation of excess pore pressure due to hydrate dissociation is known to be the most relevant process responsible for destabilizing hydrate-bearing sediments [10][11][12][13][14][15]. For example, a 1 °C increase in the temperature of hydrate-bearing sediments results in the pore fluid pressure increasing by the order of several megapascals under the no mass flux condition [15,19,20].…”